Mechanisms of arrhythmias
- Ectopic focus – irregular
- Re-entry circuits – regular rate
- VT – is it monomorphic or polymorphic? Polymorphic are more likely to degenerate to ventricular fibrillation.
- AVNRT – seen in young women on exertion – not dangerous.
- AF
- Atrial flutter
Anti-arrhythmic drugs are classified using the Vaughan-Williams system.
- Class 1A: Quinidine. Procainamide. Disopyramide. TCAs.
- Class 1B: Lignocaine. Phenytoin.
- Class 1C: Flecainide.
- Class 2: Beta-blockers (already covered, but revise).
- Class 3: Amiodarone, Sotalol
- Class 4: Calcium channel blockers (Verapamil, amlodipine, diltiazem, nifedipine).
- Others: Adenosine. Digoxin & Digibind. Magnesium.
Cardiac antiarrhythmics are all proarrhythmic and can reduce LV function
- Class 1 block fast Na channels – slow down the fast upstroke (phase 0), prolongs AP, prolong QT and widen QRS
- Class 2 – beta blockers – decrease SA node firing and slow AV node conduction prolonging PR
- Class 3 – K block – prolong phase 3, prolong action potential, prolong ventricular depolarisation and repolarisation, prolong QT and PR
- Class 4 – Ca channel block – inhibit L type Ca channel so inhibit AP generation(slows sinus rate), slows cardiac conduction esp in AV node, prolongs repolarisation, causes decreased cardiac contractility and vasodilates. In arrhythmia the AV node block helps in SVT esp AVNRT. Decreases Sinus rate helps stop abherant pacemakers.
- Verapamil and diltiazem are more cardiac selective and better for angina and arrhythmias – dec HR & contractility
- Dihydropiridines eg amlodipine and nifedipine are more peripheral and better for HTN – vasodilation (art>vein)
In fast rate class I is best (eg flecainide) but have to be sure the is no structural abnormality in the heart.
In slow rate class III is preferable.
Side effects
| 1a | Proarhytmic (tdP), hypotension, decrease LV function |
|---|---|
| 1b | Proarhytmic (shortens QT –> VT), lignocaine which slows conduction in ischaemic fibres so there are prone to re entrant tachy |
| 1c | Proarhytmic (tdP), decrease LV function |
| 2 | decrease LV function |
| 3 | Proarhytmic (tdP) |
| 4 | Proarhytmic (tdP) |
Digoxin
- Increases AV block
- Negative chronotrope, positive inotrope
- Digoxin toxicity
- Acute – nausea, vomiting
- Chronic – fatigue, visual disturbance
- Arrhythmias, VT, VF, SA node block, AV block
- Diarrhoea, abdo pain, headache, dizziness, confusion, delerium
Viva questions:
- Tell me about the Vaughan-Williams classification system
- Tell me about class 1 agents (or 2, 3 etc.)
- Tell me about Amiodarone
- SE
- Corneal deposits
- Thyroid dysfunction – hypo and hyper
- Interstitial lung disease/fibrosis
- Hepatic dysfunction
- Photosensitivity
- Peripheral neuropathies
- SE
Tell me about Lignocaine
Tell me about digoxin overdose
Tell me about Magnesium
Tell me about Sotolol
Tell me about Adenosine
Tell me about Flecanide
Tell me about verapamil