542 
HEMODYNAMICS 
horses were fasted 24 hours before surgery. A 
jugular catheter was implanted for intravenous 
administration of drugs. One hour before the 
thoracic surgery, the animals were tranquilized 
with propiopromazine hydrochloride (0.4 
/ig/kg). Surgical anesthesia was induced with 
thiopental sodium and maintained with either 
pentobarbital sodium or with halothane in all 
the acute experimental studies. An endotracheal 
tube was inserted and positive pressure respira- 
tion was maintained with a custom built large 
animal respirator. Halothane and nitrous oxide 
in combination were the maintenance anesthet- 
ics of choice in the chronic preparation. 
The thoracic cavity was opened by an incision 
at the fifth intercostal space. For acute studies 
the fourth, fifth and sixth ribs were resected to 
expose the heart and for chronic studies only 
the fifth rib was resected. The rib was freed 
from the periosteum and then transected near 
the junction of the upper proximal and middle 
third with an obstetrical wire saw. Resection 
was completed by disarticulating the rib at the 
costo-chondral junction. The periosteum and 
pleura were incised to open the thoracic cavity. 
Hexylcaine solution (5%) was injected in the 
pericardial space to decrease myocardial sensi- 
tivity. In the acute preparation, mercury-in- 
rubber gages were implanted to measure left 
ventricular circumference (LVC), left ventric- 
ular wall thickness (LVWT) and two segmen- 
tal lengths, upper segment length (USL) and 
lower segment length (LSL) . These gage place- 
ment techniques in horses have been previously 
described. ^'^^ 
External ventricular volume was calculated 
by the method previously described by Bowie. ^ 
A sum of a series of segmental volumes repre- 
sented the external ventricular volume. The vol- 
ume of each segment may be calculated if the 
height and cross sectional area of the segment 
are known. The accuracy with which each seg- 
mental volume may be calculated and the num- 
ber of segments measured determines the accu- 
racy of this approach. 
The internal volume was derived from these 
external dimensional calculations by subtract- 
ing the left ventricular muscle volume (MV), 
(obtained immediately after sacrificing the 
horse) from the calculated external volume. 
These calculated volumes — both internal and 
external — were compared with the actual vol- 
umes obtained at necropsy. The hearts were 
fixed quickly by infusing gluteraldehyde into 
the left main coronary artery until cessation of 
heart beat occurred. The hearts were then re- 
moved and the volume of the left ventricular 
muscle was determined by water displacement. 
Casts of the horse's left ventricular cavity ar- 
rested in either diastole or systole were made, 
using plastic materials (Figure 1). The volume 
of the casts were measured by water displace- 
ment and these measurements were used to ap- 
proximate the closeness of fit of the calculated 
volume to the actual volume. 
From the above dimensional measurements 
the average internal end diastolic axis ratio 
(ARi) was found to be 3.86. By assuming that 
the (ARi) remained constant during ejection, 
only the end-diastolic and end-systolic internal 
radius of the left ventricle was needed to calcu- 
late the end-diastolic, end-systolic and stroke 
volume using the following equations : 
LVEDV = 4/B7tWj EDV (ARi) (1) 
(ARi) = 3.86 
LVESV = 4/377R3i ESV (ARi) (2) 
Stroke volume (SV) = LVEDV - LVESV 
whereas Ri edv = internal radius of the left 
ventricle at the end-diastole and Ri ESV — in- 
ternal radius of the left ventricle at end-systole. 
Cardiac output was determined by multiply- 
ing the stroke volume by the heart rate. . 
Those animals which were prepared for 
chronic cardiovascular studies were instru- 
mented with a set of electromagnetic coils posi- 
tioned across the anterior to posterior internal 
diameter of the left ventricle (Figure 2). The 
principle of operation of electromagnetic induc- 
tion for motoring cardiac dimension has pre- 
viously been described by Perry and Haw- 
thorne.^* Large bore catheters (P. E. 360) 
were implanted in the left ventricle and de- 
scending thoracic aorta. At the time of record- 
ing, tip mounted micromanometers were passed 
into the left ventricle and aorta through these 
catheters to record the pressures. In these ani- 
mals, left ventricular volume estimates were ob- 
1 
