POSTURAL STABILITY IN THE DOG

Positional stability in quietly standing dogs has been evaluated in terms of force records from individual feet and the position of the center of gravity. All four dogs carried approximately 60% of their total weight on their anterior feet and roughly 50% of their weight on each side. Within-trial variations from the mean of the fraction of total weight borne on any one foot ranged from 0.3 to 4.5% with a mean of 1.4%; between-trial variability ranged from 1.1 to 2.7%. Continuous movement of the center of gravity occurred within an area measuring approximately 1.5 cm on a side. Shifts in weight distribution within and between trials occurred in a variety of patterns which could be described in terms of the behavior of pairs of transverse, longitudinal, and diagonal couples. Preliminary observations of the reactions to a sudden disturbance confirm the hypothesis that quiet stance is accomplished through central nervous regulation. It is concluded that the trained dog behaves with a sufficient level of predictability to permit its use as a subject in a further examination of the postural control system.

BARORECEPTOR INFLUENCE ON POSTURAL CHANGES IN BLOOD PRESSURE AND CAROTID BLOOD FLOW

The effects of 30° head-down and head-up tilting on mean systemic blood pressure, carotid blood flow, and heart rate were studied in 16 dogs under morphine and Nembutal anesthesia. The tilting procedure was further repeated after denervation of the carotid sinus and aortic arch baroreceptors and after administration of a dihydrogenated ergot alkaloid mixture (Hydergine). The results indicate that the drop in pressure in the head-down position is primarily due to baroreceptor activity and that the baroreceptors are necessary for compensatory vasoconstriction on head-up tilting. Carotid blood flow decreased in both tilted positions in the control animals; the possible relationship to cerebral blood flow is discussed.

EFFECT OF NEGATIVE-PRESSURE BREATHING ON LUNG MECHANICS AND VENOUS ADMIXTURE

Anesthetized dogs in the supine position show a spontaneous decrease in total respiratory compliance and an increase in venous admixture to the pulmonary circulation. Both these changes can be increased by negative-pressure breathing and reversed by positive-pressure breathing. If the changes in total respiratory compliance are due only to changes in lung compliance and these in turn result directly from the closure of alveoli, the relationship between compliance and inspiratory and expiratory pressure allows one to determine the scatter of opening and closing pressures in the alveoli. The venous admixture measures blood flow through collapsed alveoli, and its relationship to the negative pressure applied indicates the perfusion of the alveoli collapsing with each increment in negative pressure. By studying simultaneously changes in compliance and venous admixture, and using two basic assumptions, the dog's lungs can be described as a system composed of some elements receiving nearly 50% of the ventilation and 20% of the perfusion, relatively unstable mechanically, and having a very high Va/ Q ratio, while the remaining air spaces receive the same ventilation, but 80% of the perfusion.

lung compliance; atelectasis; ventilation-perfusion ratio

Submitted on October 28, 1963