Intraoperative assessement of coronary flow and coronary vascular resistance during coronary bypass surgery

The measurement of coronary graft flow rates is a well-established method of assessing graft function intraoperatively. In order further to understand the dynamics of graft function, the resistance to the flow was considered a desirable measurement intraoperatively. The coronary vascular resistance (CVR) was estimated by applying the Poiseuille-Hagen equation. The CVR was estimated at zero cardiac work (during cardioplegic arrest) using fixed perfusion flow rates and estimating the pressures produced. After going off cardiopulmonary bypass (CPB), the bypass graft flow (F) was estimated by a standard ultrasound Doppler technique. The perfusion pressure over the perfused coronary graft was then determined and the CVR in the working heart ascertained. The CVR was studied in 178 vein grafts in 59 patients undergoing coronary bypass surgery. The mean CVR in the cardioplegic heart (c-CVR) varied from 0.81 to 2.3 mmHg/ml/min for various coronary artery diameters and was significantly higher in small diameter arteries compared with larger arteries (p < 0.0002). Consequently significant high flows were found in the large vessels compared with the smaller ones (p < 0.0001). The mean c-CVR during cardioplegia of 1.57 +/- 0.06 increased significantly to 1.75 +/- 0.07 mmHg/ml/min after the procedure (p-CVR) and was attributed to the dynamic resistance of the working heart. The post-CPB graft flow was significantly and negatively correlated to the c-CVR of the arrested heart. The measurement of coronary vascular resistance reveals coronary beds at potential high risk for inadequate perfusion. Such areas are usually fed by small vessels with low flows. The working heart, in turn, increases the coronary resistance following cardioplegia during the surgical procedure.