Introduction: Until now, information on the determinants of blood flow in coronary artery bypass grafts has been limited . Most intraoperative measurements of bypass flow are performed only to determine the graft patency. Therefore, the objective of this study was to describe the diastolic pressure-flow (P-F) relationship and to assess critical occlusion pressure (COP) in arterial coronary bypass grafts.
Method: After approval by the local ethics committee fifteen patients electively planned for CABG were studied. In each patient, flow measurements in the left internal mammary artery bypass (IMAB) to the left anterior descending artery were performed intraoperatively 15 minutes after discontinuation of extracorporeal circulation and prior to antagonising heparin. The flow measurements were performed by using ultrasound and calculations based on the transit time principle (Cardiomed 4008, Quick-Fit probes (size 2.0-3.0 mm), Medistim, Norway). Simultaneously, aortic pressure (AP), coronary sinus pressure (CSP), and left ventricular end-diastolic pressure (LVEDP) were recorded. The zero flow pressure intercept as a measure of COP was extrapolated from the linear regression analysis of the instantaneous diastolic P-F relationship. In order to avoid capacitance effects of the arterial vessel on the results of this study, the analysis of P-F relationship only included data from the highest diastolic flow rate in the arterial bypass graft until the end of diastole. Linear regression analysis was performed using flow as dependent variable and aortic pressure as independent variable. A P value of <0.05 was considered statistically significant.
Results: Five consecutive heart beats were analysed in each patient. Mean diastolic flow in the IMAB was 46 ± 17mL/min, mean diastolic AP, mean heart rate, mean CSP and LVEDP were 60.5 ± 10.0mmHg, 86.5 ± 7.5 beat/min, 10.9 ± 3.1 mmHg and 14.4 ± 5.8 mmHg, respectively. Diastolic blood flow was linearly related to the respective AP in all patients (R-values 0.7 to 0.99). The regression lines had a mean slope of 2.1 ± 1.2mLmin−1 mmHg−1. Mean COP was 32.3 ± 9.9 mmHg and exceeded mean CSP and mean LVEDP by a factor of 3.1 and 2.6, respectively.
Conclusions: Our data demonstrate the presence of a vascular waterfall phenomenon  in the coronary circulation after IMAB grafting. COP in arterial grafts considerably exceeds CSP as well as LVEDP and should thus be used as the effective downstream pressure. Our data further suggest that the slope of diastolic P-F relationships provides a more rational approach to assess regional coronary vascular resistance (CVR) than conventional calculations.
1 Dole WP, Richards KL, Hartley CJ, et al. Diastolic coronary artery pressure-flow velocity relationship in conscious man. Cardiovasc Res
2 Permutt S, Riley RL. Hemodynamics of collapsible vessels with tone: the vascular waterfall. J Appl Physiol