We thank Drs. Wax and Baraka for their interest in our article on the benefits of the head-up position for preoxygenation in class III obese patients.1They draw attention to two aspects of our article.

It is interesting that Dr. Baraka et al.  2found, in a small study of similar design, no benefit in desaturation safety with preoxygenation in the 45° head-up position in pregnant women at term. Perhaps as speculated, the effect of the gravid uterus on the movement of the diaphragm has a negative impact on lung mechanics. Although the gravid uterus does decrease functional residual capacity, the mechanism and distribution of mass are considerably different to those seen in obesity, and therefore, the favorable effects we found with posture change may not be applicable. Our comments were speculative only, indicating that the head-up position may achieve a prolongation of the desaturation safety period. The gravid uterus may have a varying impact on lung mechanics depending on the posture: supine, 25°, 45°, and sitting up. This is an area for further research because severe obesity and the advanced gravid state are associated with increased difficulty in airway management and higher metabolic rates increasing the risk of hypoxia during anesthetic induction. Oxygen tensions taken in various positions may assist in optimizing preoxygenation, and further investigation into the role of position in preoxygenation should continue in several high-risk groups.

Dr. Wax correctly points out that the dissolved oxygen in blood under atmospheric conditions is trivial and unlikely by itself to alter the desaturation safety period. We agree and indicate within the discussion that the aim is to optimize lung oxygen content by achieving a posture that provides optimal respiratory mechanics, lung volumes, functional residual capacity, and arterial oxygen tension during preoxygenation. We found a strong correlation between the oxygen tension achieved and the desaturation safety period suggesting that end preoxygenation oxygen tension is an indicator of improved pulmonary oxygen reserves. We speculate that the extended desaturation safety period in head-up subjects is due to continued oxygenation of blood from increased pulmonary reserves and not directly due to the higher initial oxygen tension. In addition, we caution that after head-up preoxygenation, a change to the supine position for intubation may reduce these favorable conditions and shorten the desaturation safety period.

*Australian Centre for Obesity Research and Education, Monash University, Melbourne, Australia. john.dixon@med.monash.edu.au

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: A randomized controlled study. Anesthesiology 2005; 102:1110–5
Baraka AS, Hanna MT, Jabbour SI, Nawfal MF, Sibai AA, Yazbeck VG, Khoury NI, Karam KS: Preoxygenation of pregnant and nonpregnant women in the head-up versus supine position. Anesth Analg 1992; 75:757–9