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Anesthesiology:
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Mechanism of Benefit of Head-up Preoxygenation in Obese Patients

Wax, David B. M.D.

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To the Editor:—

I read with interest the study report in the June 2005 issue of Anesthesiology by Dixon et al.1 regarding the benefits of head-up preoxygenation in obese patients. Although I do not dispute the basic findings of the study or the benefit of head-up position in obese patients, I question the conclusions drawn in the abstract and discussion.
Although there was a strong correlation between oxygen tension and time to desaturation, it cannot be concluded that the higher arterial oxygen tension (Pao2) itself was protective. The oxygen content of blood in the form of dissolved oxygen under nonhyperbaric pressure conditions is minimal. At the preinduction Pao2 achieved after 3 min of preoxygenation in both supine and head-up subjects, hemoglobin would be expected to be 100% saturated, providing maximal blood oxygen content in both study groups. The additional time to desaturation afforded by the small increase in dissolved oxygen reserve in the head-up group is unlikely to have been significant. More likely, the benefit of head-up positioning in delaying desaturation (as well as increasing Pao2) is a result of factors of pulmonary mechanics mentioned in the study background as they relate to oxygen reserve.
Also for this reason, changing the patient’s position from head-up to supine at induction as suggested possible to ease intubation may partially or completely negate the benefits of the head-up preoxygenation despite the increased preinduction Pao2. This repositioning maneuver might be useful in a follow-up study to test this hypothesis.
David B. Wax, M.D.
Mount Sinai School of Medicine, New York, New York. david.wax@mssm.edu
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References

1. Dixon BJ, Dixon JB, Carden JR, Burn AJ, Schachter LM, Playfair JM, Laurie CP, O’Brien PE: Preoxygenation is more effective in the 25° head-up position than in the supine position in severely obese patients. Anesthesiology 2005; 102:1110–5

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