Revision of Training Models on Ultrasound-Guided Vascular Access: Presentation of an Animal Model

Simulation has been defined as the representation of something as real. It is necessary for performing the ultrasound-guided vascular cannulation technique correctly. The use of training models for diagnostic or therapeutic procedures: improves the quality of care for patients; decreases stress level that it can produce the realization of a new technique directly on the patient and; can be used as many times as the model is reproduced, also serving as a method for the resolution of some problems that may appear related to the in vivo technique. The evidence shows that simulation plays an important role in the acquisition of skills to perform invasive procedures. The use of ultrasound in vascular accesses whether peripheral or central, arterial, or venous, improves the success rate in the canalization and reduce the complications derived from the technique in certain critical situations (coagulopathy, thrombocytopenia, obesity, etc.) specially in pediatric patients given the variability of depth and diameter of its vessels with respect to the adult population. To facilitate learning in the technique of echoguided puncture, a training model is presented that is easily reproducible, economical and with a high fidelity in relation to the punctures performed on the patient.


Introduction
Simulation has been defined as the presentation of something as real, it means a situation in which some conditions are artificially created to resemble the reality [1][2][3].This is used with the objectives of studying something or training in a new medical procedure.To implement a technique such as the ultrasound-guided vascular access, a series of skills must be acquired in order to reach the required aptitudes to perform vascular cannulations in a proper manner [2][3][4][5].
These skills include the following: (a) knowledge and comprehension of the device to be used as well as their technical bases, in our study, the ultrasound machine and ultrasonography; (b) the visualization and optimization of the vascular image and of the needle, and; (c) the ability to acquire the required skills to use the ultrasound probe and to insert the needle (puncture) when performing the procedure of ultrasound-guided vascular access [6][7][8][9][10][11].
The use of simulation models as diagnostic or therapeutic procedures training models has the following advantages: (a) they increase patients' assistance quality, especially if these techniques are associated to complications and risks; (b) they decrease the stress level eventually provoked by the direct performance of a new technique on patients, and; (c) they can be used as many times as the model is reproduced, so they can be additionally used to solve some problems that could arise from the "in vivo" performance of the technique [10,12,13].
Evidence shows that simulation plays an important role in the acquisition of skills required to perform invasive procedures [13,14].The use of ultrasound scan on vascular access increases the success rate of the cannulation and reduce the complications derived from this technique.Irrespective of these vessels are peripheral or central and arteries or veins [15][16][17][18].However, the ultrasound-guided vascular access is displaced for the benefit of the classical technique ("blindly" oriented by anatomic references) by some reasons, such as the learning curve that every invasive technique requires and the ultrasound machine preparation required to perform this technique (probe sterilization, choice of the proper "pre-set," puncture plane, etc.).The preference for the classical technique occurs even when it takes the risk of complications associated, which increase under certain critical conditions (coagulopathy, thrombocytopenia, obesity, etc.).These considerations are especially relevant in pediatric patients due to their vessels' depth and diameter variability, which is higher than in adult patients [19,20].

Model types
It is noticeable that experimental, simulation, or no-human models are used infrequently in learning invasive techniques/procedures such as ultrasound-guided vascular access.Any training process on a simulation model represents an opportunity to practice the technique without taking risks and entails the learning of the use of ultrasounds.All of the above is feasible to increase patients' security when performing invasive procedures on children [8,13].
The training models usually are extremely expensive, hardly available, or not good at transmitting ultrasounds in an optimal manner.
Most of the training or experimental models than can be used to perform simulations for the ultrasound-guided vascular access training are synthetic or biological.Some of them are commercially available and other can be constructed manually by any person [11,[21][22][23].They are the following: (a) "in vivo" models performed on research animals; (b) commercially available models such as Blue Phantom® or silicon or latex models, and; (c) synthetic handcrafted models constructed by using gelatine/agar or tissue animal models constructed by using chicken thighs, turkey thighs, or chicken breast [14,24,25].Each of these models must contains tubular structures inside, that can be made on plastic, latex, or rubber, and filled with liquid, in order to simulate the vessels to be cannulated.
These models can be classified as cannulation-puncture models or puncturelocalization models depending on the use of them.

Characteristics of an ideal training model
An ideal training model to perform ultrasound-guided vascular access procedures should: • reproduce texture and resistance of human tissue; • have enough penetration surface to transmit correctly the ultrasounds; • permit the identification and localization of the different tissue structures; • obtain an optimal image; • permit different difficulty and complexity levels when the procedure is performed; • avoid, to the extent possible, the visibility of the puncture needle path; • permit the visualization of the needle; • have long average life; • be easily transportable, and reproducible in any environment; • be easily available and inexpensive.

