Adam A. Dalia and Michael G. Fitzsimons
CHAPTER 20
1. What is an intra-aortic balloon pump?
An intra-aortic balloon pump (IABP) is a device that is placed in the aorta in order to improve coronary perfusion during diastole and facilitate ejection during systole. The balloon is inflated during the diastolic phase of the cardiac cycle, displacing blood towards the aortic root and coronary arteries and increasing diastolic coronary perfusion. The balloon is deflated at the onset of systole, allowing easier ejection of blood and a reduction in myocardial work.
2. What are the components of an Intra-Aortic Balloon Pump System?
The IABP system contains a signal processor including an amplifier and display, infusion flushing system, helium gas pump, a pressure transducer, and an intra-arterial cannula with an attached polyethylene balloon. The balloon comes in various sizes chosen based on patient height with filling volumes ranging from 25 to 50 mL. The vascular access sheath ranges in size from 7 to 9 Fr.
3. What gas is used for intra-aortic balloon pump inflation and why?
Helium is less dense than oxygen at room temperature (0.164 kg/m3 vs. 1.33 kg/m3). The decreased density allows the rapid inflation of the balloon to occur under laminar flow conditions by decreasing the Reynolds number, and has the advantage that if flow within the balloon becomes turbulent, the lower density helium is preferred. Concurrently, if the balloon becomes damaged or leaks in vitro, helium is absorbed rapidly into blood.
4. How is proper placement of an intra-aortic balloon pump performed and confirmed?
The most common placement site for placement of an IABP is the femoral artery. Other sites have been described including the iliac, subclavian, axillary, and even brachial artery. A percutaneous Seldinger technique is most common to place a vascular sheath. The IABP is then advanced through the sheath until the balloon tip is just distal to the left subclavian artery and above the renal arteries.
IABPs can be placed blind, under live fluoroscopy, or in the operating room with transesophageal echocardiography (TEE) guidance. Confirmation is generally made via chest x-ray. The appropriate position of the tip is 1.5 to 2.0 cm below the aortic knob.
5. What is the mechanism of action behind an intra-aortic balloon pump?
The IABP works to improve diastolic blood pressure by increasing myocardial oxygen supply (increased coronary perfusion pressure) while decreasing myocardial oxygen consumption (decreased work through decreased afterload during systole). This is accomplished through a counter-propulsion mechanism. The IABP is triggered to inflate after aortic valve closure, which improves blood flow in the aortic root, thereby raising diastolic pressure resulting in increased coronary perfusion pressure. Balloon deflation occurs immediately preceding aortic valve opening resulting in a lower end-diastolic pressure. The pressure the ventricle must generate in order to open the aortic valve in systole is reduced.
6. Name the four ways an intra-aortic balloon pump is triggered and factors which may impair triggering.
Triggering of the IABP refers to the specific indicator for the IABP to inflate and deflate. There are four
“triggers”, the electrocardiogram (EKG), arterial waveform, synchronized with a pacing device, or an asynchronous mode. The most commonly used trigger is the EKG. The balloon is inflated with the onset of diastole occurring in the middle of the T-wave and deflates at the onset of systole which is roughly the peak of the QRS complex. Triggering utilizing an arterial waveform is another common method; balloon inflation is timed in accordance with the dicrotic notch (Fig. 20.1). Triggering may be impaired by arrhythmias, tachycardia, poor quality EKG, and electrical magnetic interference.
7. What are the clinical indications for an intra-aortic balloon pump?
General clinical indications for an IABP are to improve myocardial oxygen supply while reducing demand through improved coronary perfusion and reduction in afterload. Many potential clinical uses of the IABP have been reported (Table 20.1).
Balloon inflation trigger, halfway during T wave
Balloon inflation
Time Decreased post
augmented diastolic BP
Pressure
Augmented diastolic BP
Figure 20-1. Intra-aortic balloon pump electrocardiogram and arterial waveform tracing.
Table 20-1. Clinical Scenarios Where an Intra-Aortic Balloon Pump Has Been Used
• Cardiogenic shock secondary to acute myocardial ischemia (AMI) refractory to medical therapy
• Acute mitral regurgitation from papillary muscle rupture
• Ischemic ventricular septal defect (VSD)
• Refractory ventricular arrhythmia
• Refractory unstable angina
• Decompensated heart failure (as a bridge to definitive therapy)
• Failure to wean from cardiopulmonary bypass
• Low cardiac output syndrome following cardiac surgery
• Perioperative support for high risk CABG
• Decompensated severe aortic stenosis
• Sepsis
• Prophylactic use during high risk PCI
CABG, Coronary artery bypass grafting; PCI, percutaneous coronary intervention.
