Transient respiration locomotion decoupling in galloping thoroughbreds

The compulsory respiration locomotion coupling may sometimes be broken in healthy galloping horses, leading to a `big respiratory cycle' (BRC). This work is aimed at describing the ventilatory pattern of the BRC and at studying the influence of the individual horse, gallop speed and training. Eleven healthy Thoroughbred horses were studied during a 9 week period of incremental training intensity. Instantaneous respiratory airflow, tidal volume, respiratory rate and the 02 and C02 fraction in inspired and expired gases were continuously obtained on a breath-by-breath basis during 4 standardised treadmill exercise tests (SET) performed at 3 week intervals. The inspiratory airflow of the BRC had a bi- or triphasic shape and ils inspiratory time was 1.5-fold longer than that of a normal breath. The expiratory flow was monophasic and lasted 2.5-fold longer than the normal breath. The inspiratory, but not the expiratory peak flows, remained unchanged during the BRC. The inspiratory volume was 2.0-to 1.5-fold higher than normal, according to the gallop speed. Each BRC was followed by a`readjustment cycle' (RC), which allowed the recovery of normal respiration locomotion coupling. The sum of BRC + RC times always equalled the time for 3 normal respiratory cycles. The sum of the inspiratory volumes of BRC and RC was significantly higher than the sum of their expiratory volumes. There was a highly significant effect of horse and of training on the frequency of BRC. Lastly, during the BRC and at least the 4 following respiratory cycles, the end-tidal 02 and CO, fractions were significantly lower and higher respectively. It was hypothesised that this respiratory manoeuvre could result from a negative feedback mechanism providing, at least, transiently either a readjustment of the end expiratory lung volume or an improvement of the pulmonary gas exchange