CARDIOVASCULAR HEMODYNAMICS IN 
DOMESTIC PONIES 
James F. Amend, Harold E. Garner, John P. Rosborough and Hebbel E. Hoff 
Parameters of heart rate, systemic and pulmonary 
pressure and vascular resistance, cardiac output and 
index, stroke volume and index, and ventricular stroke 
work were determined in thirty-one unanesthetized, 
standing domestic ponies. Average heart rate for the 
group was 59 beats/min. Cardiac output (per kg body 
weight) was 113 ml per minute per kg. Average car- 
diac index was 5.1 L/min/sq meter. Systolic pressure 
averaged 144 mm Hg on the systemic side (central 
aorta), anJ 45 mm Hg in the pulmonary artery. Aortic 
diastolic pressure was 107 mm Hg; diastolic pressure in 
the pulmonary artery averaged 20 mm Hg. Systemic 
vascular resistance was 0.70 peripheral vascular resist- 
ance units (PRU), while pulmonary vascular resistance 
was 0.20 PRU. Stroke volume averaged 1.64 
ml/beat/kg, and ventricular stroke work was 7.8 
joules/stroke/kg heart weight. Additional indirect indi- 
cators of cardiac function were obtained from measure- 
ments of phases of ventricular systole, including left 
ventricular pre-ejection period (PEP), left ventricular 
ejection time (LVET), PEP/LVET ratio, isovolumic 
contraction time (ICT), LVET/ICT ratio, Q-Sl, S1-S2, 
Q-S2, and QT. Significant fits were obtained for linear 
regression equations describing LVET, PEP/LVET, 
S1-S2, and Q-S2 as functions of heart rate. No signifi- 
cant fit was found for PEP, ICT, Q-Sl, or LVET/ICT. 
QT was described by the asymptotic Bazett equation, 
where QT is equal to Bazett's constant (0.4 for the 
pony) times the square root of the R-R interval length. 
ETI was 0.593, about twice that determined in man. 
The regression equation LVET = 0.593 - 0.0036 HR 
shows that LVET shortens three times faster in the 
pony than in man for equal increments in rate. Addi- 
tional observations of importance included comparison 
of the above parameters in a pony which was found to 
have coarctation of the aorta and bilateral ventricular 
hypertrophy, and in a group of animals which showed 
hemodynamic function similar to that seen in human 
labile hypertension. 
INTRODUCTION** 
Recent investigations in the development and 
evaluation of prosthetic devices for mechanical 
* Department <if Physiology, Baylor College of Medicine, Houston, 
Texas. 
*• This study was supported in part by U.S. Public Health Service 
Grants HE-05125, GM-38193.01, GM-44624.01, and RR-05425, and by re- 
search contracts from Chemagro Corporation, Kansas City, Missouri, 
and from Hoffman-LaRoche, Inc., Nutley, New Jersey. 
circulatory assistance have employed several 
species of medium-sized domestic animals as 
surgical and physiological models ; most promi- 
nent among these has been the calf. The sheep, 
goat, and pig have also been used, but have 
proven less desirable. Notable for its absence 
from this group is the small domestic pony. The 
pony possesses many of the attributes which 
have made the calf a popular model; among 
these are availability, docility, and approxima- 
tion of man in body mass and circulatory capac- 
ity. Additionally, however, the properly selected 
small pony may be used at maturity, while the 
calf is similar to man in size only at an early 
age. Such a factor is of considerable importance 
with regard to stamina required for surgical 
procedures, the fragility of blood vessels, main- 
tenance of good health throughout a prolonged 
period of chronic experimentation, and the very 
important problem of growth, which can itself 
be a limiting factor in the life of an implanted 
device. 
The pulmonary microanatomy of the horse 
and pony has been described as more similar to 
that of man than even the non-human 
primates ;i one of the major problems of as- 
sisted circulation has been the disturbance of 
function in the pulmonary circulation. The pony 
is an athletic animal, in contrast to the calf, and 
can readily be trained to exercise. Certainly the 
stress of exercise can be the ultimate test of an 
assist device, thus the availability of an appro- 
priate animal model may be of critical impor- 
tance when such a stage of development is 
reached. Finally, the pony, as a non-ruminant, 
can be a more easily handled surgical and post- 
operative patient. 
Unlike the calf,^'^ the pony has not been 
subjected to controlled study to determine nor- 
mal values for parameters of cardiac and hemo- 
dynamic function. This report describes results 
517 
