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Effects of flow-controlled expiration on oxygenation and early recovery in horses undergoing general anaesthesia.
Abstract: FLow-controlled EXpiration (FLEX) has been shown to significantly enhance oxygenation in horses under clinical and experimental conditions. Objective: To compare pulmonary gas exchange during early recovery following general anaesthesia in horses ventilated with FLEX compared with conventional intermittent positive pressure ventilation (IPPV) and to assess recovery characteristics between ventilation modes. Methods: Randomised, crossover in vivo experiments. Methods: Six adult horses underwent two 2-h anaesthetic episodes in a randomised crossover design (FLEX vs. IPPV), separated by a two-week washout. Arterial blood gases were analysed at 60- and 120-min post-induction. Arterial blood pressure, end-tidal inhalant concentrations, respiratory rate, and haemoglobin oxygen saturation were recorded in 5-min intervals during anaesthesia. The initial recovery period was defined as the first 20 min following return of spontaneous ventilation, during which arterial blood gases, end-tidal inhalant concentrations, and oxygen saturation were recorded in 5-min intervals. Horses were recovered using head and tail rope assistance. The time to sternal recumbency, standing, and the number of standing attempts were recorded. Data were analysed using linear mixed-effects models for repeated measures and paired tests for recovery outcomes. Results: PaO was higher during general anaesthesia with FLEX (mean ± SD: 422 ± 24 vs. 202 ± 101 mmHg at 60 min; 433 ± 31 vs. 180 ± 59 mmHg at 120 min; p < 0.01). In recovery, PaO remained higher with FLEX (20 min: 114 ± 21 vs. 62 ± 11 mmHg; p < 0.01) and EtIso declined more rapidly (p < 0.01). Time to sternal and standing were shorter and attempts to stand fewer with FLEX (exploratory outcomes). Conclusions: This study enrolled a small sample size, which may limit generalisability. Recovery was recorded from one camera angle, potentially restricting full assessment of recovery behaviour. No respiratory mechanics or regional ventilation distribution were quantified. Conclusions: FLEX improves oxygenation during early recovery and is associated with faster recovery times in healthy horses.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
Publication Date: 2026-07-02 PubMed ID: 42392997DOI: 10.1002/evj.70217Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Objective Overview
- This study investigated the effects of flow-controlled expiration (FLEX) versus conventional intermittent positive pressure ventilation (IPPV) on oxygenation and recovery in horses undergoing general anesthesia.
- The researchers compared pulmonary gas exchange and early recovery characteristics between these two ventilation modes.
Background and Purpose
- Flow-controlled expiration (FLEX) is a ventilation technique designed to optimize the expiration phase during mechanical ventilation.
- Previous work indicated FLEX can improve oxygenation in horses under various conditions.
- The study aimed to directly compare:
- Pulmonary gas exchange during and after anesthesia
- Recovery characteristics such as time to stand and number of attempts
Methods
- Subjects: Six healthy adult horses underwent two separate anesthetic episodes, randomly assigned to ventilation by FLEX or IPPV.
- Design: A randomized crossover study with a two-week washout period between procedures to reduce carry-over effects.
- Duration: Each anesthesia session lasted 2 hours.
- Measurements during anesthesia:
- Arterial blood gases (oxygen partial pressure, PaO2) at 60 and 120 minutes post-induction.
- Continuous monitoring every 5 minutes of arterial blood pressure, end-tidal inhalant concentration (EtIso), respiratory rate, and hemoglobin oxygen saturation.
- Measurements during the initial 20-minute recovery period after return of spontaneous breathing:
- Arterial blood gases
- End-tidal inhalant concentration
- Oxygen saturation
- Recovery Support: Horses were assisted with head and tail ropes during recovery.
- Outcome measures during recovery:
- Time to sternal recumbency (lying on chest)
- Time to standing
- Number of attempts to stand
- Data Analysis: Utilized linear mixed-effects models for repeated measurements and paired tests comparing ventilation modes.
Results
- Oxygenation:
- PaO2 values during anesthesia were significantly higher with FLEX compared to IPPV:
- 60 min: 422 ± 24 mmHg (FLEX) vs. 202 ± 101 mmHg (IPPV)
- 120 min: 433 ± 31 mmHg (FLEX) vs. 180 ± 59 mmHg (IPPV)
- p < 0.01 indicating strong statistical significance.
- During recovery (20 min post spontaneous ventilation), PaO2 remained higher in the FLEX group (114 ± 21 mmHg vs. 62 ± 11 mmHg, p < 0.01).
- End-tidal isoflurane concentrations (EtIso) decreased faster with FLEX, suggesting quicker anesthetic washout.
- PaO2 values during anesthesia were significantly higher with FLEX compared to IPPV:
- Recovery Characteristics:
- Shorter times to sternal recumbency and standing were observed with FLEX ventilation.
- Fewer attempts were needed for the horses to stand, indicating smoother recovery.
- These findings were exploratory but suggest clinically relevant improvements.
Limitations
- Small sample size (only six horses) limits how broadly results can be applied to other populations.
- Recovery behavior was recorded from a single camera angle, which may not capture all relevant behaviors.
- No measurement of respiratory mechanics or regional ventilation distribution was performed, limiting mechanistic understanding.
Conclusions and Implications
- FLEX ventilation significantly improves oxygenation during anesthesia and early recovery in healthy horses compared to conventional IPPV.
- The higher oxygen levels and faster decline in anesthetic agents likely contribute to:
- Faster times to regain consciousness and stand
- Fewer standing attempts, improving overall recovery quality and safety.
- Though additional studies with larger populations and further measurements would strengthen evidence, FLEX appears to be a promising ventilation strategy in equine anesthesia management.
Cite This Article
APA
Mika A, Brandly J, Frampton A, Douglas H, Starr M, Benchimol H, Hopster K.
(2026).
Effects of flow-controlled expiration on oxygenation and early recovery in horses undergoing general anaesthesia.
Equine Vet J.
https://doi.org/10.1002/evj.70217 Publication
Researcher Affiliations
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
- Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Grant Funding
- Thoroughbred Education and Research Foundation
- Foundation for the Horse
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