Flow-controlled expiration improves gas exchange in anaesthetised horses undergoing orthopaedic surgery.

Abstract

Background: Flow-controlled expiration (FLEX) has been shown to significantly enhance oxygenation in horses under laboratory conditions.

Objective: This study aims to corroborate these findings by evaluating the effects of FLEX on gas exchange in a randomised clinical trial involving a large population of clinical horses undergoing orthopaedic surgery.

Study design: Prospective randomised clinical trial.

Methods: A total of 406 healthy adult horses scheduled for elective orthopaedic procedures were recruited for this prospective clinical trial. Horses were randomly assigned to FLEX or VCV (volume-controlled ventilation) groups in dorsal (VCV-D and FLEX-D) or lateral recumbency (VCV-L and FLEX-L). Arterial blood gases were measured at 30, 75, and 120 min post-induction to assess arterial oxygenation (arterial partial pressure of oxygen to inspired fraction of oxygen ratio, PaO₂/FiO₂). A global index of ventilation/perfusion matching ([PaCO₂ - ETCO₂]/PaCO₂) was also calculated. Peak airway pressure (P_peak) and tidal volume were measured to calculate dynamic respiratory system compliance (Cdyn). Data were compared with repeated-measures ANOVA.

Results: Horses ventilated with FLEX showed significantly higher PaO₂/FiO₂ (FLEX-D vs. VCV-D, 369 ± 42 vs. 198 ± 112 mmHg, p < 0.001; FLEX-L vs. VCV-L, 436 ± 38 vs. 249 ± 88 mmHg, p < 0.001). FLEX also improved Cdyn (FLEX-D vs. VCV-D, 0.81 ± 0.1 vs. 0.64 ± 0.12, p = 0.01) and the global V̇/Q̇ index ([PaCO₂ - ETCO₂]/PaCO₂) (FLEX-D vs. VCV-D, 0.11 ± 0.03 vs. 0.18 ± 0.03, p = 0.03) in dorsal-positioned but not lateral-positioned horses.

Main limitations: Anaesthesia protocols were not standardised; anaesthetists were not masked to the intervention of interest, and findings may not be generalisable to other patient populations.

Conclusions: These results confirm previous laboratory findings, demonstrating that FLEX improves oxygenation, ventilation-perfusion matching, and respiratory mechanics compared to VCV in a large clinical population of anaesthetised horses.