1 What is an intraaortic balloon pump (IABP)?

An IABP is a circulatory assist device that is intended to provide temporary support to a failing heart through synchronized actions focused on reducing afterload and augmenting diastolic pressure. The system consists of a 30- to 50-cc balloon attached to a catheter. The balloon rests within the lumen of the descending thoracic aorta, and the attached catheter snakes through the arterial vessels and out of the body. The catheter relays with an external console that monitors aortic pressure tracings and electrocardiogram (ECG) readings, inflating the internal balloon at the appropriate times. The balloon is inflated with either helium or CO₂. Although CO₂ has higher blood solubility and therefore a lower risk of embolization, helium is much more common because it is less dense and thus allows for faster filling and emptying times.

2 How is an IABP placed?

The most common site for IABP placement is through the femoral artery via the percutaneous Seldinger technique. Alternative insertion sites include the iliac, axillary, and subclavian arteries. IABP placement may even be accomplished via the ascending aorta, as is occasionally done after cardiac surgery. The ideal location for the tip of the balloon is just distal to the aortic arch, 1 to 2 cm beyond the left subclavian artery. The other end of the balloon should sit proximal to the takeoff of the celiac axis. Correct positioning of the balloon may be confirmed at the time of placement with either transesophageal echocardiography or fluoroscopy. Alternatively, confirmation may be performed after placement by chest radiograph.

3 What are the physiologic benefits of an IABP?

The IABP is designed to inflate during diastole and deflate during systole. It is beneficial in both phases of the cardiac cycle. During diastole, inflation of the balloon displaces blood proximally, increasing perfusion pressure to the coronary arteries and the critical vessels branching off the aortic arch. Perfusion and blood flow are equally improved in the aorta distal to the balloon through the vessels supplying blood to the mesentery and the lower extremities. In systole, deflation of the balloon creates a relative negative space within the aorta, which reduces afterload. As a consequence, not only is cardiac function improved because of lower systemic resistance, but the duration of isovolumic contraction, the most energy-demanding phase of the cardiac cycle, is also reduced.

4 How is IABP inflation-deflation timing coordinated?

Three possible triggers are routinely used to coordinate IABP inflation-deflation:

The ideal time for balloon inflation is at the onset of diastole, just after the closure of the aortic valve. This moment corresponds with the middle of the T wave on the ECG and with the dicrotic notch on the arterial pressure waveform. Deflation ideally occurs at the beginning of systole, which coincides with the peak of the R wave on the ECG and with the point just before the systolic upstroke on the arterial waveform. Alternatively, an option exists to run the IABP in an asynchronous mode with a preset rate, but it is not routinely used. Some recent work has also been done on real-time dicrotic notch detection and prediction, which may improve IABP timing during both regular and irregular rhythms.

5 What are the indications for IABP placement?

Clinical conditions that may warrant initiation of IABP therapy can be broken down into two categories: those conditions that would benefit primarily from increased diastolic coronary perfusion and those that would benefit mainly from afterload reduction. Balloon pumps may also be placed prophylactically in high-risk patients undergoing cardiac surgery (Box 19-1). The most common condition for which IABP therapy is started is cardiogenic shock, accounting for approximately 20% of placements.

6 What are the contraindications for IABP placement?

The contraindications for IABP placement include the following:

7 What are possible complications of IABP placement?

Complications related to the use of IABP fall into three categories: vascular, positional, and balloon related.

8 Is anticoagulation necessary in a patient with an IABP?

The standard of care is that unfractionated heparin be used for anticoagulation during the IABP use, with a partial thromboplastin time goal from 50 to 70 seconds. Data are inconclusive as to whether this actually decreases the incidence of limb ischemia; however, in animal studies an immobile, deflated balloon is subject to thrombus after 20 minutes. For this reason, a nonfunctional balloon should be removed promptly, as soon as anticoagulation has been reversed. For elective removals, it is recommended that heparin be discontinued for at least 2 hours before taking out the device.

9 How is a patient weaned from an IABP?

The amount of aid a balloon pump provides can be quantified as a ratio of native beats to assisted beats. Full support is at a ratio of 1:1; that is, every beat is augmented by the IABP. Weaning occurs by gradually reducing the augmentation ratios to 1:2, 1:4, and then 1:8. Time intervals between step-downs vary depending on the clinical situation but are usually on the order of 1 to 6 hours. During weaning, cardiac output, blood pressure, mental status, kidney function, and distal perfusion should be monitored. It may be necessary to commence inotropic support as the patient is weaned. As the augmentation ratio is reduced, the risk of thrombus formation on the balloon pump increases. Weaning may also be accomplished by gradually reducing the balloon volume.

