Cardiac Mechanical Assist Devices

FIGURE 96.1 Example of standard 7.5-French 40-mL intra-aortic balloon pulsation.

It is also important to select the appropriate-sized balloon pump for the patient. Balloon pumps are manufactured in various sizes, ranging from those appropriate for a small infant to those used in large adults (Fig. 96.4); the standard adult size is 40 mL. The manufacturer’s labeling and recommendations and the patient’s habitus should be considered in size selection.

Mechanism of Action

There are two complimentary effects of an intra-aortic balloon pump. The first is the diastolic augmentation of coronary blood flow. The balloon pump is carefully timed to the cardiac cycle so that the pump inflates during aortic valve closure and thus enhances the diastolic pressure in the proximal aorta. This allows increased coronary blood flow and enhanced myocardial oxygen delivery. The second action involves the afterload reduction at the time of cardiac systole, thereby allowing enhanced runoff for the failing ventricle. The balloon should be properly timed to deflate during aortic valve opening, thereby creating a pocket of reduced afterload and thus enhancing the ability of the heart to eject the blood during systole. This action serves to lower the left ventricular end-systolic volume. Both of these mechanisms augment left ventricular function and serve in complementary fashion to help those patients with right ventricular dysfunction. It is a common misperception that a balloon pump is not beneficial, or indicated, for patients with right ventricular dysfunction. In clinical practice, patients with right ventricular dysfunction benefit from the diastolic augmentation of coronary blood flow in the right coronary artery. Additional benefit is derived from the reduced left ventricular end-diastolic pressure and left atrial pressure, thereby allowing increased forward flow and decreased afterload for the failing right ventricle.

FIGURE 96.2 Insertion of intra-aortic balloon pulsation (IABP) from femoral approach. A: Artery is accessed and wire advanced. B: Sheath inserted over wire. C: IABP advanced over wire and through sheath to appropriate level.

FIGURE 96.3 Chest radiograph showing intra-aortic balloon pulsation in place. Distal tip is marked.

FIGURE 96.4 Different sizes of intra-aortic balloon pulsation devices.

Complications

Although complications related to balloon pump insertion and use are relatively infrequent, they can be quite serious. The first potential complication can occur during insertion of the balloon pump and involves direct trauma to the arterial insertion site. It is important that insertion occurs relatively high in the thigh in the femoral artery, near the inferior edge of the inguinal ligament. It is possible for a misplaced balloon to shear off one of the major arterial branches. Misplacing the balloon through a smaller-caliber artery may completely occlude the artery and create an ischemic limb. In arteries that are not particularly calcified, it is relatively easy to use the dilator and place the balloon pump without the use of a vascular sheath. This serves to reduce the maximal diameter of obstruction in the artery, and perhaps reduces the potential for thrombus within the artery.

There is also the potential for mishap in both placement and location of intra-aortic balloon pumps. It is important to remember that when the balloon pump is placed as part of a cardiopulmonary bypass procedure, the blood is equally oxygenated in the arterial and venous vessels because of the bypass oxygenator. Additionally, there is zero pulsatile flow at the time of insertion, making it difficult to differentiate artery from vein. This situation has led some to inadvertently place balloon pumps via the femoral vein into the right atrium. Additionally, hemodynamic instability at the time of insertion may also lead to suboptimal confirmation of balloon pump location. Improper placement can thus lead to impingement of the arch vessels, causing cerebral ischemia or thromboembolism.

In more chronic management of the balloon pump, ranging from several days to weeks or months, infectious complications become more predominant. In addition to meticulous sterile insertion techniques, balloon pumps also require daily attention from the nursing staff. Like any other percutaneously inserted central catheter, they have the potential to become a source of infection. Attention to the insertion site for signs of erythema or purulence and close monitoring of the patient’s temperature is mandatory in all patients using a balloon pump. When a patient’s hemodynamic status fails to stabilize with balloon pump therapy, a VAD is the next course of progressive therapy.

VENTRICULAR ASSIST DEVICES

Preoperative Considerations

Indications for VADs include unstable hemodynamic measurements and failure to stabilize measurements with other less invasive therapies previously discussed. Common hemodynamic parameters that are indications for a VAD placement are listed in Table 96.1.

