Transmission of Applied Pressure Through Tissues: Interstitial Fluid Pressure,Solid Tissue Pressure,and Total Tissue Pressure

Abstract

Materials and methods

Interstitial fluid pressure (P_if) of the dog was measured from capsules implanted in subcutaneous tissue of the upper and lower hindleg 4 to 8 weeks prior to use. (The capsules were a disk-shaped hollow (6 mm x 9 mm i.d.) porous polyethylene matrix (Bolab, Inc.) with an average pore size of 65 μm.) Total tissue pressure (P_tot) was measured either with small flaccid balloons inserted subcutane-ously or, more frequently, by the needle method for measuring total tissue pressure (see (3) for a discussion of these methods).

Pressure transmission was examined in two different preparations. With the first, the intact hindleg of the pentobarbital anesthetized dog was placed up to the hip in a water filled plethysmograph and P_if and P_tot measured while applying positive or negative pressure to the leg. With the second preparation, the hindlimb was totally isolated from the dog and artificially perfused with whole blood as described elsewhere (4). The isolated hindlimb was then placed entirely inside the plethysmograph with only the arterial, venous, and pressure measurement catheters extending out of the plethysmograph.

Using the intact hindlimb preparation, the response of interstitial fluid pressure to alterations in ambient pressures was determined as P_if was increased above its normal value in the intact hindleg. In order to increase P_lf, warmed Tyrode's solution was infused intravenously. When P_if approached or exceeded atmospheric pressure and the infusion was terminated, P_if at first decreased with time as if the tissues were undergoing stress-relaxation. Therefore, measurements were made about 0.5 hr after the infusion of Tyrode's solution when F_if appeared to have reached a steady value.

With changes in applied pressure, a small amount of fluid (less than 1 μ1) is displaced into or out of the pressure transducer.