Intra-Aortic Balloon Pumps and Ventricular Assist Devices
Laura Downey
▪ INTRODUCTION
Cardiac disease continues to be the number one cause of death in the United States, accounting for 34.3% of all deaths in 2006. Over 81.1 million American adults carry the diagnosis of one or more types of cardiovascular disease. As the incidence of cardiac disease grows, an increasing number of patients with heart disease undergo anesthesia and surgery every year for various procedures. These procedures range from coronary artery revascularization, coronary angioplasty, valve replacement surgery, repair of aortic aneurysms, correction and palliation of congenital heart disease, to heart transplantation.
Despite the improvement in medical and surgical interventions for cardiovascular disease, the aging U.S. population has led to a steady rise in the incidence of heart failure. Data from the 2010 Centers for Disease Control and Prevention (CDC) Heart and Stroke Update estimated that approximately 5.8 million people are living with heart failure and approximately 25% have advanced or end-stage heart failure. The 1-year mortality rate for patients with heart failure is estimated at 20%, while the 5-year mortality rate is estimated to be as high as 59% for men and 45% for women. While advances in medical and surgical therapy have been effective in reducing the mortality and morbidity from heart failure, there is a subset of patients who develop severe cardiac failure in the setting of myocardial infarction (MI), following cardiopulmonary bypass, trauma, or advanced cardiomyopathies. Medical therapies for these patients are often limited, and patients may require artificial support to maintain their cardiac output and perfusion to their vital organs while awaiting recovery or definitive surgical treatment. The most common devices used to support these patients until recovery or heart transplantation are an intra-aortic balloon pump (IABP) or a ventricular assist device (VAD).
This chapter examines at the indications, setup, operation, maintenance, and troubleshooting for these devices. Depending on the institution, these devices may be managed by anesthesia technicians, cardiovascular technicians, or perfusionists. Of note, both intra-aortic balloon pumps and various VADs often require extra training to learn the specifics of individual devices. Therefore, please follow your institution’s requirements and ensure that you are properly trained in the equipment prior to using the device on a patient.
▪ AORTIC BALLOON PUMPS
The IABP is a device that augments blood flow to the heart by inflation and deflation of a balloon that sits in the thoracic aorta (Fig. 42.1). This device is used to augment cardiac output and coronary blood flow in patients with cardiogenic shock. The indications for an IABP include (1) cardiogenic shock, (2) failure to separate from cardiopulmonary bypass, (3) stabilization of a patient prior to the operating room (OR), or (4) as a bridge to transplantation. See Table 42.1 for a listing of the indications and contraindications for IABP placement. IABPs are usually placed by cardiothoracic surgeons or cardiologists in the cardiac catheterization lab, OR, or in the intensive care unit (ICU) where appropriate monitoring and emergency equipment are available.
▪ CARDIAC CYCLE AND CORONARY PERFUSION
As IABP therapy is based on the timing of the cardiac cycle, it is important to review the two major phases of the cardiac cycle: diastole and
systole (Fig. 42.2) (see Chapter 7). These phases are important for understanding the mechanism of the IABP therapy because it uses the cardiac cycle to augment cardiac output during systole and coronary blood flow during diastole, a process called counterpulsation.
systole (Fig. 42.2) (see Chapter 7). These phases are important for understanding the mechanism of the IABP therapy because it uses the cardiac cycle to augment cardiac output during systole and coronary blood flow during diastole, a process called counterpulsation.
 ▪ FIGURE 42.1 Cutaway of heart and aorta showing placement of an IABP in aorta just distal to the subclavian artery. |
Diastole
The onset of diastole is noted by the relaxation of the ventricles. As the ventricular pressure falls below the aortic and pulmonary artery pressure, the aortic and pulmonic valves close. During the period of isovolumetric relaxation, the pressure in the ventricles is still greater than that in the atria. Therefore, the mitral and triscuspid valves remain closed and the ventricular volume does not change. At the end of diastole, the pressure is called the left ventricular end-diastolic pressure (LVEDP). Coronary artery perfusion occurs during diastole.
Coronary Perfusion
It is important to understand that coronary perfusion occurs only during diastole, when the wall tension or the LVEDP is the lowest. During systole, the pressure generated by the contraction of the myocardium may completely stop blood flow to the coronary bed. When the diastolic blood pressure is higher than the LVEDP, blood flows into the coronary arteries, resulting in coronary perfusion.
Systole
At the beginning of systole, the ventricles are full of blood from the previous diastolic filling period and the ventricles begin contracting, a period called isovolumetric contraction. As the pressure in the ventricles rises higher than the atrial pressure, the mitral valves and tricuspid valves close. The period of isovolumetric contraction accounts for approximately 90% of the myocardial oxygen consumption. Eventually, enough pressure is generated to open the aortic and pulmonary valves, leading to rapid ventricular
ejection, where approximately 65%-75% of the stroke volume is ejected. After the blood is ejected, the pressure in the ventricles drops dramatically, but blood continues to flow into the aorta until the end of systole. The end of systole is signaled by the onset of myocardial relaxation and closure of the aortic valve.
ejection, where approximately 65%-75% of the stroke volume is ejected. After the blood is ejected, the pressure in the ventricles drops dramatically, but blood continues to flow into the aorta until the end of systole. The end of systole is signaled by the onset of myocardial relaxation and closure of the aortic valve.
