BALANCING PULMONARY AND SYSTEMIC ARTERIAL FLOWS IN PARALLEL CIRCULATIONS - THE VALUE OF MONITORING-SYSTEM VENOUS OXYGEN SATURATIONS

Accurate bedside assessment of the ratio of pulmonary to systemic flow (Q(p)/Q(s) ratio, referred to as ''the flow ratio'' or ''the ratio'') plays an important role in the management of many congenital heart de fects, especially the complexes unified by univentricular atrioventric ular connections. Arterial oxygen saturation can be a misleading measu re 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 around the aorta and pulmonary trunk were adjusted to alter the flow ratio from 0.1 to 6.5. Venous oxygen saturations, me asured in the mid-inferior caval vein, were at a maximum at a ratio ab out 1, and declined rapidly with increases or decreases in the ratio b eyond a limited range. The venous oxygen saturation was found to vary much more than arterial oxygen saturation, with arterial oxygen satura tion only falling when the ratio dropped below 0.5. Oxygen delivery (O xygen Content x Cardiac output) 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 st udy suggests that systemic venous oxygen saturation provides a better estimate than does systemic arterial oxygen saturation of the flow rat io and oxygen delivery. Interventions that maximize systemic venous ox ygen saturation should maximize oxygen delivery, and determination of systemic venous oxygen saturation should be a helpful addition in mana ging children with a number of congenital heart defects.