Measurement of resistance versus flow in assessing efficiency of aortocoronary bypass grafts. Measurement of flow in saphenous bypass grafts with an electromagnetic flowmeter is complicated and poorly reproducible. Since coronary flow is largely dependent on variable factors the stable value of resistance seems more appropriate for comparison. A simple method has been developed for intraoperative measurement of resistance in the respective coronary bed. Pressure is recorded in the saphenous graft by an electromanometer during continuous flushing with known amounts of blood, and resistance is calculated instantaneously. The procedure is very simple and takes less than one minute. The quality of the saphenous vein itself can be assessed simultaneously by the same method. Resistances were measured during coronary surgery in over 500 saphenous grafts. The results were highly reproducible and comparable. Excellent flows can be expected if resistance is below 200 Peripheral Resistance Units (PRU); if this is over 800 PRU flow is very poor. 