Medical evaluation of the cardiovascular system.

The approach to pre-purchase evaluation of the equine cardiovascular system is discussed with emphasis on cardiac auscultation and on the terminology that should be used when reporting clinical findings. Innocent cardiac murmurs and arrhythmias are contrasted with those that should be considered as reliable signs of cardiovascular disease. Ultrasonography and electrocardiography are recommended to establish baseline values for future reference, to support opinions rendered by the examining veterinarians, or to provide for diagnosis and treatment.

Cardiorespiratory drift during exercise in the horse.

The purpose of the present study was to measure the time-course and degree of cardiovascular and respiratory 'drift' during constant submaximal exercise in the horse. One Thoroughbred and four Morgan mares were instrumented for simultaneous measurement of respiratory and blood gases which also enabled cardiac output (Q) to be calculated. Data were collected at rest, and at 10, 20 and 30 mins during a constant workload which elicited an initial exercising heart rate (HR) of 150 beats/min, and an approximate 15-fold increase in oxygen consumption (VO2). Significant cardiac and respiratory drift during exercise were observed over time so that ventilation increased from 750 +/- 58 to 910 +/- 49 litres/min (21 per cent increase) from the 10 to 30 min time-point (P < 0.05) and HR went from 154 +/- 4 to 173 +/- 9 beats/min (mean +/- se) over the same time period (P < 0.05). Q also rose from 142 +/- 5 to 177 +/- 17 litres/min (P < 0.05) during the same interval while stroke volume (SV) was maintained. Rectal temperature (TR) and mixed venous lactate (LA) also showed significant increases during exercise while PaO2 and PaCO2 remained constant. The results indicate a significant degree of cardiac and respiratory drift in the horse in response to strenuous submaximal exercise. At the constant exercise work rate chosen, a levelling off, or plateauing of the selected parameters of interest was not observed. Therefore if a true exercising 'steady-state' was achieved, it must have occurred very early in the exercise bout.

Electrocardiography.

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.

Effects of training on cardiorespiratory function in the horse.

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.

Atrial fibrillation in horses: factors associated with response to quinidine sulfate in 77 clinical cases.

Seventy-seven horses with atrial fibrillation (AF) were treated orally with quinidine sulfate (QS) at the University of Pennsylvania, School of Veterinary Medicine, Thirty-seven horses (48%) had adverse reactions to QS, the most common of which were nasal mucosal edema, anorexia, colic, and diarrhea. The mean highest daily QS dose was slightly greater in horses showing toxicity (P less than 0.1) and their conversion rate was less (P less than 0.05). A schedule of gradually increasing QS dose was time consuming, costly and not associated with fewer adverse reactions. Horses were compared by their response to QS: conversion for at least 12 months (Group I; 25 horses); reversion with 12 months (Group II; 8); nonconversion (Group III; 14). All horses with congestive heart failure (CHF) failed to convert. The mean age (P less than 0.05), total QS dose and highest daily QS dose (P less than 0.05) were significantly greater in Group III horses. The mean duration of signs before treatment and occurrence of adverse reactions to QS were greater in horses in Group II and III than Group I. (P less than 0.05). All horses in Groups I and II returned to performance, and many (81%) were successful. Most Group III horses (13/14) are known to have died. It was concluded that most horses with AF uncomplicated by CHF have a good prognosis when treated early with QS. Delayed QS therapy may reduce chances for conversion of AF or increase the probability of reversion.

The equine electrocardiogram with standardized body and limb positions.

Eleven-lead electrocardiograms (ECGs) were recorded from 50 Thoroughbred (TB) and 50 Standard bred (SB) horses at rest with 40 tracings being selected from each group for more complete analysis. Incomplete atrioventricular block with dropped beats commonly reported in mature horses was recorded only in the TB (15%), while wandering of the atrial pacemaker (WAP) thought also to be associated with variations in tone of the autonomic nervous system was detected almost equally in both breeds (30%). In some horses (12.5%), the initial component of the P wave (P1) did not vary as seen with WAP but had a mean manifest electric axis in the frontal plane between 0 and -64 degrees with vectors directed craniad and sinistrad. Breed differences were noted for the mean duration of the P-wave and for the PR and QT intervals while no significant differences were present for mean heart rate. Significant differences have also been reported for other ECG values (amplitude and duration) in relation to age, breed and sex. It may be of value when attempting to determine the clinical or research significance of cardiac vectors or other ECG parameters, to establish values for each of the different breeds. Interpretation of ST-segment deviations and changes in the form of the T-wave are complicated by their associations with conditions of excitement or exercise in clinically normal horses as well as in those with organic heart disease. The diagnostic significance of changes in amplitude and direction of the T-wave requires further elucidation.

Cardiorespiratory and metabolic responses to treadmill exercise in the horse.

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.

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.

Electromagnetic measurement of cardiac output during exercise in the horse.

Aortic root blood flow was measured with an electromagnetic (EM) flow meter in unanesthetized horses during rest and exercise. The cardiac output response to exercise, as determined by the EM technique, was compared with the response reported by others who used indicator dilution techniques with good agreement. The EM method will allow making measurement of cardiovascular responses to exercise in the horse, not obtainable with other techniques.

Swimming--a method to study the physiologic response of the horse to exercise.

The literature on exercise physiology in the horse with special reference to swimming is briefly reviewed. The use of a pool for swimming horses is described and a technique discussed for the collection of haematologic and haemodynamic data using this form of exercise.