Myocardial preservation

Most surgical techniques on the heart require an immobile heart. On bypass, the aorta is cross-clamped between the aortic cannula and the aortic valve, thus isolating the heart from the flow of oxygenated blood. During aortic cross-clamping, ischaemic damage to the myocardium can be minimized by attempts to reduce myocardial oxygen consumption. Currently, techniques of myocardial preservation include hypothermia to reduce basal metabolic rate and cardiac arrest to reduce oxygen requirements to a minimum, the latter usually achieved by injecting 500-1000 ml of crystalloid cardioplegic solution around the coronary arteries. Many cardioplegic solutions are available; the majority contain potassium and a membrane-stabilizing agent, e.g. procaine. Some centres infuse warm blood-based cardioplegic and reperfusate solutions continuously to minimize ischaemic and reperfusion injuries and improve delivery of oxygen and other substrates to the myocardium.

Cooling is achieved by the use of ice-cold cardioplegia and by pouring cold fluid (4°C) into the pericardial sac and into the heart chambers if they have been opened. If the heart is cooled to 15°C, it withstands total ischaemia for approximately 1 h. The technique used most commonly at present involves moderate hypothermia of

Aortic cannula

Cross-clamp

Cardioplegia

Venous drainage

CAVG

Aortic cannula

Cross-clamp

Cardioplegia

Venous drainage

CAVG

LIMA graft

Aortic cannula

Side-clamp

Venous drainage

Aortic cannula

Side-clamp

Venous drainage

CAVG

Arrangement of cross-clamp, cardioplegia and anastomoses. A A left internal mammary artery (LIMA) graft B Vein graft with side clamp on

CAVG

Arrangement of cross-clamp, cardioplegia and anastomoses. A A left internal mammary artery (LIMA) graft B Vein graft with side clamp on the body to 32°C and local cooling of the myocardium to a temperature of 15-18°C. If cross-clamping times are prolonged, car-dioplegic cooling must be repeated if spontaneous cardiac contraction resumes.

A modification to this traditional technique has been the combination of local cooling of the heart with body temperatures of 33-37°C during CPB in an attempt to limit postoperative hypothermia.

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