Selective Control of Sinoatrial and AV Nodes

In some forms of arrhythmia, certain drugs can be used that are capable of selectively facilitating and inhibiting (green and red arrows, respectively) the pacemaker function of sinoatrial or atrioventricular cells. 

Sinus bradycardia. An abnormally low sinoatrial impulse rate ( 60/min) can be raised by parasympatholytics. The quaternary ipratropium is preferable to atropine, because it lacks CNS penetrability (p.108). Sympathomimetics also exert a positive chronotropic action they have the disadvantage of increasing myocardial excitability (and automaticity) and, thus, promoting ectopic impulse generation (tendency to extra-systolic beats). In cardiac arrest, epinephrine, given by intrabronchial instillation or intracardiac injection, can be used to reinitiate heart beat. 

Sinus tachycardia (resting rate 100 beats/ min). p-Blockers eliminate sympatho-excita-tion and lower cardiac rate. Sotalol is noteworthy because of its good antiarrhythmic action (caution QT-prolongation) 

Atrial flutter or fibrillation. An excessive ventricular rate can be decreased by verapamil (p.126) or cardiac glycosides (p.134). These drugs inhibit impulse propagation through the AV node, so that fewer impulses reach the ventricles. 

Nonspecific Drug Actions on Impulse Generation and Propagation 

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