Echocardiographic studies of valvular and ventricular function in horses

Abstract

This thesis evaluates echocardiography in the assessment of the equine heart. Echocardiography is employed extensively in human medicine to differentiate the origin of cardiac murmurs and to assess ventricular performance. Such a noninvasive method would be valuable in equine medicine where routine diagnostic techniques, used successfully in other species, are of little value. Publications concerning the origins of cardiac murmurs, the indications for echocardiography in human medicine, and the limited studies on echocardiography in horses have been reviewed.The aims of Part 1 of this work were to standardise suitable images for twodimensional, M-mode and Doppler echocardiography; to measure selected intracardiac dimensions from the standardised two-dimensional and M-mode images; and to record blood flow velocities from the heart and great vessels using Doppler echocardiography in a group of normal horses and in horses with valvular disease.Eighteen standard images were defined. All but two of the images could be recorded within a narrow range of transducer location, rotation and angulation. Selected intracardiac dimensions were measured from M-mode studies derived from the standard two-dimensional images. Measurements of intracardiac dimensions were repeatable and were not significantly correlated to bodyweight or age. Horses with suspected aortic regurgitation had a significantly larger left ventricular internal dimension in diastole, measured from the right hemithorax, than normal horses. Although other significant differences were detected between groups, intracardiac dimensions were not sufficiently sensitive to differentiate horses with suspected valvular disease from normal horses.Colour flow Doppler studies revealed that valvular regurgitation was present in many normal horses. Horses with murmurs indicating tricuspid and aortic regurgitation showed larger regurgitant signals at the tricuspid and aortic valves respectively, than normal horses. Horses with murmurs indicating mitral regurgitation had a regurgitant signal of longer duration than that of the control group, ii but the size of the jet was not significantly different between groups. Colour flow Doppler echocardiography was a sensitive technique for the detection of valvular regurgitation in horses. The flow velocities recorded by pulsed wave Doppler echocardiography in horses were similar to those reported in other species. Flow velocities were significantly increased at elevated heart rates. The peak aortic velocity and acceleration were also significantly higher in horses with aortic regurgitationPart 2 of this thesis evaluates Doppler echocardiography in the assessment of ventricular performance in horses. Publications concerning measurement of cardiac output and the use of Doppler indices to assess ventricular function in other species have been reviewed. Cardiac output measured by Doppler echocardiography was compared to that measured by thermodilution in nine conscious horses. Cardiac output was modified by the infusion of inotropes and the administration of an alpha 2 agonist. The effects of these agents on selected Doppler indices were assessed.Doppler measurements of cardiac output correlated closely with measurements by thermodilution. Doppler measurements from the aortic outflow correlated more closely (r = 0.89) than those from the pulmonary artery (r = 0.77). The infusion of inotropes caused a significant increase in aortic peak velocity and peak acceleration, whereas the administration of detomidine and butorphanol resulted in a significant decrease in these variables. Other Doppler variables were also significantly altered by the administration of these agents.In conclusion, echocardiography was found to be a sensitive technique for the diagnosis of cardiac murmurs and the evaluation of ventricular performance in horses.