MYOCARDIAL VENOUS O-2 SATURATION BECOMES MORE HETEROGENEOUS DURING HYPOXIC AND CARBON-MONOXIDE HYPOXIA

The hypothesis tested was that myocardial venous O-2 saturation (SvO(2 )) heterogeneity, a measure of microregional O-2 supply/consumption ba lance, would increase under hypoxic and CO-hypoxia conditions. Since w e are able to determine both O-2 supply and the O-2 supply/consumption ratio, we could also determine whether regional myocardial O-2 consum ption was heterogeneous. Twenty open-chest anesthetized dogs were stud ied under control and four hypoxic conditions, hypoxic hypoxia induced by ventilation with either an 8% O-2 (SaO(2) = 56%) or a 6% O-2 (SaO( 2) = 40%) gas mixture for 20 min, or CO hypoxia induced by ventilation with a 1% CO gas mixture for either 7 min (SaO(2) = 67%) or 20 min (S aO(2) = 40%), Regional myocardial blood flow was measured using radioa ctive microspheres in 40 pieces (similar to 0.5 g) of the left ventric ular free wall. Arterial and venous O-2 saturations were determined wi th a four-wavelength microspectrophotometric method. A total of 28 vei ns (20-100 pm) were examined to determine SvO(2) for each condition wi thin each animal. The coefficient of variation (CV = SD/mean x 100), a n index of heterogeneity, was calculated for both flow and SvO(2) unde r each condition. Flow increased with increasing severity of hypoxia b ut its heterogeneity did not change with hypoxic or CO hypoxia. Howeve r, SvO(2) heterogeneity significantly increased with increasing severi ty of hypoxia. A linear regression of SvO(2) CV and mean SvO(2) showed a significant correlation (CV = -0.84 (mean SvO(2)) + 51.1, R = 0.59) . All possible myocardial O-2 consumptions were calculated by multiply ing all of the flows and O-2 extractions. In 53 subepicardial and sube ndocardial measurements, only 10% of the flow and O-2 supply/consumpti on heterogeneity observations could be explained by uniform O-2 consum ption if our acceptance criterion was 0.06-0.1 ml O-2/min/100 g, and 5 0% could be explained with an acceptance criterion of 0.3-0.4 ml O-2/m in/100 g. Therefore, there must be some regional myocardial O-2 consum ption heterogeneity. The increase in venous O-2 saturation heterogenei ty during hypoxia may be due to increased variation in regional myocar dial O-2 consumption or variation in the control of O-2 supply/consump tion coupling. (C) 1995 Academic Press, Inc.