Effects of tachycardia on the adequacy of subendocardial oxygen delivery in experimental aortic stenosis

Abstract

We studied the interaction of tachycardia and aortic stenosis on the adequacy of subendocardial oxygen delivery. In 18 open-chest dogs with acute supravalvular aortic stenosis, we produced subendocardial ischemia by increasing either heart rate (atrial pacing) or the severity of aortic stenosis. Ischemia was determined from ST-elevation of the intracavitary electrocardiogram. Subendocardial oxygen supply was assessed by measuring subendocardial flow (radioactive microspheres, 8 to 10 microns) and arterial oxygen content, and related to simultaneous oxygen demand [estimated from the tension time index (TTI)]. The adequacy of the supply/demand relationship in the subendocardium was estimated from the ratio DPTI × O 2 content (supply)/TTI (demand). Subendocardial ischemia occurred at aortic gradients ranging from 30 to 100 mm. Hg and heart rates from 120 to 180 beats per minute. Ischemic hearts were characterized by (1) redistribution of coronary flow away from the subendocardium (endo/epi flow ratio < 1.0), (2) reduced subendocardial oxygen delivery per unit of demand (TTI) (p < 0.01), (3) failure to lower left ventricular end-diastolic pressure with tachycardia, and (4) supply/demand ratios (DPTI × O 2 content/TTI) below 15 (p < 0.01). These findings suggest that (1) the principal determinant of subendocardial ischemia in aortic stenosis is the unfavorable alteration of the supply/demand relationship caused by the interaction between heart rate and severity of stenosis, rather than absolute heart rate or aortic gradient and (2) the adequacy of subendocardial oxygen delivery can be assessed from readily obtained measurements of blood pressure and oxygen content.