New Directions in the Dynamic Assessment of Brain Blood Flow Regulation

with progressive exercise to intensities above the ventilator threshold was only evident in the anterior circulation. Their data indicate, and support previous suggestions, that the poster and anterior cerebral circulations possess disparate blood flow regulation and CO Written by international experts, this publication provides the reader with the present knowledge and future research directions of diagnostic and therapeutic ultrasound and spectroscopy. Focused topics include Duplex ultrasound, transcranial color Duplex, MRA guided Doppler ultrasonography and near-infrared spectroscopy. New directions in the use and application of transcranial and color Duplex ultrasound are provided, as well as the use of ultrasound and arterial stiffness for measuring human vascular health and circulatory control. Novel use of ultrasound for the detection of intra-cardiac and intra-pulmonary shunts is also described along with its utility for the assessment of gastric regulation and emptying.


Introduction
The principal aim of this book chapter is to provide an overview of the utilities of transcranial Doppler ultrasound (TCD), and high resolution vascular ultrasound for the assessment of human cerebrovascular function with respect to other common measurement tools. Specifically, we aim to: (1) examine the advantages and disadvantages of TCD in the context of other imaging metrics; (2) highlight the optimum approaches for insonation of the basal intra-cerebral arteries; (3) provide a detailed summary of the utility of TCD for assessing cerebrovascular reactivity, autoregulation and neurovascular coupling and the clinical application of these measures; (4) give detailed guidelines for the appropriate use and caveats of neck artery flow measures for the assessment of regional cerebral blood flow distribution; and (5) provide recommendations on the integrative assessment of cerebrovascular function. Finally, we provide an overview of new directions for the optimization of TCD and vascular ultrasound. Future research directions -both physiological and methodological -are outlined.

Background
Maintenance of adequate cerebral blood flow (CBF) is necessary for normal brain function and survival. That the brain receives ~15% of total cardiac output and is responsible for ~20% of the body's oxygen consumption, despite being 2-3% of total body weight, is testament to its high energetic cost. This, combined with a very limited ability to store energy (the brain's total energy pool would theoretically allow it to function for ~12 minutes were energy substrate supply abolished) requires effective regulation of blood supply. Numerous pathologies such as head trauma, carotid artery disease, subarachnoid haemorrhage and stroke result in disturbances to the regulatory mechanisms controlling CBF (Hossmann, 1994;Panerai, 2009). However, the skull makes it difficult to measure parameters such as blood flow and blood velocity. Many approaches such as radioopaque tracers, radioactive markers and similar methods are inadequate because of poor temporal resolution (see See Table (appendix) for a summary of the advantages and disadvantages of other methods). Key factors that determine adequate CBF for maintenance of cerebral oxygen delivery are: (1) sensitivity to changes in arterial PO 2 and PCO 2 (cerebrovascular reactivity) and the unique ability to extract a large amount of www.intechopen.com Applied Aspects of Ultrasonography in Humans 2 available oxygen; (2) effective cerebral autoregulation (CA) that assists maintenance of CBF over a wide range of perfusion pressures, helping to prevent over/under perfusion and consequent risk of hemorrhage or ischemia; and, (3) matching of local flow to localized metabolic needs (neurovascular coupling; NVC). The high temporal resolution and non-invasive nature of transcranial Doppler ultrasound (TCD) make it a useful tool in the assessment of integrative cerebrovascular function in terms of cerebral reactivity, autoregulation, and NVC. New technologies are further increasing the utility of TCD. For example, combining TCD with microbubble contrasting agents allow for quantification of local changes in perfusion for measuring absolute volumetric flow (Powers et al., 2009). However, the interaction of ultrasound with microbubble contrast agents is complex and beyond the scope of this review; the reader is referred to (Powers et al., 2009) for a detailed review of the current state of contrast TCD technology. With or without contrast, a TCD machine is relatively inexpensive ($20,000 to $50,000 USD); moreover, TCD is easy to use and it is safe in healthy and disease states alike. For these reasons TCD is practical in the clinical setting, where it is used to assess a variety of different cerebrovascular pathologies.
The principal aim of this chapter is to summarize the utilities of TCD in the assessment of cerebrovascular function with respect to other common measurement tools. Specifically, we aim to: (1) examine the advantages and disadvantages of TCD in the context of other imaging metrics; (2) highlight the optimum approaches for insonation of the basal intracerebral arteries; (3) provide a detailed summary of the utility of TCD for assessing cerebrovascular reactivity, autoregulation and neurovascular coupling and the clinical application of these measures; and (4) provide recommendations on the integrative assessment of cerebrovascular function and avenues for future research. Kety and Schmidt (1945) were the first to quantify CBF using an inert tracer (e.g., nitrous oxide, N 2 O). The reference method for the measurement of global CBF, the Kety-Schmidt method is based on the Fick principle, whereby the arterio-venous difference of an inert tracer is proportional to the volume of blood flow through the brain (Kety & Schmidt, 1948). The tracer is infused until tension equilibrium is attained (the saturation phase) and then terminated, after which the concentration falls toward zero (the desaturation phase). Simultaneous arterio-jugular venous samples are withdrawn during either phase and CBF calculated by the Kety-Schmidt equation: Theoretically, this principle can be exploited using any freely diffusible tracer; indeed N 2 O, 133 Xe, hydrogen, and iodoantipyrine have all been utilized (Edvinsson & Krause, 2002). While this method did stimulate seminal research in cerebrovascular physiology (Kety, 1999), there are several important limitations: measurements are taken over the course of minutes, making it impossible to assess dynamic changes in CBF; only a global, but not regional, measure of CBF, cerebral metabolic rate, or blood-brain substrate exchange is possible; internal jugular and peripheral arterial lines are necessary making it quite invasive; and, finally, the value of cerebral oxygen consumption must be assumed (Kety & Schmidt, 1948). Furthermore, venous outflow from the brain may not be symmetrical, with 50% of individuals exhibiting cortical drainage of venous blood mainly through the right internal jugular vein, and subcortical largely through the left. Two decades later, the measurement of cerebral oxygen consumption was improved using radioactive inert gases 85 Kr (Lassen et al., 1963)] and 133 Xe (Harper & Glass, 1965) that allow extracranial imaging of gamma emission from the cerebral cortex. However, the aforementioned temporal resolution combined with potential problems of extra-cranial tissue sampling and inadequate desaturation are limitations that remain with these approaches. The use of Doppler ultrasound was first described as early as 1959 for assessing blood velocity in the extracranial vessels (Miyazaki & Kato, 1965). The thickness of the skull bones greatly attenuates the penetration of ultrasonic waves making noninvasive use of the technique difficult. Ultrasound was therefore limited to surgical procedures, or to use in children with open fontanels. However, Aaslid et al (1982) demonstrated that the attenuation of sound by bone within the frequency range of 1-2MHz was far less than conventional frequencies of 3-12MHz. Indeed, insonation is possible through thinner regions of the skull, termed "acoustic" windows, making it feasible to measure static and dynamic blood velocities within the major cerebral arteries. For the first time, a non-invasive measure of beat-to-beat changes in blood velocity in the vessels of the brain with superior temporal resolution than indicator-dilution techniques was available. However, it is imperative to note that TCD cannot measure CBF per se. Rather, TCD measures the velocity of red blood cells within the insonated vessel. Moreover, only the larger basal arteries provide an adequate signal for measurement of cerebral blood velocity with TCD. Because these arteries tend to deliver oxygenated blood to large regional areas of the brain, TCD gives an index of global, rather than local, stimulus-response. This is an important distinction given that local changes in CBF likely differ (Hendrikse et al., 2004;Nöth et al., 2008;Piechnik et al., 2008). Certainly, there is a notable dissimilarity in vasoreactivity across the cerebrovasculature. At least in hypercapnia, small vessels and capillaries possess much higher reactivity to CO 2 (Mandell et al., 2008), and vasculature residing within gray matter show higher reactivity than vasculature within white matter (Nöth et al., 2008;Piechnik et al., 2008). This lack of spatial resolution in brain hemodynamics is the principal limitation of TCD. There are a variety of modern imaging techniques that allow sufficient spatial resolution to discern localized brain perfusion [see See Table (appendix) for a summary and (Wintermark et al., 2005) for a detailed review].

