Spirometry in normal subjects in sitting, prone, and supine positions.

Objective: Determine whether pulmonary function testing is affected by patient positioning.

Methods: In a descriptive study with measurements made in a sequential but randomized order at a university-based pulmonary function laboratory, 20 healthy men, ages 18-50 years, were evaluated with spirometric assessment of forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), and maximum voluntary ventilation (MVV) in the sitting, supine, and prone positions. Subjects were excluded for body mass index (BMI) > 30 kg/m2 or abnormal baseline spirometry.

Results: Comparing sitting to supine and prone positions, there was a statistically significant decline in the spirometry values (reported as percent of predicted normal +/- standard error of the mean). FVC was 102% +/- 4% while sitting, 95% +/- 4% while supine, and 94% +/- 4% while prone. FEV1 was 104% +/- 3% while sitting, 96% +/- 3% while supine, and 94% +/- 3% while prone. MVV was 115% +/- 4% while sitting, 102% +/- 4% while supine, and 97% +/- 3% prone.

Conclusion: In healthy men with BMI < 30 kg/m2, changing from the sitting to supine or prone position results in statistically significant change in respiratory pattern. However, all spirometry values in each position were normal by American Thoracic Society definitions.