Cardiac function and intravascular volumes during and after maximal exertion in the horse

During attempts to measure extravascular lung water in exercise using a dual indicator technique, we noted that stroke volume (SV) increased by ∼27% from 2.0 ± 0.15 (SEM) ml/kg while standing to 26 ±0.06 ml/kg at maximal exercise Cardiac output (CO) was determined from transients in electrical impedance, produced by the injection of 50 ml hypertonic, ice-cold saline into 3 horses on a treadmill, walking and at 75 and 100 % HR_max, (Wilkins et al 1996, Physiologist 39: A50), and at 1, 10 and 20 min during recovery at the walk CO returned promptly to the walking baseline once exercise stopped, while heart rate continued to decline. Thus in recovery, SV dropped to 2.1 ± 0.09 ml/kg at 1 min and rose thereafter until at 20 min it was 2.6 ± 0.15 ml/kg, as high as at maximal exertion. For comparison, the SV when walking before exercise was 2.3 ± 0.12 ml/kg. We conclude that SV can rise as much during recovery as during exercise. Pulmonary to carotid artery mean transit time (tt) of the electrical impedance transient, ∼ 4.6 s at the walk and decreased to ∼ 2.6 s at maximal effort and recovered immediately after exercise. The central blood volume (CO × tt), in contrast to SV, tended to track the changes in pulmonary arterial pressure and CO, probably reflecting changes in lung blood volume.