High flow nasal oxygen versus face mask pre-oxygenation of pregnant patients with high body mass index - a crossover non-inferiorityrandomised controlled trial 

Obesity in pregnancy confers higher risk of airway management complications[1]. Obstetric guidelines recommend best practice pre-oxygenation as target end tidal oxygen (etO2) ≥ 90%[2]. Our prior study showed very strong evidence that high flow nasal oxygen (HFNO) was superior to face mask oxygen (FM) at achieving this threshold in pregnant patients[3]. The efficacy of HFNO & FM pre-oxygenation in pregnant patients with high body mass index (BMI), particularly obesity class II or above is uncertain. 

Hypothesis: 

That HFNO is non-inferior to FM for increasing etO2 after pre-oxygenation of pregnant patients with BMI ≥ 30 kg/m2 in late pregnancy.

Cohort: Pregnant patients ≥ 36 weeks gestation with BMI ≥ 30 kg/m2. 

Eligibility: Elective caesarean delivery, antenatal clinics. Exclusion: age < 18 years, significant nasal pathology, severe systemic disease, preeclampsia, sepsis, labour/multiple pregnancy.

Intervention: Serial simulated pre-oxygenation protocols with either HFNO followed by FM (Group HFNO-FM) or vice versa (Group FM-HFNO) in the ramped position (Figure 1). HFNO via OptiflowSwitchTM (maximum 70 l/min) & FM via tight-fitting face mask (10 l/min). 

Measurement: EtO2 measured at four time points: 5 min rest (baseline) followed by 3 min HFNO or FM, then 5 min rest (to confirm O2 washout) followed by 3 min FM or HFNO.

Primary outcome: etO2 captured via face mask (with 100% oxygen) after HFNO & after FM. 

Secondary outcomes: Proportion of participants achieving etO2 ≥ 90%, tolerance of HFNO, proportion of patients maintaining mouth closure with HFNO, correlations between primary outcome and relevant measured variables. 

Analysis: Primary outcome comparison: Paired sample t-test. Comparisons between proportions: Mcnemar’s test for paired samples. Correlations: Pearson’s correlation coefficient.

 

Results

Recruitment: Seventy pregnant participants were enrolled from 629 patients assessed. Four were excluded due to withdrawal or non-tolerance. 

Sample: The median [IQR] participant weight & BMI for Group HFNO-FM vs Group FM-HFNO were 97 [88-118] kg vs 107 [93-132] kg, & 37 [33-41] kg/m2 vs 38 [34-46] kg/m2 respectively.

Outcomes: First etO2 after HFNO was higher than after FM pre-oxygenation; mean (SD) 90 (3.2)% vs 88 (3.3)% [mean difference 1.8 (95% CI 0.7 to 2.9) %; p-value 0.002] (Figure 2). HFNO was superior to FM in achieving etO2 ≥ 90%; 50/66 (76%) HFNO vs 19/66 (29%) FM; p<0.001 (Table 1). 77% participants tolerated maximal (70 l/min) HFNO flow & 74% participants maintained mouth closure for the full HFNO protocol. There were no significant correlations between etO2 & age, gestation, weight, BMI, HFNO flow rate, duration of mouth closure, respiratory rate or minute ventilation. 

Conclusions

HFNO was superior to FM pre-oxygenation in pregnant patients with BMI ≥30 kg/m2under simulated conditions. After three minutes of pre-oxygenation the number of patients who achieved etO2 ≥ 90% was 2.5-fold greater with HFNO.