Capnographic Analysis of Minimum Mandatory Flow Rate for Hudson Face Mask: A Randomized Double-blind Study

Oxygen supplementation through Hudson type face mask is frequently used in perioperative settings. Hudson mask is a variable rate performance device with the risk of rebreathing. Studies using capnography to find out an actual fraction of rebreathing in spontaneously breathing patients are not available.

In this study, we analyzed the effects of different flow rates through Hudson mask with capnography on fractional inspired carbon dioxide (FiCO₂).

Forty patients posted for minor surgeries under monitored anesthesia care were divided into four groups. They received oxygen flow rate of 3 L/min, 4 L/min, 5 L/min, and 6 L/min as per group division, through Hudson mask.

Parameters such as pulse rate, noninvasive blood pressure, oxygen saturation, respiratory rate (RR), end-tidal carbon dioxide (EtCO₂), and FiCO₂ were noted at baseline. After starting oxygen supplementation, these parameters were monitored every minute for ten minutes. Statistical analysis was done using analysis of variance and Kruskal-Wallis test. Pearson correlation was used to establish a relation between RR and FiCO₂.

EtCO₂ and FiCO₂ were comparable in all four groups with no statistical significance. There was strong positive correlation seen between RR and FiCO₂ at a flow rate of 3 L/min while negative correlation in other groups. We conclude that FiCO₂ is similar at all flow rates while breathing through Hudson mask. Increase in RR can cause increase in FiCO₂ at lower flow rate, which is within physiological tolerance limits.