An Electrocardiogram (ECG) is a snapshot of the heart’s electrical activity. While “spot diagnosis” can be tempting, a structured approach ensures you don’t overlook critical details in the rhythm, axis, or intervals.
Before looking at the heart, check the paper settings:
Paper Speed: Usually 25 mm/sec.
Voltage: Usually 10 mm/mV.
Patient Info: Name, DOB, and crucially, the time/date the ECG was taken.
Clinical Context: Is the patient currently having chest pain?
Calculate the heart rate to determine if there is bradycardia (<60 bpm) or tachycardia (>100 bpm).
Regular Rhythm: Count the number of large squares between two R-waves and divide 300 by that number.
Irregular Rhythm: Count the number of R-waves on the 10-second rhythm strip (bottom lead II) and multiply by 6.
Determine if the heart is in Sinus Rhythm:
Is the rhythm regular or irregular?
Is there a P-wave before every QRS complex?
Is the QRS narrow (<120ms) or wide?
Clinical Tip: An “irregularly irregular” rhythm with no P-waves is the hallmark of Atrial Fibrillation (AF).
Check the direction of electrical flow by looking at Lead I and Lead aVF:
Normal Axis: QRS is positive (upwards) in both Lead I and aVF.
Left Axis Deviation: Positive in Lead I, negative in aVF (“leaving” each other).
Right Axis Deviation: Negative in Lead I, positive in aVF (“reaching” toward each other).
The PR interval represents the time taken for the impulse to travel from the atria to the ventricles.
Normal: 120–200 ms (3–5 small squares).
Short PR: May indicate Wolff-Parkinson-White (WPW) syndrome.
Long PR: Indicates First-Degree Heart Block.
Width: Should be <120 ms (3 small squares). If wide, think Bundle Branch Block or a ventricular rhythm.
Pathological Q-waves: If >25% the height of the R-wave, they may indicate a previous myocardial infarction.
Voltage: Very tall R-waves in chest leads may suggest Left Ventricular Hypertrophy (LVH).
This is the most critical step for identifying an acute heart attack.
ST Elevation: Indicates an acute STEMI (infarction). Look for the “tombstone” appearance.
ST Depression: Can indicate Ischaemia or “reciprocal changes” opposite an area of elevation.
Note: The ST segment must be compared to the isoelectric line (the flat line between beats).
T-waves: Should be upright in most leads (except aVR and V1). Inverted T-waves can signify ischemia or strain. Peaked T-waves are a classic sign of Hyperkalaemia (high potassium).
QT Interval: Calculate the QTc (corrected for heart rate). A long QTc (>450ms in men, >470ms in women) increases the risk of dangerous arrhythmias.
| Feature | Normal Value | Significance of Abnormality |
| Heart Rate | 60–100 bpm | Bradycardia or Tachycardia |
| PR Interval | 120–200 ms | Heart blocks or WPW |
| QRS Width | <120 ms | Bundle Branch Blocks |
| ST Segment | Flat (Isoelectric) | Elevation = Infarction; Depression = Ischaemia |
| T-waves | Upright | Inversion = Ischaemia; Peaked = Hyperkalaemia |
1. What is the most important lead to look at?
Lead II is generally the best for assessing rhythm (P-waves), but you must look at all 12 leads to “see” the heart from different angles to locate an infarction.
2. Can an ECG be normal during a heart attack?
Yes. An ECG can be normal in the early stages of an NSTEMI (Non-ST Elevation Myocardial Infarction). If clinical suspicion is high, the doctor will check Troponin blood tests.
3. What does “Sinus Rhythm” actually mean?
It means the heart’s electrical signal is starting where it should—in the Sinoatrial (SA) node.
4. Why is the QT interval “corrected” (QTc)?
The QT interval naturally shortens as the heart rate increases. Correcting it (usually via the Bazett formula) allows doctors to see if the interval is abnormally long regardless of how fast the heart is beating.
Life in the Fast Lane (LITFL): ECG Library – The gold standard for emergency ECG interpretation.
Geeky Medics: How to Read an ECG – A comprehensive guide for medical students and clinicians.
British Heart Foundation: What is an ECG? – Simple patient-facing explanation of the procedure.
American Heart Association: Electrocardiogram Guide – US clinical perspective on ECG use in emergencies.
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