THE EFFECTS OF THE CATECHOLAMINES AND GLUCAGON 
ON THE PERFORMANCE OF THE EQUINE HEART 
W. C. Bowie, R. C. Purohit, R. R. Adams*, K. T. Somajah, E. W. Hawthorne and J. E. Hinds' 
The equine heart because of its large size is ideally 
suited for studies of cardiac dynamics. Surgical tech- 
niques have been developed to implant various measur- 
ing devices in and around the left ventricle of this spe- 
cies. Anesthesia was achieved with thiopental sodium, 
and maintained with pentobarbital sodium or with hal- 
othane for the entire period of surgery. Mercury gauges 
were positioned on the left ventricle to record left ven- 
tricular circumference, wall thickness and upper and 
lower segmental lengths. From the above dimensional 
measurements, left ventricular volumes were calculated ; 
additionally the internal axis ratio of the left ventricle 
was derived. In further studies the circumference gauge 
was replaced with a pair of mutual inductance coils to 
measure the internal diameter of the left ventricle, 
throughout the cardiac cycle. 
By knowing the changes in radius from end-diastolic 
to end-systolic size of the left ventricle and the internal 
axis ratio, ventricular volumes were calculated. Along 
with measurement of changes in dimensions, ventricu- 
lar aortic and arterial pressures were also measured. 
Information obtained as a result of the above measure- 
ments permitted us to make the following determina- 
tions: End-diastolic volume, end-systolic volume, stroke 
volume, cardiac output, ejection fraction, stroke work, 
left ventricular end-diasystolic, end-systolic and aortic 
pressures. 
Thus in acute and chronic horses, the effects of cat- 
echolamines and glucagon were studied to evaluate 
their effects on the performance of the equine heart. In 
conscious standing instrumented horses the ventricular 
function studies were done by the rapid intravenous in- 
fusion of Tyrode's solution. The development of the 
surgical technique to implant these various transducing 
devices in chronic horses has permitted us to make such 
studies in this animal over a prolonged period of time. 
INTRODUCTION 
A historical review of cardiopulmonary stud- 
ies in horses has been published by Garner et 
al.i and documents the contributions of this 
species to our knowledge of cardiovascular phys- 
iology. The large size of the cardiovascular 
* School of Veterinary Medicine, Tuskegee Institute Tuskegee Insti- 
tute, Alabama. 
** College of Medicine, Howard University Washington, D. C. 
structures in the horse makes this species a 
unique model for hemodynamic studies involv- 
ing the simultaneous implantation of several 
transducing devices. Techniques for performing 
thoracic surgery in the horse are relatively 
recent^ and closely parallel the advances in 
equine anesthesiology.^-^ Such progress has 
made it possible for many investigators to in- 
creasingly utilize the potential offered by this 
species. Bowie et al.^ have used horses to calcu- 
late the end-diastolic, end-systolic and stroke 
volumes from direct measurements of linear di- 
mensional changes of the left ventricle in anes- 
thetized animals. Eberly et al.^"'^^ have used 
this species to evaluate cardiac output using the 
dye dilution technique as well as heart rate and 
blood pressures in awake and anesthetized ani- 
mals. The relationship of ventricular volume to 
body weight in horses and other mammals has 
been studied by Holt et al.^^ These reports 
clearly demonstrate the usefulness of horses in 
cardiovascular experimentation. 
Previous studies undertaken in our labora- 
tory have been directed toward devising meth- 
ods for the continuous measurement of left ven- 
tricular dimensional changes throughout the 
cardiac cycle^'^^ and the use of these measure- 
ments to derive estimates of chamber volume, 
muscle mass, mural stresses and indices of car- 
diac performance. The present studies were 
conducted to examine the hemodynamic effects 
of the catecholamines and glucagon in both an- 
esthetized and conscious horses. The response of 
the cardiovascular system to chemical inotropic 
stimuli in this species and its relationship to 
canine and human standards in this regard 
has not been examined previously. 
MATERIALS AND METHODS 
Random source horses were quarantined and 
conditioned for two weeks prior to surgery. The 
541 
