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

Hypothermia, commonly used for organ protection during cardiopulmonary bypass (CPB), has been associated with changes in plasma volume, hemoconcentration and microvascular fluid shifts. Fluid pathophysiology secondary to hypothermia and the mechanisms behind these changes are still largely unknown. In a recent study we found increased fluid needs during hypothermic compared to normothermic CPB. The aim of the present study was to characterize the distribution of the fluid given to maintain normovolemia. In addition, we wanted to investigate the quantity and quality of the fluid extravasated during hypothermic compared to normothermic CPB.

Two groups of anesthetized piglets were studied during 2 h of hypothermic (28 degrees C) (n=7) or normothermic (38 degrees C) (n=7) CPB. Net fluid balance (input-output) was recorded. Changes in colloid osmotic pressures of plasma (COPp) and interstitial fluid (COPi), plasma volume (PV), hemoglobin (Hb), hematocrit (HCT), mean corpuscular volume (MCV), s-osmolality, s-albumin and s-total protein was followed throughout the experiments. Fluid extravasation rate was calculated. In addition, total tissue water content was measured and compared with a control group (n=6) (no CPB).

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 x kg(-1) x min(-1) and 0.33+/-0.03 ml x kg(-1) x 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 x kg(-1) x min(-1), (1st hour) and 1.1+/-0.2 ml x kg(-1) x min(-1) (2nd hour) compared with 0.8+/-0.2 ml x kg(-1) x min(-1), and 0.1+/-(0.1) ml x kg(-1) x min(-1) during normothermia, respectively (P<0.01). The total intravascular protein and albumin masses remained constant in both groups. Following hypothermic CPB, the water content increased significantly in all tissues and organs.

During hypothermic CPB an increased extravasation of fluid from the intravascular to the interstitial space was found. As no leakage of proteins could be demonstrated, based on stable values for albumin and protein masses throughout the experiments, the extravasated fluid contained mainly water and small solutes.