Respiratory effects of the kneeling prone position for low back surgery

BACKGROUND AND OBJECTIVE: The kneeling prone position is often used for low back surgery in order to decrease intraoperative bleeding and increase the surgical exposure of the vertebral canal. The aim of this study was to assess effects of the kneeling prone position on respiratory gas exchange focusing on oxygen consumption and early changes in oxygenation. METHODS: Thirty ASA I-II patients scheduled for low back surgery in the kneeling prone position were studied. Anaesthesia was maintained with isoflurane, 1.2% end-tidal concentration. Respiratory gas exchange was measured with indirect calorimetry. RESULTS: When the patients were turned into the kneeling prone position their oxygenation was immediately improved--measured by arterial oxygen tension and arterial oxygen saturation. The oxygen uptake rate did not change from a baseline supine level of 76 mL min(-1) m(-2), but the carbon dioxide excretion rate decreased from a baseline supine value of 71 mL min(-1) m(-2) to 66 mL min(-1) m(-2) at 5 and 10 min after the kneeling prone position was adopted. Alveolar ventilation decreased in the kneeling prone position. CONCLUSIONS: The present study demonstrates that the kneeling prone position improves oxygenation and that the mechanisms involved are fast in onset. Furthermore, the prone position does not change oxygen consumption although alveolar ventilation is significantly reduced. The changes in alveolar ventilation could possibly be the result of circulatory changes caused by the prone position, but further studies are needed to clarify that hypothesis.