Studies that show improved gas exchange with mechanical ventilation in prone
compared to supine posture usually include application of PEEP as part of the
treatment protocols (1,2). The aim of this work was to study effects of PEEP on
PBF in prone and supine postures.
Methods Seven anesthetized, ventilated sheep were studied in 4 conditions in random order: prone and supine posture, without and with PEEP 5 cm H2O. This amount was chosen because it increases lung volume with minimal effects on cardiac output. PBF was measured with iv injection of 15 µ fluorescent microsphere. The lungs were flushed free of residual blood, air-dried at total lung capacity and sectioned into ~2 cm3 pieces. The pieces were weighed and assigned spatial coordinates. Fluorescence was eluted with solvent and measured with a spectrophotometer. Fluorescent signals were corrected by weight and normalized to mean flow for all pieces. The coefficient of variation was used to assess flow heterogeneity.
Results PEEP increased PBF heterogeneity in supine (47% to 54%, P<.05), but not in prone (40% to 41%). Gravitational flow gradients were only present in supine, and were larger with the addition of PEEP (11.1 %/cm vs 7.9 %/cm, P<.05).
Conclusion PEEP (5 cm H2O) does not influence PBF distribution in prone, but increases perfusion heterogeneity in supine. This could explain the improved gas exchange described in prone compared to supine posture.
References 1. Douglas et al. Am Rev Respir Dis 1977:115:559-66 2. Brussel et al. J Cardiothor Vasc Anesth 1993:7:541-6
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