Recording principles
The principles of TCD are the same as extracranial Doppler ultrasound: the Doppler probe emits sound waves that are reflected off moving red blood cells, which are subsequently detected by the transducer. The resultant Doppler-shift is proportional to the velocity of the blood (DeWitt & Wechsler, 1988;Aaslid, 1986a). Duplex ultrasound (simultaneous two dimensional B-mode and pulse-wave velocity) typically used in vascular ultrasound to measure both vessel luminal diameter and blood velocity (and therefore volumetric flow; see Section VI) is not possible with current TCD systems due to lack of resolution. Because the diameter of the insonated vessel is unknown, TCD only measures cerebral blood velocity (CBV) not absolute volumetric flow. The velocity of blood through a vessel is proportional to the fourth power of vessel radius; the measurement of CBV by TCD assumes constant diameter of the insonated vessel -this assumption has been found to be valid in various studies (Serrador et al., 2000;Bishop et al., 1986;Nuttall et al., 1996;Peebles et al., 2008;ter Minassian et al., 1998;Valdueza et al., 1997). Despite these validations, however, it remains possible that the cerebral conduit vessels do, in fact, change diameter, and as such, any TCD data should be openly interpreted with this possibility in mind. In addition, other problems remain for meaningful velocity quantification. For example, the velocity of blood through a vessel -in the presence of laminar flow -is approximately parabolic in shape, www.intechopen.com New Directions in the Dynamic Assessment of Brain Blood Flow Regulation 5 with the fastest velocity in the center of the vessel. The Doppler signal consequently represents not a single value but rather a distribution of velocities, therefore requiring mathematical manipulation to extract meaningful velocity values. Typically, a power spectrum distribution is produced from segments of ~5 seconds using a Fast Fourier transform, and maximum or mean velocity is calculated from the maximum or intensity weighted mean, respectively (see Figure 2; Lohmann et al., 2006;Aaslid, 1986b).  Aaslid et al. J Neurosurg (1982) vol. 57 (6).
A frequency of 2MHz is typically used for TCD because higher frequencies do not sufficiently penetrate the bones of the skull (DeWitt & Wechsler, 1988;Aaslid et al., 1982). Despite the increased penetration of TCD, an acoustic window in the skull is necessary for adequate insonation of intracerebral arteries. The choice of window, however, can dramatically affect the type of recording possible. For example, insonation through the transtemporal or foramen magnum windows allow use of a headpiece for securing the Doppler probe, whereas this is not possible when insonating through the optic canal.
Imaging modalities such as colour-coded Doppler and power Doppler, significantly increase the reliability of TCD as direct visualization of the target vessel facilitates better insonation angle correction (Martin et al., 1995). Studies have demonstrated that the use of colour-coded and/or power TCD facilitates: (1) an improved signal-to-noise ratio with transcranial insonation (Postert et al., 1997); (2) insonation in the presence of poor acoustic windows, particularly when combined with the use of contrast (Nabavi et al., 1999;Gerriets et al., 2000); (3) the measurement of arterial diameter threshold for collateral flow in the circle of Willis (Hoksbergen et al., 2000); and, (4) an increase in the diagnostic sensitivity for cerebral vasospasm (Sloan et al., 2004). Readers are referred to Willie et al., (2011b) for a detailed review of general TCD principles. www.intechopen.com

Regulation by arterial PCO 2
The cerebral vasculature is highly sensitive to changes in arterial blood gas pressures, in particular PaCO 2 , which exerts a pronounced effect on CBF. Alveolar ventilation, by virtue of its direct effect on PaCO 2 , is consequently tightly coupled to CBF. The response of CBF to PaCO 2 is of vital homeostatic importance as it directly influences central CO 2 /pH, which is the central chemoreceptor stimulus (Chapman et al., 1979). In response to increases in PaCO 2 , vasodilation of downstream arterioles increases CBF. This, in turn, lowers PaCO 2 by increasing tissue washout, resulting in vessel constriction, and a subsequent decrease in CBF. Functional modulation of CBF by PaCO 2 influences pH at the level of the central chemoreceptors and directly implicates CBF in the central drive to breath (reviewed by Ainslie & Duffin, 2009; also see (Fan et al., 2010a;Fan et al., 2010b;Lucas et al., 2010b). Indeed, previous studies have found a correlative link between blunted cerebrovascular CO 2 reactivity and the occurrence of central sleep apnea in patients with congestive heart failure (Xie et al., 2005), and also in the pathophysiology of obstructive sleep apnea (Burgess et al., 2010;Reichmuth et al., 2009).

Regulation by arterial PO 2
Hypoxia is a cerebral vasodilator as reflected by a proportional increase in CBF with decreasing PaO 2 in conditions of isocapnia (Reviewed in (Ainslie & Ogoh, 2010). However, the resultant hyperventilation that accompanies hypoxic exposure yields hypocapnia that induces a counteracting cerebral vasoconstriction and decreased CBF. Indeed, a threshold of <40mmHg PO 2 is required in the face of prevailing hypocapnia for cerebral vasodilation to occur (Ainslie & Ogoh, 2010). This minor sensitivity is clinically relevant given the arterial hypoxemia encountered during exercise in elite athletes (Ogoh & Ainslie, 2009a;Ogoh & Ainslie, 2009b) and at high altitude (Ainslie & Ogoh, 2010) as well as in certain pathologies such as chronic lung disease and heart failure (reviewed in Ainslie & Ogoh, 2010 (Galvin et al., 2010). The extent to which changes in cerebrovascular reactivity to hypoxia are related to pathological outcome is unknown. Moreover, unlike the CO 2 reactivity test, complex feedback (Kolb et al., 2004;Ito et al., 2008;Robbins et al., 1982) or feed-forward (Slessarev et al., 2007) gas manipulation techniques are needed to independently control PaCO 2 and PaO 2 .

