No abstract

The circulatory and respiratory changes occurring during the first 5 min. of hypoxia were studied in unanawsthetized rabbits. Mild degrees of hypoxia produced an increase in ventilation in the rabbit, without eliciting a detectable circulatory response. The early circulatory effects consisted of bradycardia, a rise in mean arterial pressure and a fall in cardiac output, indicating predominant systemic vasoconstriction. The magnitude of the bradyeardia and rise in arterial pressure were related to the fall in arterial 02 saturation. Atropine or vagotomy reduced or abolished the bradycardia, but greatly accentuated the rise in blood pressure. Denervation of the carotid baroreceptors and chemoreceptors almost abolished the bradyeardia and diminished the rise in blood pressure.IN the first few minutes after induction of acute hypoxia most mammalian species develop tachycardia, a moderate rise in mean arterial pressure and an increase in ventilation. It is generally agreed that the role of the arterial chemoreceptors is of paramount importance in the early respiratory drive during oxygen lack [Bjurstedt, 1946;Otis, 1947 and1949]. Up till recently the tachycardia which develops in the early phase of hypoxia has also been considered to be of chemoreceptor origin. Recent observations in dogs on the effects of stimulation of the arterial chemoreceptors by perfusion with venous blood cast considerable doubt on this interpretation, since these procedures usually produce bradyeardia [Bernthal, Greene and Revzin, 1951; Neil, 1956;Scott, 1958 and.The unanaesthetized rabbit develops during the first few minutes of hypoxia a pronounced bradycardia rather than the usual tachyeardia and some rise in arterial pressure. The purpose of this paper is to report the results of an analysis of the mechanism of this early circulatory response to acute hypoxia. METHODSUnanaesthetized rabbits were used in these experiments, except when otherwise stated. The operative techniques were carried out under local anaesthesia and included tracheotomy, central ear artery cannulation and right atrial catheterization

By combining vacuum extraction in a Van Slyke chamber and separation of the extracted gases in a gas chromatograph, it is possible to determine N2 content of 1.5 ml of blood or other biological fluids in less than 10 min. The 95% confidence limits are 0.44% on either side of the mean of the triplicate analysis-or 2.4 mm Pn2 in arterial blood when breathing room air. Application of the method to the problem of arterial-alveolar N2 difference yielded the following data: 1) N2 solubility in whole blood at 37.3 C varied from 0.0125 to 0.0129; 2) N2 solubility in urine is inversely related to urine specific gravity, confirming Klocke and Rahn's data; 3) changes in arterial N2 content were reflected in arm superficial venous blood and urine N2 only after a considerable period of time, indicating that either of these will give an excellent indication of the mean Pn2 over a period of time; 4) there is no systematic difference between venous blood and urine Pn2; 5) the (a-A)N2 difference in nine normal subjects varied from 3.7 to 13.1 mm Hg. Note: (With the Technical Assistance of M. Passke) Submitted on July 24, 1962

Since the alveolar temperature influences the solubility of most inert gases in pulmonary capillary blood, knowledge of the solubility in arterial blood may be used to determine the equilibration temperature, i.e., alveolar temperature. Because the partial pressure of inert gas in arterial blood cannot be deduced from the alveolar pressure, direct determination of solubility is impractical. However, if a mixture of two inert gases is used, the ratio of partial pressures in the arterial blood is equal to that in the inspired gas and the ratio of gas contents will vary with the ratio of solubility. The blood solubility ratio He/A varies by 1.34% per degree centigrade. Using an O2-He-A inspired mixture, the following points were established in five resting subjects, fully clothed. 1) The pulmonary capillary temperature (Tpc) is linearly related to the rectal temperature (Tr), with a regression line equation: Tpc = 37.5 + 2.4 (Tr – 37.1). 2) When measurements were obtained on the same subject in different days, these measurements show that variations in Tpc are in the same direction as changes in Tr, but much more pronounced. Note: (With the Technical Assistance of M. Passke) Submitted on July 24, 1962