Medicine — Cardiology, NMC MBBS licence examination syllabus (Nepal Medical Council).
Reading an ECG
The trace is not a picture of the heart. It is a record of where electricity went, and every abnormality on it is that journey going wrong somewhere specific.
The ACS chapter reads the ECG for one thing: ST elevation. This chapter covers reading a trace when you do not already know what you are looking for — which is the situation in every station and on every ward round.
Read it in the same order every time
The order matters more than the speed. Almost every missed finding comes from looking at the ST segments first, because that is where the dramatic abnormality usually is — and a rate or rhythm problem does not announce itself, so an eye that goes straight to the ST segments never sees it. Follow the sequence even when the answer looks obvious.
Rate: derive it, do not estimate it
The paper runs at a fixed speed, which makes it a ruler for time. For a regular rhythm, count the large squares between two R waves and divide 300 by that number. One square is 300, two is 150, three is 100, four is 75, five is 60, six is 50 — worth knowing without calculating.
💡 The trap is applying that method to an irregular rhythm. In atrial fibrillation every R–R interval differs, so whichever pair you happen to measure gives a different answer, and the number you report is arbitrary. Establish regularity first; if it is irregular, count complexes across a known length of strip and scale up instead.
Rhythm: three questions about the P waves
Is there a P before every QRS? If there are no P waves at all and the rhythm is irregular, atrial fibrillation is the answer until something else explains it.
Is there a QRS after every P? A P wave with nothing following it means the impulse was generated normally and blocked on the way down — the atria fired and the ventricles did not answer.
Is the PR interval constant? A PR that lengthens progressively, or that varies with no relationship to anything, both indicate disease in the conducting system. The arrhythmia chapter covers what each pattern means.
QRS width: the single most useful measurement
A narrow QRS means the beat travelled through the normal conducting system. Those fibres conduct fast, so the whole ventricle depolarises almost simultaneously and the complex is compact. A narrow complex therefore tells you the rhythm originated above the ventricles.
A wide QRS means it did not. Either the beat started inside the ventricle itself, or it arrived from above but found the conducting system blocked. In both cases depolarisation spreads muscle-to-muscle, which is far slower, and the complex smears out.
🩺 That one measurement answers two questions at once — where the rhythm started, and whether conduction below the atria is intact — which is why it is worth taking seriously rather than glancing at. A candidate who can say "narrow, therefore supraventricular" has already halved the differential before considering anything else.
Axis, briefly
The axis is the net direction of ventricular depolarisation. The quick check is leads I and aVF: if the QRS is predominantly positive in both, the axis is normal. Deviation points towards a chamber that has enlarged, a conduction block on one side, or a shift in the heart's position — it is a pointer towards other findings rather than a diagnosis on its own.
ST segments and T waves — last
ST elevation, depression, and T wave inversion are covered in the ACS chapter, including the territories each lead group represents and why posterior infarction is missed. Two points belong here rather than there.
ST changes have causes other than infarction. Pericarditis, ventricular strain, bundle branch block and electrolyte disturbance all alter the ST segment, and reading every elevated ST as an infarct produces both wrong diagnoses and unnecessary thrombolysis.
Comparison with a previous ECG changes the interpretation more than any single feature. An abnormality that is unchanged from a trace taken a year ago means something entirely different from the same abnormality appearing today.
What to recognise immediately
Most ECGs can be thought about. Three patterns cannot: ST elevation in a coronary territory, a broad-complex tachycardia, and complete heart block or a symptomatic bradycardia. Each changes what happens in the next minute rather than the next hour.
💡 The habit that prevents the worst errors: look at the patient between reading the rate and reading the rhythm. A trace showing a dangerous rhythm in someone sitting up and talking comfortably is more likely to be an artefact — movement, a loose electrode — than a rhythm about to kill them. And a normal-looking ECG in a patient who is grey and clammy does not reassure; it means the problem is somewhere else.
Clinical reasoning: four traces
Irregularly irregular, no P waves, narrow complexes. Atrial fibrillation. Narrow tells you it comes from above the ventricles; the absent P waves and irregularity give the diagnosis.
Regular, fast, broad complexes, patient unwell. Treat as ventricular tachycardia. Broad-complex tachycardia has other causes, but assuming the dangerous one and being wrong is far safer than the reverse.
Regular P waves at a steady rate, QRS complexes at a slower steady rate, no relationship between them. The atria and ventricles are firing independently — complete heart block. The arrhythmia chapter covers the urgency.
ST elevation in leads sharing a coronary territory, with reciprocal depression elsewhere. STEMI until proven otherwise. The reciprocal change is what distinguishes it from the more benign causes of ST elevation.
Commonly confused
Rate and rhythm. Rate is how fast; rhythm is the pattern and its origin. A normal rate does not mean a normal rhythm.
A wide QRS and a ventricular origin. Ventricular beats are wide, but a supraventricular beat conducted through a blocked bundle is wide too. Width tells you conduction was abnormal, not where the beat began.
Artefact and arrhythmia. Movement and poor electrode contact produce alarming traces in comfortable patients. Look at the patient.
An abnormal ECG and an unwell patient. Neither implies the other. Both need explaining.
Rapid revision
Rate, rhythm, axis, intervals, ST — in that order, every time.
300 divided by large squares for a regular rhythm; count and scale for an irregular one.
Narrow QRS = above the ventricles. Wide QRS = ventricular origin or blocked conduction.
P without QRS = blocked. No P with irregular rhythm = atrial fibrillation.
Three emergencies: ST elevation, broad-complex tachycardia, complete heart block.
Compare with an old ECG whenever one exists.
Syllabus points
Reading in a fixed order: rate, rhythm, axis, intervals, ST segments
Why looking at the ST segments first is how rate and rhythm problems are missed
Deriving rate from the paper: 300 divided by large squares between R waves
The 300, 150, 100, 75, 60, 50 sequence
Why the 300 rule fails on an irregular rhythm, and what to do instead
Is there a P before every QRS — absent P waves with irregularity
Is there a QRS after every P — a blocked impulse
Is the PR interval constant — progressive lengthening and variable PR
Narrow QRS as evidence the beat used the normal conducting system
Wide QRS as ventricular origin or blocked conduction below the atria
Axis assessed quickly from leads I and aVF
Non-ischaemic causes of ST change: pericarditis, strain, bundle branch block, electrolytes
Comparison with a previous ECG as the single most useful context
ST elevation in a coronary territory as a time-critical finding
Broad-complex tachycardia treated as ventricular tachycardia
Complete heart block and symptomatic bradycardia as immediate concerns
Distinguishing artefact from arrhythmia by looking at the patient
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