10 What are the clinical criteria for IABP removal?

Actual criteria will vary from institution to institution. Some of the recommended clinical criteria for the removal of an IABP include the following:

Systemic anticoagulation should be corrected before removal of IABP. The balloon pump may be removed percutaneously, with direct pressure being applied to the insertion site for 20 to 30 minutes after removal to assure hemostasis.

11 What does an arterial pressure tracing look like in a patient being assisted by an IABP?

The function of an IABP during diastole results in an upswing in the arterial pressure waveform during diastole (Fig. 19-1). The peak created by the IABP is known as the peak augmented diastolic pressure. The upstroke created by the IABP, if timing is correct, correlates with the dicrotic notch. When maximal augmentation by the IABP is achieved, the peak augmented diastolic pressure will be greater than the unassisted systolic pressure. In addition, both the assisted aortic end-diastolic pressure and the assisted systolic pressure should be less than their unassisted counterparts, given appropriate IABP function.

Figure 19-1 A and D, Unassisted systolic pressure. B, Diastolic pressure augmentation by IABP. C, Unassisted aortic end-diastolic pressure. C′, Assisted (post-IABP) aortic end-diastolic pressure. D′, Assisted (post-IABP) systolic pressure.

12 What are some common problems that may result in failure of the IABP to augment?

Failure to augment may be associated with inadequate inflated balloon size, as a result of either a balloon that is too small or an appropriately sized balloon that is being underfilled. A balloon that is misplaced too proximal or too distal in the aorta might not augment appropriately, as will a balloon inadvertently placed in a false passage (possibly created with insertion of the device). Inappropriate timing of inflation or deflation of the balloon, which may be either too early or too late, can impair augmentation (Table 19-1). Improper timing can often be corrected manually on the IABP console. Both tachycardia and bradycardia can result in failure to augment, as can certain arrhythmias such as atrial fibrillation. These rhythm abnormalities should be treated to allow for maximal effectiveness of the IABP.

^{Table 19-1} Results of Inappropriate IABP Timing

13 What is a ventricular assist device (VAD)?

In general, VADs are mechanical devices inserted to assist cardiac function by offloading part or all of the pumping responsibilities from the ventricle. Placement of a VAD can be done on the left side of the heart to assist in left ventricular function (LVAD) or on the right to help with the right ventricle (RVAD). The presence of both an LVAD and an RVAD is referred to as biventricular support (BiVAD). A number of different VAD constructs exist, with major differentiators being pulsatile versus nonpulsatile (continuous), external versus internal, degree of assistance provided, ability to help the left or right sides, and the length of time it can be used (Table 19-2). Some VADs are intended to be used for only 7 to 10 days, whereas others have supported patients over a 7-year period.

14 What are the end goals of VAD therapy?

When deciding the appropriateness of the placement of a VAD, multiple potential end points can be considered. A “bridge to recovery” implies temporary VAD support with the goal of removal once native heart function has returned to baseline. A “bridge to transplant” refers to the use of a VAD in a patient who ultimately will be best served by cardiac transplantation but needs temporary assistance until a heart becomes available. The term “destination therapy” means that the VAD itself is the end goal of therapy. These patients have irreversible heart conditions and, for whatever reason, are not eligible for transplantation.

15 What are common insertion sites for LVADs and RVADs?

The VAD inflow cannula permits blood flow to the pump. It may be placed in the right atrium for an RVAD or the left ventricular apex for an LVAD. After going through the VAD pumping system, blood exits the device by passing through an outflow conduit. For RVADs, these are attached and direct blood flow to the pulmonary artery. In LVADs, outflow conduits are usually connected to the ascending aorta. Newer LVADs exist that are entirely endovascular and sit across the aortic valve.

16 What are the differences between pulsatile and continuous systems?

VADs (and other bypass systems) can be divided into two classes based on how they eject blood from the pump: in a continuous or pulsatile manner. Continuous systems typically use either a roller pump or a rotary pump to propel blood. Roller pumps, while inexpensive, generally require continuous vigilance and tend to cause more trauma to blood components. As a result, they are rarely, if ever, used in VADs. The more practical rotary pumps can be further divided into subsets based on mechanism of propulsion. The two most popular subsets are axial (screwlike) and radial (better known as centrifugal) systems. Pulsatile VAD systems generally provide blood propulsion via pneumatically or electrically powered systems. They do not cause significant hemolysis and may have physiologic advantages over nonpulsatile systems. They do, however, tend to be larger than many of the continuous systems.

17 What is the difference between extracorporeal membrane oxygenators (ECMO) and a VAD?

ECMOs are very similar to VADs in terms of function and placement. The major difference is that a VAD requires that lung function be adequate, because it only helps with cardiac function, depending on the lungs to oxygenate the blood. ECMO incorporates an oxygenator that maintains gas exchange and is often used in clinical situations in which the lungs are no longer effective in maintaining oxygenation or ventilation. In addition, unlike ECMO, many VADs are created with portability in mind.

