DRUGS ACTING ON THE CIRCULATION 31 



The heart's output is the resultant of the filling, the capacity, the rate 

 and the strength of the ventricles. Powerful stimulation of the vagus 

 may arrest the heart in diastole, or may impede the passage of impulses 

 through the auriculoventricular bundle (heart block), so that the ven- 

 tricles beat more slowly than the auricles. Stimulation of the vagus 

 may cause loss of tone so that there is more relaxation in diastole and 

 less force in systole. The coronary arteries are filled from the aorta in 

 both systole (first part) and diastole and prolonging diastole may or 

 may not increase the amount of blood entering them. In general, moder- 

 ate slowing of the heart increases its blood supply and nutrition. The 

 heart is influenced by drugs as follows : 



1. Stimulation of the inhibitory apparatus leads to slowing or 

 weakening of the heart-beats, or to both. 



2. Depression of the inhibitory apparatus results in quickening or 

 strengthening the heart-beats, or both. 



3. Stimulation of the accelerator apparatus causes an increase in 

 the rate or force of the heart-beats, or both. 



4. Depression of the accelerator apparatus induces decrease in the- 

 rate or force of the heart-beats, or both. 



5. Stimulation of the heart-muscle produces increase in the rate or 

 force of the heart-beats, or both. 



6. Depression of the heart-muscle lowers the rate or force of the 

 heartbeats, or both. 



The vagus centre is stimulated by agencies increasing blood-pressure, 

 or causing asphyxia. On the other hand, agencies reducing blood-pressure 

 depress the vagus, or stimulate the accelerator nerve, or both. Thus, 

 the nitrites, as amyl nitrite, nitro-glycerin and spirit of nitrous ether, 

 quicken the heart by lowering vascular tension. External stimuli to sen- 

 sory nerves reflexly stimulate the heart, as also do many locally irritating 

 agents taken internally ; e.g., strong alcoholic or ammoniacal preparations. 



Since drugs commonly influence more than one parti of the mechan- 

 ism controlling the heart, and since it is difficult to determine the exact 

 physiological details in such complex actions, we shall content ourselves 

 with tabulating the actions of drugs ordinarily employed for their influ- 

 ence on the heart, remembering that while moderate doses produce the 

 effects enumerated below, poisonous doses often give rise to diametrically 

 opposite actions. 



(a) Druffs increasing the force of the heart-heat. 



Slow the pulse 



Digitalis Squill Strophanthus 



Adrenalin . Physostigmine Sparteine 



Do not alter rate particularly 

 Barium salts Camphor 



(b) Drugs increasing the rate of heart-beats. 

 Belladonna Stramonium 

 Atropine Cocaine 

 Hyoscyamus 



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