Balancing pulmonary and systemic arterial flows in parallel circulations: the value of monitoring system venous oxygen saturations

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Abstract
Accurate bedside assessment of the ratio of pulmonary to systemic flow (Qp/Qs ratio, referred to as “the flow ratio” or “the ratio”) plays an important role in the management of many congenital heart defects, especially the complexes unified by univentricular atrioventricular connections. Arterial oxygen saturation can be a misleading measure of the ratio, and may not reflect derangements until they are quite large. Theoretical analysis suggests that systemic venous oxygenation may be a better indicator of the ratio. To examine this, we created a widely patent atrial septal defect in neonatal piglets (weight =4–6.5 kg, n=6). Snares aruond the aorta and pulmonary trunk were adjusted to alter the flow ratio from 0.1 to 6.5. Venous oxygen saturations, measured in the mid-inferior caval vein, were at a maximum at a ratio about 1, and declined rapidly with increases or decreases in the ratio beyond a limited range. The venous oxygen saturation was found to vary much more than arterial oxygen saturation, with arterial oxygen saturation only falling when the ratio dropped below 0.5. Oxygen delivery (Oxygen Content x Cardiacoutput) was found to parallel closely systemic venous oxygen saturation, and was at a maximum at the same ratio that produced a maximum value of systemic venous oxygen saturation. The study suggests that systemic venous oxygen saturation provides a better estimate than does systemic arterial oxygen saturationof the flow ratio and oxygen delivery. Interventions that maximize systemic venous oxygen saturation should maximize oxygen delivery, and determination of systemic venous oxygen saturation should be a helpful addition in managing children with a number of congenital heart defects.