Correspondence to: Douglas C. Johnson, MD, Spaulding Rehabilitation Hospital, 125 Nashua St, Boston, MA 02114; e-mail: firstname.lastname@example.org
The study by Fu et al (November 2004)1documents well that supplemental oxygen impairs the detection of hypoventilation by pulse oximetry, as discussed in the accompanying editorial by Demers.2 We regularly perform overnight oximetry on inpatients to screen for sleep apnea/hypopnea and find that the results are very insensitive if performed using supplemental oxygen. Therefore, we perform overnight oximetry either using room air or, if the baseline awake oxygen saturation level is < 90%, using only enough oxygen to bring the awake saturation to approximately 90%.
Fortunately, it is now possible to directly assess hypoventilation using continuous transcutaneous carbon dioxide tension monitoring.3 We now routinely monitor transcutaneous carbon dioxide pressure in patients who are at high risk for hypoventilation in our ventilator weaning program. We find monitoring to be very helpful during the initial tracheostomy using a mask or during overnight periods when the patient is not receiving ventilation, as well as during bronchoscopies. The advantages of monitoring cutaneous carbon dioxide tension over monitoring with end-tidal carbon dioxide tension, which we also use, include allowing continuous measurement, not requiring deep exhalation, and making accurate measurements in patients with high dead space ventilation.
The device (CO-OXSYS Monitor, SenTec AG; Therwil, Switzerland) has been used in Europe4–5 and is now available in the United States (Aspen Medical Products Inc; Irvine, CA). The monitor allows us to closely follow transcutaneous carbon dioxide pressure using a small probe that clips on the ear lobe. It usually takes about 5 min to equilibrate and then tracks carbon dioxide pressure closely, along with oxygen saturation.
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