Myocardial tissue oxygen supply and utilization during coronary artery bypass surgery: evidence of microvascular no-reflow

The supply and utilization of oxygen by the myocardium reflect the dynamic efficiency of the microcirculation. The present study examines these parameters during coronary artery bypass surgery. We used a voltammetric microelectrode technique to assess regional variations in myocardial tissue partial pressure of oxygen (PO₂) and myocardial tissue perfusion (MTP) in patients undergoing coronary artery bypass surgery. A total of 29 myocardial regions were studied in 17 patients to assay tissue PO₂, and 13 regions in 10 patients to measure MTP. There was an increase in MTP from 53±9 ml·min^-1·100 g^-1 before cardiopulmonary bypass to 72±13 ml·min^-1·100 g^-1 after (means±S.E.M.; P = 0.05). Tissue PO₂ showed an overall increase from a baseline level of 45±8 mmHg to a final level of 88±10 mmHg (P< 0.0001). Following release of the aortic cross-clamp there was a variable time delay before a change in tissue PO₂ was observed. There was an immediate response in five regions, whereas in 20 regions the response was delayed by between 0.5 and 32 min. In the remaining four regions there was no change in tissue PO₂. The duration of the delay in response was correlated positively with the cross-clamp time (r = 0.45, P < 0.05) and negatively with the final tissue PO₂ (r =-0.5, P < 0.05). Voltammetric methods for monitoring changes in oxygen supply and utilization offer new insights into the changes that occur during ischaemia and reperfusion. A delay in the delivery of oxygen to the myocardium occurs in many patients following coronary artery bypass surgery.