Appendix A: Myocardial management during aortic occlusion

The ideal situation for the surgeon operating on the heart is to have a still heart in a bloodless field. To render the heart bloodless, the aorta must be cross-clamped to prevent coronary perfusion. The heart is then rendered ischaemic and there are two ways of coping with this ischaemia without causing lasting myocardial damage.

Cardioplegic arrest

There are various cardioplegic solutions which arrest the heart in diastole and preserve adenosine triphosphate (ATP) levels. They all have high potassium levels and may be crystalloid or contain blood. Blood cardioplegia has become popular because it can act as a buffer with a high oncotic pressure and because it can also supply oxygen to the ischaemic myocardium at a rate sufficient to meet its metabolic demands. With the patient on full cardiopulmonary bypass, the core temperature is reduced to 26-280C and the aorta is cross-clamped. Cardioplegic solution is infused into the aortic root and topical cooling with iced slush may also be employed to increase myocardial protection. The heart can be kept arrested for up to 2 hours with this technique.

There are many permutations with the type of cardioplegia, the temperature of cardioplegia, its route of delivery and whether it is given continuously or intermittently. There is no consensus agreement and the reader should refer to a specialist text for more information.

Ventricular fibrillation and intermittent aortic occlusion

This is a technique where ventricular fibrillation is induced by a small electrical charge. The heart does not eject and is

relatively still but not bloodless. To perform an operative procedure (e.g. CABG) the aorta is cross-clamped to render the heart ischaemic. The heart can tolerate short periods of intermittent ischaemia providing the heart is reperfused and allowed to beat in-between. For CABG the choice between this technique and cardioplegia depends on the surgeon's preference.

Total circulatory arrest

Cardiopulmonary bypass is instituted and the core temperature reduced to 12-180C. The metabolic rate of all the organs of the body is so low that periods of up to 30 minutes of total circulatory arrest with the pump switched off are tolerated. This technique has its main role in paediatric surgery and surgery of the ascending aorta.