Advantages and disadvantages of the different ultrasound-guided vascular access training models 3.1 Blue Phantom®
Commercially available simulator to be used as a training model of procedures in which ultrasounds are used as vascular access guides (Figure 1).
They are expensive, not-transportable, and not-changeable, although the latest models permit even simulate arterial pulse (Figure 2).The puncture needle entry point and path usually remain visible.The puncture performance sensation on this model is different from that on human tissues.They require maintenance and deteriorates after multiple puncture performances.They are not easily available nor affordable [21][22][23].

Silicone models
Commercially available models consisting of a silicone model containing a tubular structure which permits the vascular access simulation and that can be refilled after each puncture performance.They have a long average life, and they are easily transportable.However, they offer a small surface area due to their small size, they are expensive and not easily available to all [11,26,27].The puncture needle entry point and path remain visible after multiple puncture performances (Figure 3).

Agar/gelatine models
It consists of an agar or gelatine model in which a tubular elastic structure is inserted (in some cases, Penrose surgical drains of different sizes).They have been used by radiologists to train and teach ultrasound-guided procedures (Figure 4).They are easy to construct by using everyday kitchen utensils and they are ideal for hand-eye coordination learning and improvement [28,29].
However: (a) they usually show an uniform appearance of the ultrasound image (Figure 5) without identifiable muscle or tendon structures (with the exception of preparations including any component like mucilage); (b) the puncture needle entry point and path remain visible after some puncture performances; (c) their puncture performance sensation is different from that on human tissues; (d) depending on the gelatine concentration used during their construction, they can be easily damaged, and; (e) the needle sideways movements during its introduction into the agar/gelatine could be hardly controllable when trying to puncture the vessel.

Animal models
(a) "In vivo": the use of research animals results in high cost and laborious preparation when optimal conditions are required (sedation, mechanical ventilation or respiratory support, monitoring, etc.) as well as in a limited number of punctures; (b) "artificial": they are manually constructed by using pork, turkey or chicken thighs, pork-belly, tofu or sausage/cold meat piece as muscle structure, and different elastic components (urinary catheters, chest drains, metallic trocar, serum infusion systems, etc.) as vascular structure.By using them, a real sensation    of different tissues is created with respect to both the ultrasound image perspective and the sensation obtained when performing puncture and cannulation/vascular access.These models show an affordable preparation, but their construction needs a certain amount of time.In addition, they are inexpensive as well as easily transportable and manageable.Their design permits different complexity levels with respect to different vessels diameter and depth, so the difficulty level can be increased as progress is made on the training process.With respect to their disadvantages, these models' conservation needs refrigeration, their average life is short (some weeks to get their muscle structure deteriorated) and they must be carefully constructed in order to reduce their risk of air introduction into their vascular structure [8,15,25].
The main advantages and disadvantages of the main cannulation-puncture models are shown in Table 1.

Simulation software
Simulators provided with high software-algorithm-based fidelity have been used as training models on ultrasound-guided procedures.Their disadvantages include their high cost and the need for software support.Their advantages include the absolute lack of infection control problems, the possibility of changing the complexity/difficulty level.Simulators can be installed in not clinical settings, such as training rooms, and they can be available at any time of the day or night [30,31].

Puncture-localization models
These are artificial models frequently used by radiologists for the training on localization and puncture of nodular or cystic structures, mainly hepatic mammary or thyroid ones [11,26,27].These models are used to get familiarize with the ultrasound machine and with the puncture technique in ultrasound-guided procedures.The tissue/muscle component can be: piece of poultry breast, liver or tofu, sausage/ cold meat piece or surgical gloves filled with a warm water dilution containing food thickener.The vascular/nodular component can be whole/pitted olives, cheese puffs-blocks, jelly beans or gumdrops, wires or knitting needles, etc. and they must be included inside the tissue/muscle component (Figure 6).

Animal model construction and description
The training model described here consists of the following [8]:

Muscle component
Piece of bird breast that can be acquired at any grocery store/shop and with the following approximated measures (length, width, height): 10 cm × 10 cm × 3 cm (Figure 7).Frozen poultry breast is preferably used; being defrosted in a refrigerator within 24 hours before performing the vascular punctures.

