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

The measurement of coronary graft flow rates is a well-established method o f 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 per fusion flow rates and estimating the pressures produced. After going off ca rdiopulmonary bypass (CPB), the bypass graft flow (F) was estimated by a st andard ultrasound Doppler technique. The perfusion pressure over the perfus ed coronary graft was then determined and the CVR in the working heart asce rtained. The CVR was studied in 178 vein grafts in 59 patients undergoing c oronary bypass surgery. The mean CVR in the cardioplegic heart (c-CVR) vari ed from 0.81 to 2.3 mmHg/ml/min for various coronary artery diameters and w as significantly higher in small diameter arteries compared with larger art eries (p < 0.0002). Consequently significant high flows were found in the l arge vessels compared with the smaller ones (p < 0.0001). The mean c-CVR du ring 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 measureme nt 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 resistan ce following cardioplegia during the surgical procedure.