2002 OPEN FORUM Abstracts
EVALUATION OF PREDICTIVE EQUATIONS FOR PaO2 AT ALTITUDE
Demars, RRT, Steven Slaughter, RRT, Eric Feucht, MD, Robert Chatburn
University Hospitals of Cleveland,
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|
|Our Study||0.0||5.4||- 0.2||3.9||0.4||3.6|
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.