Quantification of rate and depth of pitting in human edema using an electronic tonometer

An instrument (tonometer) was developed to measure objectively the rate as well as depth of pitting of edematous limbs under a sudden local load. Displacement versus time curves were obtained in vivo in postmastectomy edema arms and also in vitro (compression of sponges) and were analyzed in terms of spring and dashpot constants. There was no significant difference between the quasi-instantaneous indentation of tissue in the edematous and normal arms (median 2.9mm), and the two correlated strongly (r = 0.91, p < 0.0001). An exponentially slowing indentation followed. The mean difference between initial and final deformation (X infinity-X0) was greater in the swollen arms (5.7mm) than in the normal arms (1.3mm, p < 0.01). The time constant of indentation (tau) was significantly greater in the swollen arms (227s) than in the normal arms (71s). There was no correlation between the duration of the edema and any of the pitting characteristics. There was a significant negative correlation between glycosaminoglycan concentration of interstitial fluid and rate constant 1/tau (r = 0.9, p < 0.01). The tonometer thus provides an objective way of quantifying the rate and depth of pitting edema.