Body posture, as a gravitational factor, has a clear impact on pulmonary ventilation and perfusion. In lung units with mismatched ventilation and perfusion, gas exchange and/or elimination of carbon dioxide can be impaired. In this situation, differences in the value of arterial and end-tidal carbon dioxide tension [Δ(PaCO₂ - P_ETCO₂)] are expected to increase. This study was conducted to observe how Δ(PaCO₂ - P_ETCO₂) changed according to the 3 different surgical positions, and to determine whether Δ(PaCO₂ - P_ETCO₂) is a reliable predictor of ventilation/perfusion mismatch when a patient is in different postural positions.

Fifty-nine patients were divided into either the chronic obstructive pulmonary disease (COPD) group (n = 29) or the non-COPD group (n = 30). PaCO₂ and P_ETCO₂ were measured during surgery in the supine, prone, and lateral decubitus positions after a 10 minute stabilization period. The Δ(PaCO₂ - P_ETCO₂) were calculated and compared among positions.

The Δ(PaCO₂ - P_ETCO₂) decreased slightly in the prone position and increased significantly in the lateral decubitus position compared with the supine position in both groups. These patterns almost corresponded with the degree of ventilation/perfusion mismatch from the results of the radiological studies. The Δ(PaCO₂ - P_ETCO₂) in the COPD group was significantly greater than that in the non-COPD group at all surgical positions.

Lateral decubitus position is associated with marked increase in Δ(PaCO₂ - P_ETCO₂), especially in patients with COPD. The Δ(PaCO₂ - P_ETCO₂) is a simple and reliable indicator to predict ventilation/perfusion mismatch at different surgical positions in patients with or without COPD.