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Pacemakers, ICDs and cardiac resynchronization devices are implanted and followed mostly by cardiac electrophysiologists. Detailed diagnostic data (pacing statistics, lead function, arrhythmia episode intracardiac electrograms etc.) are available using manufacturer-specific programmer devices or remote follow-up (Figure 1). However, patients may present with suspected cardiac or arrhythmia-related symptoms when these measures are not immediately available. Using conventional diagnostic methods basic device function can be evaluated and correlation with the clinical presentation may be assessed (McPherson, 2004). In certain cases, such as with transient events, these may be the only diagnostic clues available as current CIEDs do not have full Holter capability – only episodes of significance, as determined by the device, are stored.
Basic evaluation of CIED function requires a 12-lead ECG and review of past medical records to identify device type and settings. If prior records are not available, a simple chest X-ray may provide important clues (pacemaker, ICD, or CRT; lead locations) (Jacob, 2011). In case intermittent or transient malfunction is detected and device interrogation does not provide clear answer, Holter monitoring or an event recorder may be required. If a programmer is available, diagnostic tests should be performed according to the guidelines (Wilkoff, 2008). Patient symptoms, if any, should be assessed, whether they can be signs of a possible device malfunction.
2. Electrocardiographic evaluation of device function
A 12-lead ECG may raise the suspicion of device malfunction. Careful evaluation of patient-related factors is required as these interact with device function (Table 1). Occasionally, very advanced forms of electrophysiological abnormalities may be identified as the devices generally do not prevent natural progression of underlying pathophysiology. In case an arrhythmia or device malfunction is suspected on a telemetry recording, a full 12-lead ECG is recommended to avoid misinterpretation (Figures 2-6). Artifacts may severely impact interpretation and tracings with good technical quality should be obtained (Figures 7-10). Atrial rhythm and characteristics of atrioventricular/ventriculoatrial conduction should also be assessed (Figures 11-17).
Importance – diagnostic clues and possible interaction with devices
Should be paced unless there is oversensing or no atrial lead present
Blocked PACs should elicit different response than sinoatrial block if atrial sensing is present
May be tracked with high ventricular rate
May be undersensed, leading to ineffective atrial pacing
Variable AV conduction interval
May lead to fusion and pseudofusion beats
Complete heart block
May be intermittent or unidirectional
May lead to pacemaker-tachycardia or pacemaker syndrome
Native QRS morphology
Assess biventricular capture during cardiac resynchronizationIf atrial pacing only, may be used to identify ischemia, etc.
May trigger safety or sense response pacing or activate rate smoothing algorithms
Important ECG features that should be assessed when evaluating CIED function.
Certain conditions, such as acute heart failure may require adjustment of device settings, even without device malfunction – pacemaker algorithms do not provide optimal hemodynamics for all situations. Unfortunately, evidence-based approach is limited due to scarcity of data (Lahiri, 2011).
3. Unexpected findings with normal device function
Certain artifacts or interaction of pacemaker algorithms with underlying rhythm may lead to electrocardiographic findings, which may be difficult to distinguish from abnormal function (Balachander, 2011). P/QRS morphology, timing and response to pacing spikes should be addressed, when analyzing the ECG. With ubiquity of bipolar systems, spikes may be difficult to identify (Figure 18). In addition, myocardial depolarization has a vector, which may be isoelectric in certain leads, or may be delayed by intraatrial or intraventricular conduction delay, suggesting ineffective stimulation (Figure 19). Spike morphology may be affected be automatic signal gain function of the ECG system or issues with digital sampling (Figure 20). „Anticipated” spikes may be missing due to very small variations in heart rate, inhibiting demand pacing (Figure 21).
Variable signal morphology may be caused by fusion beats (when the resulting signal morphology is the sum of activation from the pacemaker and spontaneous/conducted activation) or pseudofusion beats (pacing occurs when the myocardium is already refractory from spontaneous/conducted activation, Figures 22-24). Identification of the pacing site is crucial to prove appropriate device function (Figures 25-28).
Occasionally, pacing mode may be difficult to identify based solely on the ECG (Figure 29). It may change due to algorithms trying to minimize right ventricular stimulation (Figures 30-32), rate smoothing function (Figure 33), or arrhythmia – mainly, atrial fibrillation (Israel, 2002).
If pacing spikes are seen during tachycardia, most common causes are atrial tachyarrhythmia tracked by the pacemaker, or true PM mediated tachycardia (caused by retrograde conduction or atrial oversensing of ventricular events, leading to endless loop tachycardia). Rate response function may also cause transient increase in pacing rate. The differential is usually difficult based on surface ECG alone, unless initiation and termination can be clearly identified. Device interrogation is strongly recommended (Ip, 2011). Transient changes in rhythm may elucidate the mechanism of a suspected malfunction (Figure 34).
Both atrial and ventricular tachyarrhythmias may raise the concern of device malfunction. If no spikes are seen, the rhythm is likely not related to pacing and patient-related issues should be suspected (Figures 35-38). Underlying rhythm should be identified: atrial fibrillation/flutter may be difficult to recognize with asynchronous pacing, but still pose a thromboembolic risk (Figures 39-40).