In the preoperative assessment, it is important to determine the likelihood that right ventricular support will be needed as a course of therapy. There are several scoring systems that are commonly used (4–6). Most of these scoring systems center on the calculation of right ventricular stroke work index and other hemodynamic indices such as transpulmonary gradient, right atrial pressure, and tricuspid annular plane excursion (7,8). Generally, practitioners should use caution if the central venous pressure is greater than the pulmonary capillary wedge pressure, or if the patient’s central venous pressure is greater than 20 mmHg after optimization. Dependence on the right ventricle to support a left-sided device in such instances may prove to be difficult. It is also important to look at the overall illness of the patient. Patients who are very debilitated at the time of implantation, with organ deterioration caused by right heart dysfunction, are more likely to require right-sided support devices. Thus, it is important to take elevated liver enzymes, abnormal coagulation parameters, and renal dysfunction into consideration.

Finally, it is important to select the device based on the goal of implantation. Devices may be implanted as a bridge to recovery, a bridge to transplantation, or as a destination therapy. The lines between bridge to recovery and bridge to transplantation can sometimes become blurred when the neurologic status of patients cannot be defined prior to implantation. These patients have been termed bridge to decision, where a short-term device may be appropriate to stabilize hemodynamics until the neurologic status and overall candidacy for transplantation is better elucidated. Finally, in patients who are not candidates for transplantation because of age or end-organ dysfunction, consideration of destination therapy may be appropriate. Destination therapy refers to permanent device implantation, intended to remain in use for the duration of the patient’s lifetime. Patients may migrate between these defined groups based on a center’s transplant protocols.

TABLE 96.1 Hemodynamic Parameters Suggesting Need for Mechanical Support

Preparation of the Patient

Before the operative implantation of the VAD, it is often useful to have a period of volume optimization, or a preoperative “tune up.” This is done to ensure that right ventricular function is as well preserved as possible for placement of a left VAD, and may include diuretic and inotropic therapy. In some cases, where the decision for biventricular support is difficult, a 24- to 48-hour period of intra-aortic balloon pumping may be a useful prognostic indicator. This helps to demonstrate the response of the right ventricular function to a reduced left ventricular end-diastolic pressure (9). During this period of optimization, it is ideal to use an arterial pressure monitor and a pulmonary artery catheter to allow fine tuning of medications and volume status.

It is essential that attention be given to antimicrobial prophylaxis during the period of preoperative optimization. This usually involves selective skin decontamination with Hibiclens scrub (Regent Medical, London, UK). Additionally, Bactroban (mupirocin) is often used to reduce the number of pathogens in the nasal passages of the patient. It may also be useful to use red cell augmentation in patients who are having semielective implants, as frequently there may be a 5- to 7-day delay before implantation, during which time erythropoiesis-stimulating drugs, such as erythropoietin, can be combined with iron supplementation to achieve a significant increase of hematocrit.

Classification of Ventricular Assist Devices

Flow Type

The devices may be classified by the type of flow:

Pulsatile: In these devices, the intermittent relocation of a pusher plate or blood sack emits a pulsatile wave similar to that of the natural heart (Fig. 96.5).
Axial flow: The term nonpulsatile is frequently applied to these devices, although this is a misnomer. These pumps actually have a central blade that rotates at a rapid rate, similar to a jet engine in an airplane (Fig. 96.6). The native function of the left ventricle does intermittently augment the inflow to the pump, which generates a pulsatile output at appropriate speeds.
Centripetal flow: These pumps have a continuous spinning impeller that generates a flow similar to axial pumps. However, the more advanced pumps may be magnetically levitated to function without bearings (Figs. 96.7 and 96.8).

Mechanism

Pneumatic: These pumps are operated by air, where intermittent external application of compressed gas through a tube to a blood sac emits the pulse of the pump.
Electric: Electric pumps are driven by batteries or AC current via an adapter. They may have the axial flow motor or the pusher plate-driven motor (see Figs. 96.5 and 96.6).