Effects of atrial pacing on regional myocardial gas tensions with critical coronary stenosis

Abstract

Changes in myocardial carbon dioxide (PmCO2) and oxygen tension (PmO2) measured by mass spectrometry have been shown to reflect quantitatively progressive degrees of regional myocardial ischemia associated with stepwise reduction in coronary blood flow. The present study utilized mass spectrometry to assess the severity of regional myocardial ischemia developing during atrial pacing in the presence of a flow-limiting proximal critical coronary artend subendocardial layers was measured by the radioactive microsphere technique. Application of a "critical stenosis" resulted in a 6-mmHg decrease in PmO2 and a 17-mmHg increase in PmCO2 in the region of the myocardium supplied by the stenosed vessel. The addition of atrial pacing resulted in a 3-mmHg further decrease in Pmo2 and a 40-mmHg further increase in PmCO2. In the region of myocardium supplied by the critically stenosed vessel MBF increased in the subepicardial layer, but decreased or remained unchanged in the subendocardial layer. The failure of myocardial blood flow to increase in deeper myocardial layers in response to the increased myocardial oxygen demand of atrial pacing would provide a mechanism for the development of subendocardial ischemia in the presence of a critical coronary stenosis.

Effects of nitroglycerin on regional myocardial ischemia induced by atrial pacing in dogs.

“…The present study employs a previously wellcharacterized, anesthetized, open-chest canine model to study the mechanism of nitroglycerin's beneficial effect on regional myocardial ischemia, induced distal to a fixed, flow-limiting coronary stenosis by atrial pacing (O'Riordan et al, 1977). This preparation is designed to model a milder degree of reversible ischemia, such as would be present during angina.…”

“…Several recent clinical studies have demonstrated that nitroglycerin can be administered safely to patients with acute myocardial infarction by intravenous infusion with consistently beneficial effects on indices of myocardial ischemia (Borer et al, 1975, Come et al, 1975, Flaherty et al, 1976. The ability of nitroglycerin to relieve ischemia in either clinical situation may result from dilating effects on either peripheral arteries or veins, or both, which might decrease determinants of myocardial oxygen demand, or from direct effects on the coronary circulation which might increase myocardial oxygen suppiy-The present study employs a previously wellcharacterized, anesthetized, open-chest canine model to study the mechanism of nitroglycerin's beneficial effect on regional myocardial ischemia, induced distal to a fixed, flow-limiting coronary stenosis by atrial pacing (O'Riordan et al, 1977). This preparation is designed to model a milder degree of reversible ischemia, such as would be present during angina.…”

A Closed Chest Animal Model for the Study of Reversible Ischemia

“…Conversely, at a given resting heart rate, blood flow through a coronary artery remains constant even up to about 85% stenosis (31) because the predominant resistance to flow resides in the arterioles of the coronary microcirculation. When pacing at increased heart rates in the presence of a major coronary stenosis, evidence indicates potentially substantial reduction in flow through the stenosed coronary artery (32), even though the level of global ventricular myocardial blood flow may not change (3) or even increase. The reason for the pairing effects appear similar to those dis cussed in a recent study in which a potent coronary vasodilator (dipyridamole) was administered in open chest dogs with extracoronary partial occluders (4).…”

Rate of rise of myocardial PCO2 during early myocardial ischemia in the dog.

“…Since the original description of the direct measurement of Pmcoj (Brantigan et al, 1972a), subsequent studies have confirmed the increase of Pmco? during myocardial ischemia or anoxia (Brantigan et al, 1972b;Furuse et al, 1973;Khuri et al, 1975aKhuri et al, , 1975bMacGregor et al, 1974;O'Riordan et al, 1977aO'Riordan et al, , 1977b. Even with lesser degrees of ischemia, it was observed (O'Riordan et aL, 1977b) that Pmcc>2 provides a much more sensitive index of ischemia than myocardial Pch measurements, since Pmcc^ changes more extensively; the magnitude of PmccHi rise also was related to the degree of S-T segment change (O'Riordan et al, 1977a, Khuri et al, 1975c.…”