ECG analysis should always include assessment of QRS and ST-T, even in patients with paced rhythms. Atrial pacing preserves normal ventricular activation, so it may be interpreted without interference from the device. Underlying conduction blocks may mimic paced beats (Figures 41-42). If artifacts limit interpretation, comparing multiple simultaneous ECG leads may be helpful (Figure 43).
In patients presenting with symptoms suspicious for pacemaker syndrome (hypotension, shortness of breath, dizziness, most commonly in an intermittent pattern), atrioventricular activation sequence and presence of ventriculoatrial conduction should be assessed. If these are compatible with PM syndrome, device settings should be adjusted (or the device should be upgraded), to restore AV synchrony and avoid atrial contraction during ventricular systole (Figures 44-45).
4. Pacemaker or lead malfunction
Most common pacemaker and lead related malfunctions, that should be promptly identified and corrected, include oversensing, undersensing and ineffective stimulation. These may be related to inappropriate settings that may be easily corrected with a programmer, however, pacemaker lead related issues (dislocation, fracture, insulation failure) may present similarly and require hardware revision. If true pacemaker dysfunction cannot be ruled out with certainty based on ECG, device interrogation should be performed – this is especially important, if the patient was exposed to factors with potential device interaction, such as MRI, therapeutic irradiation, trauma, or drugs with known effect on pacing threshold (Goldschlager, 2001).
Pure undersensing may be identified by a pacing spike that comes early compared to the anticipated timing, with appropriate capture, if the paced chamber is not refractory. Transient arrhythmias, such as premature ventricular beats, may lead to intermittent undersensing, as their intracardiac signal amplitude may be low (Figure 46). Atrial fibrillation is often undersensed and elicits different behavior in AAI and DDD systems (Figures 47-48).
Loss of capture is easily recognized, however, post-pacing artifacts should not be misinterpreted as capture (Figure 49). Complete lead fracture usually leads to exit block with no visible spikes, while lead dislocation or insulation failure may manifest in various ways (Figures 50-54).
6. Evaluation of ICD function
Implantable cardioverter defibrillators have multiple therapeutic zones (bradycardia, „physiological”, ventricular tachycardia and fibrillation - VT, VF), that should be taken into account when interpreting ECG changes. While issues due to undersensing or ineffective capture usually manifest similarly to a pacemaker, oversensing may lead to inappropriate therapy due to false VT/VF detection.
As ICD therapies may cause severe patient distress or proarrhythmia, prompt device interrogation and expert consultation is required after such events, unless appropriate device behavior is evident (Figures 55-56). Even when appropriate therapies have been delivered, the patient has to be fully evaluated and appropriate measures should be taken to reduce the risk of arrhythmia recurrence (Mishkin, 2009). In cases when inappropriate therapy is suspected and the risk of recurrence is high (atrial fibrillation with rapid ventricular rate, oversensing), a magnet may be applied to temporarily inhibit tachyarrhythmia therapies, until the device may be interrogated and appropriately reprogrammed (Figure 57). Continuous monitoring is required in the meantime as the patient will not be protected from malignant tachyarrhythmias while in magnet effect.
7. Evaluation of cardiac resynchronization devices
Consistent biventricular capture is required to maintain cardiac resynchronization. Paced QRS morphology may vary based on underlying conduction abnormalities, lead location, interventricular delay and the amount of myocardium captured by each lead, relative to each other. Typically, right axis deviation and atypical RBBB pattern is present. If interventricular delay is set greater than 0 ms, usually two pacing spikes can be seen prior to the QRS (Figure 58). In rare cases, conventional RV pacing may mimic biventricular paced QRS morphology (Figure 59). QRS morphology may change due to variable fusion with conducted beats either from variable AV delay or atrial fibrillation (Figures 60-61).
Sense response pacing is an algorithm that was designed to maintain the benefits of biventricular stimulation with premature beats or fast AV conduction – in case a ventricular event is sensed, a pacing stimulus is delivered simultaneously to decrease ventricular activation time. The resulting QRS morphology is affected by the origin of the premature beat and the amount of fusion (Figures 62-64).
Loss of left ventricular lead capture changes QRS morphology, so it becomes similar to RV pacing. A full 12-lead ECG should always be obtained during follow-up (Barold, 2011a and Barold, 2011b). Comparison with previous tracings is recommended as biventricular paced QRS morphology varies individually (Figure 65). Undersensing or oversensing may be more difficult to identify with resynchronization devices, than with conventional pacemakers, due to the algorithms designed to maintain biventricular pacing (Figure 66-67). In uncertain cases, device interrogation should be performed to prevent loss of resynchronization.
Conventional 12-lead ECG is an important tool to evaluate CIED function. A systematic approach is required to identify appropriate device function and to decide whether further investigation is necessary. As advanced devices, such as implantable cardioverter-defibrillators and cardiac resynchronization systems become more abundant, even common malfunctions and pseudo-malfunctions may be more difficult to identify, due to the presence of special pacing algorithms. In uncertain cases, review of prior patient data, device interrogation and expert consultation is required.
Attila Roka (August 17th 2012). Electrocardiographic Troubleshooting of Implanted Cardiac Electronic Devices, Current Issues and Recent Advances in Pacemaker Therapy, Attila Roka, IntechOpen, DOI: 10.5772/50983. Available from:
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