Neuronal metabolism and coupling
The effect of neural activity on CBF was demonstrated approximately 130 years ago in patients with skull defects (Mosso, 1880). Neurovascular coupling can be utilized as a sensitive method to test the function of cerebral vasculature. This activation-flow coupling describes a mechanism that adapts local CBF in accordance with the underlying neuronal activity (Girouard & Iadecola, 2006). The adaptation of regional CBF is based on local vasodilation evoked by neuronal activation, but the cellular mechanisms underlying neurovascular coupling are not fully understood. Synaptic activity has been shown to trigger an increase in the intracellular calcium concentration of adjacent astrocytes, which can lead to secretion of vasodilatory substances -such as epoxyeicosatrienoic acid, adenosine, nitric oxide, and cyclooxygenase-2 metabolites -from perivascular end-feet, resulting in increased local CBF (reviewed by Jakovcevic & Harder, 2007). Thus, astrocytes, via release of vasoactive molecules, may mediate the neuron-astrocyte-endothelial signaling pathway and play a profound role in coupling blood flow to neuronal activity (reviewed by (Iadecola & Nedergaard, 2007). Indeed, the 10-20% increase in CBF observed during aerobic exercise is likely due to combined elevations in cortical neuronal activity in addition to elevations in mean arterial pressure (MAP) and PaCO 2 (reviewed by (Ogoh & Ainslie, 2009a). Yet, even during exercise neurometabolic coupling with visual stimulation remains intact (Willie et al., 2011a). The relative contribution of neurometabolic factors (i.e., neurovascular coupling) and systemic factors (i.e., increased PaCO 2 and blood pressure) is currently unknown.

Cerebral autoregulation
CBF is traditionally thought to remain relatively constant within a large range of blood pressures (60 to 150 mm Hg), at least in non-pathological situations (Lassen, 1959). This unique characteristic of the mammalian brain is known as cerebral autoregulation (CA). If CA fails, the brain is at risk of ischemic damage at low blood pressures or hemorrhage at high blood pressure. Preceding the advent of technologies with temporal resolution capable of measuring changes in CBF over the course of seconds, CA was measured under steadystate conditions (See Section 2.1). This "static" CA is a measure of cerebrovascular regulation of gradual changes in perfusion pressure. Traditionally, static CA was believed to hold CBF constant through a MAP range of ~60-150 mmHg (Lassen, 1959), but this concept has been recently challenged with evidence to support CBF closely paralleling changes in blood pressure . Furthermore, static CA through this supposed autoregulatory range is difficult to assess because the upper range of blood pressures can only be achieved in healthy individuals using relatively high-dose pharmacological intervention. Indeed, we and others, have found that induction of hypertension using continuous phenylephrine infusion produces cardiotoxic effects on ECG, limiting MAP increases to <120 mmHg. Moreover, pharmacologically induced changes in BP also cause marked changes in ventilation; thus, the confounding influence of arterial PCO 2 needs to be considered. The extent to which a normal healthy human can cope with extreme static changes in BP is unknown for obvious reasons. Regardless, even within the "autoregulatory range", static autoregulation appears to be influenced by blood pressure (Immink et al., 2008;Lucas et al., 2010a).
Dynamic CA (dCA) describes the ability of the cerebral vasculature to resist acute changes in perfusion pressure, (due to changes in arterial blood pressure) over a short time-course of less than five seconds (Zhang et al., 1998). dCA can be quantified from either spontaneous fluctuations in MAP, or from stimulus-induced changes in MAP. With increased utilization of TCD a number of methods of MAP perturbation and dCA quantification have been developed.
That CBF appears to vary directly with MAP, suggests that arterial baroreflex regulation of peripheral blood pressure likely plays a more significant role in CBF control than previously thought . Moreover, Tzeng et al (2010a) showed an inverse relationship between cardiac baroreflex sensitivity and dCA, suggesting the presence of compensatory interactions between peripheral blood pressure and central CBF control mechanisms, directed to optimizing CBF control. Such interactions may account for the divergent changes in CA and baroreflex sensitivity seen with normal aging, and in clinical conditions such as spontaneous hypertension , autonomic failure (Hetzel et al., 2003), and chronic hypotension (Duschek et al., 2009). Some evidence also suggests that free radicalsparticularly superoxide anion observed in pathologies such as ischemic and traumatic brain injury -causes impaired CA and increased basal CBF through activation of potassium channels within vascular smooth muscle cells (Zagorac et al., 2005).

Assessment of cerebrovascular function
In this section we provide a practical overview of methods used to assess cerebral autoregulation (CA) including the use of suprasystolic thigh cuff, postural alterations, lower body negative or oscillatory pressure, the Valsalva maneuver, the Oxford technique and transfer function analysis.

Suprasystolic thigh cuffs
The rapid release of thigh cuffs inflated to suprasystolic pressures for ≥2 minutes elicits a transient hypotension (Aaslid et al., 1989;Mahony et al., 2000;Tiecks et al., 1995a). The rate of regulation (RoR), quantifies the rate at which cerebrovascular resistance (CVR), or conductance changes in response to a perturbation in MAP and can be given by Equation 1: where CVR is given by MCAv mean /MAP, and MAP by control MAP-MAP mean . t is taken as the 2.5 second period one second following thigh cuff release ( Figure 3). This time interval was originally put forth by Aaslid et al (Aaslid et al., 1989) based on two factors: (1) the change in CVR is relatively linear during this period, allowing a slope of the response to be taken; and, (2) it was thought that the latency of the baroreflex response was such that within the first 3.5 seconds of the cerebrovascular response to a hypotensive www.intechopen.com challenge, only cerebrovascular mechanisms (i.e., dCA) would be involved in regulation of CBF. However, the drop in arterial pressure following thigh cuff deflation engages the arterial baroreflex within 0.44 seconds of baroreceptor unloading by neck pressure (Eckberg, 1980) causing transient tachycardia. Although unilateral thigh cuff deflation was reported to not alter central venous pressure (Fadel et al., 2001), unpublished observations from our laboratory indicate this may not be the case for bilateral thigh-cuff release. It is unclear, consequently, how cardiac output is affected following bilateral thigh-cuff release. Some authors (Ogoh et al., 2003;Ogoh et al., 2007), but not all (Deegan et al., 2010), have reported that increases in cardiac output can augment CBF; thus, it is plausible that baroreflex function may exert a modulating influence on dynamic CBF regulation such that RoR reflects the integrated response of both the baroreflex and dCA (Ogoh et al., 2009). The thigh cuff technique is also somewhat painful, with inflation often associated with an increase in MAP that is maintained until cuff release. The influence of sympathetic nervous system activity in response to discomfort is not known, but a minimum 8-minute recovery period has been recommended following cuff deflation (Mahony et al., 2000).
Another prevalent method to quantify the dCA response to thigh cuff release, termed the autoregulatory index, was proffered by Tiecks et al (1995a). This approach uses a second order differential equation to relate changes in MAP and three predefined model parameters (T: time constant; D: damping factor; and k: autoregulatory gain) to generate ten templates of CBV-response to a non-pharmacologically induced transient hypotension ( Figure 4). Typically, rapid thigh-cuff deflation is used to induce a transient hypotension. According to the Tiecks model, the autoregulatory index assigns an integer value to each of ten template curves (0-9). These coefficients are generated using a second-order linear differential equation: where dP n is the normalized change in mean arterial blood pressure (MAP) relative to baseline MAP (MAP base ) and adjusted for estimated critical closing pressure (CCP); x2 n and x1 n are state variables (equal to 0 at baseline); mV n is modeled mean velocity; MCAv base is baseline MCAv mean ; f is the sampling frequency, and n is the sample number. The mV n generated from ten predefined combinations of parameters T (time constant), D (dampening factor) and k (autoregulatory gain) that best fit the actual MCAv mean recording is taken as an index of dynamic CA. A value of 0 represents no autoregulation where CBV passively follows perfusion pressure, and a value of 9 represents perfect CA where changes in perfusion pressure produce no alteration to CBV. The autoregulatory index has also been derived from spontaneously occurring blood pressure and cerebral blood vessel velocity fluctuations using transfer function analysis (Panerai et al., 1999a;Panerai et al., 2001). However, the validity of comparison between a linear model and that from a transient hypotensive stimulus may be questionable (see Transfer function analysis below).

