Regional myocardial blood flow and left ventricular diastolic properties in pacing-induced ischemia

The relation between left ventricular diastolic abnormalities and myocardial blood flow during ischemia was studied in eight open chest dogs with critical stenoses of the proximal left anterior descending and circumflex coronary arteries. The heart was paced at 1.7 times the heart rate at rest for 3 min. In dogs with coronary stenoses, left ventricular end-diastolic pressure increased from 8 ± 1 to 14 ± 2 mm Hg during pacing tachycardia (p < 0.01) and 16 ± 3 mm Hg (p < 0.01) after pacing, with increased end-diastolic and end-systolic segment lengths in the ischemic regions. Left ventricular diastolic pressure-segment length relations for ischemic regions shifted upward during and after pacing tachycardia in dogs with coronary stenoses, indicating decreased regional diastolic distensibility.In dogs without coronary stenoses, the left ventricular diastolic pressure-segment length relation was unaltered. Pacing tachycardia without coronary stenoses induced an increase in anterograde coronary blood flow (assessed by flow meter) in both the left anterior descending and circumflex coronary arteries, and a decrease in regional vascular resistance. In dogs with coronary stenoses, regional vascular resistance before pacing was decreased by 18%; myocardial blood flow (assessed by microspheres) was unchanged in both the left anterior descending and circumflex coronary artery territories. During pacing tachycardia with coronary stenoses, regional coronary vascular resistance did not decrease further; subendocardial myocardial blood flow distal to the left anterior descending coronary artery stenosis decreased (from 1.03 ± 0.07 to 0.67 ± 0.12 ml/min per g, p < 0.01), as did subendocardial to subepicardial blood flow ratio (from 1.04 ± 0.09 to 0.42 ± 0.08, p < 0.01). In dogs without coronary stenoses, subendocardial myocardial blood flow increased during pacing tachycardia and subendocardial to subepicardial blood flow ratio did not change.In conclusion, during pacing tachycardia in the presence of coronary stenoses, subendocardial perfusion decreased markedly in the ischemic region, possibly reflecting impaired regional myocardial relaxation and subendocardial compression due to increased left ventricular end-diastolic pressure