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

ObjectiveTo 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 designRandomized crossover design.AnimalsA total of six nonpregnant Brogna ewes weighing 34–47 kg, undergoing thoracolumbar magnetic resonance scan.MethodsIn medetomidine-sedated sheep, anaesthesia was induced with propofol and maintained with isoflurane 1.1% ± 0.1% and an inspired oxygen fraction (FiO₂) of 0.4. Animals were placed in left lateral recumbency and, after 10 minutes of spontaneous breathing, 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/alveolar oxygen partial pressure (PAO₂) and PaO₂/FIO₂ during spontaneous breathing before mechanical ventilation (T₀), after 20 minutes of ventilation (T₂₀) and during spontaneous breathing at extubation (T_ext).ResultsBoth ventilatory strategies improved the arterial oxygen content although four animals in group PEEP showed oxygen content compatible with hypoxia compared with 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 T_ext compared with T₀ in group ARM. At extubation F-shunt was statistically lower in sheep that underwent an ARM. Mechanical ventilation improved PaO₂/PAO₂ and PaO₂/FIO₂ but they did not differ between groups.Conclusionsand clinical relevance The stepwise ARM evaluated in this study improved oxygenation indices and decreased F-shunt. This effect was maintained at extubation compared with sheep that were ventilated with only PEEP 5 cmH₂O.