Complete upper airway collapse and apnoea during tethered swimming in horses.
Authors: Jones S, Franklin S, Martin C, Steel C
Journal: Equine veterinary journal
Summary
# Complete Upper Airway Collapse During Swimming in Horses Whilst horses are commonly exercised in water for rehabilitation and fitness, little is known about their actual breathing mechanics when submerged. Jones and colleagues used endoscopy, heart rate monitoring and video analysis to observe ten fit racehorses during tethered swimming, revealing a striking pattern: all animals exhibited complete upper airway collapse during post-inspiratory apnoea lasting approximately 1.6 seconds, including closure of the external nostrils, nasopharynx and rima glottidis, with bilateral arytenoid adduction occurring in all cases and epiglottic retroversion in two horses. The breathing cycle comprised a brief 0.51-second inspiration followed by the apnoeic phase and a short forced expiration, cycling at roughly 28 breaths per minute, yet notably this pattern did not occur during overground treadmill exercise in the four horses examined on both surfaces. Although post-exercise plasma lactate levels indicated substantial metabolic effort (median 4.71 mmol/L), only two horses showed mild exercise-induced pulmonary haemorrhage, suggesting the airway collapse itself does not inevitably cause tissue damage during this single bout. The mechanism remains unclear—whether serving a buoyancy function or representing a mammalian dive reflex—but the findings suggest swimming imposes fundamentally different respiratory demands than land-based work, which has important implications for designing effective aquatic rehabilitation programmes and for recognising that normal upper airway function during trotting does not predict behaviour in the water.
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Practical Takeaways
- •Swimming-induced upper airway collapse appears to be a normal physiological response in horses rather than pathological obstruction, likely related to buoyancy control or the mammalian dive reflex
- •This finding should be considered when assessing horses with suspected respiratory issues during swimming versus overground exercise, as airway closure during water work does not indicate dysfunction
- •Monitor for post-swimming haemorrhage and respiratory signs, though the study found only mild EIPH in 2 of 10 fit racehorses
Key Findings
- •All 10 horses exhibited complete upper airway collapse during tethered swimming, including closure of external nares, nasopharynx, and rima glottidis, with mean apnoea duration of 1.59±0.53 seconds
- •Breathing during swimming occurred at 28±5 breaths/min with a brief inspiration (0.51±0.08s) followed by apnoea and forced expiration (0.42±0.5s), with no locomotor-respiratory coupling observed
- •No upper airway collapse was observed during overground exercise in the 4 horses examined on track, suggesting collapse is specific to the swimming environment
- •Post-swim plasma lactate was elevated at 4.71 mmol/L (median) and grade 1 exercise-induced pulmonary haemorrhage was detected in 2 horses