The purpose of this study was to measure cardiovascular, respiratory, and metabolic responses to graded treadmill exercise in the horse. A five-stage treadmill test up to 90% of predicted maximal heart rate was administered to five sedentary horses. The highest measured level of exercise produced a sixfold increase in cardiac output and a 41% elevation of stroke volume over standing values. Left ventricular, arterial, and right atrial pressures as well as the maximal time derivative of left ventricular pressure were all elevated during exercise. Under the same two conditions hematocrit (Hct) increased from 33 +/- 2 to 47 +/- 1%, oxygen uptake (VO2) from 1.1 +/- 0.1 to 32.7 +/0 2.1 1.min-1, and respiratory exchange ratio (RQ) increased from 0.83 +/- 0.02 to 0.96 +/- 0.01. In addition, three of the horses were willing to trot at a sixth stage (14 km.h-1, 11.5% grade), which elicited a VO2 of 39.8 +/- 0.31.min-1 and an RQ of 0.98 +/- 0.01. In response to the sympathetic stimulation of exercise, the horse is able to elevate Hct, hemoglobin, and oxygen-carrying capacity by emptying the spleen, thus increasing total circulating red blood cells by as much as 50%. This enabled three of the five horses used in the present study to increase aerobic metabolism almost 40-fold above resting or twice that seen in humans. Therefore, in the horse "blood-boosting" is an efficient mechanism for increasing aerobic metabolism. This finding lends further support to the view that the oxygen transport system limits maximal aerobic capacity in humans.

The effects of human social contact on the electrocardiogram (EKG) and general behavior of 2 horses were explored. Petting elicited a slowing of heart rate, while a person entering and exiting elicited transient, but often marked heart rate increases. In both horses transient periods of T wave inversion occurred in the EKG during human contact. In one of the horses the frequency of dropped heart beats increased dramatically during successive trials of human petting. In this horse a total of 29 dropped heart beats were observed of which 23 occurred while a person was with the horse, although this contact occupied only 15 percent of the total observation period. These findings parallel previous observations made with dogs. Similar observations of changes in the heart rate and rhythm of patients in coronary care units and a shock‐trauma unit during social contact with other humans underscore the need to develop empirical analogues of these reactions. The horse appears to offer an ideal model for understanding the psychosocial, hemodynamic, and central nervous system mechanisms involved in these reactions.

Electrocardiography is currently the most accurate method of documenting cardiac arrhythmias and conduction abnormalities. This article discusses techniques of recording electrocardiograms and the systematic evaluation of electrocardiograms to ensure accurate interpretation.

The effects of 5- and 10-wk treadmill exercise training on cardiorespiratory function were evaluated in the horse. Cardiac output (Q), heart rate (HR), and stroke volume (SV), as well as left ventricular (LVP), aortic (AoP), and mean right atrial (RAP) pressures and the peak first time derivative of LVP (LV dP/dtmax), were measured at rest and at five different levels of exercise up to 90% of initial predicted maximal HR (HRmax). Oxygen uptake (VO2) and respiratory exchange ratio (R) were also obtained under the same conditions. At rest, although HR was no different after training, LV dP/dtmax was lower at 10 wk (2,369 +/- 502 vs. 1,615 +/- 302 mmHg/s). At the different measured work loads during exercise, Q and VO2 remained unchanged with training, whereas there were consistent trends toward lower LVP, AoP, and LV dP/dtmax and significant reductions in HR and R (both P less than 0.05). In contrast, SV and mean RAP were elevated (P less than 0.05) during exercise after the 10-wk training program. Although the observed changes in cardiorespiratory function in response to a training program in the horse are generally similar to those reported for other species, our data also suggest a training-induced increase in venous pressures during exercise as measured by elevated mean RAP values.