CHANGES IN OXYGENATION VARIABLES DURING PROGRESSIVE HYPOTHERMIA IN ANESTHETIZED PATIENTS

Because deliberate hypothermia is becoming commonly used during neuros urgery, this study was performed to investigate the effects of a progr essive reduction of body core temperature (T) on whole body oxygenatio n variables in patients undergoing elective intracranial surgery. In 1 3 patients (Hypothermic Group), T was reduced to 32.0 degrees C using convective-based surface cooling. In six patients (Control Group), T w as maintained at 35.5 degrees C during the entire study period. The ca rdiac index (CI) was determined with a pulmonary artery catheter by th ermodilution. Whole body oxygen delivery (DO2) was calculated from CI and arterial oxygen content. Whole body oxygen consumption (VO2), carb on dioxide production (VCO2), and energy expenditure (EE) were determi ned by ventilation gas analysis (indirect calorimetry). Mixed venous o xygen tension at 50% saturated hemoglobin (P-50), and whole body oxyge n extraction ratio (O2ER) were calculated. Repeated-measures analysis of variance and the Mann-Whitney test were used for statistical analys is. Data are expressed as means +/- SD. VO2 (from 100 +/- 13 to 77 +/- 11 ml.min(-1).m(-2)), VCO2 (from 75 +/- 7 to 57 +/- 7 ml.min(-1). m(- 2)), EE (from 667 +/- 67 to 509 +/- 66 kcal.d(-1).m(-2)), P-50 (from 2 3.8 +/- 1.7 to 20 +/- 0.9 mm Hg), and O2ER (from 0.29 +/- 0.05 to 0.22 +/- 0.03%) decreased significantly in the Hypothermic Group between 3 5.5 and 32.0 degrees C (p < 0.05). None of these variables changed in the Control Group and at 32.0 degrees C VO2, VCO2, EE, P-50, and O2ER were significantly lower in the Hypothermic Group than in the Control Group. DO2 remained unchanged in both groups. We conclude that progres sive hypothermia in anesthetized patients reduces metabolic rate but d oes not change DO2. The significant decrease in O2ER may partly be rel ated to a leftward shift of the oxyhemoglobin dissociation curve, as e videnced by the decrease in P-50.