Observations on respiratory flow strategies during and after intense treadmill exercise to fatigue in Thoroughbred racehorses

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Reasons for performing study:

Locomotor-respiratory coupled (LRC) breath types are a feature of galloping exercise in horses. Differences in breath type have been demonstrated during exercise in particular the ‘big respiratory cycle’ (BRC) and ‘flow hesitation’. To investigate breath types during recovery and quantitatively investigate BRCs during exercise to understanding the mechanism driving BRCs.


To investigate the occurrence of different breath types during and after intense treadmill exercise, and test the hypothesis that large breaths (BRCs) were a function of respiratory frequency.


Six trained and clinically normal Thoroughbred horses were exercise tested on a treadmill (slope 10%). Breathby-breath pulmonary ventilation was measured continuously during exercise and recovery using a Quadflow mask.


Five different breath types were identified, and classified as normal monophasic, normal biphasic, deglutition, effort pause, or large breaths. Exercising at 10 m/sec, the number of large breaths was significantly related to Rf (r = -0.86, P = 0.03). During 120 sec after exercise there were 2 distinct populations of breaths, large and normal monophasic.


BRC type breaths are a normal feature of ventilation during and after intense exercise. In recovery there are two distinct breath populations. During exercise BRC frequency is inversely associated with respiratory frequency and highly dependant on the individual horse. From intense exercise to recovery, high flow rates and LRC limited tidal volumes are replaced by high tidal volumes and progressively decreasing flow rates. There is a temporal association between BRC occurrence and PECO2-.

Potential relevance:

Breath types and the physiological mechanism for driving each type is important in the clinical interpretation of respiratory disease or dysfunction. The demonstration of BRC association with PECO2 may help understand the driving mechanism for the BRC. In pulmonary function testing, breath type is important in quantitative results. The demonstration that high tidal flows with limited tidal volumes during intense exercise being replaced by high tidal volumes and progressively decreasing flows in recovery has potential clinical relevance.

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