Postural alterations
Because of the confounders inherent to both pharmacological and non-physiological methods of BP alteration (e.g., thigh-cuff release) postural maneuvers to alter blood pressure have been utilized for CA assessment. The simple act of standing from a sitting, supine, or squat position is enough to elicit a transient drop in BP of ~35 mmHg and associated drop in CBV Murrell et al., 2009;Murrell et al., 2007). RoR , ARI, and transfer function analysis (Claassen et al., 2009) have all been used to quantify the dCA response to postural changes in BP.

Valsalva maneuver
Forced expiration against a closed glottis regularly occurs during normal daily activities such as during defecation and lifting. The Valsalva maneuver has been well described in the literature (Smith et al., 1996;Tiecks et al., 1996) and consists of four phases: (1) increased MAP due to increased intrathoracic pressure; (2a) impaired atrial filling and resultant drop in MAP, followed by; (2b) a baroreceptor mediated tachycardia and increase in MAP; (3) release of strain and drop in intrathoracic pressure which decreases MAP; and (4) baroreceptor mediated sympathetic activity that drives MAP above baseline in the face of transient hypotension. Due to impaired atrial filling, combined with raised intracranial pressure induced by increased intrathoracic pressure during strain, there is a marked decrease in cerebral perfusion pressure during the onset of phase 2a. This drop in perfusion pressure provides an adequate stimulus for measurement of dCA (Tiecks et al., 1996;Tiecks et al., 1995b;Zhang et al., 2002). The Valsalva maneuver may, however, be confounded by its inherent physiological complexity. Changes in PaCO 2 are likely to occur over the course of the breath-hold, and although it has been suggested that there is sufficient time delay between changes in end-tidal PCO 2 (PetCO 2 ) and subsequent changes in CBF to preclude the breath-hold from effecting measured values of CA, this is not known for certain (Hetzel et al., 2003). Furthermore, changes in intrathoracic pressure likely vary between individuals, and throughout the maneuver there are changes in intracranial pressure, venous outflow pressure and resistance. And though there is a period of relatively stable intracranial pressure, the possibility of these changes confounding measures of CA certainly exist. Nonetheless, Tiecks et al (1996) described the Valsalva ARI as the ratio between the relative changes in cerebral blood flow velocities and blood pressure, calculated as:

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ARI Valsalva = ((CBFV(iv)/CBFV(i)/(BP(iv)/BP(i)) where I and IV signify phases one and four of the Valsalva response, respectively. CBF is modified proportionally more than BP, implying that autoregulation is preserved, if the ratio is found to be >1. Because minor variations in expiratory pressure results in changes in the various stages of the maneuver, it is imperative to standardize the technique between and within subjects.

Transfer function analysis
Transfer function analysis (TFA) for the assessment of dynamic CA is based on analysis of the coherence, frequency and phase components of spontaneous changes in MAP, and the resultant degree to which these changes are reflected in CBV (Zhang et al., 1998). It is thought that CA acts as a high pass-filter, effectively dampening low-frequency oscillations (<0.07Hz) (Panerai et al., 1998;Zhang et al., 1998). An advantage to this method is the ability to complete the measurement in a subject at baseline without the need for any pharmacological or physiological manipulation of BP. However, the corollary is that TFA cannot be used to analyze the CA response to a transient and directional change in BP (i.e., it cannot distinguish between "upward" and "downward" fluctuations in BP). Furthermore, TFA assumes that the dynamic autoregulatory responses to spontaneous fluctuations in BP are linear. This is to say that TFA assumes CA is equally effective in attenuating changes in cerebral perfusion in response to both hyper and hypotensive changes in MAP; however, this may not be the case (Aaslid et al., 2007;Tzeng et al., 2010b). If hysteresis is a natural characteristic of CA (i.e., differential CA depending on directionality of the blood pressure change), than the assumptions of linear techniques such as TFA may not be valid. An extension of TFA is impulse response analysis, whereby spontaneous changes in MAP are inversely transformed back to the time domain (Panerai, 2008). In other words, the impulse response function is an inverse algorithm of the Fast Fourier analysis of frequency shifts of blood velocity, as a time domain function. This allows time-domain models such as the ARI to be applied to spontaneous data (Czosnyka et al., 2009). Again, if the CA response is not linear, comparison of ARI's generated from linear models, to those generated from a transient hypotensive or hypertensive stimulus, may also not be valid. A limitation of CA assessment using spontaneous data is the small magnitude and inconsistency of spontaneous pressure oscillations (Taylor et al., 1998).See Table (appendix).

The Oxford technique
The Oxford technique is the method of using vasoactive drug injections (most often phenylephrine hydrochloride and sodium nitroprusside; the modified Oxford technique) to provoke baroreflex responses, and has been widely used since (Smyth et al., 1969) utilized bolus angiotensin injected intravenously during wakefulness and sleep; the R-R interval response to changes in arterial BP provides an index of cardiac baroreflex sensitivity. Despite its prevalence in baroreflex research until recently, the technique had not been utilized for assessment of CA, despite providing some distinct advantages in CA quantification. Blood pressure can be raised or lowered, allowing both positive and negative CA gains to be assessed independently -an important consideration given www.intechopen.com New Directions in the Dynamic Assessment of Brain Blood Flow Regulation 13 evidence that CA may be more effective at dealing with increases in blood pressure than decreases in blood pressure (Aaslid et al., 2007;Tzeng et al., 2010b). The Oxford technique is also largely painless, reducing the influence of pain induced sympathetic activity. dCA is quantified by taking the slope of the linear regression between CBV and MAP -the slope is inversely proportional to the efficacy of cerebral autoregulation in maintaining CBV. This is to say, a slope of zero would imply perfect autoregulation where CBF remains constant across the entire range of MAP, while a gain equal to 1 would reflect the total absence of autoregulation (Tzeng et al., 2010b). The limitations inherent to any pharmacological approach remain manifest with this technique, and although there is supportive evidence (Greenfield & Tindall, 1968) the assumption that there is no direct drug-effect on the cerebral vasculature has been questioned (Brassard et al., 2010). However, direct effects of PE and SNP on cerebral vasculature are considered unlikely given that the blood-brain barrier normally prevents endogenous circulating catecholamine from binding to  1 -adrenoreceptors in small cerebral vessels (Ainslie & Tzeng, 2010;MacKenzie et al., 1976;McCalden et al., 1977).

Oscillating blood pressure
A criticism of TFA is that it analyses relatively small natural swings in blood pressure.
Coherence between pressure and CBF is often low (<0.5) making it difficult to ascertain the statistical and TFA model reliability, as well as the causal relationship between these variables. These difficulties have been partially overcome by inducing large-amplitude blood pressure oscillations through either repeated squat-standing or oscillatory lower body negative pressure at frequencies associated with CA (Claassen et al., 2009;Hamner et al., 2004). See Figure 5 for detailed explication of this technique.

