SummaryBody water content and distribution were determined in 16 children aged 2 wk to 28 months before and after open-heart surgery for correction of congenital cardiac defects. Operative procedures were performed using hypothermia and extracorporeal oxygenation. On the day before and the day after surgery, total body water was estimated as the antipyrine space (APS); extracellular water, as the corrected bromide space (CBS), and plasma volume, as the 10-min T-1824-albumin space. Intracellular water (ICW) was assumed to be the difference between APS and CBS; interstitial water was calculated from plasma volume and CBS. Before initiation and after completion of extracorporeal circulation, a pectoral muscle biopsy was performed, and a blood sample was obtained. Muscle total water (TW) content was determined by desiccation, extracellular water (ECW) was estimated as the corrected chloride space, and ICW was assumed to be the difference between TW and ECW. Plasma sodium, potassium, chloride, glucose, and osmolality contents were determined by routine methods. All studies were not completed in all patients.Although APS and CBS increased in eight of 11 children, mean APS (± S.E.) before and after surgery (662 ± 28.0versus714 ± 37.2 ml/kg) and mean CBS (335 ± 30.5versus358 ± 15.5 ml/kg) were not statistically different. Mean ICW changed neither in relation to body weight (328 ± 28.0versus355 ± 34.2 ml/kg) nor in relation to APS (ICW/APS ratio = 0.48 ± 0.040versus0.49 ± 0.010). Neither mean PV (54 ± 4.0versus56 ± 2.8 ml/kg) nor mean blood volume (92 ± 5.4versus90 ± 5.5 ml/kg) changed significantly. Mean interstitial water increased by 9 to 68% over preoperative values in all but one patient (238 ± 10.4versus305 ± 13.4 ml/kg;P< 0.01).Muscle composition was not affected by the procedure. Mean TW was 79 ± 1.3 ml/100 g before extracorporeal circulation and 78 ± 0.8 ml/100 g afterwards whereas ECW averaged 32 ± 4.4 and 36 ± 3.4 ml/100 g, and ICW averaged 48 ± 4.6 and 42 ± 2.9 ml/100 g. Mean ICW/TW ratios were 0.60 ± 0.055 and 0.54 ± 0.040 ml/100 g.Although mean plasma sodium (142 ± 3.5versus139 ± 2.4 mEq/liter) and potassium (3.3 ± 0.16versus3.6 ± 0.15 mEq/liter) concentrations did not change appreciably during extracorporeal circulation; mean plasma chloride content decreased (108 ± 2.9versus100 ± 2.0 mEq/liter;P< .002). Plasma glucose averaged 60 mg/dl more at completion of the procedure, increasing from a mean of 223 ± 25.6 mg/dl to a mean of 283 ± 5.3 mg/dl (P< 0.05). Plasma osmolality increased in five of eight children, but mean osmolalities were similar before and after extracorporeal circulation (301 ± 8.9versus303 ± 5.4 mOsm/kg).These data suggest that a child's organism does not react complacently to the invasive procedures associated with open-heart surgery. Further research into effects of these procedures and into means of minimizing undesirable homeostatic disturbances is warranted.