Studies on fluid extravasation related to induced hypothermia during cardiopulmonary bypass in piglets

Background: Hypothermia, commonly used for organ protection during cardiopu lmonary bypass (CPB), has been associated with changes in plasma volume, he moconcentration and microvascular fluid shifts. Fluid pathophysiology secon dary to hypothermia and the mechanisms behind these changes are still large ly unknown. In a recent study we found increased fluid needs during hypothe rmic compared to normothermic CPB. The aim of the present study was to char acterize the distribution of the fluid given to maintain normovolemia. In a ddition, we wanted to investigate the quantity and quality of the fluid ext ravasated during hypothermic compared to normothermic CPB. Methods: Two groups of anesthetized piglets were studied during 2 h of hypo thermic (28 degreesC) (n=7) or normothermic (38 degreesC) (n=7) CPB. Net fl uid balance (input-output) was recorded. Changes in colloid osmotic pressur es of plasma (COPp) and interstitial fluid (COPi), plasma volume (PV), hemo globin (Hb), hematocrit (HCT), mean corpuscular volume (MCV), s-osmolality, s-albumin and s-total protein was followed throughout the experiments. Flu id extravasation rate was calculated. In addition, total tissue water conte nt was measured and compared with a control group (n = 6) (no CPB). Results: During hypothermic compared with normothermic CPB, the average net positive fluid balance from 10-120 min of extracorporeal circulation was 1 .35 +/-0.06 ml (.) kg(-1 .) min(-1) and 0.33 +/-0.03 ml (.) kg(-1) min(-1) respectively (P <0.0001). We found a marked increase in fluid extravasation during hypothermic CPB. The extravasation rate during hypothermia was 1.8 +/-0.2 ml (.) kg(-1) min(-1), (1st hour) and 1.1 +/-0.2 ml (.) kg(-1) min(- 1) (2nd hour) compared with 0.8 +/-0.2 ml (.) kg(-1) min(-1) and 0.1 +/- (0 .1) ml (.) kg(-1) min(-1) during normothermia, respectively (P <0.01). The total intravascular protein and albumin masses remained constant in both gr oups. Following hrrothermic CPB, the water content increased significantly in all tissues and organs. Conclusion: During hypothermic CPB an increased extravasation of fluid from the intravascular to the interstitial space was found. As no leakage of pr oteins could be demonstrated, based on stable values for albumin and protei n masses throughout the experiments, the extravasated fluid contained mainl y water and small solutes.