Dynamic evaluation of fluid shifts during normothermic and hypothermic cardiopulmonary bypass in piglets

Background: Edema, generalized overhydration and organ dysfunction commonly occur in patients undergoing open-heart surgery using cardiopulmonary bypa ss (CPB) and induced hypothermia. Activation of inflammatory reactions indu ced by contact between blood and foreign surfaces are commonly held respons ible for the disturbances of fluid balance ("capillary leak syndrome"). We used an online technique to determine fluid shifts between the intravascula r and the interstitial space during normothermic and hypothermic CPB. Methods: Piglets were placed on CPB (fixed pump flow) via thoracotomy in ge neral anesthesia. In the normothermic group (n=7), the core temperature was kept at 38 degreesC prior to and during 2 h on CPB, whereas in the hypothe rmic group (n=7) temperature was lowered to 28 degreesC during bypass. The CPB circuit was primed with acetated Ringer's solution. The blood level in the CPB circuit reservoir was held constant during bypass. Ringer's solutio n was added when fluid substitution was needed (falling blood level in the reservoir). In addition to invasive hemodynamic monitoring, fluid input and losses were accurately recorded. Inflammatory mediators or markers were no t measured in this study. Results: Cardiac output, s-electrolytes and arterial blood gases were simil ar in the two groups in the pre-bypass period. At start of CPB the blood le vel in the machine reservoir fell markedly in both groups, necessitating fl uid supplementation and leading to a markedly reduced hematocrit. This extr a fluid need was transient in the normothermic group, but persisted in the hypo thermic animals. After 2 h of CPB the hypothermic animals had received 7 times moro fluid as compared to the normothermic Figs. Conclusion: We found strong indications for a seater fluid Extravasation du ring hypothermic CPB compared with normothermic CPB. The experimental model using the CPB-circuit reservoir as a fluid gauge gives us the opportunity to study further fluid volume shifts, its causes and potential ways to opti mize fluid therapy protocols.