Abstract
The problem of the causation of the respiratory variations in blood pressure is an old one, but one that has resisted solution surprisingly long. Examination of the literature on the subject shows that diametrically opposite observations have been made upon the relation between the phases of respiration and the rise and fall of blood pressure. About half of the workers have found an inspiratory rise, and half an inspiratory fall.
In examining the factors heretofore mentioned in explaining the phenomenon it is apparent that the possibility that factors tending to change blood pressure might not be immediately effective, has been neglected. In other words, the possibility that the effects of inspiration might show up in expiration, has been neglected.
In experiments in which the inferior vena cava, the superior vena cava and the azygos vein were clamped separately and in combination, it was found that there was absolutely no sign of these changes on the arterial pressure trace for a period, on the average, of three heart beats. Very evidently there is a latent period of three heart beats in this process.
The changes in arterial pressure were similarly recorded when variations were artificially produced in the thoracic cavity. Similar latent periods were observed.
These experiments were repeated upon animals with denervated hearts and the same observations made, exc'luding a nervous factor.
It is evident that the effects of inspiration may show up entirely in expiration and the effects of expiration in the succeeding inspiration. The heart-respiration ratio determines in what phase of respiration the various changes will appear.
Many published arterial pressure tracings have been examined and it has been found that this explanation is quite valid in interpreting them.
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