Different Flow Rates on Gas Exchange in Children During Apnea

Introduction Patients undergoing general anesthesia with neuromuscular blockade experience a period of apnea between cessation of mask ventilation and initiation of mechanical ventilation. This interval is particularly critical in infants and toddlers, who are prone to rapid desaturation because of higher oxygen consumption and lower functional residual capacity than older children and adults [1]. Accordingly, hypoxemia during induction and airway manipulation is associated with an increased incidence of postoperative complications and serious adverse events, including death [2,3].

Apneic oxygenation refers to the continuous insufflation of oxygen into the airway during apnea [4]. Several studies have shown that this approach effectively maintains oxygenation and reduces the risk of hypoxemia during tracheal intubation [5,6]. An ongoing challenge in pediatric anesthesia is to ensure adequate oxygenation while limiting lung volume loss and carbon dioxide accumulation during apnea, particularly during airway manipulation. Therefore, techniques that extend the duration of safe apnea without introducing additional risks are of considerable clinical relevance [7]. Accordingly, apneic oxygenation has gained attention as an adjunctive technique, with similar benefits reported in children using both low-flow and high-flow nasal oxygen, although pediatric data remain more limited [8].

Multiple studies have examined low-flow apneic oxygenation in children, showing that it reduces the incidence of hypoxemia and extends the duration of safe apnea, in some reports by up to several minutes, compared with no apneic oxygenation. Comparative data on CO2 kinetics across different low-flow rates in children are limited [9-11]. Therefore, this study aimed to examine the effect of different flow oxygen rates applied during a standardized 6-minute period of apnea on the increase in arterial partial pressure of CO2 (PaCO2) in anesthetized, paralyzed children.

Sample size No previous data were available for the primary outcome, the rise in PaCO₂ between the 2nd and 3rd minutes of apnea (ΔPaCO₂, mmHg), so a pilot study of five children per group was performed. These pilot children were a separate, non-randomized cohort and were excluded from the modified intention-to-treat analysis. The mean ± SD ΔPaCO₂ was 4.2 ± 1.6, 3.4 ± 1.5 and 2.8 ± 1.4 mmHg at 2, 4 and 8 L/min. The largest difference (2 vs 8 L/min) was 1.4 mmHg, a one-third reduction, considered clinically meaningful as it would delay hypercapnia and respiratory acidosis during apnea; the pooled SD for these two groups was 1.5 mmHg (Cohen's d = 0.93). For three pairwise comparisons with 1:1:1 allocation, a two-sided alpha of 0.0167 (0.05/3) and 80% power, at least 26 children per group were required (independent-samples t-test, G*Power 3.1.9.7), used as an approximation for the three-group ANOVA design. Allowing for 15% loss of data, 30 children were enrolled per group (90 total).