In cases of acute myocardial infarction complicated by cardiogenic shock, the intra-aortic balloon pump (IABP) is used for temporary support when coronary revascularization (either percutaneous or surgical) is planned (31). It is contra-indicated in the presence of aortic valve insufficiency or aortic dissection.
FIGURE 8.2 Intra-aortic balloon counterpulsation showing balloon inflation during diastole (left panel), and balloon deflation during systole (right panel). The arrows indicate the direction of blood flow. The effects on the aortic pressure waveform are shown by the dotted lines.
1. Method
a. The intra-aortic balloon is an elongated polyurethane balloon that is inserted percutaneously into the femoral artery and advanced up the aorta until the tip lies just below the origin of the left subclavian artery (see Figure 8.2).
b. A pump attached to the balloon uses helium (a low density gas) to rapidly inflate and deflate the balloon. Inflation begins at the onset of diastole, just after the aortic valve closes (using the R wave on the ECG as a trigger), and deflation begins at the onset of ventricular systole, just before the aortic valve opens.
2. Effects
The hemodynamic effects of IABP support are demonstrated in Figure 8.2.
a. The aortic pressure waveform on the left shows that balloon inflation during diastole increases the peak diastolic pressure, and subsequently increases the mean arterial pressure (equivalent to the integrated area under the aortic pressure tracing).
b. The increase in peak diastolic pressure increases coronary blood flow (which occurs predominantly during diastole). The increase in mean arterial pressure obviates the need for a vasopressor, but is not accompanied by an increase in systemic blood flow (31).
c. The aortic pressure tracing on the right shows that balloon deflation creates a suction effect that decreases aortic pressure when the aortic valve opens. This reduces the impedance to left ventricular outflow (i.e., afterload) and augments ventricular stroke output. The decrease in LV afterload is accompanied by a decrease in cardiac work and myocardial O2 consumption.
d. In summary, the IABP increases cardiac stroke output while increasing myocardial O2 delivery (via increased coronary blood flow) and reducing myo-cardial O2 consumption. Despite these beneficial effects, numerous studies have shown no consistent survival benefit with IABP support (31,32).
3. Complications
a. Limb ischemia is reported in 3–20% of patients (33,34), and can appear while the balloon is in place or shortly after removal. Most cases are the result of in-situ thrombosis at the balloon insertion site.
b. Loss of distal pulses alone does not warrant removal of the balloon as long as sensorimotor function in the legs is intact (35). Loss of sensorimotor function in the legs should always prompt immediate removal of the device.
c. Surgical intervention is required in 30–50% of cases of limb ischemia (35).
d. Fever is reported in 50% of patients during IABP support, but bacteremia is reported in only 15% of patients (36).
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Chapter 9
Systemic Infection and Inflammation
The most significant discovery in critical care in the past 20–30 years is the prominent role played by inflammation in the pathogenesis of multiorgan dysfunction in critically ill patients. This chapter presents four disorders that involve inflammatory injury in major organs: sepsis, septic shock, anaphylaxis, and anaphylactic shock.
I. CLINICAL SYNDROMES