8. What are the absolute and relative contraindications for an intra-aortic balloon pump?
There are few absolute contraindications for an IABP. Aortic dissection, severe aortic insufficiency, and patient refusal are generally accepted as situations where an IABP should never be used. Most contraindications are relative and should be considered on a patient-by-patient basis (Table 20.2).
9. How and when are patients weaned from an intra-aortic balloon pump?
Consideration to wean a patient from an IABP should occur after hemodynamic stability has been established. The weaning process is done gradually over 6 to 12 hours. Baseline hemodynamic data should be established (Table 20.3). Sufficient organ perfusion is generally reflected by a cardiac out- put (CO) $2.0 L/min/m2), normal central venous pressure (CVP), normal serum level lactate, adequate urine output (.0.5 mL/kg/min), acceptable mixed venous saturation SVO2, and the absence of acid- base disturbances or significant cardiac arrhythmias. When these parameters are met the weaning process begins. An incremental decrease of the ratio of IABP-supported heartbeat to unassisted heartbeat. Initially the ratio is 1:1. IABP support is decreased to 1:2, 1:4, and 1:8. Hemodynamic parameters, pharmacologic support, and organ perfusion are monitored with each decrease. The IABP is removed and support ceased when the ratio is 1:8 to prevent thrombosis formation.
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10. How can improper timing of intra-aortic balloon pump inflation or deflation be harmful?
There are four variations of ill-timed balloon inflation/deflation: early inflation, late inflation, early deflation, late deflation. Early inflation occurs when the balloon inflates while the heart is still in the systolic phase and the aortic valve is open. Early inflation results in a forced closure of the aortic valve and impairs ventricular ejection. Late inflation occurs well after the closure of the aortic valve. Blood in the root of the aorta has run off and inflation does not increase diastolic perfusion as effectively. Early deflation occurs when the balloon is deflated early in diastole and full advan- tage of increased diastolic perfusion does not occur. Late deflation occurs when the balloon remains inflated into the early systolic phase of the cardiac cycle. Ventricular work and myocardial oxygen consumption are increased as isovolumic contraction is prolonged.
11. What are potential complications of the intra-aortic balloon pump?
Complications associated with the IABP are generally either vascular or hematologic. Vascular complications include vessel or aortic dissection, limb ischemia, or peripheral embolization of aortic debris. Risk factors for vascular complications include hypertension, diabetes, peripheral vascular disease, smoking, and female sex. Hematologic complications with include thrombocy- topenia and bleeding. Improper IABP positioning may compromise blood flow to the renal, splanchnic, or hepatic vessels. Additional complications included infection.
12. What is the current literature consensus on intra-aortic balloon pump?
The true efficacy of the IABP is difficult to study due to variable practices, bias, different placement times, and frequency of some potential indications. The most common traditional use of the IABP is in management of myocardial infarction complicated by cardiogenic shock. Recent studies do not Table 20-2. Contraindications for an Intra-Aortic Balloon Pump
Absolute Contraindications
• Severe aortic regurgitation
• Aortic dissection
• Patient or proxy refusal Relative Contraindications
• Iliac artery stents
• Iliofemoral grafts/stents
• Abdominal aortic aneurysm
• Structural pathologies of the aorta (major reconstructive surgery)
• Septic shock
• Severe bleeding disorder
• Severe bilateral peripheral vascular disease.
Table 20-3. Physiologic Effects of IABP
PARAMETER EFFECT
Diastolic blood pressure Increase
Cardiac output Increase
Cerebral perfusion Increase
Urine output Increase
Skin temperature Increase Systolic blood pressure Decrease
Heart rate Decrease
Wedge pressure Decrease
LV wall stress Decrease
Pulmonary blood volume Decrease
1. IABP decreases myocardial demand by decreasing afterload during systole and improves myocardial oxygen supply by increasing diastolic blood pressure, thereby increasing coronary perfusion pressure.
2. IABP is absolutely contraindicated in severe aortic regurgitation, aortic dissection, and competent patient or proxy refusal.
3. The most common complications of IABP are vascular, limb ischemia having the highest incidence.
KEY POINTS: INTRA-AORTIC BALLOON PUMP
support the generalized use of an IABP in this setting over percutaneous coronary intervention (PCI).
Data does not support the generalized use of an IABP for high-risk PCI or prior to coronary artery bypass grafting (CABG). Very limited data mostly in the form of case reports and small case series indicate that the IABP may be useful to help stabilize patients with mechanical complications of myocardial infarction such as papillary muscle rupture and acute mitral regurgitation or post- myocardial infarction ventricular septal defect associated with hemodynamic instability or shock.
There is also limited but supporting observational data that an IABP may help stabilize intractable ventricular arrhythmias.
ACKNOWLEDGEMENT
The authors wish to acknowledge Drs. Joseph L. Weidman, MD, and Michael N. Andrawes, MD, for the valuable contributions to the previous edition of this chapter.
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