Evaluation of methodologies for measurement of interstitial fluid pressure (Pi): physiological implications of recent Pi data.

As one of the determinants of net transcapillary filtration, filling of initial lymphatics and interstitial compliance, interstitial fluid pressure (Pi) is of great physiological interest. Several methods have been developed to measure Pi, all measuring the fluid equilibration pressure, i.e., the pressure in a saline-filled tube brought into contact with the interstitium. The methods designed to establish such contact may be characterized as acute and chronic. With acute methods, i.e., needle without infusion, wick catheters, WIN, and micropipettes, measurements are made within minutes to a few hours after insertion of the device. The chronic methods are various capsules (perforated, porous, or Teflon rings) implanted into the tissue 4 to 6 weeks before measurements. A general finding in previous studies from different laboratories and in different species have been that the chronic methods give more strongly negative (subatmospheric) Pi than the acute methods, and that the chronic methods give much greater pressure responses to changes in IFV. By direct comparison of acute and chronic methods in the same tissue and site, all give similar Pi in steady state conditions, while an acute over- and dehydration results in far more pronounced pressure changes recorded with chronic than with acute devices. It is proposed that these transient pressure differences recorded by acute vs. chronic methods result from different physical properties of the capsule lining compared with that of the surrounding skin, in addition to a possible osmometer effect of the capsule lining. Recent data show that such methodological problems have resulted in great variation in the estimation of compliance of the interstitium and have overemphasized the importance of Pi as an edema-preventing mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)