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Home / Equine Research Bank / Equine Vet J / Flow-controlled expiration improves gas exchange in anaesthe...
Equine veterinary journal2025; doi: 10.1111/evj.70079

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

Abstract: 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. Methods: 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, PaO/FiO). A global index of ventilation/perfusion matching ([PaCO - ETCO]/PaCO) was also calculated. Peak airway pressure (P) 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 PaO/FiO (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 ([PaCO - ETCO]/PaCO) (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. Conclusions: 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.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2025-08-21 PubMed ID: 40841837DOI: 10.1111/evj.70079Google Scholar: Lookup
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Summary

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Overview

  • This study evaluates the effect of flow-controlled expiration (FLEX) on gas exchange and respiratory mechanics in anaesthetised horses undergoing orthopaedic surgery.
  • It confirms that FLEX improves oxygenation and ventilation efficiency compared to conventional volume-controlled ventilation (VCV) during anaesthesia.

Research Background

  • Previous laboratory studies showed FLEX could improve oxygenation in horses under anaesthesia.
  • FLEX is a ventilation technique where expiration flow is controlled rather than being passive, potentially improving lung function and gas exchange.
  • This study aimed to validate these promising results in a clinical setting with a large number of horses undergoing actual surgery.

Study Design and Methods

  • Type: Prospective randomised clinical trial involving 406 healthy adult horses scheduled for elective orthopaedic surgery.
  • Horses were randomly assigned to two ventilation methods:
    • FLEX (flow-controlled expiration)
    • VCV (volume-controlled ventilation, conventional method)
  • Positioning during surgery:
    • Dorsal recumbency (on back), groups named FLEX-D and VCV-D
    • Lateral recumbency (on side), groups named FLEX-L and VCV-L
  • Data collection:
    • Arterial blood gases measured at 30, 75, and 120 minutes after anaesthesia induction to determine oxygenation status via PaO2/FiO2 ratio.
    • A ventilation/perfusion (V̇/Q̇) matching index calculated as ([PaCO2 – ETCO2]/PaCO2), indicating efficiency of gas exchange.
    • Respiratory mechanics evaluated by measuring peak airway pressure and tidal volume to calculate dynamic lung compliance (Cdyn).
  • Statistical analysis: Repeated measures ANOVA was used to compare results across groups and time points.

Key Findings

  • Oxygenation:
    • FLEX significantly improved arterial oxygenation in both dorsal and lateral recumbency compared to VCV:
      • Dorsal position: FLEX-D had PaO2/FiO2 ratio of 369 ± 42 mmHg vs VCV-D 198 ± 112 mmHg (p < 0.001)
      • Lateral position: FLEX-L had 436 ± 38 mmHg vs VCV-L 249 ± 88 mmHg (p < 0.001)
  • Respiratory mechanics (dynamic compliance):
    • FLEX improved Cdyn in dorsal but not lateral recumbency (0.81 ± 0.1 vs 0.64 ± 0.12, p = 0.01)
  • Ventilation/perfusion matching:
    • Better V̇/Q̇ matching with FLEX was observed in dorsal recumbency (0.11 ± 0.03 vs 0.18 ± 0.03, p = 0.03), indicating improved efficiency of gas exchange.
    • Improvement was not statistically significant in lateral recumbency.

Interpretation and Implications

  • FLEX systematically enhances oxygenation and lung mechanics compared to standard volume-controlled ventilation.
  • These benefits are more pronounced in horses positioned on their backs (dorsal recumbency) than on their sides (lateral recumbency).
  • Improved ventilation/perfusion matching suggests that FLEX provides more effective distribution of ventilation and blood flow in the lungs during anaesthesia.
  • Clinical relevance:
    • Can improve safety and outcomes for horses undergoing orthopaedic surgery requiring anaesthesia by optimizing respiratory function.
    • Potential to reduce hypoxemia (low blood oxygen) risks during surgery.

Limitations

  • Anaesthesia protocols were not standardized across all horses, potentially introducing variability.
  • Anaesthetists were aware of the ventilation mode being used, raising risk of observational bias.
  • Findings may not directly apply to different patient populations (e.g., horses with respiratory disease) or other surgical procedures.

Conclusions

  • This large clinical trial supports previous experimental results by demonstrating that flow-controlled expiration improves important lung function parameters in anaesthetised horses.
  • FLEX leads to better oxygenation, improved lung compliance, and enhanced ventilation-perfusion matching compared to conventional volume-controlled ventilation.
  • These outcomes suggest FLEX is a beneficial ventilation strategy during equine orthopaedic surgeries under general anaesthesia.

Cite This Article

APA
Hopster K, Soares JHN, Levine D, Ortved K, Driessen B, Araos J. (2025). Flow-controlled expiration improves gas exchange in anaesthetised horses undergoing orthopaedic surgery. Equine Vet J. https://doi.org/10.1111/evj.70079

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Hopster, Klaus
  • Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Soares, Joao Henrique Neves
  • Department of Radiological and Surgical Sciences, School of Veterinary Medicine at University of California, Davis, Davis, California, USA.
Levine, David
  • Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Ortved, Kyla
  • Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Driessen, Bernd
  • Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Araos, Joaquin
  • Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA.

Grant Funding

  • University of Pennsylvania

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