Effects of ischaemia on vasculature

The effects of ischaemia on the structure and function of coronary vessels have been evaluated most thoroughly in the open-chest anaesthetised dog. In the beating heart the flow into the capillary bed is controlled by precapillary sphincters in the terminal arterioles. Short periods (up to 20 min) of ischaemia result during reperfusion in vasodilatation which is mediated through increased tissue adenosine levels. Longer periods of ischaemia (60 or more min) result in degenerative vascular changes and no-reflow during reperfusion. Endothelial cells swell, lose their pinocytotic vesicles and form spherical cytoplasmic protrusions into the capillary lumina. Prolonged ischaemia (over 3 hours) causes ruptures in microvascular walls. Such vessels become permeable to large tracers, e.g. carbon particles, and reperfusion results in haemorrhage. Capillaries in ischaemic myocardium are collapsed by compression resulting from intra- and extracellular oedema and contracture of myocytes. Erythrocytes plug collapsed microvessels. However, in vitro observations have shown that no-reflow may result from ischaemia even without erythrocyte plugging. Capillary compression is the main determinant of no-reflow after ischaemia. Degenerative ischaemic changes in vascular walls further contribute to vascular incompetence in vivo.