← Full ECG interpretation algorithm
Reasoning GP · Algorithm · 1-page summary

ECG interpretation and when to escalate

For: adults (16 and over) with a 12-lead ECG in primary care. Not for: children: adult thresholds do not apply, so seek paediatric advice for any abnormal paediatric ECG.
Reviewed October 2026 · next review April 2027
NICE CG109CG95NG196Resus Council UKDVLA
Core rule: treat the patient, not the trace. If they are unwell, ABCDE and 999 first, and never delay transfer to record or send an ECG. A normal ECG does not exclude ACS, PE, paroxysmal arrhythmia or severe hyperkalaemia.

0Stop — call 999 now — shock, syncope, ischaemia, heart failure

  • Ongoing possible cardiac chest pain, whatever the ECG; ST elevation or presumed new LBBB with ACS symptoms → local STEMI pathway 999
  • Broad-complex tachycardia (QRS ≥120 ms): treat as VT; irregular, broad and fast = possible pre-excited AF → 999
  • Any arrhythmia with adverse features; complete heart block, Mobitz II or pauses over 3 s with symptoms; QTc ≥500 ms with syncope or presyncope → 999
  • While waiting: no AV-nodal blockers (adenosine, verapamil, diltiazem, beta-blocker, digoxin) in an irregular broad-complex tachycardia; send the ECG with the crew.

1Triage — same day, not 999 if stable

FindingAction
Pain <12 h,
now pain-free		ECG abnormal or none → emergency referral; normal, or pain 12–72 h ago → Same-day hospital (CG95)
Hyperkalaemia
changes		Peaked T, long PR, lost P, broad QRS, sine wave, at any K⁺ → Emergency
CHB / Mobitz II,
no symptoms		Risk of asystole → Emergency assessment today
New AFFast, poorly tolerated or heart failure → Same-day; reliably <48 h, well → same-day discussion
QTc ≥500 ms,
no symptoms		Stop the cause, check K⁺, Mg²⁺, Ca²⁺ → Same-day advice
Pre-excitationSymptomatic → Same-day; well → Urgent cardiology

2Screen — TLoC + any of these → within 24 h (CG109)

  • Conduction: complete RBBB or LBBB, any degree of heart block; inappropriate persistent bradycardia; paced rhythm
  • QT: long QT (QTc >450 ms) or short QT (QTc <350 ms)
  • ST/T: any ST-segment or T-wave abnormality, abnormal T-wave inversion, pathological Q waves
  • Other: any ventricular arrhythmia (including ectopics), Brugada pattern, ventricular pre-excitation, LVH or RVH, sustained atrial arrhythmia
  • Also 24 h: heart failure, exertional TLoC, FH sudden death <40, new breathlessness, murmur; consider if over 65 with no prodrome → ≤24 h

3Assess — read in the same order every time

☐ Right patient, time, symptoms during recording  ☐ 25 mm/s, 10 mm/mV (small square 40 ms, large 200 ms)  ☐ Lead reversal?  ☐ Old ECG to compare

☐ Rate 60–100  ☐ Rhythm: P before every QRS?  ☐ Axis −30° to +90°  ☐ PR 120–200 ms  ☐ QRS <120 ms  ☐ ST isoelectric  ☐ T upright I, II, V3–V6  ☐ QTc measured yourself

STEMI (4th UDMI): J-point ST elevation in 2 contiguous leads ≥1 mm; V2–V3 ≥2 mm men ≥40, ≥2.5 mm men <40, ≥1.5 mm women.

4Classify — common patterns

  • AF / flutter — irregularly irregular, no P; sawtooth, 2:1 at ~150 /min → confirm on 12-lead.
  • Heart block — PR >200 ms (1st) · progressive PR then drop (Mobitz I) · constant PR, dropped QRS (Mobitz II) · P and QRS independent (complete).
  • BBB — RBBB rSR′ V1 (often benign if isolated); LBBB usually means heart disease and hides ischaemia.
  • LVH — voltage ± strain → BP, murmur, echo. WPW — short PR, delta wave.
  • Brugada — coved ST ≥2 mm, negative T in V1–V2; fever unmasks. Pericarditis — widespread concave ST elevation, PR depression.
  • Athlete — brady, 1st-degree, Mobitz I, voltage can be training; T inversion, LBBB, Q waves are not.

5Treat today — doses: NICE/SPC, local formulary

  • Stable new AF: beta-blocker (not sotalol) or rate-limiting CCB (no CCB in HFrEF); CHA₂DS₂-VASc ≥2 → offer a DOAC; men with 1 → consider; ORBIT. Do not wait for cardiology (NG196).
  • Long QTc on a medicine: stop or switch, correct K⁺/Mg²⁺/Ca²⁺, repeat and measure yourself.
  • Brady or 1st-degree block, no symptoms: review rate-slowing drugs; U&E, TFTs. Document benign findings.

6Refer routinely

  • Urgent cardiology pre-excitation in a well patient, spontaneous type 1 Brugada, confirmed long QT not explained by drugs, cardiomyopathy pattern, symptomatic Mobitz I, new asymptomatic LBBB.
  • Routine asymptomatic bifascicular block, LVH (echo), frequent or complex ectopics (monitoring and echo).
  • Primary care fit sinus bradycardia, isolated RBBB, 1st-degree block, isolated ectopics, early repolarisation.

7Never reassure by

  • A normal ECG — repeat it if symptoms persist or recur (CG95).
  • The machine report — read and measure the trace yourself.
  • A trace with no old ECG to compare — "new" changes urgency.
  • Calling a broad-complex tachycardia SVT with aberrancy.
  • Adult thresholds in a child.
  • An abnormal ECG left in an inbox without a named reviewer.

8Safety-net — say it, then document it

999 “Call 999 for chest pain or pressure, a blackout, severe breathlessness, or a racing or very slow heartbeat with feeling faint or unwell.” Same-day “Contact us today for new or worse palpitations, dizziness or near-fainting, breathlessness on exertion, new ankle swelling, or any new symptom after a medicine change.”

Driving: arrhythmia that has caused or may cause incapacity → stop driving; DVLA decides. Family: ask about sudden death under 40.

✓Safe to leave primary care only if — all five, documented

No block 0–2 row applies Systematic read recorded in your own words Compared with old ECG Tier, action and named reviewer documented Safety-net and DVLA advice given; review date set
Summary of the full Reasoning GP ECG interpretation algorithm, which holds the systematic read, patterns and evidence. Sources: NICE CG109, CG95, NG196; Resuscitation Council UK ALS (2021; 2025 update); Fourth Universal Definition of MI (2018); AHA/ACCF/HRS ECG standards (2009); international athlete ECG criteria (2017); UK Kidney Association (2023); DVLA. Clinical reference — follow local emergency and cardiology referral arrangements and check doses: NICE/SPC, local formulary.