Role of red blood cells in the coronary microcirculation during cold blood cardioplegia.

The role of red blood cells during cold blood cardioplegia was studied using an intravital microscope in 13 isolated canine hearts perfused with diluted blood containing potassium chloride. The coronary microcirculation on the left ventricular epicardial surface was observed while the perfusate temperature was varied between 37 degrees C and 10 degrees C. Considerable sludging of red blood cells occurred during hypothermia. The percentage of capillaries perfused by red blood cells (percentage change) significantly decreased as perfusate temperature was reduced (100, 56, and 31% at 37, 20, and 10 degrees C respectively). This was caused by occlusion of microvessels due to sludging and by functional closure due to hypothermia. There was incomplete recovery of perfusion of capillaries at the end of rewarming (60%). The diameters of venules were reduced to 76% of control value at 10 degrees C because of the decrease in the numbers of feeding capillaries, but this value returned to 91% at the end of rewarming. Coronary vascular resistance (mmHg.ml-1.min-1 (kPa.s.litre-1] significantly decreased from 2.0(0.2) at 37 degrees C to 1.2(0.12) at 10 degrees C, but it increased to 2.4(0.24) at the end of rewarming. The finding in this study that sludging occurred which was slow to clear is a definite disadvantage of perfusion with red blood cells during hypothermia.