FURTHER STUDIES ON A-V CONDUCTIVITY

¹ From the Cardiovascular Department, Michael Reese Hospital, Chicago, Ill.

1. The effect of various procedures on A-V conductivity was studied by noting their influence on the ventricular rate in faradically maintained auricular fibrillation. Innervated and denervated heart preparations were employed to differentiate between direct and indirect influences.

2. Asphyxia at first produced enhancement and later, as it became more advanced, depression of A-V conductivity. These effects are shown to be due primarily to a direct action on the heart. Nerve influences tend to moderate them. Both anoxemia and hypercapnia, which are the two components of asphyxia, share in both these effects. Evidence is given to show that the enhancing action of asphyxia on A-V conduction is not propagated by means of epinephrine.

3. Experiences with hypocapnia, hypercapnia, and the results of intravenous injection of NH₄Cl and NaHCO₃ suggest that slight deviations of the blood pH to either side of the control level in these experiments enhance A-V conduction, and that more extreme deviations of acidity and alkalinity both lead to depression of A-V conductivity.

4. Paredrine hydrobromide, through a direct action on the heart, enhances A-V conduction, its effect in the innervated heart being modified by reflex activity.

5. Barium chloride enhances A-V conduction through a direct effect upon the heart. Minor changes which occur occasionally in the innervated heart depend upon the opposite reflex vagal action of barium.

6. Atropine sulfate enhances A-V conduction in the innervated heart through its vagus action but depresses A-V conduction in the denervated heart through a direct action on the conducting tissues.

7. Quinidine sulfate, in the denervated heart, depresses A-V conduction through a direct action. In the innervated heart, this is occasionally masked by a reflex action which may actually lead to a resulting slight enhancement of A-V conduction.

8. The indirect actions of paredrine hydrobromide, barium chloride and quinidine sulfate appear to be the result of a reflex accompanying the blood pressure changes they produce. The indirect effects of these drugs may require abolition in some way when the therapeutic direct effects of the drug is sought.