Beat-by-beat forearm blood flow with Doppler ultrasound and strain-gauge plethysmography.

Simultaneous Doppler ultrasound estimates of brachial artery mean blood velocity (MBV) and venous occlusion strain-gauge plethysmography measures of forearm blood flow (FBF) were performed to determine the beat-by-beat relationship between the two methods and provide a method for flow calibration of Doppler MBV estimates. Such a calibration of Doppler MBV eliminates the need for knowledge of vessel cross-sectional area and angle of insonation while allowing for the quantification of limb blood flow. Six healthy subjects (5 men and 1 woman) performed 40 s of isometric forearm contraction at 35% maximal voluntary contraction with arterial inflow occluded. This resulted in elevated blood flow on relaxation and cuff deflation, and simultaneous beat-by-beat Doppler MBV and strain-gauge FBF measurements were then made over a period of 2-3 min as flow gradually decreased to resting levels. The r2 values for the fitted regression lines over a wide range of flows ranged from 0.87 to 0.98, and the mean square error terms ranged from 0.88 to 3.07 ml.100 ml-1.min-1. Significant day-to-day variation of the fitted regression parameters within subjects indicated that quantitative estimates of FBF from Doppler MBV require a calibration to be performed for each experiment. The finding of a strong linear relationship between Doppler MBV and venous occlusion strain-gauge plethysmography, as well as the marked beat-by-beat effect of cuff inflation on blood flow, confirms the importance of calibration on the same beats, not on adjacent segments of beats.