Control of myocardial oxygenation-Effect of atrial pacing

Myocardial oxygenation is controlled by alterations in both oxygen supply and oxygen demand of the heart. The factors determining oxygen supply and demand in controlled and stressful conditions are discussed on a regional basis. Evidence is presented to show that the subendocardial (ENDO) region of the left ventricle has a lesser oxygen supply versus its demand compared to the subepicardial (EPI) region. The effects of stress are shown to worsen the disparity in myocardial oxygenation of the ENDO in relation to the EPI. Data is presented on the stress of atrial pacing. Experiments were conducted on 31 anesthetized open chest dogs. Electrodes were placed in the ENDO and EPI to measure tissue PO₂ and blood flow by hydrogen clearance. The hearts were paced by left atrial stimulation at 150, 175, 200, and 225 beats/min. Tissue PO₂ fell in a stepwise manner at every pacing rate. ENDO PO₂ fell to a greater extent than EPI PO₂ at every pacing rate. Pacing increased regional blood flow in the EPI more at every pacing rate. It can be concluded that the increases in flow observed did not totally compensate for the increased demand for oxygen in the heart brought on by the stress of pacing. This effect is considerably greater in the ENDO. These results are discussed in terms of several clinical findings.