Systemic oxygen transport in patients with congenital heart disease

The physiology of oxygen delivery was studied in 118 stable patients from 3 months to 20 years old with congenital heart disease. During cardiac catheterization, oxygen consumption (VO2), arterial and venous blood gases and oxygen saturations (range 41% to 98%), hemoglobin concentration, diphosphoglycerate (2,3-DPG), and P50 levels were measured, and then cardiac output, systemic oxygen transport (SOT), arterial and venous oxygen contents, and the VO2/SOT ratio (fractional O2 extraction) were calculated. P50 averaged 31 mm Hg, compared with 27 mm Hg in 10 control children (p less than .01). The composite O2-hemoglobin dissociation curve in vivo was broad: Po2 varied from 37 to 65 mm Hg at 80% saturation. P50, 2,3-DPG, hemoglobin concentrations, and O2 saturation varied widely and inconsistently with Po2 and arterial and venous O2 content, but resulted in clustering of the arterial oxygen content near 165 +/- 23 (SD) ml/liter over a wide range of Po2 and hemoglobin concentrations. SOT varied in direct relation with flow (r = .82, p less than .001), but not with oxygen content, Po2, or P50. VO2 varied widely at normal or high levels of SOT, but decreased linearly at SOT levels below 400 ml/min/m2. Oxygen extraction varied inversely with venous O2 content, rising to about 50% and plateauing below venous contents of 100 ml/liter. O2 extraction did not correlate with Po2, arterial O2 content, or P50. These data suggest that: O2 saturation cannot be predicted or calculated accurately from measured Po2, but must be measured directly, 2,3-DPG, hemoglobin concentration, and P50 fluctuate to stabilize arterial oxygen content, SOT is determined primarily by cardiac output in subjects who are adapted chronically, O2 extraction rises, due to a fall in venous O2 content, to maintain VO2 as transport falls, below a critical level of SOT, O2 extraction ceases to rise and VO2 falls with further reduction in transport.