518 
HEMODYNAMICS 
of such a cardiovascular examination in a num- 
ber of small domestic ponies, and discusses both 
properties of the cardiovascular system and 
suitability of the animal as a surgical and phys- 
iological model. 
MATERIALS AND METHODS 
Selection, Preconditioning, Housing, and 
Handling 
The experimental group was composed of 
thirty-one ponies. Fourteen were males, thir- 
teen were females, and four were geldings. 
Ages of the animals ranged from eight months 
to nine years, and weights from 53.6 to 161.5 
kg. Animals were selected so that, in most 
cases, weight did not exceed 100 kg and age did 
not exceed three years. Docility, minimal train- 
ing, and ease of handling were additional con- 
straints. Each pony was quarantined for two 
weeks prior to delivery, and treated during this 
time to eliminate intravascular or gastrointes- 
tinal parasites. Upon arrival in the laboratory, 
ponies were placed in box stalls^ and allowed 
twenty-four to forty-eight hours during which 
to accommodate to the unfamiliar surround- 
ings. 
Anesthesia and Surgery 
Ponies were premedicated with acetylproma- 
zine or Rompun (Bay Va 1470) prior to induc- 
tion of anesthesia. A combination of 0.2% pen- 
tothal sodium and 5% guaiacol glyceryl ether 
was given intravenously until loss of equilib- 
rium occurred. An endotracheal tube was then 
passed, and surgical anesthesia established with 
a combination of oxygen, nitrous oxide, and hal- 
othane. The left carotid artery was exposed, 
and two No. 14 Teflon catheters were placed in 
it and advanced until their tips lay in the arch 
of the aorta. A large bore polyvinylchloride 
catheter was placed in the left jugular vein. 
Upon completion of the surgery, ponies were 
given an intravenous infusion of 5% dextrose 
(One L) containing Butazolidin (600 mg), Liq- 
uamycin (500 mg) , and sodium heparin (10,000 
units). An intramuscular injection of tetanus 
antitoxin was also given as a precaution. The 
catheters were filled with heparinized saline 
and capped. Forty-eight hours of post-operative 
recovery were allowed before any measure- 
ments were taken. 
Physiological Measurements 
All data were obtained from unmedicated 
standing ponies, using only mild restraint 
which was provided by a small, mobile cart.^ 
Parameters were recorded on magnetic tape, 
and displayed on a three-channel Physiograph. 
Heart rate was measured from the R-R interval 
of the electrocardiogram, which was recorded 
from silver-silver chloride disk electrodes in a 
chest-to-back configuration. Blood pressure was 
obtained from one of the indwelling carotid 
catheters, using a Statham AA (0-300 mm Hg) 
pressure transducer. A 125 cm No. 9 french 
right heart sampling catheter was passed 
through the jugular implant to the right ventri- 
cle and pulmonary artery. Pulmonary artery 
pressure was measured from the right heart 
catheter using a Statham BB (0-50 mm Hg) 
transducer. Cardiac output was determined by 
dye dilution, using indocyanine green dye and a 
Waters densitometer. Dye was injected into the 
pulmonary artery via the right heart catheter, 
and blood was withdrawn through the densi- 
tometer cuvette from one of the aortic catheters ; 
the other was used to simultaneously monitor 
blood pressure. 
Cardiac output, stroke volume, vascular re- 
sistance, and stroke work, together with related 
indices, were calculated by an IBM 7094 com- 
puter, using modifications of equations described 
by Liu.'^ Simultaneous tracings of electrocardio- 
gram, phonocardiogram, and the aortic pres- 
sure wave, displayed at 100 mm/sec on a Honey- 
well Visicorder, were used for measurement 
of left ventricular ejection time, pre-ejection 
period, isovolumic contraction time, electro- 
mechanical lag, mechanical systole, electrome- 
chanical systole, and related ratios. Measure- 
ment or calculation of these phases of ventricular 
systole was performed according to methods 
described by Tavel.'^ The relationship of these 
parameters to heart rate was derived from lin- 
ear regression analysis, using an IBM 7094 
computer. Comparisons of cardiac and hemo- 
dynamic function in normotensive and hyper- 
tensive ponies were made by first matching a 
