THE EFFECTS OF CHANGES IN INTRAPULMONARY AIR-PRESSURE ON THE PULMONARY AND AORTIC CIRCULATION OF THE DOG

¹ Department of Physiology, University of Edinburgh

1. The results in the. dog on the rate and character of respiration and on the blood-pressure, in the pulmonary and systemic circulation of increased or decreased intrapulmonary pressure, i. e. of enlargement or diminution of the pulmonary alveoli, have been reinvestigated.

2. The effects on respiration were determined by Hering and Breuer in 1868. Our results, like those of most subsequent observers, are in agreement with theirs, and show that the effect of increase of intrapulmonary pressure is. to cause inhibition of respiration, while decrease of intrapulmonary pressure causes the respirations to be increased in rate and generally also in extent.

3. These results on respiration are not appreciably affected by denervation of the carotid sinuses, but appear to be abolished, or at any rate to be greatly diminished, by double vagotomy in the neck. Abolition of the Hering-Breuer effect as the result of vagotomy is, however, not quite complete, for even after double vagotomy a perceptible effect is still produced on the rate and extent of respiration as a result of increased or decreased intrapulmonary pressure.

4. The effect of increasing intrapulmonary air-pressure is to produce a marked rise of pulmonary blood-pressure lasting as long as the period of increased air-pressure, usually increasing gradually during that period, and rapidly declining to normal on readmitting air at atmospheric pressure.

5. The effect on systemic blood-pressure is at first to produce a fall; this is generally followed by a rise. The rise may be considerable, and is usually increased temporarily, often strikingly so, on readmission of air at atmospheric pressure, although soon coming back to the original level. The rise is abolished by section of the vagi.

6. The effect of decreasing intrapulmonary air-pressure is to cause a rapid fall in pulmonary blood-pressure to a level considerably below normal. The fall lasts during the period of decreased intrapulmonary pressure, and rapidly rises to normal on readmitting air at atmospheric pressure.

7. The effect on the systemic blood-pressure is less striking and is variable, since it depends on the flow of blood through the pulmonary capillaries. The pressure usually rises during the period of reduced intrapulmonary pressure.

8. The respiratory oscillations of blood-pressure, especially the pulmonary, are increased during the diminution of intrapulmonary air-pressure. (These oscillations are, of course, absent in both pulmonary and systemic blood-pressure tracings during the inhibition caused by increase of intrapulmonary air-pressure.)

9. Denervation of the carotid sinuses does not appreciably influence these results.

10. Section of both vagi, although altering the appearance of the tracings of blood-pressure owing to the effect of that operation on respiration, does not affect the general result of increasing or decreasing intrapulmonary air-pressure on either the pulmonary or systemic circulation. As before, the pulmonary blood-pressure tends to rise, and the systemic blood-pressure to fall, when the intrapulmonary pressure is increased; when it is decreased the opposite effects are seen.

11. We conclude that the changes in pulmonary blood-pressure are produced mechanically, and are the direct effects of the alterations in intrapulmonary pressure on the flow of blood in the pulmonary capillaries. Increase of intrapulmonary air-pressure tends to obstruct or diminish, decrease of intrapulmonary air-pressure to facilitate, the passage of blood from the right to the left side of the heart. This will-indirectly affect the systemic blood-pressure, which must follow, at a certain but variable interval of time, the changes in the pulmonary system. But the pressure in the systemic circulation is liable to be influenced by the nerves of the carotid sinus and by the depressor fibres of the nerve of Ludwig and Cyon, which, in the dog, runs in the vagus. The effects on the systemic pressure are therefore not purely mechanical and are liable to variation.

12. An appearance of duplication, perhaps caused by a lack of synchronisation between diaphragmatic and costal respiration, is occasionally manifested during a period of changed intrapulmonary air-pressure, especially after section of the vagi.