Effects of Acutely Increasing Systemic Vascular Resistance on Oxygen Tension in Tetralogy of Fallot

Abstract

Effects of phenylephrine hydrochloride on the degree of hypoxia were studied in six cyanotic patients with tetralogy of Fallot. Baseline aortic and right atrial oxygen saturations, blood gas tensions, pH, and indicator-dilution curves were obtained under stable conditions. Phenylephrine was infused while systemic arterial pressure was monitored continuously. Aortic and right atrial blood samples were measured at each 10 mm Hg systolic pressure increment and decrement and analyzed for pH, PO2, PCO2, and oxygen saturation. A 40 mm Hg peak systolic aortic pressure rise was the arbitrary end point for phenylephrine infusion and the indicator-dilution curve repeated. Aortic oxygen tensions rose in all from 4 to 28 mm Hg (mean, 14 mm Hg). The net right-to-left shunt decreased from 10% to 40% (mean, 25%) with phenylephrine infusion as compared to the shunt under basal conditions. The presumed mechanism related to the increase in systemic vascular resistance relative to right ventricular outflow obstruction with resultant augmentation of pulmonary blood flow. It is concluded that acutely raising systemic vascular resistance in patients with tetralogy of Fallot using phenylephrine infusion has a beneficial effect on systemic arterial oxygen level and might be the medical treatment of choice for protracted hypoxic spells.