The Adequacy of Subendocardial Oxygen Delivery

Abstract

The interactions of determinants of subendocardial oxygen delivery (flow and oxygen content) and oxygen demand were studied. In open chest dogs, oxygen requirements were increased with aortic stenosis and oxygen delivery reduced with normovolemic anemia. We estimated potential subendocardial flow from a Diastolic Pressure Time Index (DPTI) and oxygen demands from the Tension Time Index (TTI), and detected ischemia with endocardial (Endo) and epicardial (Epi) electrocardiograms (ECG). Subendocardial oxygen delivery per unit of TTI was significantly lowered and Endo ECGs showed ischemia (ST elevation) when Endo/Epi flow ratios (microsphere method) fell below 1.0 (P < 0.001). Ischemia occurred without anemia (Hgb > 10 g %) in severe aortic stenosis; with mild anemia (Hgb 5-10 g %) and mild aortic stenosis; and with severe anemia (Hgb < 5 g %) without aortic stenosis. Ischemic Endo ECGs occurred with mild supravalvar aortic stenosis and mild anemia despite a 63% (92 to 150 cc/100g/min) average increase in subendocardial flow while Epi ECGs remained normal. Altered flow distributions and abnormal ECGs were always predictable from the ratio: DPTI x O₂ content (Supply)/TTI (Demand). These determinants of Supply and Demand are readily obtained from measurements of hemoglobin, saturation, and blood pressure.