ECG Interpretation Course Module: Building a Sequence You Never Abandon

Module orientation

This is the foundation module of the electrocardiography strand, taken before the arrhythmia and infarction modules, and it is aimed at residents, cardiology trainees and physicians who read tracings daily without ever having been taught a formal method. The premise is uncomfortable: most interpretation errors are not knowledge failures. The clinician who overlooks a long QT usually knows the normal range and simply stopped searching once atrial fibrillation had been found. The module exists to install a sequence that continues after the first abnormality, especially on tracings that look unremarkable.

Learning objectives

• Perform a fixed nine-step reading sequence identically on every tracing, including normal ones.
• Detect technical faults — gain, speed, lead reversal and artefact — before committing to any interpretation.
• Resolve rhythm through four ordered questions rather than by pattern recognition alone.
• Quote the reference values for axis, intervals and voltage without hesitation.
• Complete a deliberate second pass targeting the abnormalities that hide behind an obvious one.

Block 1 — Before interpretation begins

The step most often skipped comes first. Confirm calibration and speed from the pulse at the start of the trace: 25 mm per second and 10 mm per millivolt are standard, and machines are frequently left on other settings after a paediatric or tachycardia recording. At half gain, hypertrophy criteria become meaningless; at double speed, every interval appears stretched.

Then check lead placement. Reversal of the arm electrodes produces a negative P wave and QRS in lead I with global negativity in that lead, imitating dextrocardia — with the distinguishing feature that precordial progression stays normal. Precordial electrodes placed a space or two too high generate poor R wave progression and can manufacture an anteroseptal infarct that does not exist. Finally, consider artefact: tremor and poor electrode contact imitate flutter and ventricular tachycardia convincingly, and the resolving observation is that genuine QRS complexes usually march through the noise at their own regular rate. Participants who want extended practice on technically flawed tracings use the clinical ECG workshop.

Block 2 — Rate and rhythm

For a regular rhythm, divide 300 by the number of large squares between consecutive R waves, giving the familiar sequence of 300, 150, 100, 75, 60 and 50. For an irregular rhythm that method is invalid; count complexes across a six-second strip and multiply by ten. Atrial and ventricular rates are reported separately whenever they may differ, because a single number discards the diagnosis in complete heart block, second-degree block and flutter with variable conduction.

Rhythm is then resolved by four questions asked in order. Is it regular? Are P waves present and what is their morphology? Does every P conduct, and is every QRS preceded by a P? Is the QRS narrow or broad? The governing rule for broad complex tachycardia is clinical rather than electrocardiographic: assume ventricular tachycardia until proven otherwise, particularly with prior infarction or structural disease, where pre-test probability is overwhelming. Supporting features include atrioventricular dissociation, capture and fusion beats, duration beyond about 160 ms and an extreme axis.

Block 3 — Axis and atria

Axis is determined quickly from leads I and aVF, with lead II used to confirm suspected left deviation.

The normal P wave is under 120 ms wide and under 2.5 mm tall, upright in leads I, II and aVF and inverted in aVR. Left atrial abnormality gives a notched P in lead II beyond 120 ms or a terminal negative deflection in V1 at least 1 mm deep and 40 ms wide, and supports elevated filling pressures when the echocardiogram is equivocal — a link explored in the echocardiography course material. Right atrial abnormality gives a peaked P above 2.5 mm in the inferior leads.

Block 4 — Intervals

Intervals are where satisfaction of search does its greatest damage, so they are measured explicitly rather than eyeballed. PR conduction runs from 120 to 200 ms; a short interval with a delta wave indicates pre-excitation, and progressive prolongation with dropped beats separates the two forms of second-degree block. The QRS is under 120 ms when conduction is normal; broadening demands a decision between bundle branch block, fascicular combinations, pre-excitation, hyperkalaemia, sodium channel blockade and ventricular origin.

QT measurement deserves the discipline the module imposes: measure in the lead with the longest apparently normal interval, correct for rate, and treat a corrected value above roughly 450 ms in men and 470 ms in women as prolonged, with values beyond 500 ms carrying real arrhythmic risk. Drug interactions, electrolyte disturbance and bradycardia should be sought whenever it is long. A short QT is rarer and no less significant.

Block 5 — The QRS and repolarisation

Assess voltage against hypertrophy criteria while remembering that they are insensitive and body-habitus dependent, and always confirm calibration first. Look for pathological Q waves, at least 40 ms wide or deeper than a quarter of the following R, and describe R wave progression across the precordial leads. Then read the ST segments and T waves lead by lead in territorial groups rather than sweeping across the tracing, since a two-lead abnormality in one territory is easily lost in a general impression.

The distinction the module drills hardest is between primary repolarisation change, which reflects ischaemia or metabolic disturbance and appears in a coronary distribution, and secondary change, which follows an abnormal QRS in hypertrophy, bundle branch block and pacing and is discordant with the main deflection. Infarct-specific patterns are taken further in the companion clinical ECG interpretation course book.

Block 6 — The second pass

The final step is procedural rather than intellectual: having reached a diagnosis, go back and complete the sequence. The second pass exists to catch the long QT behind the atrial fibrillation, the epsilon wave behind the ectopy, the delta wave behind the tachycardia, the Brugada morphology in V1 and V2 behind the right bundle appearance, and the subtle hyperkalaemic T wave behind the bradycardia. Comparison with any previous tracing is part of this pass, and it changes management more often than any other single habit taught in the module.

Self-check

• Lead I is globally negative with normal precordial progression. Most likely explanation? Arm lead reversal, not dextrocardia.
• Irregular rhythm at first glance. Can you use the 300 rule? No; count over six seconds and multiply by ten.
• Broad complex tachycardia in a 70-year-old with prior anterior infarction. Working diagnosis? Ventricular tachycardia until proven otherwise.
• Corrected QT of 510 ms. Immediate actions? Review medications, check potassium, magnesium and calcium, and consider bradycardia as a contributor.
• Why is the second pass mandatory? Because the first abnormality found suppresses further searching, which is how coexisting high-risk patterns are missed.

How this module is taught in the course

The module is taught as supervised repetition rather than as a lecture. Participants read aloud through the nine steps on twenty tracings while a facilitator interrupts whenever a step is skipped, which is initially frequent. A reference card carrying the values above is issued for ward use, and a nightly drill of five tracings is expected for the following month. Core reading is the ECG course book, with rhythm material extended in the arrhythmia course text and revision support from the cardiology quick revision course book and the diagnosis and imaging collection. A continuous prose version of the sequence is published as the guide on how to read an ECG systematically.

Module FAQ

Does a fixed sequence slow you down?

Initially yes, for roughly a fortnight. After that it is faster than unstructured reading, because it removes backtracking and second-guessing.

Should machine interpretation be read first?

No. Read the tracing, commit to your own interpretation, then compare. Reading the algorithm first anchors you to its errors, which cluster in exactly the high-risk patterns.

How is competence assessed?

By an observed reading of ten unseen tracings in which the sequence must be demonstrated aloud, not merely the diagnosis reached.

What follows this module?

The infarction module and then the arrhythmia strand. Both assume the sequence taught here is already automatic.

Reviewed by Dr A M Thirugnanam, MD, MSICP, FSCAI, Ph.D., Senior Interventional Cardiologist — who insists that trainees on his unit read every tracing aloud through this sequence before offering a diagnosis.