Cerebrovascular reactivity
Cerebrovascular reactivity gives an index of reactivity of the intracranial vessels in response to a stimulus -typically either pharmaceutical (e.g., acetazolamide) or through ventilatory alterations of PaCO 2 . There is differential reactivity to CO 2 across the cerebral vasculature. Cerebrovascular CO 2 reactivity assessed by TCD gives a global measure of reactivity compared to more sophisticated techniques such as pulsed arterial spin labeling MRI and positron emission tomography that both allow a specific brain area to be assessed. Typically, a hypercapnic stimulus is utilized to assess reactivity, and using TCD to assess CBV reactivity can be given by: Similarly, volitional hyperventilation can be utilized decrease PetCO 2 , such that reactivity to hypo and hypercapnia can be assessed. From a clinical perspective, impairment of cerebrovascular reactivity to CO 2 -as assessed by TCD -has been linked to such pathologies as obstructive and central sleep apnea (Burgess et al., 2010;Reichmuth et al., 2009), carotid artery stenosis (Widder et al., 1994), hypertension , congestive heart failure (Xie et al., 2005), and cerebral ischemic events (Wijnhoud et al., 2006). It is also an established independent predictor of ischemic stroke (Markus & Cullinane, 2001;Silvestrini et al., 2000;Vernieri et al., 2001). 1 Hz maneuvers occur, respectively. Note: 1) the large and coherent oscillations in MAP and MCAv during these maneuvers relative to resting conditions; 2) despite the strong hemodynamic effects, there is no distortion of MCAv and MAP waveforms; 3) end-tidal PCO 2 is well maintained; 4) the influence of 'targeting' 0.05Hz and 0.01Hz frequency ranges on blood pressure (BP) and MCAv power spectral densities (panels (D) and (E) respectfully). For example, the repeated squat-stand maneuvers at 0.05 results in a 40-fold increases in BP spectral power (compared with spontaneous VLF oscillations), while at 0.1 Hz, a 100-fold increase occurs relative to spontaneous LF oscillations. These augmented oscillations in BP led to 20-, and 100-fold increases in MCAv spectral power at 0.05, and 0.1 Hz, respectively. Thus increases in MCAv oscillations are relatively smaller than increases in BP oscillations at 0.05 Hz but not at 0.1 Hz, indicating more effective damping at the lower frequencies. Importantly, the coherence between BP and MCAv is typically much higher for repeated squat-stand maneuvers than for spontaneous oscillations (e.g., range (n=8): 0.6 to 0.99 vs 0.2 to 0.6, respectively). Thus, large oscillations in BP and MCAv induced during repeated squat-stand maneuvers not only provided strong and physiologically relevant hemodynamic perturbations, but also led to improved estimation of transfer function to assess dynamic cerebral autoregulation at the very low and low frequencies.
Acetazolamide can also be used to assess cerebrovascular reactivity. Acetazolamide inhibits carbonic anhydrase, the enzyme responsible for reversible catalyzation of H 2 CO 3 formation from CO 2 + H 2 O. Consequently it increases tissue PCO 2 , leads to metabolic acidosis, and increased CBF. Although the exact mechanisms of acetazolamide-induced increases in CBF are not fully understood they likely involve both metabolic factors and direct as well as indirect vascular effects (Pickkers et al., 2001); and reviewed by Settakis et al., 2003). The use of acetazolamide for cerebrovascular reactivity assessment necessitates intravenous administration. However, in this form the drug can be costly, and depending on the country, difficult to procure. Regardless, confounds associated with cerebrovascular reactivity quantification using acetazolamide are not well understood, but it may directly effect the cerebral vasculature and can indirectly drive increased ventilation, which when combined with the need for intravenous administration and high-cost, makes acetazolamide less utilized than alteration of inspired CO 2 . Regardless of the stimulus used, when assessing cerebrovascular reactivity, an absolute measurement of CBV is not as important as resolution of beat-to-beat changes in CBF from a pre-stimulus baseline.

Neurovascular coupling
Functional hyperemia describes the increased CBF to active areas of the brain where the demand for both nutrient delivery, and clearance of metabolic by-products is increased. The functional anatomy of the brain allows this neurovascular coupling to be easily and reliably examined by measurement of the sensorimotor or cognitive stimulatory effects on CBV -a method termed functional TCD (fTCD; Figure 6). This technique was first utilized by Aaslid et al (1987) who showed that blood velocity in the PCA changed with visual stimulation (see Hubel & Wiesel, 2005 for a comprehensive report of visual system physiology), but there are numerous studies in the neuro-cognitive literature that demonstrate consistent CBF changes in response to cognitive, verbal, and motor tasks (Rosengarten et al., 2003;Aaslid, 1987;Deppe et al., 2004;Klingelhöfer et al., 1997;Silvestrini et al., 1993;Stroobant & Vingerhoets, 2000).

Estimation of intracranial and critical closing pressure using TCD
The critical closing pressure is the theoretical pressure at which blood flow within the cerebral vessels drops to zero, due to failure of the transmural pressure across a vessel to counteract the tension created by the vessel's smooth muscle. Measures of cerebrovascular resistance or compliance assume proportional linearity between blood flow and pressure, and that flow through a vessel ceases when the pressure is zero. Aaslid et al. (Aaslid et al., 2003) demonstrated in humans that flow stops due to vessel collapse when perfusion pressure remains positive, making CCP a potentially better measure of cerebrovascular tone. CCP can be estimated by extrapolation of the CBV -blood pressure relationship to the pressure at which zero flow would theoretically occur. However, regardless of whether the entire pressure and velocity waveforms are used (Aaslid et al., 2003), or the systolic and diastolic values only , this technique typically yields an underestimate of CCP. Indeed, in some individuals the estimated CCP may even be negative, which is difficult to interpret physiologically. Furthermore, most studies have used peripheral blood pressure recordings that do not take into account pulse wave amplification in the periphery, which further contaminate CCP estimation. The reader is referred to (Panerai, 2003) for a detailed review of the concept.

Clinical applicability of TCD
The low cost, excellent temporal resolution, and bedside availability of TCD make it an ideal tool for clinical diagnosis of acute and chronic cerebrovascular diseases. The principle area of clinical application of TCD is the assessment of pathologies that alter blood velocity within the intracranial arteries or veins. We particularly focus on vasospasm, stenosis, intracranial occlusions, thrombosis, critical closing pressure, brain death, and patent foramen ovale.

