Atrial Fibrillation. 



385 



It is the opinion of the writer that an adequate and satisfac- 

 tory explanation of the change to regular rhythm will include a 

 consideration not only of changes in the condition of circus con- 

 traction, but also of events in the immediate neighborhood of the 

 normal pacemaker. This becomes more evident when we remem- 

 ber that the change in rhythm takes place quite suddenly at a 

 time when the rate of flutter or fibrillation has been slowed to 

 approximately twice the normal sinus rate. 



Furthermore, the experimental evidence that we now have 

 confirms our view, suggested by the experiments of Lillie on con- 

 duction and our own records of the effect of quinidin on the human 

 heart, that quinidin not only increases the refractory interval but 

 also slows the conduction rate. In fact the conduction rate 

 seems, at least in some cases, 2 to be much more markedly affected 

 than does the refractory interval. This is quite at variance with 

 previous explanations of the effect of quinidin. 



It is more in accord with known facts to believe that quinidin 

 restores normal rhythm in the fibrillating or fluttering atrium, 

 not by prolonging the refractory period to such an extent that 

 circus movement is no longer possible, but by decreasing the con- 

 duction rate and so slowing up the circus movement to such an 

 extent that the rhythmic function of the atrial muscle can reassert 

 itself. A review of our knowledge of events prior to the sudden 

 change in mechanism will make this more evident. 



There is a fair amount of experimental evidence that in the 

 condition known as atrial fibrillation there is a continuous circuit 

 or circus movement taking place in the atrium at a rate somewhere 

 in the neighborhood of 500 per minute. It is clear that with such 

 a circus movement stimulating the atrium approximately 500 

 times a minute the sinu-atrial node, with its normal rate of 60- 

 100 per minute, has very little chance of obtaining control and 

 thus the condition of fibrillation is self-perpetuating. If we give 

 quinidin to a patient with atrial fibrillation we know that the rate 

 of this circus movement is slowed very appreciably. It is obvious 

 that if the circus movement could be made slower than the sinus rate 

 the sinus, if normal, would regain control and normal rhythm 

 would be established. A more careful consideration of the prob- 

 lem will reveal that it will not be necessary to slow the rate of the 



