Intracardiac Devices, Catheters, and Cannulas

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Intracardiac Devices, Catheters, and Cannulas




image General Concepts


The close anatomic relationship between the esophagus and the mediastinal contents (as well as the descending aorta) allows for transesophageal echocardiographic (TEE) imaging of the various intracardiac devices inherent to the practice of modern cardiac surgery and anesthesiology. The following is a non-exhaustive list of potential applications of TEE in this regard for confirmation of the desired position of:



TEE can also be helpful in detecting obstruction of cannulas and conduits and ensuring the presence of adequate blood flow in the aortic arch during bypass when alternative arterial cannulation sites (e.g., axillary artery) are used.


Ultrasound imaging depends critically on the angle of intercept between the structure or device of interest and the ultrasound beam, with the best visualization obtained when the object of interest lies directly perpendicular to the ultrasound beam. Objects that lie parallel to the beam are poorly visualized because of the lack of reflectivity back to the transducer of ultrasound waves intercepting objects at parallel angles. However, flow through such parallel objects can be detected and quantitated with Doppler methodologies, including color flow mapping.


As with other “tubes” (e.g., aorta), if the ultrasound beam cuts perpendicularly through a cannula in its short axis, that section will be displayed as a circular “donut,” and if the ultrasound beam cuts perpendicularly through a cannula in its long axis, a longitudinal “pipe” consisting of two parallel lines is displayed. An oblique angle of intercept will result in at least partial deflection of the beam at an oblique angle away from the transducer, resulting in poor image resolution. For this reason, if the beam intersects a cannula at an oblique angle, an oblique section with a variable appearance will be displayed.


Most medical devices are made of high-density plastic or metal, which, while highly echogenic, tends to create artifacts in addition to the echocardiographic image of the device itself. On occasion, devices themselves are not well visualized, and only the presence of artifacts suggests that something is present in that location. The most commonly seen artifacts associated with echocardiography of medical devices are shadowing, side lobes and grating lobes, and reverberations. Such artifacts can be mistaken for abnormalities of the cardiac chambers, valves, or great vessels, and the echocardiographer must be aware of this possibility.



• Shadowing occurs when the ultrasound beam fails to penetrate a strong reflector, resulting in an absence of signal in the far field beyond the structure or object. It appears on the echo image as a dark area (shadow) past a bright structure.


• Reverberations occur when a strong reflector causes ultrasound waves to bounce back and forth between the structure or object of interest and the transducer face, creating multiple signals at evenly spaced intervals. On the echo image, they look like comet tails or a series of copies of an object into the far field of the image.


• Side lobe and grating lobe artifacts occur when laterally directed ultrasound emanations return from real structures in the periphery with the main lobe, causing central misplacement of a signal from an actual peripheral structure (e.g., a curvilinear line across the display). The intensity of such reflections is generally far less than those of the main lobe signals, and the artifacts can often be identified when they appear to cross through cardiac structures. Grating lobes are unique to phased-array transducers but are relatively uncommon given the design of modern transducers. Both side lobes and grating lobes can create diagnostic confusion.


Owing to the likelihood of artifacts, it is best to confirm the position of devices in at least two different views to ensure proper placement; three-dimensional (3D) imaging can also be helpful. The easiest way to obtain a different view without changing the position of the probe is by advancing/rotating the multiplane angle. Cannulas imaged in their long axis (tubular appearance) will be imaged in the short axis (circular appearance) or vice versa when the multiplane angle is changed by 90 degrees.



image Cannulas



Cannulation for Cardiopulmonary Bypass


Typically, the aortic cannula is the first one placed by cardiac surgeons when cannulating for cardiopulmonary bypass (CPB). The aortic cannula is commonly placed in the distal part of the ascending aorta, often reaching the proximal arch, but it is generally not possible to visualize it by TEE because this segment of the aorta is coursing anterior to the left bronchus, and the large difference in acoustic impedance between blood/tissue and the air in the bronchus results in reflection of ultrasound waves. In some patients with slightly differing anatomy (or when the cannula is very long or placed very high), it might be possible to image the routine aortic cannula ( Fig. 22-1), and one could certainly use epiaortic scanning if necessary.



Atheromatous plaques have been correlated with strokes and cognitive dysfunction. 1 The ME aortic valve long-axis (LAX) view and upper esophageal (UE) aortic arch LAX view provide an idea of the degree of atheromatous burden near the cannulation site, but only visualization of the actual cannulation and cross-clamp sites with epiaortic scanning by the surgeon can rule out significant disease in those sites, and it is important to do so. 2


TEE also allows for assessment of the aorta for the presence of dissection after cannulation. An aortic dissection might extend proximally into the ascending aorta or distally into the arch or descending aorta. The ME aortic valve LAX view, UE aortic arch LAX view, and ME aortic arch short axis (SAX) view allow imaging of the aorta proximal and distal to the aortic cannula. Once CPB is commenced, assessing the aorta for the presence of flow in the aortic arch and descending aorta is recommended to avoid the possible catastrophic outcome of malperfusion, especially when a non-central cannulation has been performed (e.g., cannulation of axillary or femoral artery). This is typically done in the aortic arch LAX view and descending aorta LAX view using color Doppler. The presence of laminar flow might require the Nyquist limit to be adjusted to visualize flow.