Vasospasm
Vasospasm is observed as a complication of subarachnoid hemorrhage with an incidence ranging between 30% and 70% depending if the vasospasm is symptomatic or angiographic, respectively. Because blood velocity within a vessel is inversely proportional to its crosssectional area, the primary pathological condition that affects flow-velocity is vasospasm, which is therefore detectable with TCD (Aaslid et al., 1982). Vasospasm can remain asymptomatic, but the factors leading to symptom presentation are largely unknown. Although diagnosis of vasospasm requires the presence of hyperaemia in addition to increased blood flow velocities (see the Lindegaard index, below), at least within the MCA, threshold values of MCAv are fairly well accepted. Velocities between 120 and 200 cm/s are indicative of a reduction in lumen diameter between 25% and 50%, and serious vasospasm and lumen diameter reduction greater than 50% is indicated with velocities above 200 cm/s (Tsivgoulis et al., 2009). Hyperaemia must also be present to diagnose vasospasm; the Lindegaard Index is a ratio between the mean flow velocity in the MCA and that in the ICA, where values greater than 6 indicate severe vasospasm, between 3 and 6 indicate moderate vasospasm, and less than 3, hyperaemia (Rasulo et al., 2008). The disadvantage of using the Lindegaard ratio is that it assumes a dichotomous condition -where there is either vasospasm or not -which may be misleading in certain patients. A promising diagnostic criterion is the use of a daily increase in the systolic pressure of more than 50 cm/sec; this avoids dichotomous classification of vasospasm, informs about the physiopathological trend towards vasospasm, thereby allowing the early identification of patients at risk. To further increase the accuracy of transcranial Doppler in the identification of cerebral vasospasm, thresholds in mean velocities of more than 160 cm/sec have accurately diagnosed cerebral vasospasm (Mascia et al., 2003).

Stenosis
Typically TCD does not provide sufficient data for accurate identification of stenosis of a cerebral vessel, particularly in the posterior vessels that are more tortuous and have greater anatomic variability. Diagnosis of stenosis using TCD requires: (1) acceleration of flow velocity through the stenotic segment, 2) decrease in velocity below the stenotic segment, (3) bilateral asymmetry in flow, and (4) disturbances in flow (i.e., turbulence and murmurs) (Rasulo et al., 2008). Diagnosis of stenosis using TCD has greater sensitivity and specificity in the anterior than in the posterior circulation due to the lower anatomic variability and relative ease of insonation of the anterior vessels.

Intracranial occlusion
TCD has excellent utility in diagnosis of occlusion within the cerebral vessels with sensitivity and specificity over 90% -particularly in patients where cerebral ischemia is present (Camerlingo et al., 1993). Diagnosis is through absence or a profound reduction of flow at the normal position and depth, and/or consequent lack of signal for the vessels in the immediate vicinity of the occluded region. Furthermore, due to its non-invasiveness, TCD can easily be used to track the progression of an occlusion both before and after treatment (Rasulo et al., 2008). Furthermore, recent data suggest an independent effect of the ultrasound in augmenting the thrombolysis of the occlusion in patients with acute MCA thrombosis (Eggers et al., 2003;Eggers et al., 2009). The Clotbust trial (Alexandrov et al., 2004) demonstrated that the presence of residual flow signal, dampened waveform, and microembolic signals prior to thrombolysis was associated with increased likelihood of complete recanalisation after thrombolysis. Furthermore, in patients with acute ischemic stroke, continuous TCD significantly increased tissue plasminogen activator-induced arterial recanalization (Alexandrov, 2009). See (Alexandrov, 2006) for a review of the use of TCD in thrombolytic treatment of stroke.

Sickle cell disease and risk of arterial thrombosis
Sickle cell disease is associated with an increased risk of stroke in children (Adams et al., 1998). Level I evidence has been established for the use of TCD in the diagnostic screening of patients with sickle cell anemia. A MCAv threshold of 170 cm/sec was identified as indicative for the need of blood transfusion, such that a 30% reduction in circulating hemoglobin-s was achieved (Adams et al., 1997). A randomized trial subsequently demonstrated that application of the above Doppler criteria yielded a 92% absolute reduction in the risk of stroke in children (Adams et al., 1998). Additionally, reference CBV values were recently outlined for the purpose of screening for intracranial vessel narrowing in children with sickle cell disease (Krejza et al., 2000).

Brain death
Electroencephalography or angiography can be utilized for clinical diagnosis of brain death. Angiography is typically preferred because EEG gives little information regarding brainstem function and signals can be difficult to attain within the intensive care unit. However, angiography requires injectable contrast media, and cannot be completed bedside at all. Typically, increased intracranial pressure concomitant with brain death reduces diastolic blood flow velocity in the intracerebral vessels. Further increases in intracranial pressure produce reversed flow during diastole in the circle of Willis, and finally, spiked and reverberating flow is considered indicative of brain death (de Freitas & André, 2006;Ropper et al., 1987;Tsivgoulis et al., 2009).

Shunt and emboli detection
In the presence of right-to-left cardiac shunt microbubbles injected into the venous circulation -that are largely filtered out in the lungs -will appear in the cerebral circulation within 5-15 seconds. There are reports of up to 100% sensitivity in right-to-left shunt detection (Droste et al., 2002); however, it seems unlikely that the TCD technique can differentiate between various forms of shunt. For example, that microbubbles appear in the systemic circulation could be indicative of a patent foramen ovale, pulmonary arteriovenous malformation, or an atrial septal defect. But given that up to 60% of the normal population may present with right-to-left shunt of either intracardiac or pulmonary arteriovenous malformations (Woods et al., 2010), an inexpensive means of screening such as TCD, as part of a diagnostic battery, may be very useful.
Both gaseous and solid microemboli can be detected using TCD through recognition of irregularities within the Doppler signal (Padayachee et al., 1987;Deverall et al., 1988;Ringelstein et al., 1998). Although these microemboli are often clinically silent, their detection may be of prognostic value in assessing risk of stroke, and of use during cardiac or vascular surgeries where gaseous emboli may originate from the oxygenator. The detection of emboli using TCD is complicated and relies on 10 technical parameters and Ringelstein et al. provide a detailed description of the technique (Ringelstein et al., 1998). There is some difficulty in distinguishing between gaseous and solid emboli. This is of clinical importance as each has distinct clinical relevance, particularly in cases where both types of emboli may be present (e.g., mechanical heart valve patients, patients with carotid stenosis (reviewed in: (Rodriguez et al., 2009;Markus & Punter, 2005).

Utility of assessment of neck artery blood flow
Measurement of CBF by quantification of inflowing blood through the neck is not a new technique, but has not seen the prolific utilization of TCD or MRI, for example. Nonetheless, the technique has recently seen a resurgent popularity because it facilitates estimation of both flow proper (as opposed to blood velocity), and regional distribution of CBF during a variety of conditions (exercise, standing, environmental stress, etc.) not possible with MRI. The technique is nonetheless especially prone to measurement error, both for technical reasons and because of the apparent ease with which an untrained individual can pick up a vascular ultrasound probe, image a carotid artery, and believe the resulting measurement is accurate.
The aim of this section is to outline the virtues and caveats of the measurement of CBF via quantification of neck artery blood flow using high-resolution vascular ultrasound. First, we will discuss the vascular anatomy providing blood to the head. Then, we will address principals of linear vascular ultrasound, methods of analysis, sources of measurement error and our perspective on the appropriate use of the technique. Finally, the utility of the technique will be discussed in conjunction with an overview of the current literature.

The arteries of the neck
Early Greek physicians termed the principal arteries of the neck Karatides, after the adjective for stupefying, because their compression yielded unconsciousness. Such were these vessels' importance recognized early, even before the physiological function of blood was understood. Indeed, the carotid arteries are the principle conduit for blood transport to the brain, carrying approximately 70% of global CBF.

