Chest
Volume 106, Issue 1, July 1994, Pages 187-193
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Clinical Investigations in Critical Care
Variability of Arterial Blood Gas Values Over Time in Stable Medical ICU Patients

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The spontaneous variability of arterial blood gas and pH values (ABGs) was examined in a group of 28 typical stable medical ICU patients under a variety of ventilatory conditions. In each patient, 13 ABG specimens were measured at 5-min intervals during a 1-h study period using a new bedside, extravascular fluorescent blood gas monitor. For all patients, the mean coefficient of variation (C) was 6.1 percent for Po2 and 4.7 percent for Pco2. The average SD for pH was 0.012. We conclude that the spontaneous variability for ABG values over a 1-h period is substantial and that this variability should be taken into account when making clinical decisions based on ABG values.

Section snippets

Patient Selection

The study group consisted of 28 clinically stable patients from the medical ICU at the Long Beach VA Medical Center. The study was approved by the Institution's Human Subjects Committee and informed consent was obtained from each patient the start of each study. The study patients had an arterial catheter placed by the primary team and were judged to be clinically stable. They are representative of typical patients from our medical ICU (Table 1). The patients were not consecutive since

Results

During the monitoring period, the patients were clinically stable as shown by the minimal percentage changes (±SD) in pulse, mean arterial blood pressure, and respiratory rate which were −1.6 (±5.3) percent, 5.0 (±9.6) percent, and −1.0 (±11) percent, respectively. Inspection of the 84 individual PO2, Pco2, and pH plots vs time revealed no consistent upward or downward trends in any patient.

The comparisons of the CDI blood gas monitor values with the Instrumentation Laboratories 1306 blood gas

Discussion

Our results provide additional information regarding the degree of spontaneous variability that can be expected from blood gas and pH measurements obtained from typical medical ICU patients, ventilated and oxygenated under a wide range of conditions. We performed multiple measurements, using an accurate monitoring system, with frequent calibration checks to validate our data.

One might question whether this monitor is accurate enough to examine the issue of spontaneous variability. For PO2, the

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Presented at the 58th Annual Scientific Assembly, American College of Chest Physicians, Chicago, October 25-29, 1992.

Manuscript revision accepted November 29.

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