522 
HEMODYNAMICS 
Perhaps the most significant observation to 
be made in this study was that of the occur- 
rence of spontaneous hypertension in a small 
number of the ponies studied. Such spontaneous 
hypertension in animals has been observed by 
Smirk and Hall;^'' more recently Okamoto and 
Aoki have succeeded in developing an inbred 
strain of laboratory rats in which spontaneous 
hypertension is virtually universal.^* The im- 
plications of routine availability of such a prep- 
aration for hypertension research are certainly 
of major importance. The possibility that the 
domestic pony population contains a large ani- 
mal model for hypertension research deserves 
considerable attention and investigation, partic- 
ularly because the hemodynamic performance 
of this small group was found to show such 
marked similarity to hemodynamic function in 
human labile hypertension. 
The relatively young age of these animals 
(2.2 years for the hypertensive group) suggests 
that perhaps this hypertension may bear some 
relationship to juvenile forms of human hyper- 
tensive disease. The occurrence of elevated car- 
diac output in juvenile patients who exhibit la- 
bile or intermittent hypertension has been 
noted. 1" Among the general population of per- 
sons with labile hypertensive disease, a high 
output state with an absence of change in pe- 
ripheral resistance has been observed. ^^"^^ Most 
recently, Frohlich et al.," in studies of labile 
hypertension, found increased heart rate, sys- 
tolic and diastolic pressures, cardiac indices, 
and rate of ejection of the left ventricle. These 
changes bear a strong resemblance to those ob- 
served in the group of hypertensive ponies (see 
Table IV). Further evidence for early-stage in- 
volvement with the disease is the fact that no 
ventricular hypertrophy was observed in the 
hypertensive group. This data describes only a 
small group of animals, and as such does not 
provide conclusive determination of the nature 
of hypertension in the domestic pony, nor does 
it provide an indication of the magnitude of oc- 
currence of the disease in the pony population. 
It does, however, suggest that further investi- 
gation is warranted, in order to exploit what 
may be a unique opportunity for the expansion 
of hypertension research. 
SUMMARY 
The size, maturity, behavior, trainability, and 
exercise capacity of the pony suggest that it can 
be an excellent laboratory animal for a broad 
range of physiological studies. Its resemblance 
to man in cardiovascular performance indicates 
in particular that it may be well suited for re- 
search in the mechanical assistance of the circu- 
lation. The presence of a hypertension syn- 
drome in a small group of ponies suggests that 
a new resource for hypertension research may 
become available, if sufficient effort is applied to 
systematic screening and evaluation of the pony 
population. Certainly the domestic pony can be 
a unique resource in cardiovascular research. 
ACKNOWLEDGMENTS 
Halothane (Fluothane) was generously sup- 
plied by Dr. Robert Nuttall, Ayerst Laborato- 
ries. The dye dilution cardiac output determina- 
tions were made possible by loan of a Waters 
densitometer; for this assistance the authors 
are grateful to the Department of Anesthesiol- 
ogy, Baylor College of Medicine. The senior au- 
thor is particularly appreciative of time allowed 
from other duties by Dr. Paul A. Klavano, 
Chairman, Department of Physiology and 
Pharmacology, College of Veterinary Medicine, 
Washington State University. 
REFERENCES 
1. McLaughlin, R. F., Tyler, W. S., and Can- 
ada, "R. O. a Study of the Sub-Gross Pulmonary 
Anatomy of Various Mammals Am. J. Anat. 
108:149, 1961. 
2. Weber, K. T., Dennison, B. H., Fuqua, J. M., 
Speaker, D. E., and Hastings, F. W. 
Hemodynamic Measurements in Unanesthetized 
Calves J. Surg. Res. 11:383, 1971. 
3. KuiDA, J. T., Brown, A. M., Lange, R. L., and 
Hecht, H. H. Cardiovascular Studies on Normal 
Calves Am. J. Physiol. 200:247, 1961. 
4. Garner, H. E., Rosborough, J. P., and Amend, J. 
F. Laboratory Housing for the Pony in an Urban 
Research Facility Lab. Animal Sci. 21:269, 1971. 
5. Rosborough, J. P., Garner, H. E., and Amend, J. 
F. A Laboratory Restraining Device For the Pony 
Am. J. Vet. Res. 32:1097, 1971. 
6. Liu, C. K. Formulas used in Cardiac Catheteriza- 
tion In Luisada, A., (ed.) Cardiology, New York, 
McGraw-Hill, 1959, pp. 339-346. 
