Reading an ECG: A Seven-Step Routine for Ward and Clinic

Forty tracings in the tray, a pre-operative list to clear, and no cardiology service to bounce the doubtful ones to. In that setting an ECG routine is not an academic ritual; it is the only thing that stops the abnormal tracing hiding among the normal ones at four in the afternoon. The routine below is built around what each part of the tracing lets you decide, and it is deliberately sequenced so that the findings which change management tonight surface before the ones that merely populate a report.

Why a fixed order beats pattern recognition

Experienced readers recognise patterns instantly, which is exactly the problem. Once a striking abnormality captures attention, the second abnormality on the same tracing is missed at a rate that has been demonstrated repeatedly. A fixed sequence protects against that, and it also produces reports that are comparable between clinicians and over time. The cost is about ninety seconds per tracing. The routine matters most on the tracings that look boring.

The decision points

1. Decide whether you trust the recording

Calibration at 10 mm per millivolt, paper at 25 mm per second, and a filter setting that has not amputated the ST segment. Half standardisation is used more often than clinicians realise and will convert genuine hypertrophy into a normal voltage report. Limb lead reversal produces an inverted P and QRS in lead I with a normal precordial progression, a combination that should trigger a repeat rather than a diagnosis of dextrocardia. Then find a previous tracing. Comparison changes interpretation more often than any single measurement on the current one.

2. Decide whether the rate explains the patient

Rate is trivial to calculate and is worth pausing over only when it is inconsistent with the clinical state. A rate of 105 in a comfortable patient at rest, or a rate of 48 in a breathless one, is a finding rather than a number. In irregular rhythms count over a full six seconds rather than dividing into a single interval.

3. Decide whether the rhythm needs anything doing tonight

Work through three questions in order: is there organised atrial activity, is the ventricular response regular, and is there a fixed relationship between the two. That sequence separates sinus rhythm, atrial fibrillation, flutter with variable conduction, atrial tachycardia and the junctional and ventricular rhythms without needing a differential list. In broad complex tachycardia, treat the rhythm as ventricular in origin until proven otherwise, particularly in any patient with structural heart disease; the alternative error is far more dangerous than the diagnostic one. Systematic rhythm interpretation at the bedside is covered in our arrhythmia bedside reference.

4. Decide whether conduction is safe to leave unmonitored

Measure the PR interval and classify any block precisely, because the management divide runs between block above and below the atrioventricular node. Progressive PR lengthening with a dropped beat in an asymptomatic patient is usually benign. Sudden non-conducted P waves with a constant PR, block with a broad escape, or alternating bundle branch patterns indicate infranodal disease and a pacing conversation. Then assess QRS duration: a broad complex demands an explanation, whether bundle branch block, pacing, pre-excitation, hyperkalaemia or a sodium channel blocking drug.

5. Decide how much weight to give the chamber signals

Axis, atrial abnormality and voltage criteria for hypertrophy are supportive rather than decisive. Voltage criteria have modest sensitivity and are distorted by body habitus, chest wall and lung disease, so hypertrophy suggested on a tracing should be confirmed by imaging rather than reported as fact. Left axis deviation with a small septal q pattern, right axis deviation in a breathless patient, and biatrial enlargement all direct the next investigation rather than concluding it. Poor R wave progression deserves a considered comment rather than a stock phrase, since prior infarction, lead placement, hypertrophy and chronic lung disease all produce it.

6. Decide whether repolarisation is ischaemic, structural or pharmacological

Judge ST deviation against the QRS that produced it. Discordant changes proportional to a broad or high-voltage complex are secondary and expected; changes disproportionate to the complex or concordant with it are not. Look at T wave morphology as well as polarity, since symmetry and base width carry the ischaemic signal. Then measure the QT and correct it, remembering that the common correction formula overcorrects at fast rates and undercorrects at slow ones. A corrected interval beyond 500 milliseconds in a patient on multiple prolonging agents is an action point, not an observation: review the drug chart, replace potassium and magnesium, and arrange monitoring. Broader rhythm risk assessment is set out in the heart failure, rhythm and risk hub.

7. Decide what the tracing means for this patient, then say it

Finish by answering the referral question in one sentence. A report that lists findings without a conclusion transfers the decision back to the person least equipped to make it. If the tracing is normal for the clinical context, say so plainly, since an equivocal report generates unnecessary referrals and anxious patients.

When to escalate, transfer or call for help

• Same shift, with monitoring: complete heart block, Mobitz type two block, a broad escape rhythm, sustained broad complex tachycardia, or a corrected QT beyond 500 milliseconds with syncope.
• Urgent referral even when the patient feels well: pre-excitation with atrial fibrillation, a coved right precordial pattern, epsilon waves with right precordial T inversion, and voltage criteria with deep T inversion suggesting apical hypertrophic disease. These patients look well until the moment they do not, and the diagnostic detail sits in our electrophysiology essentials volume.
• Escalate the tracing, not the patient: photograph an equivocal ECG and send it to a colleague. Building that route before you need it is one of the more valuable things a doctor in an isolated post can do.
• Arrange further imaging rather than referral when voltage or repolarisation findings suggest structural disease in a stable patient; a targeted echocardiogram answers more than a cardiology appointment three months away, and the diagnosis and imaging hub outlines the sequence.

What to document

• Date, time and clinical context, including symptoms at the moment of recording.
• Rate, rhythm, axis, and the measured PR, QRS and corrected QT intervals as numbers.
• Whether a previous tracing was available, and what changed.
• Technical limitations, including any non-standard calibration or suspected lead placement error.
• An explicit conclusion answering the referral question, and the action taken or planned.
• For any drug-related interval finding, the drugs reviewed and the electrolytes checked.

Bench card

The seven-step order, condensed: trust the recording, check the rate against the patient, define the rhythm, classify conduction, weigh the chamber signals, judge repolarisation against the QRS, then state a conclusion. Practice sets that build this into a reflex are collected in the clinical ECG workshop and the rapid cardiology clinical reference. A stepwise tutorial version of the same routine is published as this systematic approach to reading an ECG.

Questions from the floor

Should I rely on the machine interpretation at the top of the tracing?

Use it as a prompt, never as a conclusion. Automated algorithms are reasonable at rate and interval measurement and unreliable at rhythm classification, particularly in atrial fibrillation with a regularised response, in paced rhythms and in artefact. Delete the machine statement from the record if you disagree with it, and write your own.

How much does a previous ECG really add?

More than any other single input. Established T wave inversion, an old bundle branch block or long-standing Q waves change the disposition of a chest pain patient entirely. Where old tracings are inaccessible, ask the patient to photograph any they hold; in international practice this succeeds more often than requesting records.

Which QT correction should I use in atrial fibrillation?

Average the measured interval over several beats spanning the range of RR intervals rather than trusting a single corrected value. When the decision concerns a prolonging drug, err towards caution, correct electrolytes and monitor rather than relying on the arithmetic.

How do I keep the routine when the department is overwhelmed?

Shorten the report, not the sequence. The order takes seconds; abandoning it is what produces the missed second finding. Reading in the same order every time also makes it far easier to teach juniors and to audit your own accuracy afterwards.

Reviewed by Dr A M Thirugnanam, MD, MSICP, FSCAI, Ph.D., Senior Interventional Cardiologist — set out for clinicians who are the final reader of every tracing in their department.