# ECG Interpretation: Arrhythmias

> Arrhythmia ECG interpretation for Cardiac Technology: SVT, atrial fibrillation, flutter, VT versus SVT, heart block degrees and emergency drug doses.

- Canonical URL: https://prepelephant.com/topics/allied/cardiac-technology/ecg-interpretation-arrhythmias-ct
- Exam / course: Allied Health · Subject: Cardiac Technology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "ECG Interpretation: Arrhythmias", PrepElephant, https://prepelephant.com/topics/allied/cardiac-technology/ecg-interpretation-arrhythmias-ct

## 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.