ECG Interpretation: Arrhythmias
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Direct answer
A rhythm strip asks three questions in fixed order — is it fast or slow, is the QRS narrow (under 120 ms, supraventricular) or wide, and is it regular or irregularly irregular — and the answers sort almost every arrhythmia into its box. Regular narrow-complex tachycardia at 150-250 per minute is usually AV nodal re-entry tachycardia (AVNRT), the commonest supraventricular tachycardia, broken in seconds by intravenous adenosine 6 mg then 12 mg; an irregular narrow tachycardia with no visible P waves is atrial fibrillation; sawtooth flutter waves at about 300 per minute conducting 2:1 give a ventricular rate near 150. Any wide-complex tachycardia in an adult is ventricular tachycardia until proven otherwise, and bradyarrhythmias are graded by the Mobitz system, with Mobitz II and complete heart block demanding pacemaker support.
What you must remember
- AVNRT signature: regular narrow complex, rate 150-250, P waves buried in the QRS — look for the pseudo-r′ in V1 or pseudo-S in II, a favourite exam image; AVRT (accessory pathway) shows a retrograde P stying just after the QRS.
- Adenosine protocol: 6 mg rapid intravenous push with flush, then 12 mg; warn the patient of chest heaviness and flushing; contraindicated in asthma and second- or third-degree heart block without a pacemaker.
- Atrial fibrillation: absolutely irregular R-R intervals, no P waves, fibrillatory baseline; new-onset with instability calls for synchronised cardioversion, otherwise rate control.
- Atrial flutter: atrial rate near 300, classically 2:1 conduction for a ventricular rate of 150 — a regular narrow tachycardia at exactly 150 should make you hunt for sawtooth waves in II, III, aVF.
- Wide-complex tachycardia rules: favour VT with AV dissociation, fusion or capture beats, concordance across the precordium, extreme left-axis deviation, or a history of myocardial infarction; treat as VT when unsure.
- Torsades de pointes: polymorphic VT with twisting axis on long QT — intravenous magnesium sulphate 2 g is the drug, and QT-prolonging drugs are stopped.
- Heart block ladder: Mobitz I (Wenckebach) shows progressive PR stretching then a dropped QRS, usually benign and nodal; Mobitz II drops QRS complexes without warning and is infra-nodal; third-degree block shows complete dissociation with a junctional escape of 40-60 (narrow) or ventricular escape of 20-40 (wide).
A worked example: the regular tachycardia at 180
A 34-year-old with a regular narrow-complex tachycardia at 185 and a blood pressure of 110/70. Record a 12-lead during the episode and search for P waves: a pseudo-r′ in V1 or a pseudo-S in the inferior leads clinches AVNRT; a retrograde P in the ST segment suggests orthodromic AVRT. Vagal manoeuvres come next — a Valsalva, or carotid massage after auscultating for bruits — terminating many AVNRTs at no cost. If the rhythm persists, adenosine is pushed fast through a proximal cannula with a running flush while the technologist prints continuously — the moment of termination shows the mechanism. If the strip suddenly widens, the clock stops: adenosine is harmless in VT and diagnostic, but a pre-excited tachycardia (AF with a delta wave conducting down an accessory pathway) must never receive verapamil, diltiazem or digoxin — blocking the AV node pushes conduction down the pathway and can degenerate into ventricular fibrillation.
Where the exam sets its traps
Question banks recycle four confusions. The rate-150 regular tachycardia called "SVT" is often flutter with 2:1 block, so scan for flutter waves before reaching for adenosine. A wide-complex tachycardia in a 60-year-old with prior infarction is VT even when the patient looks comfortable — over 80 per cent of wide-complex tachycardias in such patients are VT, and treating it as SVT with verapamil is the classic error. Mobitz I versus II is tested by the behaviour of the PR interval before the dropped beat: stretching means Wenckebach (watchful waiting), identical PR with a sudden drop means Mobitz II (pacing referral). Finally, the irregularly irregular strip in a treated patient may be atrial premature beats or atrial tachycardia with varying block, classically from digoxin toxicity.
Frequently asked questions
What ECG feature distinguishes AVNRT from orthodromic AVRT?
AVNRT buries the retrograde P wave inside the QRS (pseudo-r′ in V1, pseudo-S in II), whereas AVRT places it in the early ST segment because the impulse must travel over the accessory pathway.
Which drugs are forbidden in pre-excited atrial fibrillation?
Verapamil, diltiazem and digoxin — all block the AV node, forcing conduction down the accessory pathway at rates that can precipitate ventricular fibrillation. Synchronised direct-current cardioversion is the safe option in the unstable patient.
How do you tell Mobitz I from Mobitz II second-degree block?
Mobitz I shows progressive PR prolongation before the dropped QRS with a shortened PR after the pause; Mobitz II shows constant PR intervals with abrupt drops. Mobitz II is infra-nodal, carries a risk of progression to complete block, and indicates pacemaker therapy.
Why is a ventricular rate of exactly 150 suspicious?
Because atrial flutter typically runs near 300 per minute with 2:1 conduction. A regular narrow tachycardia at 150 warrants a deliberate search for sawtooth flutter waves, especially in the inferior leads, before labelling it SVT.
What is the escape rhythm in complete heart block?
A junctional escape at 40-60 per minute is narrow and reasonably stable; a ventricular escape at 20-40 is wide, unreliable and syncopal (Stokes-Adams attacks). Either way a temporary pacemaker discussion begins immediately.