Ventricular assistance may salvage viable myocardium after a severe ischemic insult. The concept of an "all or none" phenomenon of myocyte necrosis has been replaced by the concept of a gradation of injury. The resultant functional and structural changes depend on the duration of the coronary occlusion, the extent of collateral flow, the metabolic and hemodynamic conditions during wich the myocardium is made ischemic, and the timing and nature of reperfusion.
Among the mechanical devices developed to assist the left ventricle until more definitive therapy can be undertaken, the Intra-aortic balloon pump(IABP)has been in use the longest, since Kantrowitz reported the first successful clinical use in 1967.
The IABP is placed in the descending thoracic aorta via the femoral artery. Its inflation and deflation are timed to the cardiac cycle (generally synchronized with the ECG). The balloon inflates in diastole immediately following aortic valve closure. The augmentation of diastolic pressure, to a level higher than systolic pressure, increases coronary perfusion as well as that of other tissues. The balloon deflates at the end of diastole, immediately before left ventricular contraction, abruptly decreasing the afterload and improving left ventricular ejection.
IABP is a particularly attractive therapeutic option because it decreases myocardial oxygen demand while simultaneously increasing oxygen supply and maintaining mean aortic pressure. The systolic unloading leads to a reduction in afterload and thus in left ventricular end-diastolic pressure, which helps reduce oxygen demand in ischemic segments. This is also the only method that will augment diastolic aortic pressure without increasing oxygen demand, as is the case with catecholamine-mediated vasoconstriction.
IABP indications can be divided into absolute, relative an controversial. 1) Absolute indications : inability to wean from cardiopulmonary bypass; postcardiotomy low output syndrome; acute ventricular septal defect secondary to myocardial infarction (MI); acute mitral regurgitation secondary to MI; failed angioplasty with hemodynamic compromise or evidence of ischemia. 2) Relative indications : severe left ventricular dysfunction in a surgical candidate; development of ischemic ECG changes in postcardiotomy patient; development of electrical instability in a postcardiotomy patient. 3) Controversial indications : cardiogenic shock secondary to MI; unstable angina (it can improve perfusion through subtotal stenosis); major noncardiac operations in patients with known severe coronary artery disease.
Absolute contraindications for IABP are aortic insufficiency (that will worsen with diastolic augmentation); aortic dissection (the balloon can increase the size of the false channel and further damage the aorta); irreversible cardiac disease; and chronic end-stage disease. Relative contraindications include abdominal aortic aneurysm; severe peripheral vascular disease; and tachyarrhytmias (rapid,irregular rythms make synchronized inflation of the balloon during diastole difficult and reduce the efficacy of intra-aortic balloon support).
Indications for initiation for IABP include cardiogenic shock or hemoynamic failure due to primary pump failure or to mechanical lesions. Hemodynamic data that indicate the need for counterpulsation are hypotension(<90mmH systolic pressure, <60 mmHg mean pressure, or >30mmHg below previous basal levels), increased pulmonary artery wedge pressure (>16 to 18 mmHg) and a low cardiac index(<2.0 liters/min er square meter).
Complications of IABP arise from its insertion or from the device itself. Those from the insertion include dissection of the aorta or the common femoral artery (which can result in vascular rupture or paraplegia), dislodgment of plaque or thrombus-forming emboli to major arteries, and perforation of the common femoral, iliac artery or aorta. Those from the device include obstruction of the common femoral artery with resultant peripheral ischemia, systemic infection, occlusion of a major artery, thrombocytopenia, hemorrhage, hemolysis, and rupture of the balloon with gas embolization. Major vascular complications in patients with cardiogenic shock are common and may occur in up to one-third of patients.
Weaning from IABP should begin in 24-48 hours of reperfusion or medical stabilization, if surgery is not planned. This is accomplished by gradually reducing the proportion of cardiac cycles during which the balloon inflates. When the patient's own circulation can be maintained at a pumping frequency of every fourth or greater cardiac cycle, the balloon can usually be removed successfully. It should not be completely stoped in situ because of the danger of thrombus formation.
In selected cases the balloon can be placed from an axillary cut-down, the removal of wich usually requires a return to operating room.
It should be remembered that while this device can clearly improve hemodynamics in the short term, they will not improve survival by itself, reaffirming the importance of definitive therapy (survival of cardiogenic shock patients with IABP without reperfusion therapy is very low, averaging 20%; with reperfusion it is about 50%).
Back to CARDIOLOGY
Back to SPECIALTIES
Back to MEDSTUDENTS HOMEPAGE