Determination of low blood flow limits for arteriovenous carbon dioxide removal

Arteriovenous carbon dioxide removal (AVCO₂R) has been shown to achieve total carbon dioxide (CO₂) exchange. To determine optimal blood and gas flow parameters that can provide maximal gas exchange and evaluate the utility of AVCO₂R at reduced blood flow, the authors used a low resistance membrane gas exchanger within an arteriovenous shunt in mechanically ventilated sheep. Adult female sheep (n = 5) were anesthetized and underwent placement of the gas exchange device in a simple arteriovenous shunt created between the carotid artery and common jugular vein. CO₂ removal was determined as the product of the sweep gas flow (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 ventilator settings, stepwise reduction in blood flow were made to determine the resultant changes in arterial PaCO₂ at a sweep gas flow of 3 L/min. CO₂ removal increased proportionally to blood flow to a maximum of 1,417 ± 26 ml/min (19% of cardiac output) and gas flow to 3 L/min. 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 ≤ 75 mmHg) resulted only when blood flow was decreased to below 500 ml/min. Optimizing AVCO₂R blood and gas flow maximizes CO₂ removal and allows a significant reduction in minute ventilation. In cases of severely limited blood flow, lung rest can still be realized at moderate hypercapnia. At flow rates achievable by percutaneous access, extracorporeal AVCO₂R can be used to achieve lung rest during mechanical ventilation.