Supplemental oxygen impairs detection of hypoventilation by pulse oximetry.

STUDY OBJECTIVE

This two-part study was designed to determine the effect of supplemental oxygen on the detection of hypoventilation, evidenced by a decline in oxygen saturation (Spo(2)) with pulse oximetry.

DESIGN

Phase 1 was a prospective, patient-controlled, clinical trial. Phase 2 was a prospective, randomized, clinical trial.

SETTING

Phase 1 took place in the operating room. Phase 2 took place in the postanesthesia care unit (PACU).

PATIENTS

In phase 1, 45 patients underwent abdominal, gynecologic, urologic, and lower-extremity vascular operations. In phase 2, 288 patients were recovering from anesthesia.

INTERVENTIONS

In phase 1, modeling of deliberate hypoventilation entailed decreasing by 50% the minute ventilation of patients receiving general anesthesia. Patients breathing a fraction of inspired oxygen (Fio(2)) of 0.21 (n = 25) underwent hypoventilation for up to 5 min. Patients with an Fio(2) of 0.25 (n = 10) or 0.30 (n = 10) underwent hypoventilation for 10 min. In phase 2, spontaneously breathing patients were randomized to breathe room air (n = 155) or to receive supplemental oxygen (n = 133) on arrival in the PACU.

MEASUREMENTS AND RESULTS

In phase 1, end-tidal carbon dioxide and Spo(2) were measured during deliberate hypoventilation. A decrease in Spo(2) occurred only in patients who breathed room air. No decline occurred in patients with Fio(2) levels of 0.25 and 0.30. In phase 2, Spo(2) was recorded every min for up to 40 min in the PACU. Arterial desaturation (Spo(2) < 90%) was fourfold higher in patients who breathed room air than in patients who breathed supplemental oxygen (9.0% vs 2.3%, p = 0.02).

CONCLUSION

Hypoventilation can be detected reliably by pulse oximetry only when patients breathe room air. In patients with spontaneous ventilation, supplemental oxygen often masked the ability to detect abnormalities in respiratory function in the PACU. Without the need for capnography and arterial blood gas analysis, pulse oximetry is a useful tool to assess ventilatory abnormalities, but only in the absence of supplemental inspired oxygen.