Are daytime arterial blood gases a good reflection of nighttime gas exchange in patients on long-term oxygen therapy?

OBJECTIVE: Compare nighttime and daytime arterial blood gas values in patients undergoing long-term oxygen therapy (LTOT). METHODS: We studied 39 LTOT patients with chronic airflow limitation. Oxygen from an oxygen concentrator was administered via nasal prongs until daytime blood oxygen saturation (measured via pulse oximetry [S(pO2)]) was > or = 90%. Arterial blood samples were drawn at 6:00 PM, while the subject breathed room air, and also during oxygen administration at night (3:00 AM), early in the morning (7:00 AM), and at noon. S(pO2) was measured throughout the night. RESULTS: Mean patient age was 70 +/- 7 yr. All patients suffered severe chronic airflow limitation (mean forced expiratory volume in the first second 28 +/- 9% of predicted). The mean oxygen flow administered was 1.41 +/- 0.6 L/min. Mean overnight S(pO2) was 92 +/- 2.5%, with 21.5 +/- 28% of recording time under 90%. There were statistically significant differences between P(aO2), P(aCO2), and pH obtained at 3:00 AM and noon and between 7:00 AM and noon, while the patients breathed the same oxygen concentration. The differences between the 3:00 and 7:00 AM values were not significant. In 23 patients (59%) we observed a P(aCO2) increase > 10 mm Hg and/or a pH decrease to < 7.33 during that period, indicating poor response to LTOT. CONCLUSIONS: Daytime arterial blood gas measurements do not reflect nighttime gas exchange. However, samples taken early in the morning (7:00 AM) do seem to reflect arterial blood gases during the night and can therefore be used for setting and monitoring nighttime oxygen flow.