Cardiopulmonary bypass (CPB) is a critical component of open-heart surgery, enabling oxygenation and circulation while the heart is temporarily stopped. One key factor in managing CPB is the sweep gas flow rate, which governs the gas exchange efficiency—especially the removal of carbon dioxide (CO2). This study, conducted at Acibadem Altunizade Hospital in Turkey, aimed to clarify the optimal sweep gas flow rate to maintain arterial partial pressure of CO2 (PaCO2) within physiological limits, thus minimizing the risk of respiratory alkalosis and its impact on cerebral perfusion.
The randomized trial involved 84 adult patients undergoing elective open-heart surgery. They were assigned into three groups, each managed with a different sweep gas flow rate:
- Group 1: Constant sweep gas flow at 1.35 L/m²/min
- Group 2: Reduced from 1.35 to 1.2 L/m²/min before rewarming
- Group 3: Reduced from 1.35 to 1.0 L/m²/min before rewarming
Blood gas levels and regional cerebral oxygen saturation (rSO2) were monitored throughout surgery using near-infrared spectroscopy (NIRS). The six sampling time points included pre-anesthesia, various CPB stages, and immediately after bypass.
Findings showed that all groups experienced a reduction in PaCO2 due to hypothermia. However, differences emerged during the rewarming phase. Group 3, which had the lowest final sweep gas flow, maintained PaCO2 within normal physiological ranges and exhibited significantly better rSO2 compared to Groups 1 and 2. Group 1, with the constant high flow, had the lowest PaCO2 and highest pH—indicating respiratory alkalosis. A significant correlation was found between changes in PaCO2 and rSO2 levels (r = 0.45, P < 0.001), suggesting that minimizing alkalosis helps preserve cerebral autoregulation.
The study also analyzed the physiological dynamics of CO2 during rewarming. Typically, CO2 production increases due to elevated metabolic rates, while solubility decreases due to temperature rise. However, if the sweep gas flow remains too high during rewarming, it can create an excessive gradient, stripping CO2 too effectively and resulting in alkalemia. This imbalance may impair cerebral blood flow and potentially lead to neurological issues. Group 3’s reduced sweep gas flow allowed a modest and expected rise in PaCO2, avoiding respiratory alkalosis and ensuring better cerebral oxygen delivery.
Despite theoretical concerns, no clinical signs of cerebral hypoxia or neurological complications were observed across any group. This absence of adverse outcomes emphasizes the safety of modifying sweep gas flows during surgery.
The study reinforces earlier findings that while oxygen partial pressure (PaO2) remains relatively stable regardless of sweep gas flow, PaCO2 is highly sensitive to changes in flow rate. A high sweep gas flow rate may not benefit oxygenation but can dangerously lower PaCO2. Prior literature and simulation studies align with this observation, confirming a linear relationship between sweep gas rate and CO2 removal efficiency.
A significant takeaway from this research is that maintaining a constant high sweep gas flow may not be optimal throughout CPB. Instead, tailoring the rate according to the surgical phase—especially during rewarming—can better align PaCO2 levels with physiological norms, support cerebral autoregulation, and potentially reduce postoperative neurological risks.
The study also highlights some limitations. The primary one is the lack of a definitive guideline on how much sweep gas flow should be reduced during different CPB phases. Another limitation is the sole use of frontal rSO2 monitoring, which may not fully reflect global cerebral oxygenation and can be influenced by external factors like skin thickness and temperature.
In conclusion, the research advocates for a dynamic approach to managing sweep gas flow during CPB. Rather than relying on a fixed rate, adjusting the flow preemptively before rewarming may prevent harmful alterations in CO2 levels and maintain better neurological stability. These findings can inform perfusion strategies and enhance safety protocols in cardiac surgeries involving CPB.
Study ranking = 4 (high quality randomized clinical trial, though limited by sample size and single-center design)
