Post-ejection wall thickening as a marker of successful short term hibernation

Short term hibernating myocardium is characterised by a decrease in contractile function in proportion to the reduced blood flow, the recovery of creatine phosphate despite ongoing ischaemia, a recruitable inotropic reserve, and the absence of necrosis. During acute myocardial ischaemia systolic wall thickening decreases and post-ejection wall thickening develops. The extent of post-ejection thickening during severe ischaemia correlates with the recovery of contractile function during reperfusion. Whether the extent of post-ejection wall thickening can also distinguish short term hibernating myocardium from more severely ischaemic, infarcting myocardium and thus predict the amount of viable tissue was tested in 13 anaesthetised pigs.

The left anterior descending coronary artery (LAD) was cannulated and perfused at constant flow. After control measurements of regional myocardial blood flow (with radiolabelled microspheres) and wall thickening (sonomicrometry), coronary inflow was reduced to produce a reduction in regional contractile function by 60-100%. After 85 minutes of ischaemia, dobutamine was infused into the LAD for five minutes to determine the extent of inotropic reserve. Transmural biopsies were taken to measure regional myocardial creatine phosphate content and infarct size was determined after two hours of reperfusion by staining with triphenyl tetrazolium chloride.

The extent of post-ejection wall thickening after 85-90 minutes of ischaemia correlated with the myocardial creatine phosphate content (r = 0.812, n = 11, p < 0.01) and the extent of the dobutamine recruitable inotropic reserve (r = 0.783, n = 7, p < 0.05). A negative correlation existed between the extent of post-ejection wall thickening and % infarct size (r = -0.699, n = 10, p < 0.05 for the transmural piece of tissue containing the ultrasonic crystals; r = -0.743, n = 10, p < 0.05 for the area of the left ventricle at risk). Finally, post-ejection wall thickening after 85-90 minutes of ischaemia correlated with the recovery of contractile function at 30 minutes reperfusion (r = 0.657, n = 10, p < 0.05).

The extent of post-ejection wall thickening may indicate the amount of viable tissue after 85-90 minutes of low flow ischaemia. The greater the post-ejection wall thickening, the more myocardium is successfully hibernating.