Venous Cannulation


For procedures not involving opening of the heart chambers (e.g., aortocoronary bypass), a two-stage venous cannula is generally inserted through the right atrial appendage (RAA), with the distal orifice directed into the IVC and the proximal orifice positioned in the RA. It is ideal for the tip of the IVC cannula to be located within the IVC, but one does not want it entering a hepatic vein ( Fig. 22-2). It is convenient to image the cannula and locate the tip starting from the ME bicaval view. One follows the cannula from its entrance through the RAA into the IVC. Advancement and slight rotation of the probe to the right will optimize the image of the cannula within the IVC. Non-standard views of the IVC and hepatic veins at whatever multiplane angle is optimal can be used to determine the location of the IVC cannula tip. Ideally, the tip should reside in the lumen of the IVC. If the tip of the cannula enters a hepatic vein, it should be repositioned to avoid poor venous drainage and potential abdominal distention.



For procedures where heart chambers are opened (e.g., valve surgery), the SVC and IVC are often each cannulated directly with two separate cannulas (bicaval technique). The SVC can usually be visualized in its long axis to the right of the display in the ME bicaval view and also in its short axis in the ME aortic valve SAX view. Figure 22-3 shows a cannula in the SVC.



An alternate site of venous cannulation of the RA is through the femoral vein, using a Seldinger technique. Femoral vein cannulation requires a long multistaged cannula that courses from the IVC to the SVC. Correct placement into the SVC begins first by imaging the guidewire as it comes up the IVC, crosses the RA (use ME bicaval view), and enters the SVC ( Fig. 22-4). The cannula is then advanced over the wire. Care should be taken when there is a patent foramen ovale (PFO) to ensure the wire does not inadvertently enter the LA. For optimal venous drainage, it is necessary to confirm the tip of the cannula ultimately lies in the SVC.




Cannulas for Partial Cardiopulmonary Bypass


Partial left heart bypass used for repair of descending aortic aneurysms requires placement of a venous inflow cannula in the LA (often from a left pulmonary vein) ( Fig. 22-5) and a femoral arterial (FA) outflow cannula. In this “LA-FA bypass,” the heart and lungs maintain oxygenation of the blood in the body, the native cardiac output from the LV perfuses only the upper body, and the outflow from the LA-FA bypass provides what is called distal perfusion (oxygenated blood is drained from the LA and returned to the lower body distal to the aneurysm to perfuse the spinal cord and viscera from below). The conduct of the partial bypass then requires a balancing of the upper and lower circulations by the perfusionist. For example, if the upper pressure is too high, a higher flow is used to drain the upper body and perfuse the lower until the pressures equalize. If the upper body pressure is too low, lower flows are used until the pressure in the circulations are optimally balanced.



The ME four-chamber and ME two-chamber views are used to confirm proper position of the LA cannula. Improper positioning of the LA cannula tip into a right pulmonary vein will restrict the amount of blood that can be perfused into the lower body and will also volume overload the upper body circulation. The LA cannula can also be improperly positioned into the LV through the mitral valve, with the possibility of inducing severe mitral regurgitation and reduced cardiac output to the upper body.



Cannulas for Cardioplegia


Unless there is significant aortic insufficiency, antegrade cardioplegia is generally given via a small catheter positioned in the root of the aorta proximal to the cross-clamp. Although the cardioplegia cannula is not commonly imaged, TEE is helpful in evaluating and assessing the degree of ventricular distention that occurs during infusion of antegrade cardioplegia in the setting of aortic regurgitation (which will also result in poor cardioplegia distribution down the coronaries). When the ventricle is distended, the cardioplegia is not fully distributed in the endocardium, and the wall tension is increased in the LV (which increases the myocardial oxygen demand), rendering myocardial protection suboptimal. The ME LAX view allows visualization of the degree of cardioplegia regurgitation and the volume inside the ventricle. The TG midpapillary SAX view is used to assess the degree of LV distention.

Related posts:

  1. Principles and Physics: Transducer Characteristics
  2. Principles and Physics: Principles of Ultrasound
  3. Echocardiographic Evaluation of Pericardial Disease
  4. Myocardial Ischemia and Aortic Atherosclerosis

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Jun 12, 2016 | Posted by in CARDIOLOGY | Comments Off on Intracardiac Devices, Catheters, and Cannulas

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