Impact of Different Sweep Gas Flow Rates on Respiratory Alkalosis and Cerebral Oxygenation during Cardiopulmonary Bypass

Abstract

There is no consensus on the ideal sweep gas flow volume for achieving targeted blood partial gas pressures during cardiopulmonary bypass (CPB). The sweep gas flow rate is one of the oxygenator's main gas exchange variables. High sweep gas flow rates can lead to respiratory and hypocapnic cerebral alkalosis, which can cause neurological complications.

This study included 84 patients aged > 18 years who were scheduled to undergo elective open-heart surgery with CPB. Before rewarming, the participants were randomly assigned to one of the three groups based on their sweep gas flow rates (Group 1, 1.35 L/m²/minute; Group 2, 1.2 L/m²/minute; and Group 3, 1 L/m²/minute). During the surgery, arterial blood gases were sampled at six different time points, and regional cerebral oxygen saturation (rSO₂) levels were monitored bilaterally on the forehead.

The study found that all groups experienced a decrease in partial pressure of arterial carbon dioxide (PaCO₂) levels after the onset of hypothermia, which decreased to below the normal range at a moderate hypothermia level of 32°C. During both the baseline and hypothermic periods, the PaCO₂ were similar between the groups; however, after rewarming, Group 3 had significantly higher PaCO₂ than Groups 1 and 2 (P< 0.001). During the same period, Group 3 had significantly higher rSO₂ levels than Groups 1 and 2 (P = 0.005). For all patients, there was a significant correlation between delta-PaCO₂ and delta-rSO₂ levels after rewarming (r = 0.45, P< 0.001).

This study demonstrated that low sweep gas flow prevented alkalosis and preserved cerebral autoregulation.