The changes in ECG ST segment and mechanical function of regional ischemic myocardium during afterload reduction in isolated dog hearts with coronary stenosis.

Abstract

By measuring ECG ST segment deviation and regional mechanical dysfunction, we assessed the effects of an alteration in afterload pressure on regional myocardial ischemia due to a partial coronary occlusion of the left circumflex coronary artery (CFX). Eight isolated, perfused and paced dog hearts were loaded with an artificial arterial system which simulated the aortic input impedance of the dog arterial tree. Afterload pressure was altered by stepwise changes in peripheral resistance (Rp), while left ventricular end-diastolic pressure (LVEDP) and heart rate were kept constant. Coronary perfusion pressure (CPP) was kept equal to mean aortic pressure (AoP). ECG and myocardial systolic segment shortening (SS) were measured in both areas perfused by the CFX and the left anterior descending coronary artery (LAD). In the presence of CFX stenosis, mean AoP decreased from 96 +/- 7 to 46 +/- 7 mmHg following a decrease in Rp and cardiac output increased progressively from 465 +/- 30 to 1055 +/- 100 ml/min. In this situation, CFX coronary blood flow decreased from 75 +/- 2 to 28 +/- 6 ml/100 g/min. Epicardial ECG in the ischemic region showed two types of ST segment deviation after CFX stenosis: ST segment elevation and ST segment depression. Moreover, following afterload reduction, these individual ECG ST segment changes showed further deviations. The levels of mean AoP, below which further ST segment deviations significantly occurred, were as follows: 71 +/- 6 mmHg in ST segment elevation cases and 52 +/- 7 mmHg in ST segment depression cases. Myocardial systolic segment shortening in the ischemic region also significantly decreased following CFX stenosis (91 +/- 8% of pre-ischemic control, p less than 0.05). Then, during afterload reduction, SS in the CFX area appeared to be bimodal and it definitely decreased when mean AoP was 46 +/- 7 mmHg. To enhance the sensitivity of detection of further mechanical dysfunction throughout afterload reduction following regional myocardial ischemia, we calculated percentage values of the SS in the CFX area to the SS in the LAD area at each Rp. By this normalization, mechanical dysfunction was found when mean AoP was below 67 +/- 5 mmHg. These results suggest that reduction in afterload pressure below a certain level aggravated the regional ischemia, despite an increase in cardiac output, and to detect this change in the ischemic region during afterload reduction, ECG ST segment deviation seems to be a useful indicator.(ABSTRACT TRUNCATED AT 400 WORDS)