Variability of Arterial Blood Gas Values in Stable Patients in the ICU

doi:10.1378/chest.84.1.14

Chest

Volume 84, Issue 1, July 1983, Pages 14-18

https://doi.org/10.1378/chest.84.1.14 Get rights and content

To establish guidelines for the interpretation of changes in arterial blood gas (ABC) values, we studied 29 clinically stable ICU patients for spontaneous variability in PaO₂, PaCO₂ and pH. ABCs were sampled six times over a 50-minute period, during which all patients received a fixed FIo₂ of 0.5 via endotracheal tube and underwent no therapeutic interventions. Each sample was analyzed in duplicate with careful attention to method of collection and measurement. The range separating the lowest and highest PaO₂ varied from 1 to 45 mm Hg (16.2±10.9 mm Hg [mean ± SD]). For PaCO₂ this range was from 1 to 8 mm Hg (3.0±1.9 mm Hg). Coefficient of variation for PaO₂ and PaCO₂ averaged 5.1±3.2 percent (mean ± SD) and 3.0±1.5 percent respectively. pH varied within 0.03±0.02 units. Percentage change in PaO₂ between sequential intra-patient samples averaged 5.3±2.8 percent (mean ± SD) and 7.1±7.9 percent over ten- and 50-minute intervals, respectively. Various clinical features were analyzed by multiple regression analysis for their relation to PaO₂ variation. Only leukocyte count and mean arterial oxygen content were statistically significant associations (p <0.05), but together explained less than 35 percent of the variation observed. Because considerable spontaneous variation occurs, even in stable patients, clinicians should base therapeutic decisions on trends in PaO₂ values rather than on isolated changes interpreted without appropriate clinical correlation.

Section snippets

Patient Selection

The study group consisted of 29 consecutive patients in the intensive care units of Harborview Medical Center who were judged by their physicians to be clinically stable and who were able to give informed consent (patient or family member) for the study. Descriptive data are provided in Table 1. Patients were accepted for the study if within the preceding four hours systolic blood pressure varied by less than 10 percent, temperature varied by less than 0.5°C, and mental status remained

ABG Variability

Arterial oxygen saturation exceeded 94 percent in all patients (FIo₂ = 0.5). Mean PaO₂, PaCO₂ and pH values across patients ranged from 71.6 to 257.7 mm Hg, from 24.4 to 73.5 mm Hg, and from 7.36 to 7.56 units, respectively (Tables 2 and 3). The intra-patient ranges (highest value obtained over six samplings minus lowest value) in PaO₂, PaCO₂ and pH extended from 1 to 45 mm Hg, from 1 to 8 mm Hg, and from 0.01 to 0.08 units respectively. (Corresponding mean ± SD: PaO₂, 16.2±10.9 mm Hg; PaCO₂,

DISCUSSION

In initiating this study it was our concern that, in the intensive care setting, altered ABG values alone often prompt clinicians to make important therapeutic decisions, even without additional evidence for clinical change. In part, this practice results from a lack of knowledge of the range of fluctuation around the mean ABG value that can occur in seriously ill but stable patients.

These data provide an indication of the spontaneous variation observed over brief intervals in such subjects.

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Supported in part by NIH Grant No. HL00248.

Manuscript received August 9; revision accepted February 1.

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Chest

Clinical InvestigationsVariability of Arterial Blood Gas Values in Stable Patients in the ICU

Section snippets

Patient Selection

ABG Variability

DISCUSSION

Br J Anaesth

Sampling and storing of blood for determination of acid-base status

Scand J Clin Lab Inv

Variations in arterial blood gas measurements due to sampling techniques

Respir Care

Accuracy and reproducibility of arterial blood gas and pH measurements

Acta Anaesth Scand

Suctioning protocol

Applied respiratory physiology

A systematic error in the determination of blood Pco2

Am Rev Respir Dis

Clinical Investigations
Variability of Arterial Blood Gas Values in Stable Patients in the ICU