JAMES AMEND, HAROLD GARNER, JOHN ROSBOROUGH, AND HEBBEL HOFF 
521 
Table V. — Comparison of Ventricular Systolic Time 
Intervals in Normal and Spontaneously Hypertensive 
Domestic Ponies 
Parameter 
Normal 
Hypertensive 
Change % 
Heart rate (Beats/min) 
61.1 
49.9 
— 2.3 
Pre-Ejection Period (sec) 
0.076 
0.091 
+ 19.7 
Left Ventricular Ejection 
0.393 
0.379 
— 3.5 
Time (sec) 
Ejection Time Index 
0.576 
0.586 
+ 1.6 
PEP / LVET Ratio 
0.193 
0.227 
+ 17.6 
Isovolumic Contraction 
0.037 
0.052 
+40.5 
Time (sec) 
LVET / ICT Ratio 
10.62 
7.29 
—31.4 
Electromechanical Lag 
0.046 
0.046 
+ 2.2 
(Q-Sl; sec) 
Mechanical Systole 
0.430 
0.431 
+ 0.2 
(S1-S2; sec) 
Electromechanical Systole 
0.470 
0.477 
+ 1.5 
(Q-S2; sec) 
Electrical Systole (QT; sec) 
0.430 
0.432 
+ 0.5 
time intervals. Table V presents this compara- 
tive data. Of the intervals measured, only PEP 
and ICT were altered ; both were prolonged. Be- 
cause Q-Sl remained unchanged, the prolonga- 
tion of PEP was clearly the result of the in- 
creased ICT. This is in contrast to the changes 
reported in human essential hypertension, 
where an increased PEP was not due to a 
change in ICT, but rather to an extended 
Q-S1.15 
Table VI contains data describing changes in 
hemodynamic function and ventricular systolic 
time intervals in a pony which was found at 
necropsy to have coarctation of the aorta with 
Table VI. — Changes in Hemodynamics and Phases of 
Ventricular Systole in a Domestic Pony with Spontane- 
ously Occurring Cardiovascular Disease 
Parameter Normal Ponies Diseased Pony 
Cardiac Output/kg Body 
107 
119 
Weight (ml/min/kg) 
Systemic Blood Pressure (mm Hg) 
155/105 
115/60 
Systemic Vascular Resistance (PRU) 
0.73 
0.27 
Left Ventricular Work/kg 
6.40 
2.35 
Heart (joules/stroke/kg) 
Pulmonary Artery Pressure (mm Hg) 
45/20 
20/5 
Pulmonary Vascular Resistance 
0.19 
0.06 
(PRU) 
Right Ventricular Work/kg 
1.20 
0.36 
Heart (joules/stroke/kg) 
Heart Rate (Beats/min) 
49.0-50.0 
49.8 
Pre-Ejection Period (sec) 
0.075 
0.069 
Left Ventricular Ejection 
0.396 
0.466 
Time (sec) 
Ejection Time Index 
0.593 
0.639 
Isovolumic Contraction Time (sec) 
0.036 
0.022 
Electromechanical Lag (Q-Sl; sec) 
0.040 
0.037 
Mechanical Systole (S1-S2; sec) 
0.432 
0.477 
Electromechanical Systole 
0.472 
0.614 
(Q-S2; sec) 
bilateral ventricular hypertrophy. This ani- 
mal is presented here not so much to provide in- 
formation describing cardiovascular effects of 
aortic coarctation as to indicate the nature of 
the animal as a resource in cardiovascular re- 
search. The animal showed increased cardiac 
output, decreased vascular resistance, and re- 
duced ventricular work, prolonged LVET, and a 
PEP which was shortened by the presence of a 
decreased ICT. 
DISCUSSION 
The purpose of this study was the documen- 
tation of cardiovascular function in normal, un- 
anesthetized domestic ponies, as a necessary 
preliminary step in the utilization of the animal 
as a man-sized animal model for cardiovascular 
research. Results indicate that for most param- 
eters of cardiovascular function, including both 
hemodynamic determinations and measure- 
ments of ventricular systolic time intervals, the 
differences among pony, man, and calf are 
rather minimal. 
The dominant difference appears to be the 
relatively greater cardiac output of the pony. 
This xS not a phenomenon of size difference, as 
normalized values also demonstrated the in- 
crease, and in fact, even though some of the 
ponies were large (120-150 kg), the mean 
weight of the group was 94.0 kg, not signifi- 
cantly different from weights reported for 
three-month-old calves. ^ It is possible that the 
larger output found in the ponies is a reflection 
of the general maturity of the group (mean age 
1.9 years) as compared with the group of 
calves. In this regard, it is also interesting to 
note that heart weights in the two species did 
not significantly differ in the studies compared 
(this report; citation 2), even though the pon- 
ies were mature and the calves were not. 
The fact that differences were so small, in 
spite of the maturity of the pony group, sug- 
gests that the domestic pony may provide an 
animal model which does not introduce the vari- 
able of growth into the conduct of chronic stud- 
ies. As indicated earlier, the progression of 
growth can be a problem of major proportions 
in research involving long term evaluation of a 
prosthetic device. 
