Flow-controlled expiration (FLEX) significantly improves arterial oxygenation in horses compared to volume-controlled ventilationNew breathing technique improves oxygen in horses during surgery

Equine veterinary journal Published July 5, 2026 Study authors: Hopster Klaus, Soares Joao Henrique Neves, Levine David, Ortved Kyla, Driessen Bernd, Araos Joaquin PubMed ↗ DOI ↗ Editorial oversight: Dr. Lars van Dijk, PhD · Surgical, Procedural & Diagnostic

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Key Takeaway

Note that FLEX improves arterial oxygenation and respiratory compliance in horses compared to VCV during surgery.

This randomized clinical trial evaluated the impact of flow-controlled expiration (FLEX) versus volume-controlled ventilation (VCV) on 406 healthy adult horses undergoing elective orthopaedic procedures. The primary outcome was arterial oxygenation (PaO2/FiO2), with secondary outcomes including global index of ventilation/perfusion matching and dynamic respiratory system compliance.

In dorsal-positioned horses, the FLEX group achieved significantly higher arterial oxygenation (369 ± 42 mmHg) compared to the VCV group (198 ± 112 mmHg; p < 0.001). Similarly, lateral-positioned horses in the FLEX group showed superior oxygenation (436 ± 38 mmHg) than those in the VCV group (249 ± 88 mmHg; p < 0.001). Furthermore, dorsal-positioned horses under FLEX had higher dynamic respiratory system compliance (0.81 ± 0.1 vs. 0.64 ± 0.12; p = 0.01) and a lower ventilation/perfusion matching index (0.11 ± 0.03 vs. 0.18 ± 0.03; p = 0.03).

Safety data were not reported, and the study noted several limitations: anesthesia protocols were not standardized, and anesthetists were not masked to the intervention. While FLEX appears to improve oxygenation and respiratory mechanics in horses, these results may not be generalizable to other patient populations.

If your horse needs surgery, how it breathes during anesthesia could make a big difference. A new study tested a technique called flow-controlled expiration (FLEX) against standard volume-controlled ventilation (VCV) in 406 healthy horses having elective orthopedic procedures.

The results were striking. Horses placed on their backs (dorsal position) who got FLEX had an average arterial oxygen level of 369 mmHg, compared to just 198 mmHg with standard ventilation. That difference was highly significant. For horses lying on their side (lateral position), FLEX also outperformed VCV: 436 mmHg versus 249 mmHg.

FLEX also improved lung function in other ways. It led to better lung compliance (how easily the lungs expand) and better matching of air and blood flow in the lungs. These are all markers of healthier breathing during anesthesia.

However, the study has limitations. The anesthesia protocols weren't standardized, and the people giving the treatments knew which method they were using. Also, these findings come from horses, so they may not apply directly to humans or other animals. Still, for equine surgery, this technique looks promising.

What this means for you:

Flow-controlled expiration significantly improves oxygenation in anesthetized horses.

Common questions

What is flow-controlled expiration (FLEX)?

FLEX is a new way to help a horse breathe during anesthesia. It controls how air flows out of the lungs, which may improve oxygen levels and lung function compared to standard methods.

How much better was FLEX than standard ventilation?

In horses on their backs, FLEX gave an average oxygen level of 369 mmHg versus 198 mmHg with standard ventilation. In horses on their side, it was 436 mmHg versus 249 mmHg.

Is this study about humans or horses?

This study was done on 406 healthy horses having elective orthopedic surgery. The results are specific to horses and may not apply to other animals or people.

What are the limitations of this study?

The anesthesia protocols were not standardized, and the people giving the treatments knew which method they were using. Also, the findings may not apply to other patient populations.

Study Details

Study typeRct

EvidenceLevel 2

PublishedJul 2026

PMID40841837

View Original 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, 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. 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.

Flow-controlled expiration (FLEX) significantly improves arterial oxygenation in horses compared to volume-controlled ventilationNew breathing technique improves oxygen in horses during surgery

Common questions

Study Details

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Flow-controlled expiration (FLEX) significantly improves arterial oxygenation in horses compared to volume-controlled ventilationNew breathing technique improves oxygen in horses during surgery

Common questions

Study Details

Intraosseous route for out-of-hospital cardiac arrest linked to lower outcomes than intravenous access

Using a bone needle instead of a vein line lowers survival chances for cardiac arrest patients

Clinical research that matters. Delivered to your inbox.

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