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Ventilatory dynamics and control of blood gases after maximal exercise in the Thoroughbred horse

¹Departments of Anatomy and Physiology, ³Kinesiology, Kansas State University, Manhattan, Kansas 66506; and ²Department of Medicine, University of California, San Diego, La Jolla, California 92093

Submitted 15 September 2003 ; accepted in final form 4 February 2004

Despite enormous rates of minute ventilation (E) in the galloping Thoroughbred (TB) horse, the energetic demands of exercise conspire to raise arterial PCO₂ (i.e., induce hypercapnia). If locomotory-respiratory coupling (LRC) is an obligatory facilitator of high E in the horse such as those found during galloping (Bramble and Carrier. Science 219: 251–256, 1983), E should drop precipitously when LRC ceases at the galloptrot transition, thus exacerbating the hypercapnia. TB horses (n = 5) were run to volitional fatigue on a motor-driven treadmill (1 m/s increments; 14–15 m/s) to study the dynamic control of breath-by-breath E, O₂ uptake, and CO₂ output at the transition from maximal exercise to active recovery (i.e., trotting at 3 m/s for 800 m). At the transition from the gallop to the trot, E did not drop instantaneously. Rather, E remained at the peak exercising levels (1,391 ± 88 l/min) for 13 s via the combination of an increased tidal volume (12.6 ± 1.2 liters at gallop; 13.9 ± 1.6 liters over 13 s of trotting recovery; P < 0.05) and a reduced breathing frequency [113.8 ± 5.2 breaths/min (at gallop); 97.7 ± 5.9 breaths/min over 13 s of trotting recovery (P < 0.05)]. Subsequently, E declined in a biphasic fashion with a slower mean response time (85.4 ± 9.0 s) than that of the monoexponential decline of CO₂ output (39.9 ± 4.7 s; P < 0.05), which rapidly reversed the postexercise arterial hypercapnia (arterial PCO₂ at gallop: 52.8 ± 3.2 Torr; at 2 min of recovery: 25.0 ± 1.4 Torr; P < 0.05). We conclude that LRC is not a prerequisite for achievement of E commensurate with maximal exercise or the pronounced hyperventilation during recovery.

exercise recovery; locomotory-respiratory coupling; blood acid-base; equid