We thank Dr. Rothstein for his interesting comments regarding our article.1As he correctly points out, the effect of added elastic and/or resistive load on respiratory drive may differ in the awake and anesthetized states. In the awake state, an increase in elastic and/or resistive load is compensated by an increase in neuronal inspiratory drive. This compensatory neuronal drive may be absent or diminished during general anesthesia,2and this may be particularly so in infants who are highly susceptible to anesthesia-induced attenuation of neuronal input. Continuous positive airway pressure (CPAP) may itself reduce inspiratory drive through the Herring-Breuer inflation reflex.3The resulting hypercapnia will depend on the reduction in minute alveolar ventilation. In previous studies in anesthetized children, the impact of CPAP on minute ventilation has been clinically insignificant.4Keidan et al.  studied the effect of CPAP (6 cm H2O) on work of breathing and respiratory indices in healthy spontaneously breathing children (median age, 1.0 yr) during halothane–nitrous oxide anesthesia.4Application of CPAP via  a facemask significantly decreased the work of breathing but had no significant effect on inspiratory tidal volume, inspiratory minute volume, or end-tidal carbon dioxide tension. To the extent that CPAP relieves existing upper airway narrowing, CPAP can also improve gas exchange, resulting in a reduction of hypercapnia and improved oxygenation. It was the purpose of our study to determine the interaction of propofol anesthesia and CPAP on upper airway caliber and configuration in infants. We considered it neither relevant to the study hypothesis nor ethically justifiable to perform arterial puncture for blood gas analysis in our infant subjects. Given the relatively brief duration of CPAP application, any increase in arterial carbon dioxide tension was likely small; indeed, after removal of CPAP, we observed no outpouring of carbon dioxide as determined by end-tidal measurement.

*The Hospital for Sick Children, University of Toronto, Toronto, Ontario, Canada. mark.crawford@sickkids.ca

Crawford MW, Rohan D, Macgowan CK, Yoo SJ, Macpherson BA: Effect of propofol anesthesia and continuous positive airway pressure on upper airway size and configuration in infants. Anesthesiology 2006; 105:45–50
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