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Applied Aspects of Ultrasonography in Humans 20 Fig. 7. Diagram of the right carotid arteries and vertebral artery.
The carotid system originates with the common carotid arteries (CCA), that branch from the aortic arch and brachiocephalic trunk on the left and right sides, respectively. The CCA bifurcates into the external (ECA) and internal (ICA) carotid arteries. The position and morphology of the carotid bifurcation exhibits some variation, lying somewhere between the level of the thyroid cartilage and hyoid bone in the majority of individuals. The ECA is most often positioned either anteromedial or medial to the ICA (Al-Rafiah et al., 2011), giving off the superior thyroid artery and ascending pharangeal artery within the first two centimeters distal from the bifurcation. There is, however, variation in the loci of these arteries origins, occasionally branching from the CCA or ICA, and more often from the bifurcation itself. The ICA normally does not give off any branches until after entering the base of the skull through the foramen lacerum. The ophthalmic artery and a number of smaller arteries branch prior to the circle of Willis, where the ICA terminates to form the middle, anterior, and posterior communicating arteries. Because in the majority of individuals there are no branches off of the ICA prior to entering the skull, ICA blood flow is an accurate metric of CBF.
The vertebral arteries (VA) arise as the most proximal branches off the subclavian arteries, then course through the foremen of the transverse processes of C6-C2, into the spinal canal and skull to join bilaterally forming the basilar artery. The vessel can be imaged proximal to entering C6, and between each of the vertebra until entering the skull. There are numerous anastomoses with the VA both extra-and intracranially. The VA communicates with branches of the deep cervical artery, and inferior thyroid artery extracranially, and upon entering the skull gives off branches to the cerebellum before joining to form the basilar artery (BA). A number of arteries project from the BA to supply the cerebellum and pons before the BA bifurcates to form the poster circle of Willis as the posterior communicating arteries. The figure 8 below provide typical ultrasound example of the different waveforms commonly observed in the ICA, ECA and VA. ; 2) ICA waveform should have a more gradual upstroke (slower acceleration) in systole, and an elevated enddiastolic velocity. This is because it supplies a low resistance vascular bed (i.e., the brain). Thus flow should be above the baseline for the entire cardiac cycle; and 3) The VA waveform should have a gradual upstroke and relatively high end-diastolic velocity as it also supplies a low resistance vascular bed.

Technical aspects of vascular ultrasound
Neck artery ultrasound accurately quantifies global and regional CBF non-invasively and with high spatial and temporal resolution; indeed, it is the only known method of CBF measurement possessing these attributes. The principal limitation is consequently that of the operator, not the technique per se. But in fact the technique is so easily confounded by user error that this limitation obviates flippant dismissal. For example, a one degree error in the angle of insonation; a 0.1mm error in diameter measurement; and, one cm/s error in mean blood velocity each yield a 3, 4, and 4% error in the measurement of flow. It is obvious that very minor operator errors during insonation and during subsequent analysis quickly compound and can easily produce significant inaccuracies (Schoning et al., 1994). For every 1-degree error in the insonation angle an approximate 3% error in velocity, and therefore in flow, results. But, whereas the velocity and flow error are linearly related, inaccuracies in the measurement of diameter have an exponentially larger effect (because the diameter is squared to calculate luminal area). The appropriate measurement of diameter is a topic of extensive debate within other fields reliant on ultrasonic measure of blood flow (Black et al., 2008;Green et al., 2011). It is unfortunate, then, that the accurate measurement of luminal diameter presents a number of problems that have largely been unaddressed since the invention of the technique. The majority of investigators utilize calipers typically part of the ultrasound software to manually measure from one luminal surface to the other. Some authors have accounted for the pulsatile nature of most arteries by measuring both systolic and diastolic diameters, and calculate a mean diameter based on a third to two-thirds systolic-diastolic ratio (Sato et al., 2011;Sato & Sadamoto, 2010). Other authors have discordantly reported the ICA, ECA, and VA to be without pulsatile changes throughout the cardiac cycle (Scheel et al., 2000a;Scheel et al., 2000b;Schoning & Hartig, 1996;Schoning et al., 2005a;Schoning et al., 1994).

Integrative assessment and future directions
Cerebrovascular function is clearly regulated by an array of functionally integrated processes. Measurement of only one process will provide an inadequate representation of this complex physiology. We therefore suggest that in both the research and clinical setting, assessment of cerebrovascular function with TCD should ideally include measures of four principle factors: (1) baseline CBV; (2) cerebrovascular reactivity; (3) cerebral autoregulation; and (4) neurovascular coupling. Furthermore, there is evidence that CBF and CA varies with time of day, exercise, body position, caffeine, food intake, and menstrual cycle; as such, these variables should be carefully standardized for each of the following metrics both between and within subjects or patients.

Baseline cerebral blood flow and velocities in arteries of interest
Because normative data for blood flow velocities is well known within the literature for the MCA (Aaslid et al., 1984;Ringelstein et al., 1990) and related neck arteries (Scheel et al., 2000b;Schoning et al., 1994); thus, baseline blood flow and CBFv should always be collected for all arteries of interest. Indeed, diagnosis of atypical flow patterns within the cerebral circulation is indicative of a number of conditions such as intracerebral stenosis or occlusion (Aaslid, 1986b;Aaslid, 2006). Because in a normal population cerebrovascular function is largely determined by its ability to adjust to changes in perfusion pressure, it is largely the change from baseline following a stimulus that is of greatest importance in assessing autoregulation.

Cerebrovascular reactivity
Cerebrovascular reactivity is an accepted independent predictor of ischemic stroke (Markus & Cullinane, 2001). That it has been linked with a spectrum of vascular pathologies justifies its inclusion when assessing cerebrovascular function. Arterial CO 2 can be increased through inhalation of a hypercapnic gas or simple re-breathing paradigm. Ainslie & Duffin (2009) have recently detailed the assessment and related interpretation of cerebrovascular reactivity. Furthermore, PetCO 2 should always be measured because of its direct effects on cerebrovascular calibre. Moreover, spontaneous measures of dCA are affected differentially by hypercapnia and hypocapnia (Panerai et al., 1999b).

Cerebral autoregulation
Quantification of cerebral autoregulation (CA) is clearly an important factor in any study assessing cerebrovascular function. Despite caveats inherent to each methods of CA quantification most studies to date have applied only one measure of CA, and generally in the hypotensive range. Moreover, recent studies have largely focused on spontaneous CA analysis using TFA. However, we recommend that it is critical to 'force' the BP challenge in order to reliably engage CA. Driving BP oscillations using either squat-stand, or oscillating lower body negative pressure at the frequency of interest will enhance the reliability and validity of TFA measures (Claassen et al., 2009;Hamner et al., 2004). Like the Oxford method, this approach also permits the assessment and separation of the cerebral responses to both hypotension and hypertension. Given the apparent difference in efficacy of CA to managing falling or rising perfusion pressures, and the distinct consequences to dysregulation (i.e., ischemia versus hemorrhage, respectively), this is an important consideration.

