Comparative evaluation of three ambulatory plethysmographic devices as regards accuracy and handling in daily practice

Abstract

We compared a computed strain gauge plethysmograph (SGP1) with a Gutmann strain gauge plethysmograph (SGP2) and a photoplethysmograph (PPG) in 24 normal and 12 radiologically proven postphlebitic limbs with respect to assessment of the calf pump function, discrimination between normal and pathologic values, and handling. Recovery time (RT50), refilling volume (RV) and the Index (RT50xRV) were measured at the ankle and the calf in each limb after 20 dorsiflexions of the feet in the sitting position. The PPG device could only measure RT50. There was a strong positive correlation in RT50, RV and Index values between SGP1 and SGP2 at the ankle and at the calf (p less than 0.001), but none in RT50 ankle and calf values between PPG and SGP1 (p = 0.1) and PPG and SGP2 (p = 0.1). The comparison between normal and postphlebitic limbs revealed significant differences for all three values RT50, RV and Index only for SGP1 ankle measurements (p less than 0.02, p less than 0.01, respectively). Ankle RT50 values were significantly longer than calf RT50 values for both strain gauge devices (p less than 0.01), where PPG results were not influenced by the site of measurement. The average number of tests required to obtain a valid curve in each limb at the ankle and calf was lowest for SGP1 measurements. Vein calf pump function is easily and accurately assessed by SGP1 ankle measurements. However a threshold limit between normal and pathologic values can not be defined either for RT50 or for RV values in the individual case, and the creation of an Index adds little to solve this problem.