Class I antiarrhythmics
Class I antiarrhythmics
Class I antiarrhythmics are called membrane stabilising drugs. These agents interfere with the sodium channel and are grouped into Class Ia or Class Ib, according to their effect on the sodium channel and cardiac action potentials.
Class Ia agents block the fast sodium channel. This depresses the phase 0 depolarization (reduces Vmax), which prolongs the action potential duration by slowing conduction in the SA and AV nodes. Agents in this class also cause a delay in the refractory period of atria and ventricles. Quinidine, disopyramide and procainamide are registered drugs for this group. Indications for quinidine are (supra)ventricular tachycardia and prophylaxis of atrial fibrillation and flutter. Disopyramide is drug of choice for prophylaxis of vagal atrial fibrillation. The drug has a parasympathetic effect, meaning that it does not influence the sinus rhythm or AV node conduction.
Class Ib antiarrhythmic agents are sodium channel blockers. They have fast onset and offset kinetics, meaning that they have little or no effect at slower heart rates, and more effect at faster heart rates. Class Ib agents reduce the duration of both the action potential and the refractory period. Conduction of the action potentials becomes quicker, which helps to stop re-entries. Class Ib agents are indicated for the treatment of ventricular tachycardia and symptomatic premature ventricular beats, as well as for prevention of ventricular fibrillation. Lidocaine has a direct action on the bundle of His and Purkinje fibers. Phenytoin is used to treat digoxin-induced ventricular arrhythmias.
Class Ic antiarrhythmics depress the depolarization, thereby inhibiting the conduction. They hardly affect the duration of the action potential. They increase the threshold for depolarisation and prolong the refractory period. Class Ic agents have the most potent sodium channel-blocking effects. Class Ic agents are indicated for life-threatening ventricular tachycardia or ventricular fibrillation, and for the treatment of atrial fibrillation. Examples from this group are flecainide and propafenone.
Mr B (72) is a patient with a new onset of supraventricular tachycardia secondary to WPW syndrome. Currently his BP is 86/68 mmHg, HR 220 bpm. What is the treatment of choice for this patient?
Extra info: In addition to the patient being 72, he is hemodynamically unstable. Surgical ablation is considered the safest of all options and thus the treatment of choice. Quinidine and procainamide are good drugs to convert supraventricular tachycardias, but have many adverse effects in elderly people. Lidocaine would unlikely to be helpful, since it works more for ectopic beats.
Which of the following does not belong to the properties of the class IA antiarrhythmics?
Electrophysiologic properties possessed by lidocaine include:
Individual antiarrhythmic agents can alter the surface electrocardiogram. Characteristics of quinidine include:
Of the following antiarrhythmic agents, the one which most closely resembles procainamide with respect to electrophysiologic effects and mechanism of action is:
24 hours after an acute MI, a 46-year-old male is being treated with a continuous intravenous drip of an antiarrhythmic drug to suppress frequent multifocal premature ventricular contractions. He develops generalized seizure activity. The seizure activity can be most readily explained by:
After cardioversion, Mr Q's ECG still shows many premature ventricular contractions. What drug can be given intravenously to control his heart rate?
Extra info: Quinidine and disopyramide are not likely to work in this patient because their action is primarily for supraventricular arrhythmias. Flecainide could be helpful, but is not available for intravenous administration.