The Science Journal of the American Association for Respiratory Care

2002 OPEN FORUM Abstracts

EVALUATION OF PREDICTIVE EQUATIONS FOR PaO2 AT ALTITUDE

Bradley Demars, RRT, Steven Slaughter, RRT, Eric Feucht, MD, Robert Chatburn RRT
University Hospitals of Cleveland, Cleveland, Ohio

BACKGROUND: Dillard et al has published equations for predicting PaO2 at altitude for patients with COPD and those with normal lung function. (Chest 1995;107:352). The purpose of this study was to validate the published results and estimate the error of predicting values for individual subjects.

METHOD: Outpatients with interstitial lung or pulmonary vascular disease and concerns about hypoxia during flight were studied. Steady state PaO2 was measured at sea level and during a hypoxic inhalation test, breathing 15.1% oxygen (simulating PaO2 at 8,000 ft). Gas was delivered with a Downs high flow generator on a nitrogen cylinder entraining room air. FIO2 was measured with a MaxO2 oxygen analyzer calibrated with 5.1%, 10%, 15.1% and 19.8% precision blended gas. Dillard regression equations predicted PaO2 at altitude using PaO2 at sea level only, PaO2 with FEV1 % predicted or PaO2 with FEV1/FVC % predicted. Error intervals (Respir Care 1996;41:1090) were defined as mean difference (predicted PaO2 ? actual PaO2 at simulated altitude) plus or minus 2.659 standard deviations of the difference (based on Dillard?s sample size of 42).

Results: Nine patients were studied. Results were comparable with those published by Dillard et al. Data below give predicted minus measured in mm Hg:

  PaO2 sea level with FEV1 with FEV1/FVC
Data Source Mean SD Mean SD Mean SD
Our Study 0.0 5.4 - 0.2 3.9 0.4 3.6
Dillard Study 0.0 5.1 0.0 4.8 0.0 4.5


Because the Dillard data were confirmed, we used their results (with a larger n) to construct error intervals for prediction error when estimating the PaO2 at altitude for an individual patient. Intervals indicate range of values for 95% of estimates at 99% confidence level:

Conclusions: Our results were nearly identical to those of the Dillard study and confirmed that including FEV1/FVC reduces prediction error. However, predictions for individual patients may be in error by as much as 10 mm Hg at best. Therefore, if the predicted PaO2 is marginal, a hypoxic inhalation test may be indicated.

OF-02-024

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