Ventilatory responses of the horse to exercise: effect of gas collection systems

Experiments were undertaken to determine whether respiratory masks worn by horses exercising strenuously on a treadmill may interfere with normal gas exchange. Four collection systems, two flow-through systems and two incorporating one-way valve systems with subject-generated airflow were studied. Six horses performed standard treadmill exercise tests consisting of a 2-min warm up followed by galloping 1 min each at 8,9, and 10 m/s. Each horse exercised six times while wearing each of the four respiratory masks. Each flow-through system was used twice with flow rates of 2,360 and 3,840 l/min for one system, and 3,840 and 6,300 l/min for the other. Arterial blood gas tensions were measured during exercise at each speed for each system and were compared with values measured when the horses performed the same test without wearing a mask. Hypercapnia developed during exercise with each of the respiratory masks except with the 6,300-l/min flow-through system. All horses became hypoxemic during every exercise test, but it was most severe when systems incorporating one-way valves were used. This, plus the degree of hypercapnia observed and a suboptimal heart rate-O2 uptake relationship, indicated that such systems severely impede ventilation and suggest that experiments performed while utilizing them do not represent the normal exercise condition.