Mechanics of Breathing in Horses at Rest and During Exercise

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Abstract
The respiratory mechanics together with the rate of work of breathing were studied in five healthy adult Standardbred horses at rest and during different levels of exercise on a treadmill (0.8-2.3 ms−1, 7% slope). In three of the horses the stride frequency was also determined. The ventilatory response to exercise increased linearly with treadmill speed up to 9.6 times the resting values and was sustained more by an increase in respiratory frequency than by an increase in tidal volume. At the most elevated work load, respiratory frequency and tidal volume averaged 5.2 and 1.4 times the resting values, respectively. Lung resistance decreased during the most intense level of exercise to about 30% of the average value observed at rest, while lung dynamic compliance remained unchanged. The rate of work of breathing (Wresp, in W) increased exponentially with ventilation (Ve, in 1 min−1) according to the equation: Wresp=5.263Ve1.5897 × 10−4. In the three animals investigated a definite locomotor-respiratory coupling was observed at a trot and in two animals also during walking. This observation, together with other considerations based on available data on the energetics of respiration, suggests that the efficiency of the equine respiratory system is relatively high. In addition, when compared with other smaller mammals (man and dog), horses are, in mechanical terms, more economical breathers.