Vascular component
Tubular structure made in elastic material (modeling balloon) filled with 10 ml of water with water-soluble colorant and sealed on both sides by using knots (Figure 8).Both components simulate the muscle and the vascular structures of pediatric patients.The development of this model is based on the introduction of an eight French thoracic drain together with its puncture trocar passing longitudinally through the muscle component and at different depth levels.This simulates the different depths at which vessels are located in children depending on their age and weight.After that, the trocar is retired, and the drain plastic piece remains inside the muscle structure.In the drain distal part, the elastic tubular structure distal end is sutured in the knot area.When pulling the drain in the opposite direction, the device stays inside the muscle structure and then, it is prepared to be visualized and cannulated in an ultrasound-guided manner in this experimental model (Figure 9).
A system of clamps fixed at different length permits the application of different tension values on the elastic structure.Depending on these different tension values, three different diameter ranges can be obtained, which are comparable to pediatric patients' vessels diameters.Thus, different ultrasound-guided vascular access difficulty levels can be obtained (Figure 10).
To perform puncture and cannulation, a 3-French and 11-cm-length catheter is used, with a 30-cm radiopaque guide and a 5.5-mm needle.Each unit of this model permits the performance of more than 100 punctures without resulting   deteriorated at a cost of approximately 3 €.The model is replicable and reusable, and it lasts for approximately 6 hours at room temperature.When sessions last less than 6 hours, the model can be stored in an airtight container and refrigerated again.This increases its durability without affecting neither the visualization quality nor the puncture technique.
Ultrasound gel or aqueous solution (double-distilled water or saline 0.9%) are recommended to get a better visualization of the vascular structures.
By using a linear probe ultrasound machine, choosing the "preset vascular," and 2-cm depth, the vascular structure in the training model as well as its correlation with the real image "in vivo" in the patient can be observed (Figure 11).
After visualizing the vessel, the depth and diameter of the elastic tubular structure, which is similar to the pediatric patient's vessel, are determined.

Considerations to be highlighted with respect to the training models on ultrasound-guided vascular access
Ultrasound-guided vascular access is a tool to decrease the probability of making errors, the risk of complications, and the number of attempts when performing a vascular access [10,12,13,32].
This technique requires a proper training in order to optimize the learning curve which is characteristic of every invasive procedure [17,18].
Every simulation model, even the most rudimentary and crude, is a useful tool to improve the results of ultrasound-guided vascular access.It will make it possible to minimize the probability of complications and, with respect to the professionals performing this technique, their trust in this technique performance will be increased [11,14,24].
The simulation model described by our research team to perform ultrasoundguided vascular access has been used in more than 800 punctures [8,17,18].After testing most of the previously described models, it could be said that the structure provided by this model is closely similar to pediatric patient's muscular and vascular structures, and that it resembles these patients' vessels anatomy, depth, and diameter with respect to the children's weight.This model improves the learning curve to acquire the skill required by this technique when it is performed by doctors trained in vascular access by following the instructions of the classical technique ("blindly" oriented by anatomic references), by doctors not trained in vascular access or by nurses to cannulate peripherally inserted central catheter [14,24].This model is inexpensive, easy to prepare and transportable, and it permits the visualization of the vascular structures, the measurement of these structures, the ultrasound-guided cannulation, and the visualization of the needle from the different ultrasound axes.It also permits maneuvering in response to access difficulties, such as needle replacement/reinsertion, and checking the correct cannulation by visualizing the guide inside the vascular structure [8].
In addition, this model reproduces the different depth and diameter ranges of children's vessels.So, taken together with the above, each simulation performed by using this model can be used to develop the skills required to perform ultrasoundguided vascular accesses [19,20].

Figure 2 .
Figure 2. Ultrasound image of internal jugular vein and carotid artery (Blue Phantom® model).

Figure 4 .
Figure 4. Gelatine model: Penrose drains using different water-soluble colorants, which are fixed to different gelatine layers (image courtesy of Dr. Vicente Roqués).

Figure 5 .
Figure 5. Ultrasound image of transversal axis (left) and longitudinal axis (right) within the gelatine model.

Figure 9 .
Figure 9. Left: Puncture trocar passing longitudinally through the model muscle tissue.Middle: Drain suture on the side of the vascular structure which is isolated distal portion (details in the upper-right corner).Right: Placement of the vascular structure inside the muscle structure, and after the traction of the drain sutured to the vascular structure.

Figure 10 .
Figure 10.Vessel's diameters (D1-D3) depending on the stretching degree of the elastic structure with its clamps (arrows) and their ultrasound representation.

Figure 11 .
Figure 11.Experimental training model image (left) with respect to the "in vivo" real image (right).

Figure 12 .
Figure 12.Left: Puncture needle in the transverse axis-out of plane.Right: Vascular structure ultrasound image in the transverse axis-out of plane.

Figure 13 .
Figure 13.Vascular visualization and image of the puncture needle in the oblique axis-in plane.

Figure 14 .
Figure 14.Vascular visualization and image of the puncture needle in the longitudinal axis-in plane.

Table 1 .
Training models for ultrasound-guided vascular access: "pros and cons."

Table 2 .
Depth and diameter measurements (in centimeters: cm) of the main central vessels within the pediatric population (n = 300).