SAMPLING OF INTERSTITIAL FLUID AND MEASUREMENT OF COLLOID OSMOTIC-PRESSURE (COPI) IN PIGS - EVALUATION OF THE WICK METHOD

The wick method for sampling of interstitial fluid from subcutis was a pplied in fluid balance studies in young pigs. Colloid osmotic pressur e was measured in serum (COPs) and interstitial fluid (COPi) using a m embrane colloid osmometer. Our aims were to determine the 'true' COPi and to find the optimal duration of wick implantation. In series I (n = 6) a 'crossover' experiment was performed using wicks soaked in diff erent priming solutions (nondiluted and diluted serum protein solution s or isotonic salt solution). Circulatory arrest was induced lust befo re wick insertion in order to eliminate the vascular part of the acute inflammation. Tn series II (n = 6) wicks were removed in sequence aft er 60, 90, 120 and 180 min sampling time in anaesthetized pigs in vivo . COPs, COPi and haematocrit (HCT) together with haemoglobin (Hgb), se rum albumin and total protein concentrations were determined in the sa me animals. In series I average COPs and COPi were 13.7 (1.4) and 7.2 (1.4) mmHg respectively (SD). In series II the optimal wick implantati on times were estimated to be 60-90 min for wicks soaked in diluted pr otein solution, and 90-120 min for dry and saline-soaked wicks. COPs a veraged 13.0 (0.7) mmHg, HCT 30.0 (1.6)%, Hgb 8.3 (0.9) g/dl, s-albumi n 22.7 (0.6) g/l and s-protein 47.3 (2.3) g/l. Compared to commonly re ported reference values, we found surprisingly low values for most of the measured variables. This may be related to the fact that we used i mmature pigs. An analysis of the validity of the wick method based on our own results and published reports is presented. We conclude that s ampling of interstitial fluid with subcutaneous wicks is easy to perfo rm in young pigs. However, the COP-values measured in wick fluid have to be carefully evaluated especially when sampling is performed in viv o.