544 
HEMODYNAMICS 
LVP 
Figure 2. — Diagramatic representation of a pair of 
coils implanted in the left ventricle of the horse. 
LVID = Left ventricular internal diameter coils. 
LVP = Left ventricular pressure transducer. 
with short acting barbiturates and maintained 
with halothane had an average cardiac output 
of 23.78 Hters/min. The heart rates of all anes- 
thetized animals varied from 48 to 97 beats per 
minute, with mean value of 72 ±: 4. The in- 
creased heart rate in anesthetized horses was 
obvious when compared to the normal standing 
horses, whose mean heart rate was 40 beats per 
minute. Horses under the influence of pentobar- 
bital sodium had higher heart rates than those 
maintained on halothane. Studies of Barlow and 
Knott^s with dogs have shown that pentobarbi- 
tal sodium causes a marked increase in mean 
arterial pressure, heart rate, cardiac output and 
peripheral resistance, with a decrease in aver- 
age total blood volume and stroke volume. Our 
studies and those of Eberly and coworkers^^'^i 
show that the anesthetics pentobarbital and 
halothane both cause a marked decrease in the 
cardiac output, even though the heart rate is 
increased. 
The cardiovascular values for six conscious 
standing horses are listed in Table II. The 
mean cardiac output for this group was 53.30 li- 
ters per minute. This value is slightly higher 
than that reported by Eberly and co-workers.^^ 
Tyrode's Solution Infusion 
Tyrode's solution was rapidly infused into 
two conscious horses in an attempt to describe 
the normal ventricular function curve for this 
species. Analysis was made of the relationship 
between left ventricular volume and pressure 
and left ventricular volume and stroke work 
(Figures 4 and 5) . The left ventricular internal 
end-diastolic radius and end-systolic radius for 
each horse at control recording and at the maxi- 
mum response are listed in Tables Ilia and Illb 
along with the calculated ventricular volumes. 
Rapid infusion of Tyrode's solution resulted in 
an increase in left ventricular internal radius at 
end-diastole from 3.938 cm to 4.85 cm (horse 
#044) and from 4.4 to 4.8 cm (horse #011). 
Likewise, end-systolic diameter rose from 3.30 
cm to 4.13 cm in horse #044 and from 3,4 cm to 
3.7 cm in horse #0011. 
In an attempt to evaluate the response of the 
dP/dt-mm.Hg/sec 
'90MM/ it^ 
Figure 3. — Illustrates the parameters normally re- 
corded in a conscious standing horse. Whereas 
LVID = Left ventricular internal diameter. 
LVP = Left ventricular pressure. 
AP = Aortic pressure. 
EKG = Lead II of the electrocardiogram. 
