PH4.10 | PH4.10 | Antiarrhythmic Drugs — Summary & Reflection

KEY TAKEAWAYS

Antiarrhythmic drugs are classified by the Vaughan-Williams system into four classes: Class I Na-channel blockers (Ia/Ib/Ic, with flecainide/Ic absolutely contraindicated post-MI — CAST lesson); Class II beta-blockers (first-line rate control in AF and SVT; post-MI mortality reduction); Class III K-channel blockers (amiodarone — pan-channel, first-line for VF cardiac arrest and most serious arrhythmias, significant organ toxicities; sotalol — QT monitoring mandatory); Class IV non-DHP CCBs (verapamil, diltiazem — SVT and AF rate control; contraindicated in HFrEF, WPW + AF, and with beta-blockers). Adenosine (A1 receptor → AV block → SVT termination; very short half-life — rapid IV bolus required) is first-line acute for SVT. The WPW + AF contraindication — never give verapamil, diltiazem, digoxin, or adenosine in WPW + AF (accelerates accessory pathway → VF) — is one of the most important clinical safety lessons in arrhythmia pharmacology. QT prolongation (Class Ia and III) with hypokalaemia or drug combinations risks torsades de pointes — managed with IV MgSO4, K correction, and stopping the offending drug.

REFLECT

Return to the opening case: the 34-year-old woman with AVNRT and a WPW history on her old ECG.

1. Your colleague offered verapamil — explain precisely why this is dangerous in this patient, citing the electrophysiological mechanism.
2. Adenosine is used instead and terminates the SVT. What is its mechanism — and how does it achieve AV nodal block in seconds while its effect lasts only 10–30 seconds?
3. She converts to sinus rhythm, and you notice that at rest the ECG now clearly shows delta waves and a short PR interval (confirming WPW). She asks: 'Is there any long-term drug I can take to prevent this happening again?' What do you advise?
4. Later, her ECG during another tachycardia episode shows irregular rapid AF with some wide QRS complexes suggesting pre-excited beats — this is WPW + AF. Which drugs can you safely use?
5. If the patient becomes haemodynamically unstable during WPW + AF, what is the definitive treatment?

Think through each decision using the ion-channel and accessory-pathway mechanisms you have studied in this SDL.