Drugs for Arrhythmia and Cardiac Physiology

Vocabulary-style practice flashcards covering cardiac conduction, action potentials, ECG intervals, arrhythmia types, and the Vaughan-Williams classification of anti-dysrhythmic drugs.

Sinoatrial (SA) node

The key structure in the right atrium responsible for generating a regular heartbeat, often called the pacemaker of the heart because its self-excitation generates the action potential that initiates contraction.

Effective refractory period (ERP)

The duration of time during which a cardiac cell is unable to respond to a new excitation or initiate a new action potential.

Phase 4 (Non-pacemaker cells)

The resting phase where the resting potential is maintained at $-90\,mV$ by a constant outward leak of $K^+$ through rectifier channels while $Na^+$ and $Ca^{2+}$ channels remain closed.

Phase 0 (Depolarization in cardiomyocytes)

Phase where fast $Na^+$ channels open, allowing $Na^+$ to leak into the cell and rapidly depolarize the Transmembrane Potential (TMP) to $0\,mV$ or slightly above.

Phase 2 (Plateau phase)

A period where L-type $Ca^{2+}$ channels are open, creating a constant inward current of $Ca^{2+}$ that balances $K^+$ leak to maintain a potential just below $0\,mV$.

Funny current ($I_f$)

A spontaneous phase 4 depolarization found in pacemaker cells (SA and AV nodes) involving $Na^+$ entry through slow HCN channels and $Ca^{2+}$ through T-type channels.

P wave

The portion of the electrocardiogram (ECG) that represents atrial depolarization.

QRS complex

The portion of the ECG representing ventricular depolarization, occurring simultaneously with atrial repolarization.

PR interval

The time between the onset of the P wave and the onset of the QRS complex; an increase in this interval indicates a delay in AV conduction.

QT interval

The time measured from the onset of the QRS complex to the completion of the T wave; drugs that delay ventricular repolarization will increase this interval.

Reentry

The most common cause of arrhythmias, occurring when a unidirectional block leads to an abnormal conduction pathway that causes re-excitation of cardiac muscle.

Atrial fibrillation

An arrhythmia with an atrial rate of approximately $350\,beats/min$, characterized by multiple atrial ectopic foci and an irregular pulse.

Ventricular tachycardia (VT)

Arrhythmias originating from the ventricular myocardium or His-Purkinje system with a heart rate faster than $100$ (or $120$) $beats/min$.

Ventricular fibrillation (VF)

A life-threatening medical emergency where the ventricular rate is usually greater than $300\,beats/min$, preventing the heart from pumping blood and leading to cardiac arrest.

Class IA antiarrhythmic drugs

Sodium channel blockers like Quinidine, Procainamide, and Disopyramide that slow Phase 0 depolarization and Phase 3 repolarization, leading to an increased ERP and QT prolongation.

Cinchonism

A set of adverse effects associated with large doses of quinidine, including blurred vision, tinnitus, headache, disorientation, and psychosis.

Class IB antiarrhythmic drugs

Weak sodium channel blockers such as Lidocaine and Mexiletine that rapidly associate and dissociate from channels and shorten phase 3 repolarization.

Class IC antiarrhythmic drugs

Strong sodium channel blockers like Flecainide and Propafenone that slowly dissociate from channels, markedly slow conduction, and have minimal effect on the QT interval.

Class II antiarrhythmic drugs

Beta-blockers (e.g., Metoprolol, Esmolol) that treat arrhythmias by decreasing SA node automaticity, AV conduction, and myocardial contractility.

Class III antiarrhythmic drugs

Potassium channel blockers (e.g., Amiodarone, Sotalol, Dofetilide) that prolong the duration of the action potential and the effective refractory period by diminishing outward $K^+$ current during repolarization.

Amiodarone

A Class III drug with a long half-life ($25-110\,days$) that blocks $K^+$, $Na^+$, and $Ca^{2+}$ channels; it is structurally similar to thyroxine and can cause pulmonary fibrosis or thyroid dysfunction.

Class IV antiarrhythmic drugs

Non-dihydropyridine calcium channel blockers (Verapamil, Diltiazem) that decrease the inward $Ca^{2+}$ current, slowing spontaneous phase 4 depolarization and conduction in the AV and SA nodes.

Adenosine

An endogenous metabolite with an extremely short duration of action ($10-15\,seconds$) used intravenously to convert acute supraventricular tachycardias by depressing SA and AV node activity.