Comparison of the efficacy of two ventilatory strategies in improving arterial oxygen tension and content in anaesthetized sheep

Abstract Objective To compare F-shunt and oxygen content indices in sheep ventilated with a positive end expiratory pressure (PEEP) of 5 cmH2O alone or preceded by a stepwise alveolar recruitment manoeuvre (ARM). Study design Randomized crossover design. Animals A total of six non-pregnant Brogna ewes weighing 34 to 47 kg, undergoing thoracolumbar magnetic resonance scan. Methods In medetomidine-sedated sheep, anaesthesia was induced with propofol and maintained with isoflurane 1.1 ± 0.1% and a fraction inspired of oxygen of 0.4. Animals were placed in left lateral recumbency and, after 10 minutes of spontaneous breathing, they were mechanically ventilated with 5 cmH2O of PEEP with (group ARM) or without (group PEEP) a stepwise recruitment manoeuvre. Maintaining a fixed driving pressure of 15 cmH2O, PEEP was increased from 0 to 20 cmH2O every 3 minutes in 5 cmH2O increments. In each sheep, arterial blood samples were collected to measure arterial gases and to calculate F-shunt, PaO2/PAO2 and PaO2/FIO2 during spontaneous breathing before mechanical ventilation (T0), after 20 minutes of ventilation (T20) and during spontaneous breathing at extubation (Text). Results Both ventilatory strategies improved the arterial oxygen content although four animals in group PEEP showed oxygen content compatible with hypoxia compared to group ARM. F-shunt values were not statistically different at any time point in sheep that underwent only PEEP ventilation while they decreased at T20 and Text compare to T0 in group ARM. At extubation F-shunt was statistically lower in sheep that underwent an alveolar recruitment manoeuvre. Mechanical ventilation improved PaO2/PAO2 and PaO2/FIO2 but they did not differ between groups. Conclusions and clinical relevance The stepwise alveolar recruitment manoeuvre evaluated in this study improved oxygenation indices and decreased F-shunt. This effect was maintained at extubation compared to sheep that were ventilated with only PEEP 5 cmH2O.