614 



RESPIRATORY UNDULATIONS. [BOOK n. 



respiration. Similar undulations may be observed in the pulse 

 tracings taken from man. 



J 



FIG. 85. COMPARISON OF BLOOD-PRESSURE CURVE WITH CURVE OP 

 INTRA-THORACIC PRESSURE. (Dog.) 



a is the blood-pressure curve taken by means of a mercury manometer ; it shews 

 the respiratory undulations, the slower beats on the descent being very marked. 

 b is the curve of intra-thoracic pressure obtained by connecting one limb of a 

 manometer with the pleural-cavity. Inspiration begins at i, expiration at e. 

 With the beginning of inspiration (i) the expansion of the chest causes a marked 

 fall of the mercury in the intra-thoracic manometer ; but the effect soon diminishes, 

 since the lessening of intra-thoracic pressure does not bear on the manometer alone 

 but on the lungs also ; and as the lungs expand more and more the fall in the 

 mercury becomes less and less until towards the end of inspiration the curve 

 becomes very nearly a straight line. Conversely, the return of the chest at the 

 beginning of expiration (e) produces at first a marked rise of the mercury in the 

 manometer ; but this soon ceases as the air leaves the chest and the lungs shrink, 

 whereupon the mercury falls slowly. 



When these undulations of the blood-pressure curve are 

 compared carefully with the respiratory movements or with the 

 variations of intra-thoracic pressure, what is most commonly 

 observed is that while the blood-pressure, on the whole, rises 

 during inspiration and falls during expiration neither the rise nor 

 the fall is exactly synchronous with either inspiration or expiration. 

 Fig. 85 shews two tracings from a dog taken at the same time, 

 one, a, being the ordinary blood-pressure curve from the carotid, 

 and the other, 6, representing the condition of the intra-thoracic 

 pressure as obtained by carefully bringing a manometer into 

 connection with the pleural cavity. On comparing the two 

 curves it is evident that neither the rise nor the fall of arterial 

 pressure coincides exactly either with inspiration or with ex- 

 piration. At the beginning of inspiration (i) the arterial pressure 

 is seen to be falling ; it soon however begins to rise, but does not 

 reach the maximum until some time after expiration (e) has 

 begun ; the fall continues during the remainder of expiration, and 

 passes on into the succeeding inspiration. This suggests the idea 

 that, while inspiration tends to increase and expiration to diminish 



