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Optimal time interval for preoxygenation in pregnant patients: Determination of the EI90 between high-flow nasal oxygen and standard facemask tidal volume preoxygenation using a biased-coin sequential allocation method
Abstract Number: T2A-283
Abstract Type: Original Research
Introduction: After a fixed interval of 3 min of tidal volume (TV) breathing, high-flow nasal oxygen (HFNO) at 50-70L/min was found to be unreliable at achieving a mean end-tidal oxygen concentration (ETO2) ≥90% compared to standard facemask (FM) preoxygenation in pregnant patients. [1,2] Whether adequate preoxygenation could be achieved with HFNO using a longer interval is not currently known. We aimed to determine the optimal time interval for 90% of parturients (EI90) with TV breathing preoxygenation using either FM or HFNO for women undergoing elective cesarean delivery.
Methods: Following ethics committee approval and informed consent, healthy, term, non-laboring parturients received 100% oxygen (O2) via FM or HFNO (OptiflowTM). Patients were placed in ramped position with left uterine displacement. Baseline values were recorded after 30s of TV breathing of 21% O2 at 12 L/min. Patients then continued TV breathing of 100% O2 at 15 L/min via FM or 50L/min HFNO at a time interval determined using a 9:1 biased-coin sequential allocation design to determine EI90. The primary outcome was effective preoxygenation, defined as an ETO2≥90%. Secondary outcomes included proportion of patients achieving ETO2≥90, patient satisfaction and tolerance of the interventions.
Results: To date we studied 19 women on FM and 17 on HFNO. The estimated EI90 was 3.7 min (95% CI 3.4-6.3) for FM using isotonic regression analysis with pooled-adjacent-violators algorithm. The EI90 for HFNO could not be estimated as it reached a response plateau, but it is clear that 8 min is insufficient to achieve 90% EtO2 for most women. The mean(SD) ETO2 achieved was 90.5% (4.8%) for FM and 79.6% (9.2%) for HFNO. 15% of the patients in the HFNO could not tolerate the intervention. The proportion of patients reliably achieving ETO2 90% was 89.5% in FM and 17.6% in HFNO.
Discussion: The optimal time interval for TV preoxygenation in healthy parturients via FM is approximately 3.7 min. HFNO consistently failed to achieve ETO2≥90% and extending the time interval to 8 min does not lead to significant gain in ETO2 levels with the HFNO technique.
1. Shippam et al Anaesthesia 2019
2. Tan et al BJA 2019