Cardiorespiratory adjustments to tethered-swimming in the horse

The cardiorespiratory and metabolic responses to various levels of tethered-swimming were evaluated in 5 sedentary horses. Cardiac output (Q) and heart rate (HR) correlated highly (r = 0.89 and 0.94 respectively) with work effort (WE) expressed as kg pulled . kg body wt-1 . 10-2. While swimming, stroke volume (SV) was reduced at the lowest workloads, but increased with increasing WE so that at the highest workloads it had returned to the on-land standing SV. Pressures in the pulmonic as well as on both sides of the systemic circulation were considerably elevated by this form of exercise, although only mean carotid artery pressure (CAP) correlated highly (r = 0.83) with WE. During tethered-swimming plasma lactic acid (LA) rose exponentially from 1 to 10 mmol . 1-1 with increasing HR over the range 150-200 beats . min-1. Oxygen uptake (VO2) increased linearly (r = 0.95) from 25-112 ml . kg-1. min-1 over the We range of 3.0-7.8 kg pulled . kg body wt-1. 10-2. The aerobic capacity of the equine species would appear to be twice that of man. The greater increase in VO2 in the exercising horse cannot be explained solely on the basis of increases in Q. Therefore alterations in hematocrit, hemoglobin and oxygen extraction appear to play a more important role in the horse during exercise than they do in man.