Effect of body position on arterial oxygen tension in the elderly

Background: It is well known that body position can have an effect on gas exchange though the magnitude of this effect has not been studied thoroughly in the elderly.

Objectives: This study analyzes the effect body position change has on arterial oxygen tension (PaO(2)) and arterial carbon dioxide tension (PaCO(2)) in healthy elderly.

Methods: We tested 46 "lung-healthy" elderly, including 30 women and 16 men, 67-88 years of age. Blood was drawn from the radial artery first in the sitting position and subsequently in the supine position. Spirometry was performed.

Results: Mean (SD) sitting PaO(2) was 10.53 kPa (1.22), whereas mean supine PaO(2) was 9.85 kPa (1.33). The difference between sitting and supine PaO(2) was 0.68 kPa (0.86) and was statistically significant. Sitting PaCO(2) was 5.06 kPa (0.47) and supine PaCO(2) was 5.05 kPa (0.54). The difference between sitting and supine PaO(2) correlated positively with FEV(1)/FVC %, negatively with the corresponding difference between sitting and supine PaCO(2), and negatively with BMI.

Conclusions: We conclude that the significant difference in PaO(2) in sitting and supine positions clearly shows that the position needs to be considered both when attempting to establish reference values and when evaluating gas exchange in elderly persons. The positional changes in oxygenation are related to the corresponding change in PaCO(2), and to FEV(1)/FVC % and BMI.