Chest

Chest

Volume 126, Issue 5, November 2004, Pages 1552-1558
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Clinical Investigations
SURGERY
Supplemental Oxygen Impairs Detection of Hypoventilation by Pulse Oximetry

https://doi.org/10.1378/chest.126.5.1552Get rights and content

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 (Spo2) 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 (Fio2) of 0.21 (n = 25) underwent hypoventilation for up to 5 min. Patients with an Fio2 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 Spo2 were measured during deliberate hypoventilation. A decrease in Spo2 occurred only in patients who breathed room air. No decline occurred in patients with Fio2 levels of 0.25 and 0.30. In phase 2, Spo2 was recorded every min for up to 40 min in the PACU. Arterial desaturation (Spo2 < 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.

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Materials and Methods

The Institutional Review Board of the University of South Florida College of Medicine, Tampa, approved the study protocol, and consent was obtained in patients scheduled to undergo surgical procedures.

Phase 1

There were no intergroup differences in age or weight (Table 1). There were no differences between the Vt measured at the start of the hypoventilation trial (initial) and the Vt measured at the end of the trial (final). The final RR and Ve were approximately 50% of initial values. Initial and final arterial blood analysis data collected during induced hypoventilation are shown in Tables 234. Every patient had an increase in Paco2 and Petco2 and a decrease in arterial pH and Pao2 (p < 0.001).

Discussion

In the phase 1, the effect of hypoventilation on Spo2 was modeled in patients under general anesthesia and mechanical ventilation. Increases in Petco2 and Paco2 occurred in every patient who underwent deliberate hypoventilation. These changes were accompanied by an immediate decrease in Spo2 in patients with Fio2 of 0.21, but not in patients with Fio2 levels of 0.25 or 0.30. Although a significant decrease in mean Pao2 occurred during 10 min of hypoventilation in patients with Fio2 levels of

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This work was done at the H. Lee Moffitt Cancer Center and the University of South Florida College of Medicine.

Support was provided solely by departmental sources.

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Copyright © 2004 The American College of Chest Physicians. Published by Elsevier Inc. All rights reserved.