EFFECTS OF HYPOTHERMIA ON TISSUE O2 EXTRACTION CAPABILITIES IN DOGS

Abstract

S112 Although hypothermia decreases tissue O2 demand, it might also decrease tissue O2 extraction capabilities by its effects on blood viscosity and P50. This study tested this hypothesis in anesthetized dogs during cardiopulmonary bypass (CPB). METHODS: Sixty mongrel dogs (29.5 +/- 5.5kg) were anesthetized with thiopental (20mg/kg), ketamine (5mg/kg+0.2mg/kg.min) and pancuronium (0.1mg/kg + 2mg/h) and mechanically ventilated with air. After splenectomy and sternotomy, an aortic and a right atrial cannula were inserted. After heparinization (4 mg/kg), CPB was started at a continuous flow of 120 ml/kg/min and aorta was clamped. Gas flow and FiO2 were adapted at the beginning of the experiment to obtain a PaCO2 around 35 mmHg and a PaO2 between 150 and 200 mmHg. The dogs were randomized in three groups according to the temperature maintained during the CPB. In the first group, temperature was maintained at 38[degree sign]C (group NT, N=23), in the second one at 31.5[degree sign]C (group MHT, N=18) and in the third one at 25[degree sign]C (group SHT, N=19). Mixed venous O2 saturation (SvO2) was continuously recorded. When the heart was arrested, a 30 min period was allowed to achieve steady-state, defined by a stable mean arterial pressure (MAP) and SvO2. The flow of the CPB was then progressively reduced by 10 ml/kg/min decreases until 20 ml/kg/min. At each step, after a 10 min period to achieve a new steady state, MAP, pump flow, arterial and mixed venous blood gases were measured. O2 delivery (DO2) and O2 consumption (VO2) were calculated from standard formulas. In each dog, critical DO2 was determined from a plot of VO2 Vs DO2 (DO2crit) using a least sum of squares technique. Critical O2 extraction (O2ERcrit) was obtained by dividing VO2crit by DO2crit. Data at critical point were compared by ANOVA followed by a Tukey HSD test. RESULTS: (mean +/- SD) (Figure 1)Figure 1CONCLUSIONS: 1) Hypothermia decreased significantly VO2crit, resulting in a significant reduction in DO2crit. 2) Despite this decrease in VO2crit, O2ERcrit also decreased, indicating an impairment of tissue O2 extraction capabilities during hypothermic cardiopulmonary bypass.