TABLE 42.1 INDICATIONS AND CONTRAINDICATIONS FOR IABP USE | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ▪ FIGURE 42.2 The cardiac cycle. Changes in aortic pressure, left ventricular pressure, left atrial pressure, left ventricular volume, the electrocardiogram (ECG), and heart sounds. (From Porth CM. Pathophysiology Concepts of Altered Health States. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2005, with permission.) |
▪ HOW DOES AN IABP WORK: COUNTERPULSATION HEMODYNAMICS
The IABP is a mechanical device that increases blood flow to the coronary arteries during diastole and increases cardiac output during systole. The device is a flexible catheter with a balloon at the end that sits in the descending aorta just distal to the left subclavian artery. The balloon augments blood flow by volume displacement and pressure changes associated with rapidly injecting helium gas in and out of the balloon chamber, a principle called counterpulsation (Fig. 42.3). During diastole, the balloon is filled with helium (approximately 40 mL depending on the balloon size), thus causing the blood to be displaced by the increased balloon volume. This causes the aortic pressure to increase, which increases the driving pressure into the coronary arteries. The result is improved blood flow to the coronary arteries. During systole, the reverse occurs. The balloon deflates and the blood flows forward to fill the evacuated space. The fall in pressure decreases the amount of pressure the failing left ventricle (LV) has to generate, thus decreasing the oxygen demands of the heart and increasing cardiac output by as much as 40%. The balloon inflation can be triggered by the
patient’s electrocardiogram, a pacemaker, a set rate, or the patient’s blood pressure. Key Points:
patient’s electrocardiogram, a pacemaker, a set rate, or the patient’s blood pressure. Key Points:
 ▪ FIGURE 42.3 Counterpulsation. The image on the left shows thorax with intra-aortic balloon catheter introduced via the femoral artery. The image in the center shows aorta with catheter inflated as in diastole. The image on the right shows aorta with catheter deflated as in systole. (LifeART image copyright (c) 2012 Lippincott Williams & Wilkins. All rights reserved.) |
During diastole, the balloon inflates and augments coronary perfusion.
At the beginning of systole, the balloon deflates and increases cardiac output while decreasing myocardial oxygen consumption.
The balloon is inflated with helium, a gas that can be easily absorbed by the body without damage in the case of balloon rupture.
▪ SETUP AND PLACEMENT
Prior to placement, a thorough physical exam is performed to assess the circulation to both legs and determine the best side for insertion. The femoral artery is accessed with a needle and a guide wire is placed through the femoral artery and into the thoracic aorta (see Chapter 34). The puncture site is then dilated with successive placement of a dilator/sheath combination until the balloon can be threaded through the puncture site into the central aorta. The balloon sits in the aorta, approximately 2 cm from the left subclavian artery and above the renal artery branches. A chest x-ray or fluoroscopy is used to confirm proper placement. Daily chest x-rays demonstrate the tip at the 2nd and 3rd intercostal spaces. Depending on the institutional practice, patients may be fully anticoagulated while the IABP is in place, in order to prevent clot formation on the balloon. However, anticoagulation may increase the risk of bleeding, especially in the postsurgical setting.
While IABPs are usually placed by cardiothoracic surgeons or cardiologists, there is a wide variation in the technicians who may set up and prepare the IABP kits: perfusionists, scrub technicians, anesthesia technicians, and cardiovascular techs. Before setting up an IABP, be sure to read the instruction manual for details specific to the model you will be using as there are many different companies that manufacture IABP systems and each one will operate in a slightly different way. The general setup includes the following:
Electrocardiogram (ECG) leads
Arterial blood pressure waveform monitoring
Balloon volume monitoring
Electric console to adjust triggering and inflation timing of the balloon
Battery backup power
Gas reservoir
Details of equipment setup for IABPs are listed in Table 42.2.
▪ OPERATION AND MANAGEMENT OF AN IABP
For the optimal effect of counterpulsation, the inflation and deflation of the balloon must be correctly timed to the cardiac cycle (Fig. 42.4). This is usually achieved by triggering the balloon’s inflation based on the patient’s ECG signal or the arterial waveform. Usually, the triggering signal is from the R wave on the ECG. The balloon is set to inflate in the middle of the T wave, coinciding with the aortic valve closure and beginning of diastole. Deflation occurs prior to the R wave, noted on the arterial waveform just before the arterial upstroke (Fig. 42.5). The balloon augmentation usually starts at a beat ratio of 1:2, which means every other beat is augmented by the IABP. This allows the provider to compare the patient’s inherent ventricular beats with the augmented beats and adjust the timing as necessary. A health care provider trained in IABP therapy will likely assess the effectiveness of the IABP and adjust the timing appropriately. Figure 42.6 demonstrates an arterial waveform generated by a correctly positioned and timed balloon.
TABLE 42.2 PLACEMENT OF AN IABP | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
