Background: Oxygenation without tidal breathing, i.e. apnoeic oxygenation in combination with extracorporeal carbon dioxide removal, might be an option in the treatment of acute respiratory failure. However, ventilation with 100% O₂, which is potentially toxic, is considered a prerequisite to ensure acceptable oxygenation. We hypothesized that trapping nitrogen (N₂) in the lungs before the start of apnoeic oxygenation would keep the alveolar O₂ at a non-toxic level and still maintain normoxaemia. The aim was to test whether a predicted N₂ concentration would agree with a measured concentration at the end of an apnoeic period.
Methods: Seven anaesthetized, muscle relaxed, endotracheally intubated pigs (22-27 kg) were ventilated in a randomized order with an inspired fraction of O₂ 0.6 and 0.8 at two positive end-expiratory pressure levels (5 cm and 10 cm H₂O) before being connected to continuous positive airway pressure using 100% O₂ for apnoeic oxygenation. N₂ was measured before the start of and at the end of the 10-min apnoeic period. The predicted N₂ concentration was calculated from the initial N₂ concentration, the end-expiratory lung volume, and the anatomical dead space.
Results: The mean difference and standard deviation between measured and predicted N₂ concentration was -0.5 ± 2%, P = 0.587. No significant difference in the agreement between measured and predicted N₂ concentrations was seen in the four settings.
Conclusions: This study indicates that it is possible to predict and keep alveolar N₂ concentration at a desired level and, thus, alveolar O₂ concentration at a non-toxic level during apnoeic oxygenation.
2011 The Authors Acta Anaesthesiologica Scandinavica, 2011 The Acta Anaesthesiologica Scandinavica Foundation.