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

3.5 DISCUSSION:
The present studies have characterised the ischaernic preconditioning phenomcnon in the human
myocardium and have disclosed the following important resultsý (1) there is a graded narrow
window of protection by preconditioning with 4-5 min of ischaernia being the most effective
period, (n) the number of preconditioning cycles in itself does not influence protection, and Oli)
as shown in other animal species, there are two windows of protection, the first (< 2 hr) being
more protective than the second (=24 hr). These results have significant clinical implications and
warrant further discussion.
Intensity of the preconditioning stimulus
These studies are the first to demonstrate that maximal protection of the human myocardium by
ischaernic preconditioning is obtained with an ischaernic stimulus of 4-5 min. It was not
surprising that shorter periods of ischaernia. resulted in a decrease or loss protection since studies
in animals have shown that the ischaernic period should be greater than 2 min to achieve
protection [218,321 ]. However, the loss of protection with ischaernic periods beyond 6 min was
unexpected since several investigators have reported that 10 min of ischaernia preconditions the
heart of a number of animal species [26,104,2851.
The findings that 4-5 min of ischaernia is the optimal time for preconditioning is supported by
studies performed in the course of percutaneous transluminal coronary angioplasty (PTCA)
[75,3071. In these studies, coronary arteries are typically occluded for 2 min by balloon inflation
with 5 min apart. Consistently in all studies the severity of myocardial ischaernia, assessed by
changes in S-T segment shifts and angina symptoms, are less during the second and third balloon
inflation than during the first inflation. This suggests that a total of 4-6 min of ischaernic
preconditioning also confers maximal protection in the clinical setting.
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