Almost every ECG finding that gets missed was visible on the trace. It was missed because the reader went straight to the part of the ECG that matched their expectation and never systematically examined the rest. Interpretation becomes reliable when the reading order stops being decided by the trace and starts being decided in advance.
On this page
1. Confirm the technical quality first
Before interpreting anything, check paper speed and calibration. A 50 mm/s trace read as 25 mm/s halves every calculated rate and doubles every interval. Half-standard calibration will make voltage criteria for hypertrophy disappear. Then check lead placement: right-arm and left-arm reversal produces a negative P wave and a negative QRS in lead I with a normal V6, and is far more common than dextrocardia. Baseline wander and tremor artefact should be corrected and the trace repeated rather than interpreted around.
2. Rate
For a regular rhythm, divide 300 by the number of large squares between consecutive R waves, or 1500 by the number of small squares when precision matters. For an irregular rhythm, count the QRS complexes across a ten-second rhythm strip and multiply by six — averaging two adjacent intervals in atrial fibrillation gives a figure that can be badly wrong.
3. Rhythm
Three questions, always in this order. Is it regular? Is there a P wave before every QRS and a QRS after every P? Is the QRS narrow or broad? Those three answers narrow the possibilities before any pattern recognition is applied. Resist naming the rhythm before answering them; naming first is how supraventricular tachycardia with aberrancy gets called ventricular tachycardia and, more dangerously, how ventricular tachycardia gets called the reverse.
4. Axis
Leads I and aVF give the quadrant in two seconds. Both positive is normal. Lead I positive with aVF negative is left axis deviation — check lead II, because a positive lead II keeps it within normal limits. Lead I negative with aVF positive is right axis deviation. Both negative is extreme axis. Axis is not an academic exercise: left axis deviation with a right bundle branch block pattern is bifascicular block, and a new right axis in a breathless patient should raise pulmonary embolism.
5. P wave morphology
Look at lead II and V1. A P wave taller than 2.5 mm in II suggests right atrial enlargement; a bifid P wave broader than 120 ms, or a deep terminal negative deflection in V1, suggests left atrial abnormality. Absent P waves with an irregular baseline is atrial fibrillation; a sawtooth baseline is flutter, and the flutter rate is usually close to 300 with the ventricular rate a division of it.
6. PR interval
Normal is 120 to 200 ms. Short with a delta wave is pre-excitation, which matters enormously if that patient later develops atrial fibrillation. Long is first degree block. Progressive lengthening before a dropped beat is Mobitz I; a dropped beat without warning is Mobitz II, which is a different clinical problem with a different disposition.
7. QRS width and morphology
Above 120 ms, decide which bundle. A dominant R wave in V1 with a broad S in I and V6 is right bundle branch block; a broad notched R in V6 with a deep S in V1 is left bundle branch block. Then look for pathological Q waves, and for poor R wave progression across the praecordial leads. Check voltage criteria for hypertrophy, remembering that they perform poorly in thin young patients and in obesity for opposite reasons.
8. ST segments
Examine every lead against the TP baseline, not against your impression of the trace. Elevation must be assessed with its reciprocal change, because reciprocal depression is what distinguishes infarction from pericarditis and from early repolarisation. Widespread depression with elevation in aVR should prompt thought about left main or severe three-vessel disease rather than a single culprit. Depression that is horizontal or downsloping carries very different weight from upsloping depression at high rates.
9. T waves and QT
Inverted T waves in leads with pathological Q waves suggest established infarction; deep symmetrical inversion in the anterior leads in a pain-free patient is the Wellens pattern and is a warning, not reassurance. Biphasic and hyperacute T waves change fast and are worth repeating the trace for. Measure the QT and correct it — Fridericia rather than Bazett outside 60 to 100 beats per minute, since Bazett over-corrects at speed. The Academy’s QTc calculator gives both.
10. The deliberate second look
Return to the leads that get skipped: aVR, III, and the right praecordial leads. Then ask two questions. Does this trace explain the patient in front of me? And is there a previous ECG? A comparison trace converts an ambiguous finding into either reassurance or an emergency more often than any single feature on the current one.
Building the habit
The sequence only works when it is automatic, which takes volume rather than understanding. Read every ECG you can get hold of in this order, including the normal ones — normals are what calibrate the eye. Practise against the image of the month and the quiz, and bring the traces that puzzled you to the Wednesday ECG and arrhythmia session.
Systematic teaching on the same material is in the diagnosis and imaging reading list and the ECG mastery module of the course programme. A printable one-page version of this sequence is among the free downloads.