INFLUENCE OF RESPIRATION ON THE BLOOD-PRESSURE 293 



fall of pressure the increased intrathoracic pressure due to the inflation 

 is an important factor. When the respiration is stopped in collapse, 

 instead of a fall a steady rise of pressure occurs (as in Fig. 84, p. 188). 

 This ultimately merges in the elevation due to asphyxia, which shows 

 itself sooner than in inflation. When the tracheal cannula is closed in 

 natural respiration, no initial fall of pressure takes place (Fig. 130). 



Besides the mechanical effects of the respiratory movements on 

 the circulation, it may be influenced by changes in the cardio- 

 inhibitory and vaso-motor centres synchronous with the rhythm of 

 the respiratory centre. In many animals (the dog, for instance) 

 and in man, it can be very easily made out that the rate of the heart 

 is greater during inspiration, especially towards its end, than in 

 expiration. The phenomenon is especially distinct in deep and 

 slow respiration. It is caused by a rhythmical rise and fall in the 

 activity of the cardio-inhibitory centre, synchronous with the 

 respiratory movements, for the difference disappears after division 

 of both vagi. The normal respiratory oscillations of blood-pressure 

 are not due in any great degree to such changes in the rate of the 

 heart, for they persist after section of the vagi, and they are seen in 

 animals like the rabbit, in which in ordinary breathing little or no 

 variation in the rate of the heart is connected with the phases of 

 respiration. The most probable explanation of the respiratory 



Fig. 130. Blood-Pressure Tracing: Rabbit, under Chloral. Natural respiration 

 stopped at I in inspiration, at E in expiration. The mean blood-pressure is 

 scarcely altered; but the respiratory waves become much larger owing to the 

 abortive efforts at breathing. Time tracing shows seconds. 



variations in the pulse-rate is that the respiratory centre, when it is 

 discharging itself in inspiration, sends out impulses as a sort of over- 

 flow along fibres connecting it with the cardio-inhibitory centre. 

 These increase the tone of that centre, but, owing to the long latent 

 period of the cardio-inhibitory apparatus, the inhibition does not 

 reveal itself till the succeeding expiration. It may be, however, that 

 the impulses discharged from the respiratory centre in inspiration 

 diminish the tone of the cardio-inhibitory centre, and thus lead to 

 acceleration of the heart towards the end of the inspiratory phase. 

 In certain pathological conditions the influence of the respiration on 

 the pulse-rate is exaggerated (so-called ' respiratory arhythmia '). 



Traube-Hering Curves. Rhythmical changes in the activity of 

 the vaso-motor centre, also associated with periodic discharges from 