18 What are the indications for VAD placement?

No consensus criteria exist for placement of a VAD. As a result, indications may vary from institution to institution. However, guidelines do exist to assist in optimizing patient selection. Examples of parameters for consideration include the following:

Consideration may also be given to patients with cardiac function that is better than the parameters listed above but who have unpredictable, life-threatening ventricular arrhythmias.

19 Are there any contraindications for VAD placement?

Absolute contraindications include abdominal aortic aneurysm > 5 cm, active systemic infection or high chronic risk of infection, severe pulmonary dysfunction (FEV₁ < 1 L or fixed pulmonary hypertension), impending or actual renal or hepatic failure (including portal hypertension), inability to tolerate anticoagulation, or neurologic or psychosocial inability to manage the device. Clearly, a coexisting terminal condition contraindicates VAD placement in a patient. Relative contraindications include age > 65 years (unless minimal other risk factors); chronic kidney disease; severe chronic malnutrition; morbid obesity; or uncorrected aortic regurgitation, mitral regurgitation, or mitral stenosis.

20 What are the significant potential complications that arise in patients with VAD?

Potential complications are not infrequent and can be quite serious. They include infection, neuroembolic events, and bleeding. As model design has improved, complication rates have decreased. Infection rates lessened with exploration of alternative implantation possibilities, which sometimes eliminated the need for external or peritoneal device components. Also, innovations such as sintered (ridged) titanium materials, which promote pseudointimal layer formation on device components, as well as investigation into axial flow mechanics, helped to decrease the incidence of thromboembolic complications. Secondary organ failure (respiratory, renal) does remain a concern.

21 Why is it that some patients who have an LVAD placed subsequently require an RVAD?

Placement of an LVAD offloads some or all of the work from the left ventricle (LV). It can increase cardiac output and, as a result, can increase the venous return to the right side of the heart. This increase in right ventricular preload, along with relative emptying of the LV, can result in right ventricle (RV) distention, worsening RV contractility, and increased tricuspid regurgitation, all of which decrease RV performance. In addition, significant cytokine release can take place during the perioperative period. These cytokines can mediate pulmonary vasoconstriction, placing further stress on the RV. It is important to monitor vigilantly for impending right-sided heart failure, because appropriate pharmacologic intervention may help avoid additional surgical intervention.

22 List the major considerations for intensive care unit management of a patient directly after an LVAD placement

As mentioned above, optimization of right-sided heart function is critical in the immediate postoperative period after LVAD placement. Inotropic support should be provided as needed to assure optimal right-sided heart functionality. In addition, decreasing right-sided heart afterload through the pulmonary vasculature can assist in right side of the heart performance. Milrinone, epoprostenol, and nitric oxide all have their roles in decreasing pulmonary vascular resistance in the postoperative period. With continuous-flow LVADs, it is also important to be aware of and know how to manage suction events. These events can occur when the LVAD motor speeds are high and LV venous return is low and are a result of a collapse of the ventricle.

23 How is weaning attempted with a VAD?

When a VAD is used as a bridge to recovery, intermittent assessments of cardiac function are performed to determine the necessity of continued VAD use. Methods of assessment include echocardiography, radionucleotide studies, and exercise stress testing. As cardiac function returns, the amount of support, defined by the VAD flow rate, can gradually be reduced. As VAD flows are decreased, the RV is allowed to fill with blood. Indicators of successful weaning include ability to maintain cardiac output without increases in central venous or pulmonary artery pressures. Once VAD support is weaned to 1 to 1.5 L/min, patients can usually tolerate discontinuation of the device.

24 Do percutaneous options exist for ventricular assist systems?

Several devices can be inserted by cardiologists in the catheterization laboratory or percutaneously by cardiac surgeons in the operating room. One such device is the tubelike Impella, which is situated across the aortic valve and functions via a nonpulsatile axial flow mechanism. Another is the TandemHeart. This VAD’s inflow cannula is placed through the femoral vein, and then, via transseptal puncture, the tip of the cannula is positioned in the left atrium. Blood taken from the left atrium is pumped by an external centrifugal pump back into circulation via a cannula placed in the femoral artery. Flow rates up to 5 L/min can be achieved, and this temporary device is typically used as a bridge to recovery or bridge to bridge therapy.

25 What is the REMATCH study?

The REMATCH (Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure) study refers to a landmark New England Journal of Medicine article from Columbia University in 2001, entitled “Long-term Use of a Left-Ventricular Assist Device for End-stage Heart Failure.” The article described a 3-year study in 129 patients with end-stage heart failure who were ineligible for cardiac transplantation and were randomly assigned to two groups: optimal medical management or LVAD therapy. Data showed significant increases in 1- and 2-year survival rates of the LVAD cohort over the medical therapy cohort (52% vs. 25% and 23% vs. 8%, respectively), as well as an improved quality of life.