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

Objective: To compare F-shunt and oxygen content indices in sheep ventilated with a positive end-expiratory pressure (PEEP) of 5 cmHO alone or preceded by a stepwise alveolar recruitment manoeuvre (ARM).

Study Design: Randomized crossover design.

Animals: A total of six nonpregnant Brogna ewes weighing 34-47 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 an inspired oxygen fraction (FiO) of 0.4. Animals were placed in left lateral recumbency and, after 10 minutes of spontaneous breathing, mechanically ventilated with 5 cmHO of PEEP with (group ARM) or without (group PEEP) a stepwise recruitment manoeuvre. Maintaining a fixed driving pressure of 15 cmHO, PEEP was increased from 0 to 20 cmHO every 3 minutes in 5 cmHO increments. In each sheep, arterial blood samples were collected to measure arterial gases and to calculate F-shunt, PaO/alveolar oxygen partial pressure (PAO) and PaO/FIO during spontaneous breathing before mechanical ventilation (T), after 20 minutes of ventilation (T) and during spontaneous breathing at extubation (T).

Results: Both 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 T and T compared with T in group ARM. At extubation F-shunt was statistically lower in sheep that underwent an ARM. Mechanical ventilation improved PaO/PAO and PaO/FIO but they did not differ between groups.

Conclusions: and 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 cmHO.