An implantable colloid osmometer has been developed for measurement of interstitial fluid colloid osmotic pressure (COPi). Amicon hollow fibres with a cutoff at MW 10,000 were used as a semipermeable membrane. The osmometer has a length of 10-15 mm, an outer diameter of 0.35 mm, and is connected to a pressure transducer via a PE 20 catheter. The equlibration time was about 10 min. In vitro comparison to an ordinary membrane osmometer showed a coefficient of correlation of 0.99. When the osmometer is implanted in tissue, the pressure transducer read the sum of -COPi(COP measured as a negative hydrostatic pressure, but defined as positive) and interstitial fluid hydrostatic pressure (Pi, positive or negative). In other words, the sum of the "interstitial Starling pressures" is measured directly. After perforation of the membrane, only Piwill be recorded. With intact membrane, implantation in rat subcutis (18 measurements in 8 rats) gave an average pressure of -10.8 (SD, 1.4) mm Hg. In skeletal muscle (17 measurements in 7 rats), the average pressure was -8.6 (SD, 2.1) mm Hg. With perforated membrane, pressures between -1 and 0 were obtained in both tissues, giving a COPiof about 10 mm Hg in subcutis and 8 mm Hg in muscle. An increased pressure (less negative) could be recorded continuously or by reimplantation, while furosemide diuresis reduced the pressure. © 1979.
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