Comparison of methods for measurement of interstitial fluid pressure in cat skin/subcutis and muscle.

Interstitial fluid pressure (IFP) in cats was measured with four techniques, i.e., hollow perforated and porous polyethylene capsules, wick in needle (WIN), and micropipettes. During control conditions, skin IFP of -1.5 +/- 0.4 (SD) mmHg (n = 53) was obtained with micropipettes, whereas pressures recorded in subcutis with perforated and porous capsules were -1.6 +/- 0.9 (n = 26) and -1.6 +/- 0.8 mmHg (n = 13), respectively. These were all significantly different from the -1.2 +/- 0.5 mmHg (n = 50) obtained in subcutis with WIN. In skeletal muscle, control IFP of -0.5, -0.5, and -1.1 mmHg was measured with micropipettes, WIN, and porous capsules, respectively. During peritoneal dialysis skin and muscle IFP recorded with micropipettes and WIN was reduced by 3-3.5 mmHg, whereas pressure in porous and perforated capsules fell by 7 and 10 mmHg, respectively. Intravenous Ringer infusion caused a marked transient rise in capsular pressures, not reflected by micropipettes and WIN, but similar pressures were obtained 210 min after infusion. In conclusion, all techniques reflect true IFP under steady-state conditions. Both capsules apparently act like osmometers in acute overhydration or dehydration and are, in addition, sensitive to pressure changes in local veins and are therefore not suitable for measurement of changes in IFP that take place in less than a few hours.