The effects of posture, airway pressure and anesthesia on regulation of the regional ventilation and perfusion distribution in healthy humans

Abstract

Gas exchange has been observed to vary with posture in adult respiratory distress syndrome (ARDS) patients. In this thesis, the effect of posture on theregional distribution of ventilation (V) and perfusion (Q) in the lungs under normal breathing with and without continuous positive airway pressure (CPAP) and during general anesthesia with mechanical ventilation was studied. Additionally, endogenously produced nitric oxide (NO) may influence the effect of posture on the Q distribution. This was also studied. Since inhalation anesthesia, known to impair pulmonary gas exchange, is sometimes used in ARDS patients we investigated the effect of inhaled sevoflurane on the V and Q regional distribution. All studies were performed on healthy volunteers with Single Photon Emission Tomography (SPECT). Isotope tagged albumin macro aggregates, trapped in the pulmonary capillaries were used to measure regional Q and isotope labeled aerosol or labeled ultrafine carbon particle aerosol to measure regional V.

Prone posture, compared to supine, favors a more uniform Q distribution in the anterior to posterior direction both awake and during mechanical ventilation. CPAP, compared to normal breathing, redistributes Q to more dependent parts of the lungs, resulting in a less beneficial V/Q matching. We found indications of a higher release of endothelial NO in dorsal, compared to ventral, parts of the lungs. This may contribute to the more uniform Q distribution seen in prone compared to supine posture. NO from paranasal sinuses contributes to a more homogenous Q distribution in upright position. Inhalation anesthesia with sevoflurane decreases Q distribution heterogeneity but increases V/Q matching heterogeneity. No significant influence on the V distribution was observed in any of the studies.

In conclusion, all studied factors in healthy volunteers have an effect on the Q regional distribution but not on V, suggesting that variations in V/Q ratio distribution are a consequence of changes in the intrapulmonary Q distribution.