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How to Read an ECG - Systematic Approach

ECG waveform showing P, Q, R, S, T waves with PR interval, ST segment, and QT interval labeled

Standard ECG waveform from lead II (Costanzo Physiology, 7th Ed.)

What is an ECG?

The ECG (electrocardiogram) measures tiny voltage differences on the body surface that reflect the electrical activity of the heart. Because the atria and ventricles depolarize and repolarize in a specific sequence and timing, potential differences between different parts of the heart can be detected by surface electrodes. - Costanzo Physiology, 7th Ed.

Paper Speed & Calibration

Before reading anything, confirm the standardization:

Paper speed: 25 mm/sec
1 small box = 1 mm = 0.04 sec
1 large box = 5 mm = 0.20 sec
Amplitude: 10 mm = 1 mV

A standard ECG records 10 seconds, with leads arranged in groups (I, II, III; aVR, aVL, aVF; V1-V3; V4-V6), each group showing 2.5 seconds. A rhythm strip runs below for the full 10 seconds. - Goldman-Cecil Medicine

The 14-Step Systematic Approach

Harrison's Principles (22nd Ed.) recommends analyzing these 14 parameters in every ECG:

Standardization/technical features (lead placement, artifacts)
Rhythm
Heart rate
PR interval / AV conduction
QRS interval
QT/QTc intervals
Mean QRS electrical axis
P waves
QRS voltages
Precordial R-wave progression
Abnormal Q waves
ST segments
T waves
U waves

Always compare with prior ECGs when available.

Step-by-Step Breakdown

1. Heart Rate

Method 1 (regular rhythm): Count large boxes between two R waves and divide:

1 large box = 300 bpm
2 large boxes = 150 bpm
3 large boxes = 100 bpm
4 large boxes = 75 bpm
5 large boxes = 60 bpm

Method 2: HR = 60,000 ÷ RR interval (in msec)

Method 3 (10-sec strip): Count total QRS complexes × 6

Normal resting heart rate: 50-100 bpm (Goldman-Cecil notes 50-90 may be more physiologically normal). - Goldman-Cecil Medicine, Harriet Lane Handbook

2. Rhythm

For sinus rhythm, confirm:

Every QRS complex is preceded by a P wave
P-wave axis is normal: P wave upright in leads I and aVF
Normal respiratory variation in R-R interval (sinus arrhythmia) without morphologic changes

If P waves are absent, irregular, or not preceding QRS complexes, consider arrhythmias (AFib, junctional rhythm, AV blocks, etc.). - Harriet Lane Handbook

3. Axis (QRS Axis)

Normal QRS axis: -30° to +90°

Quick method - look at leads I and aVF:

Lead I	aVF	Axis
Positive	Positive	Normal (0° to +90°)
Positive	Negative	Left axis deviation
Negative	Positive	Right axis deviation
Negative	Negative	Extreme axis ("northwest")

The hexaxial reference system uses all six limb leads to pinpoint the axis more precisely. - Goldman-Cecil Medicine, Harriet Lane Handbook

4. Normal ECG Intervals (Adults)

Parameter	Normal Range
Heart rate	50-100 bpm
P wave duration	< 120 ms (< 0.12 sec)
PR interval	90-200 ms (0.09-0.20 sec)
QRS duration	75-110 ms (0.075-0.11 sec)
QTc (males)	390-450 ms
QTc (females)	390-460 ms

Goldman-Cecil Medicine (Table 42-1)

5. The Waveforms Explained

P Wave

Represents atrial depolarization
Duration < 120 ms; amplitude < 3 mm
Abnormal P: broad (intra-atrial conduction delay), tall or biphasic (atrial enlargement)
Atrial repolarization is not seen - it is buried in the QRS complex

PR Interval

From onset of P wave to onset of QRS
Reflects conduction time through atria + AV node + His-Purkinje
Prolonged PR (> 200 ms) = 1st degree AV block (delayed AV conduction)
Short PR (< 120 ms) = ventricular pre-excitation (e.g. WPW), junctional rhythm, or enhanced AV nodal conduction

QRS Complex

Represents ventricular depolarization
Capital letters (Q, R, S) = large deflections ≥ 5 mm; lowercase (q, r, s) = small deflections < 5 mm
Q/q waves = initial negative deflections; S/s waves = negative deflections following R
R-wave progression: R waves should grow from V1 to V5/V6 (transition zone, where R = S, is typically at V3-V4)
Wide QRS (> 110 ms) = bundle branch block or ventricular origin

ST Segment

From end of QRS (J point) to start of T wave
Normally isoelectric (flat, at baseline)
Elevation = injury/infarction (STEMI), pericarditis, early repolarization
Depression = ischemia, digoxin effect, posterior MI (reciprocal)

T Wave

Represents ventricular repolarization
Normally upright in I, II, V4-V6; inverted in aVR
Peaked T = hyperkalemia, early ischemia
Flattened/inverted T = ischemia, LVH strain, electrolyte disturbance

QT Interval

From onset of QRS to end of T wave
Includes ventricular depolarization + repolarization
Must be corrected for heart rate using Bazett's formula:

QTc = QT (sec) / √RR (sec)
Best measured in leads II, V5, V6 (use the longest value)
Prolonged QTc raises risk for torsades de pointes

U Wave

Small positive deflection after T wave (best seen in V2-V3)
Prominent U waves: hypokalemia, bradycardia
Do not include U wave when measuring QT interval

Common Pitfalls

Errors of omission are the most common mistake - always be systematic
Computerized ECG reports have major limitations and must always be reviewed by a clinician
Always check lead placement artifacts before interpreting
Always compare with prior ECGs

Quick Reference Summary

Rate:     Count R-R boxes (300/150/100/75/60)
Rhythm:   P before every QRS? Upright P in I and aVF?
Axis:     I positive + aVF positive = normal
PR:       0.12-0.20 sec (3-5 small boxes)
QRS:      < 0.12 sec (< 3 small boxes)
QTc:      < 0.45 sec men / < 0.46 sec women
ST:       Flat at baseline (elevation or depression = abnormal)
T wave:   Upright I, II, V4-V6

Sources: Harrison's Principles of Internal Medicine 22E | Goldman-Cecil Medicine | Costanzo Physiology 7th Ed. | Harriet Lane Handbook 23rd Ed.

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