Myocardial Oxygen Tension and Relative Capillary Density in Isolated Perfused Rat Hearts

Oxygen plays an important role in cardiac function. Many methods have been applied to measure tissue oxygen tension (P 2). Electron paramagnetic resonance (EPR) oximetry appears to have some significant advantages for use in the beating heart. This study presents the serial measurement of myocardial P 2by EPR oximetry in the isolated crystalloid perfused heart during changes of influent P 2, coronary flow rate, oxygen consumption and end-diastolic pressure. Baseline myocardial P 2was 198±12 mmHg (mean± ). Myocardial P 2increased as expected with increased delivery (concentration or flow) or decreased consumption. With increasing flow rate, myocardial P 2increased in a sigmoid fashion. A critical flow or pressure was reached when myocardial P 2rapidly increased to a higher level. Increased left ventricular end-diastolic pressure caused local vascular compression and resulted in a decrease of myocardial P 2. Myocardial capillary density in the intact contracting heart was calculated to be 2300±110/mm2, using local myocardial P 2and a cylindrical model for oxygen diffusion in tissue. Relative capillary density did not change with mild to moderate hypoxia, increased with increasing flow and increasing oxygen consumption and decreased with elevated diastolic pressure. We conclude that the application of EPR oximetry with LiPc to the isolated heart provides accurate and dynamic evaluation of local myocardial P 2in the contracting heart. Using various models of oxygen delivery and diffusion in tissue, these data may also be used to serially follow capillary density.