General Principles

Cardiopulmonary bypass (Fig. 3.1) is a process by which systemic venous blood is taken from the patient, transferred to a pump oxygenator and delivered back to the arterial circulation of the patient. Cardiopulmonary bypass (CPB) can be performed either at normothermia or by cooling the patient to 30°C, 28°C or even as low as 15-18°C in preparation for complete circulatory arrest. Cardioplegia is induced by a high potassium containing solution which causes electromechanical arrest of the myocardium. Electromechanical arrest decreases myocardial oxygen utilization substantially; it is said that there is a 90% decrease in myocardial oxygen utilization by simply keeping the heart at a standstill. An additional 10% decrease in myocardial oxygen consumption is achieved by cooling the myocardium. The bypass circuit at Harbor-UCLA consists of a single venous cannula in two stages for coronary bypass surgery in which inferior vena cava blood goes through the cannula and a proximal port in the right atrium drains the remainder of the systemic venous blood coming from the superior vena cava. In the case of mitral valve replacement or situations in which the right heart must be entered, the venous cannulation consists of a superior vena cava cannula and a separate inferior vena cava cannula. In the case of mitral valve replacement, this is useful because it keeps all of the hardware out of the field of the mitral valve and permits increased mobilization of the left atrium and increased visualization of the mitral valve. In the case of right heart surgery, as for example, tricuspid valve or atrioseptal defect surgery, this is important because without bi-caval cannulation, once the right heart is entered air will be aspirated into the venous cannula and cause an air lock, shutting off the pump.

The venous line drains down to a reservoir. The amount of venous drainage can be regulated by the height of the reservoir; i.e. the higher the reservoir the less the venous drainage, and the lower the reservoir with respect to the patient the higher the amount of venous drainage (i.e. the effect of gravity). The reservoir blood then enters a hollow fiber membrane pump oxygenator (Fig. 3.1) with a temperature regulating device in the proximal portion of the system and the oxygenator just distal to this. The entire heater/warmer oxygenator circuit is just distal to the roller pump head. (In the case of bubble oxygenators, the oxygenator is proximal to the roller pump head.) Once the blood passes through the membrane

Fig. 3.1. Cardiopulmonary bypass circuit.

Fig. 3.1. Cardiopulmonary bypass circuit.

where the CO2/O2 exchange takes place, the blood travels through a 40 micron filter and then back into the arterial circuit of the patient. The filter serves to remove particulate and gaseous emboli. The arterial circuit has a purge line which can remove gross air.

Cooling in cardiopulmonary bypass is done at approximately 1°C per minute. The danger of rapid cooling is that if the cool blood enters the warm patient, gaseous emboli can form within the patient, but this should not happen if the gradient between the infused blood and the patient is less than 10°C. The advantages of cooling are that it decreases the metabolic requirement of the body organs, in particular, the brain and the heart. However, disadvantages are that it may increase bleeding after coming off bypass because of stunning of the coagulation enzyme systems, and it may induce myocardial edema by impairment of enzyme systems.

Circulatory arrest is another modality which is used in tiny infants who require cardiopulmonary bypass and then removal of their cannulas after achieving circulatory arrest to prevent clutter of the operative field. It is also used in specific situations: transverse aortic arch aneurysms, inferior vena cava surgery, or descending thoracic/thoracoabdominal aortic surgery where placement of an aortic cross-clamp or venous cross-clamp is awkward, and there is a need to arrest the circulation to keep the surgical field clear of blood to permit suturing.

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