Preoxygenation Is More Effective in the 25° Head-up Position Than in the Supine Position in Severely Obese Patients: A Randomized Controlled Study

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Class III obese patients have altered respiratory mechanics, which are further impaired in the supine position. The authors explored the hypothesis that preoxygenation in the 25° head-up position allows a greater safety margin for induction of anesthesia than the supine position.


A randomized controlled trial measured oxygen saturation and the desaturation safety period after 3 min of preoxygenation in 42 consecutive (male:female 13:29) severely obese (body mass index > 40 kg/m2) patients who were undergoing laparoscopic adjustable gastric band surgery and were randomly assigned to the supine position or the 25° head-up position. Serial arterial blood gases were taken before and after preoxygenation and 90 s after induction. After induction, ventilation was delayed until blood oxygen saturation reached 92%, and this desaturation safety period was recorded.


The mean body mass indexes for the supine and 25° head-up groups were 47.3 and 44.9 kg/m2, respectively (P = 0.18). The group randomly assigned to the 25° head-up position achieved higher preinduction oxygen tensions (442 ± 104 vs. 360 ± 99 mmHg; P = 0.012) and took longer to reach an oxygen saturation of 92% (201 ± 55 vs. 155 ± 69 s; P = 0.023). There was a strong positive correlation between the induction oxygen tension achieved and the time to reach an oxygen saturation of 92% (r = 0.51, P = 0.001). There were no adverse events associated with the study.


Preoxygenation in the 25° head-up position achieves 23% higher oxygen tensions, allowing a clinically significant increase in the desaturation safety period—greater time for intubation and airway control. Induction in the 25° head-up position may provide a greater safety margin for airway control.

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