Neurovascular coupling
Neurovascular coupling (NVC) is likely involved in maintenance of adequate nutrient and oxygen supply to specific regions of the brain. It is a relatively simple addition to any experimental design, and should be measured if time permits. The regional task specificity within the brain allows coupling to be easily assessed; simple sensory stimuli (e.g., turning on a light) provides adequate stimulus to assess neurovascular coupling in the cortex by measurement of CBV in the PCA (Aaslid, 1987). Coupling between neural activity is decreased in a number of pathologies, including hypertension, Alzheimer's disease, ischemic stroke (reviewed by Girouard & Iadecola, 2006), and in long-term smokers (Woods et al., 2010).

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Applied Aspects of Ultrasonography in Humans 24

TCD and neck blood flow utility
It is clear that there is tremendous utility in the assessment of cerebrovascular function with TCD. Surprisingly, integrative assessment of cerebrovascular function is currently lacking and no studies to date have combined the assessment of these four fundamental measurements. Given that regulation of cerebrovascular function involves the complex integration of each of the above factors, only experimental designs that incorporate holistic assessment of multiple mechanisms can hope to clarify the complex physiology responsible for maintaining brain O 2 and nutrient delivery in such narrow margins.
In the clinical setting, depending on the key measurement question, the implementation of a simplified protocol that encompasses each of these metrics is prudent. Measurement of MCAv, PCAv, ECG, beat-to-beat BP, and end-tidal gases should be recorded throughout.
The following protocol would likely be adequate: 5 minutes supine baseline; 2 minutes eyes closed; 2 minutes of reading followed by 5-10 cycles of 20 seconds eyes closed followed by 40 seconds reading to assess NVC; 2 minutes of either rebreathing or breathing of a 5% CO2 gas mixture to assess cerebrovascular reactivity; and finally one BP stimulus -either using the Oxford technique, thigh cuffs, Valsalva maneuver, or squat-stand cycles. Such a protocol, with one or two skilled experimenters, can be completed within 45-60 minutes on both healthy subjects and a variety of patient groups (Figure 9).
The prototype for the assessment of both blood velocity and vessel diameter was introduced in 1974 ( Barber et al., 1974). Several years later pulsed-Doppler ultrasound was utilized for the quantification of blood velocity in the internal carotid and vertebral arteries during changes in inspired CO 2 (Hauge et al., 1980). This early technology lacked B-mode functionality to allow visualization of the vessel, and measurement of diameter. Consequently, the diameter of the vessel had to be assumed constant for the velocity to be indicative of flow, a problem overcome several years later following diagnostic utilization of combined B-mode and Doppler (Duplex) scanning for identification of artherosclerotic pathology (Blackshear et al., 1979;Strandness, 1985). The "logical progression" came soon after when Leopold et al (1987) applied Duplex scanning to the internal carotid artery to quantify CBF and CO 2 reactivity. The method's principal use has since been for the evaluation of neck artery pathology, however a handful of studies have utilized the technique for the quantification of CBF decline through aging (Schoning & Hartig, 1996;Schoning et al., 1994) and to assess cerebrovascular reactivity to CO 2 and O 2 (Fortune et al., 1992;Hauge et al., 1980;Leopold et al., 1987;Schoning et al., 1994). The results of these studies are broadly consistent with studies assessing cerebrovascular reactivity and global brain blood flow using indicator dilution techniques (Ainslie & Duffin, 2009;Battisti et al., 2010;Fan et al., 2008) (Ackerman et al., 1973;Grubb et al., 1974;Harper & Glass, 1965), and TCD.
Use of ICA blood flow has been extended to the bedside diagnostic assessment of myriad cerebrovascular pathologies. There is a small body of literature describing insonation of the ICA for the assessment of periventricular leukomlacia (Kehrer & Schoning, 2009), cerebral vasculitis (Kamm et al., 2008;Kuker et al., 2008), evaluation of cerebral circulatory arrest (Schoning et al., 2005b), and for establishing normative values for CBF during natal development (Kehrer et al., 2005;Kehrer & Schoning, 2009). Ultrasonic study of the vertebral arteries has also received attention with respect to risk identification involved with chiropractic cervical spine manipulation (Bowler et al., 2011;Mitchell, 2005), though many of the conclusions herein remain speculative. Fig. 9. (A) A complete resting assessment of intracranial blood flow using TCD should include blood velocities of middle cerebral artery (MCA), posterior cerebral artery (PCA), anterior cerebral artery (ACA) and basilar artery. The probe(s) can then be fixed on the arteries of interest for the remainder of the protocol. Electrocardiogram, end-tidal gases, and beat-to-beat blood pressure should be measured concomitantly with cerebral blood velocity metrics throughout the experiment. (B) The neurovascular coupling protocol should consist of 2 minutes each of resting with eyes open and closed, followed by 5-10 cycles of 20 seconds eyes closed and 40 seconds of reading with concurrent measurement of MCA and PCA velocities. (C) Assessment of cerebrovascular reactivity in both hypercapnic and hypocapnic ranges can be carried out in <4 minutes of rebreathing or inhalation of 5% CO 2 , or, hyperventilation, respectively. (D) Dynamic cerebral autoregulation can be assessed using any of: supra-systolic thigh-cuff release (Section 5.1), the Valsalva maneuver (Section 5.3), the Oxford technique, or oscillatory blood pressure perturbations. The Oxford technique (Section 5.5) or transfer function analysis using either squat-stand cycles or oscillating lower body negative pressure at 0.05 and 0.1 Hz are recommended for reasons detailed in Section 5.6 and Figure 5.
More recently neck artery blood flow quantification has seen a resurgent application in physiology research. Sato et al conducted two studies (Bowler et al., 2011;Mitchell, 2005;Sato et al., 2011;Sato & Sadamoto, 2010) utilizing the technique to quantify changes in regional head perfusion during varying degrees of semi-recumbent cycling exercise. They measured blood flow in the external, internal, and common carotid arteries, as well as in the vertebral artery to show that the well-documented increase-plateau-decrease with progressive exercise to intensities above the ventilator threshold was only evident in the anterior circulation. Their data indicate, and support previous suggestions, that the poster and anterior cerebral circulations possess disparate blood flow regulation and CO 2 sensitivities. In this respect the possibility that the arteries of the neck were directly involved in the regulation of CBF was raised by their data (Hellström et al., 1996).
From an integrative systems approach, numerous factors independently, synergistically, and sometimes antagonistically participate in the regulation of CBF. In addition to the traditional mechanisms describing CBF regulation (e.g. autoregulation, partial pressure of arterial carbon dioxide), a variety of other factors, such as cardiac output, the arterial baroreflex and chemoreflex control, are very likely involved in this complex regulatory physiology. Research exploring these complex interactions, especially in relation to neurovascular coupling, is currently lacking. Future studies with particular focus on these integrative physiological mechanisms are clearly warranted in both health and disease states.

Conclusions
Many methods are available for the assessment of CBF, but the high temporal resolution, non-invasiveness, and relative low-cost of TCD make it functional in both clinical and research settings. The ability to assess cerebral reactivity, CA, and neurovascular coupling, makes TCD extremely useful for the assessment of integrative cerebrovascular function. Four principle components of cerebrovascular regulation can, and should, be assessed using TCD, as collectively these provide insight into a complex physiology. Measurement of (1) velocities within the major cerebral vessels; (2) measurement of ICA, ECA and VA blood flow, (3) assessments of autoregulation, (4) cerebrovascular reactivity and (5) neurovascular coupling together facilitate holistic appraisal of cerebrovascular function.