Animal models of arteriovenous carbon dioxide removal (AVCO(2)R) have achieved lung rest during treatment of severe respiratory failure, with total CO2 removal at arteriovenous shunt flow rates of 10% to 25% of cardiac output (GO). Previously, no statistically significant changes were reported in heart rate, cardiac output, mean arterial pressure, or pulmonary arterial pressure during prolonged (7 days) AVCO(2)R with shunt flows to 25% of CO. In this study, to determine the effect of various shunt levels on organ blood flow, colored microspheres were used in a conscious ovine model of AVCO(2)R. A low resistance 2.5 m(2) oxygenator was placed in a simple carotid-to-jugular arteriovenous circuit. The AVCO(2)R flow (Qb) was incrementally increased to 5%, 10%, 15%, 20%, and 25% of baseline CO. After equilibration, colored microspheres were injected into a left atrial catheter while reference blood was withdrawn from an arterial line at a constant rate. Organ blood now obtained by measuring microspheres in the tissues, showed approximately a 10-20% decrease at a 5% shunt, but remained relatively unchanged thereafter at up to a 25% shunt, and was well tolerated without hemodynamic sequelae or evidence of end organ ischemia. It was concluded that AVCO(2)R can achieve lung rest during respiratory failure at flow rates of 10-25% CO, with a resultant mild decrease in critical organ blood flow that appears well tolerated.