DETERMINATION OF LOW BLOOD-FLOW LIMITS FOR ARTERIOVENOUS CARBON-DIOXIDE REMOVAL

Arteriovenous carbon dioxide removal (AVCO(2)R) has been shown to achi eve total carbon dioxide (CO2) exchange. To determine optimal blood an d gas flow parameters that can provide maximal gas exchange and evalua te the utility of AVCO(2)R at reduced blood flow, the authors used a l ow resistance membrane gas exchanger within an arteriovenous shunt in mechanically ventilated sheep. Adult female sheep (n = 5) were anesthe tized and underwent placement of the gas exchange device in a simple a rteriovenous shunt created between the carotid artery and common jugul ar vein. CO2 removal was determined as the product of the sweep gas fl ow (100% oxygen) and its exhaust CO2 content. Gas and blood flow were varied independently, and incremental reductions in minute ventilation (MV) were made while maintaining normocapnia. At maximally reduced ve ntilator settings, stepwise reductions in blood flow were made to dete rmine the resultant changes in arterial PaCO2 at a sweep gas flow of 3 L/min. CO2 removal increased proportionally to blood flow to a maximu m of 1,417 +/- 26 ml/min (19% of cardiac output) and gas flow to 3 L/m in. Normal PaO2 and PaCO2 could be maintained with minimal ventilator support (MV = 16% baseline MV) at a blood flow of 500 ml/min or higher . At these maximally reduced ventilator settings, moderate hypercapnia (PaCO2 less than or equal to 75 mmHg) resulted only when blood flow w as decreased to below 500 ml/min. Optimizing AVCO(2)R blood and gas fl ow maximizes CO2 removal and allows a significant reduction in minute ventilation. In cases of severely limited blood flow, lung rest can st ill be realized at moderate hypercapnia. At flow rates achievable by p ercutaneous access, extracorporeal AVCO(2)R can be used to achieve lun g rest during mechanical ventilation.