Cardiodynamic variables and ventilation during treadmill exercise in ponies.

We assessed the relationship of ventilation (VE) to cardiodynamic variables and CO2 transport in seven normal ponies during treadmill exercise. At 1.8, 3, and 6 mph, respectively, 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 three ponies at the same work loads, cardiac output (Qc) increased from approximately 12 l/min at rest to 19.7, 28.1, and 40.3 l/min between 30-60 s (P less than 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 less than 0.05) and then decreased to 11.8, 12.2, and 15.8 Torr by 8 min of the three respective work intensities. At all work loads, VE increased proportionately faster than these cardiovascular variables in the 1st min. For example, at 6 mph VE increased 470%, whereas Qc and HR increased only 230%. Thereafter, VE generally continued to increase at 3 and 6 mph, whereas MRVBP, Qc, and HR decreased. Therefore, the basic assumption of a cardiodynamic hyperpnea that VE and Qc are equivalently coupled at the exercise onset is rejected for this species. Mixed venous CO2 content (C-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.(ABSTRACT TRUNCATED AT 250 WORDS)