Cardiodynamic variables and ventilation during treadmill exercise in ponies

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Abstract
The relationship of ventilation (.ovrhdot.VE) to cardiodynamic variables and CO2 transport in seven normal ponies during treadmill exercise was assessed. At 1.8, 3, and 6 mph, respectively, .ovrhdot.VE increased from 15 l/min at rest to 43, 51, and 86 l/min by 1 min and 48, 68, and 125 l/min by 8 min. In 3 ponies at the same work loads, cardiac output (.ovrhdot.Qc) increased from .apprx. 12 l/min at rest to 19.7, 28.1, and 40.3 l/min between 30-60 s (P < 0.05) and then decreased by about 20% to a steady-state by 3-4 min. Heart rate (HR) shows a similar biphasic response during exercise. Mean right ventricular pressure (MRVBP) increased from 9.9 to 15.9 Torr at 1.8 mph, 15.2 Torr at 3 mph, and 23.5 Torr at 6 mph by 1 min (P < 0.05) and then decreased to 11.8, 12.2, and 15.8 Torr by 8 min of the 3 respective work intensities. At all work loads, .ovrhdot.VE increased proportionally faster than these cardiovascular variables in the 1st min. For example, at 6 mph .ovrhdot.VE increased 470%, whereas .ovrhdot.Qc and HR increased only 230%. .ovrhdot.VE generally continued to increase at 3 and 6 mph; MRVBP, .ovrhdot.Qc, and HR decreased. The basic assumption of a cardiodynamic hyperpnea that .ovrhdot.VE and .ovrhdot.Qc are equivalently coupled at the exercise onset is rejected for this species. Mixed venous CO2 content (C.hivin.VCO2) at 3 and 6 mph, respectively, decreased slightly from 61.6 and 62.3 vol% at rest to 59.6 and 61.9 vol% by 45 s and then increased to 63.3 and 63.5 vol% by 7 min. Thus CO2 delivery to the lung (.ovrhdot.Qc .times. C.hivin.VCO2) demonstrates a qualitatively similar biphasic response as .ovrhdot.Qc in the 1st min of exercise. In 6 ponies, arterial PCO2 (PaCO2) decreased from 43.3 Torr at rest to 41.3 Torr (1.8 mph), 40.6 Torr (3 mph), and 38.7 Torr (6 mph) at 1 min of exercise (P < 0.04), recovering only partially by 3 min. Mixed venous PCO2 (P.hivin.VCO2) follows a similar temporal pattern as C.hivin.VCO2. Apparently, MRVBP, pulmonary CO2 flow, PaCO2, and P.hivin.VCO2 do not provide the primary stimuli for the exercise hyperpnea in ponies.

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