Comparison of blood flow measured by plethysmograph and flowmeter during steady state forearm exercise

Forearm blood flow was determined at rest and during steady state grip exercise by the venous occlusion technique with a single strand mercury in rubber strain gauge plethysmograph and was compared with blood flow simultaneously determined by an electromagnetic flowmeter around the brachial artery before, during, and after intermittent isotonic grip exercise in 8 subjects. The comparison of plethysmographic flow (y axis) with simultaneously measured flowmeter flow (x axis) yielded a correlation coefficient of 0.797, a slope of 1.15 and a y intercept of 9.62. When flow was normalized per 100 cc of forearm, the correlation coefficient was 0.823, the slope 1.21, and the y intercept 0.871. The overestimation of the flowmeter blood by the plethysmograph could be explained by flow through skin vessels which bypassed the flowmeter. When averaged plethysmographic flow was compared to total planimetered flowmeter flow during exercise, there was a 1.7 times overestimation by the former technique, which was constant over the range of exercise employed.