Extracorporeal circulation

Extracorporeal circulation use in the adult ICU is extremely rare. The most common setting in which extracorporeal circulation is utilized is in intra-operative cardiopulmonary bypass

(CPB). Still, familiarity with the process is useful in caring for patients post-operatively. Indications for extracorporeal circulation include:

• cardiac surgery (valve replacement or repair, coronary bypass grafting);

• operation on the aortic arch or ascending aorta;

• heart or heart-lung transplantation;

• cardiopulmonary support;

CPB is performed by the following technique. After anticoagulation with heparin, venous blood is drained from the right atrium or from both vena cavae by special cannulae into the CPB circuit. This circuit consists of a reservoir, heat exchanger, oxygenator, and a pump. The reservoir collects the blood. The heat exchanger allows the patient's body temperature to be controlled, usually employing a moderate degree of hypothermia (25-28°C) in order to reduce the metabolic rate. (Deep hypothermia (18-20°C) significantly reduces metabolic rate so that bypass can be temporarily interrupted. This technique, known as circulatory arrest, is useful for the surgical treatment of aneurysms of the aortic arch.) The oxygenator facilitates gas exchange. The pump maintains a blood flow rate of around 2.4 and 2.8 l/min/m2; perfusion pressures are usually around 60 mmHg providing adequate tissue perfusion and oxygenation. On completion of the operative procedure, the patient is warmed and bypass is discontinued by allowing the heart gradually to take over the circulation. Anticoagulation is reversed by means of protamine.

Although CPB diverts returning venous blood from the systemic circulation away from the heart, coronary blood flow continues and the heart continues to beat. A dry, still, operating field is achieved by cross-clamping the aorta and inducing diastolic arrest by perfusing the heart with a special cold potassium-rich cardioplegia solution. Cessation of mechanical activity and low temperature reduces the rate of myocardial metabolism so that ischemia can be better tolerated. Cardioplegia infusions are repeated half-hourly and ice is applied topically to the heart to keep its rate of metabolism low. Ischemia times of up to 2 h are generally well tolerated although longer ischemia times increase the chance of damaging the myocardium.

CPB is now routinely performed with a much lower morbidity than in the past. It does, however, evoke systemic effects including: activation of the systemic inflammatory response, platelet destruction, dilution of clotting factors, red cell hem-olysis, transient renal and hepatic insufficiency, and elevated amylase levels (usually without symptoms of pancreatitis).

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