A Concept of Negative Interstitial Pressure Based on Pressures in Implanted Perforated Capsules

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Abstract
Over 200 perforated plastic capsules were implanted in different tissues of the dog, and the wounds were allowed to heal. After one month, pressures measured by inserting a needle through the skin and then through a perforation of the capsule into its cavity were always negative in normal tissues, averaging-6.4 mm Hg. The pressure was always positive in edematous tissues. Evidence is presented to indicate that the pressure measured in the capsule is equal to, or nearly equal to, the interstitial fluid pressure in the tissue spaces surrounding the capsule. On the other hand, pressure measurements made by a needle technic failed except in rare instances to give negative values in normal tissues but in edematous tissues gave almost exactly the same values as those recorded by the capsules. Also, pressures measured by the capsules changed in accordance with Starling's law of the capillaries when (a) venous pressure was raised, (b) when arterial pressure was lowered, (c) when the tissues were dehydrated by intravenous infusion of dextran, or (d) when the tissues were hydrated by intravenous infusion of saline. Pressures measured by a needle technique failed to change in accordance with this law in these same experiments. Therefore, it is concluded that needle pressure determinations in non-edematous tissue do not measure the interstitial fluid pressure.