520 
HEMODYNAMICS 
Table III. — Ventricular Systolic Time Intervals in 
Normal Domestic Ponies 
Parameter 
Range of Values 
Mean Value 
Heart rate ( beats/min ) 
41.9 - 
- 93.0 
59.3 
Pre-Ejection Period (sec) 
0.056- 
-0.129 
0.078 
Left Ventricular Ejection 
0.237- 
-0.453 
0.360 
Time (sec) 
Ejection Time Index 
0.538- 
-0.644 
0.600 
PEP / LVET Ratio 
0.141- 
-0.311 
0.217 
Isovolumic Contraction Time 
0.017- 
-0.069 
0.03.') 
(sec) 
LVET / ICT Ratio 
6.9 - 
- 22.9 
10.2 
Electromechanical Lag (Q-Sl; 
0.034- 
-0.060 
0.043 
sec) 
Mechanical Systole (S1-S2; 
0.271- 
-0.483 
0.401 
sec) 
Electromechanical Systole 
0.305- 
-0.543 
0.445 
(Q-S2; sec) 
Electrical Systole (QT; sec) 
0.287- 
-0.476 
0.401 
heart rate changed (PEP, ICT, Q-Sl) were 
also found to be very similar to values described 
for man." Parameters M^hich varied vi^ith heart 
rate (LVET, S1-S2, Q-S2, QT) showed simi- 
larity to data reported for man as long as the 
range of heart rates remained close to 70-90 
beats per minute. At lower rates, however, 
these intervals lengthened much more rapidly 
than those of man, for equal decrements in 
heart rate. The ETI for the pony was 0.593; 
that of man is 0.373.^^ Thus the regression line 
describing the relationship between LVET and 
heart rate in the pony, beginning as it does at 
an intercept (ETI) value greater than that 
found for man, must show a steeper slope if 
similar LVET values are to be found in the two 
species at higher heart rates. In fact, LVET is 
predicted to be identical in pony and man at 88 
beats per minute; the slope of the line for the 
pony is then 0.0036 (negative) . For an increase 
in heart rate of ten beats per minute, LVET in 
the pony may be expected to shorten by 0.036 
sec, while man shortens by 0.013 sec." 
An observation of particular interest was the 
occurrence of apparent spontaneous hyperten- 
sion (consistent mean aortic pressures in excess 
of 135 mm Hg) in seven of the ponies studied. 
Comparison of data available from these ani- 
mals with data from a selected matched group 
of normal ponies drawn from the total popula- 
tion showed a number of interesting results. 
Table IV presents similarities and differences 
in hemodynamic parameters in normotensive 
and hypertensive ponies. The normal group was 
chosen, in addition to age, sex, and body weight. 
for similarity in heart rate, in order to mini- 
mize effects of rate-dependent variables. There- 
fore no significant difference in heart rate is 
found between the two groups. Blood pressures 
are different, however, with systolic, diastolic, 
and mean pressures all equally elevated. In all 
absolute and normalized parameters of cardiac 
output and stroke volume, the hypertensive 
group showed increased values. Systemic vascu- 
lar resistance was approximately equal in the 
two groups, but left ventricular ejection rate 
was increased in the hypertensive ponies, as 
was left ventricular work. Plasma volume 
showed an apparent increase in the hyperten- 
sive group ; there were, however, only three de- 
terminations in the normal group and only one 
hypertensive animal was successfully used for 
this measurement. This combination of hemo- 
dynamic changes, that is, elevated cardiac out- 
put with no change in vascular resistance, in- 
creased left ventricular ejection rate, and 
equally elevated systolic and diastolic blood 
pressures has been described in human labile 
hypertension." The ponies did not, however, 
show labile pressure changes, but were appar- 
ently consistent, at least when measurements 
were obtained. 
Changes were also observed in the hyperten- 
sive ponies with regard to ventricular systolic 
Table IV. — Comparison of Hemodynamic Parameters in 
Normal Domestic Ponies 
and in 
Ponies with Spontane- 
ously Occurring Hypertension 
Parameter 
Normal 
Hypertensive 
Change % 
Heart rate (beats/min) 
66.0 
65.8 
- 0.3 
Cardiac Output (L/min) 
10.7 
13.4 
-h20.1 
Cardiac Index (L/min/sq meter) 4.95 
6.35 
+28.1 
Cardiac Output/kg Body 
106 
134 
+26.4 
Weight (ml/min/kg) 
Stroke Volume (ml/beat) 
163 
204 
+25.1 
Stroke Index (ml/beat/sq 
75.0 
96.6 
+28.6 
meter ) 
Stroke Volume/kg Body 
1.62 
2.05 
+27.3 
Weight (ml/beat/kg) 
Mean Blood Pressure 
118.6 
146.0 
+23.1 
(mm Hg) 
Systolic Blood Pressure 
146.7 
180.7 
+23.2 
(mm Hg) 
Diastolic Blood Pressure 
99.4 
122.7 
+23.4 
(mm Hg) 
Systemic Vascular Resistance 
0.68 
0.65 
— 4.4 
(PRU) 
Left Ventricular Ejection 
214 
271 
+26.6 
Rate (ml/sec/M2) 
Left Ventricular Work 
2.97 
4.64 
+B6.2 
(joules/stroke) 
Plasma Volume (ml/kg) 
50.7 
61.7 
+21.7 
