A 60-Second Systematic ECG Read for Non-Cardiologists

The reason ECGs get misread is rarely obscure knowledge. It is that the reader starts wherever their eye lands — usually the most abnormal-looking complex — and never systematically covers the rest. A fixed sequence, run the same way every time, catches far more than expertise applied unevenly. What follows is a sequence that takes about a minute once it is habitual, followed by the small set of patterns worth pattern-matching for immediately.

Key points

• Always run the same order: patient and comparison, rate, rhythm, axis, intervals, morphology (P/QRS/ST/T), then a deliberate look for the can't-miss patterns.
• Confirm calibration and paper speed before interpreting anything — a non-standard setting changes every measurement you are about to make.
• Never trust the machine's interpretation. It is a prompt, not a read, and it is least reliable exactly where the stakes are highest.
• An old ECG for comparison changes management more often than any single new finding.
• The read is not finished until you have specifically asked about ischaemia/infarct patterns, dangerous bradycardia and conduction block, pre-excitation, long QT, and hyperkalaemia.
• Correlate with the patient. A concerning ECG in an asymptomatic patient and an unremarkable ECG in a patient with ongoing chest pain both demand a next step.

Before you start: five seconds of setup

Check the name, date, and time — reading yesterday's tracing as today's is a recurring source of error. Check the calibration marker: standard is 10 mm per mV, and a half-standard tracing will make voltage look falsely low. Check paper speed at 25 mm/s. Then find the old ECG. Nothing you are about to measure is as useful as knowing whether it is new.

Step 1: Rate

For a regular rhythm, count large squares between consecutive R waves and divide 300 by that number — the familiar 300, 150, 100, 75, 60, 50 sequence. For an irregular rhythm, count the QRS complexes across the rhythm strip and multiply by the appropriate factor for the strip length, which gives an average rate rather than an instantaneous one.

Classify immediately: bradycardia under 60, tachycardia over 100. Then ask whether the rate explains the patient's symptoms, or whether the patient's condition explains the rate. Sinus tachycardia is a sign, not a diagnosis, and hunting for its cause is more useful than treating the number.

Step 2: Rhythm

Three questions in order.

• Regular or irregular? March out the R–R intervals. Irregularly irregular with absent organised P waves is atrial fibrillation until proven otherwise.
• Narrow or wide QRS? Under about 120 ms is narrow and implies supraventricular origin with normal conduction. Wide means ventricular origin, aberrant conduction, pre-excitation, pacing, or a metabolic cause.
• Is there a P before every QRS, and a QRS after every P? This single question separates sinus rhythm from the atrioventricular blocks and from dissociation.

Sinus rhythm requires upright P waves in lead II and inverted in aVR, each followed by a QRS at a constant interval. If lead II P waves are not upright, question lead placement before questioning the rhythm — limb lead reversal is common and produces a characteristic global inversion in lead I.

The high-stakes call here is the wide-complex tachycardia. In an unstable patient, treat it as ventricular tachycardia; that is both the most common cause and the one where a wrong assumption is lethal. Features favouring VT include atrioventricular dissociation, fusion or capture beats, very wide complexes, and extreme axis deviation. A history of structural heart disease or prior infarct shifts the odds strongly toward VT.

Step 3: Axis

The fast method: look at the net QRS deflection in leads I and aVF. Both positive is normal axis. Lead I positive with aVF negative is left axis deviation — check lead II, and if it is also negative the deviation is genuine. Lead I negative with aVF positive is right axis deviation. Both negative is extreme axis.

Axis rarely changes management alone, but it flags left anterior fascicular block, right ventricular strain or hypertrophy, lateral infarction, and it is a supportive clue in wide-complex tachycardia and in acute right heart strain.

Step 4: Intervals

PR interval (start of P to start of QRS), normally about 120–200 ms. Long and constant is first-degree block. Progressive lengthening with a dropped beat is Mobitz I. Constant PR with sudden non-conducted P waves is Mobitz II — a different animal entirely, often requiring pacing. A short PR with a slurred QRS upstroke is a delta wave and pre-excitation.

QRS duration, normally under about 120 ms. Widened, look at V1: an RSR' pattern with a wide S in I and V6 is right bundle branch block; a broad, notched, monophasic R in I and V6 with a dominant S in V1 is left bundle branch block. New left bundle branch block in a patient with ischaemic symptoms is a red flag and merits urgent evaluation.

QT interval, which must be corrected for rate. As a rough bedside screen, the QT should be less than half the preceding R–R interval at normal rates; anything that looks long deserves formal correction. Prolongation raises the risk of torsades de pointes and should trigger a review of medications, potassium, magnesium, and calcium.

Step 5: Morphology

P waves. Tall and peaked in II suggests right atrial enlargement; broad and notched, or with a deep terminal negative component in V1, suggests left atrial abnormality.

QRS. Look for pathological Q waves — greater than 40 ms wide or more than a quarter of the R wave height — grouped in a contiguous territory, indicating prior infarction. Assess voltage for hypertrophy, and check R wave progression across the precordial leads; poor progression suggests prior anterior infarction, lead misplacement, or chronic lung disease.

ST segments. This is where you slow down. Compare each ST segment to the TP or PR baseline. Elevation or depression must be assessed by territory: II, III, aVF inferior; V1–V4 anteroseptal; I, aVL, V5–V6 lateral. Reciprocal depression in an opposing territory strongly supports true infarction over a benign mimic. Concave, diffuse elevation with PR depression suggests pericarditis; a convex or straight elevation confined to a territory with reciprocal change suggests occlusion.

T waves. Inversion in a territory, hyperacute broad-based T waves, and biphasic T waves in V2–V3 (a Wellens pattern suggesting critical proximal left anterior descending stenosis) all matter. Compare against the old tracing.

The deliberate can't-miss pass

After the systematic read, spend ten seconds specifically hunting these. They are the patterns where a miss changes outcome.

• ST elevation infarction in any territory, with reciprocal change. Add posterior leads when there is tall R with ST depression in V1–V3, and right-sided leads in inferior infarction to detect right ventricular involvement.
• Hyperkalaemia: peaked narrow T waves, flattened P waves, widening QRS, progressing to a sine wave. Act on the ECG before the laboratory result returns.
• High-grade atrioventricular block: Mobitz II or complete heart block with atrioventricular dissociation.
• Pre-excitation: short PR with delta wave, particularly with atrial fibrillation, where nodal-blocking agents are dangerous.
• Long QT, especially with bradycardia or contributing drugs.
• Brugada pattern: coved ST elevation with T inversion in V1–V2.
• Acute right heart strain: sinus tachycardia, right axis, right bundle branch block, T inversion in V1–V4 and inferior leads — supportive of pulmonary embolism in the right clinical context, never diagnostic alone.

Closing the loop

Finish by stating your read in a fixed form: rate, rhythm, axis, intervals, notable morphology, and comparison to prior. Then ask the question that makes the ECG clinically useful — does this change what I do in the next hour? A single ECG is a snapshot of a dynamic process. If symptoms are ongoing and the tracing is unrevealing, repeat it in fifteen minutes and add extra leads. Serial tracings catch evolving occlusion that a single one misses.

To drill this sequence against progressively harder tracings, with cited sources and a virtual patient whose ECG evolves as you decide, practise at app.medicaltraining.ai.

Educational content for healthcare professionals. It is not medical advice and does not replace clinical judgement or local protocols.