ischaemic preconditioning of the human heart. - Leicester Research ...

The tolerable duration of unmodified ischaemic arrest is considered to be under 20
minutes. If this length of time is exceeded irreversible myocardial injury may occur.
There is evidence that by modifying the metabolic state of the myocardium during
ischaemia it may be possible to delay or reduce irreversible damage.
The principle of myocardial protection using cardioplegia solutions is based on the
fact that energy expenditure by the myocytes is almost totally devoted to contraction.
Thus eliminating the contractile function, by arresting the heart in diastole (using high K
concentrations), the energy utillsation is reduced by almost 90%. This enables the
surgeon to subject the heart to otherwise intolerably prolonged ischaernIc periods, which
may be extendible to 2-3 hours in complicated procedures. Dunng prolonged periods ot
ischaemia in order to improve protection of the myocardium even further, cooling of the
heart to 4'C, as well as body cooling to 22-28'C adds greatly to cardioplegic solution
[21,30,1791.
There are potential problems with the use of cardioplegiaý due to critical coronary
arterial stenoses the distribution of the cardioplegia may
be heterogeneous leaving areas
of myocardiurn at greater fisk of ischaernic damage. Also the cold solutions can cause
damage
to both the myocardium and the endotheliurn [5 1 J.
In addition to the use of chemical cardioplegia solutions, the application of brief
intermittent ischaernic episodes to the heart enables the performance of aorto-coronary
bypass surgery. This is performed while on cardiopulmonary bypass and combined with
cooling to 32"C. Dufing aortic cross-clamping (Ischaemia) the distal coronary
anastomosis is performed, which is followed by removal of the cross-clamp and
reperfusion, during which time the proximal anastomosis of the graft to the aorta is