Ecg interpretation

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Here is a complete, systematic approach to ECG interpretation, based on Harrison's Principles of Internal Medicine (22nd ed.) and Medical Physiology (Boron & Boulpaep).

ECG Interpretation: Complete Systematic Approach

Electrophysiologic Basis

The ECG records depolarization and repolarization of the working atrial and ventricular myocardium. The impulse follows this pathway:

SA node → Atria → AV node → His bundle → Right & Left bundle branches → Purkinje fibers → Ventricular myocardium

The left bundle further divides into the left anterior fascicle and left posterior fascicle.

ECG Paper & Calibration

Dimension	Small box (1 mm)	Large box (5 mm)
Time (horizontal)	0.04 s	0.20 s
Voltage (vertical)	0.1 mV	0.5 mV

Standard calibration: 1 mV = 10 mm (10 small boxes tall). Always confirm the calibration pulse at the left margin of each row.

The 14-Step Systematic Checklist

(Per Harrison's 22nd ed. — missing any step is a common source of errors)

Step 1 — Standardization & Technical Features

Confirm 1 mV calibration pulse is present
Check lead placement (limb lead reversal is the most common artifact; right arm–left arm reversal inverts lead I and swaps aVR/aVL)
Look for artifacts: muscle tremor, AC interference, baseline wander

Step 2 — Rhythm

Ask three questions:

Where is the pacemaker? — P waves before every QRS = sinus origin
What is the conduction path?
Is the pacemaker regular and at the correct speed?

Normal sinus rhythm (NSR): Regular P waves, positive in I and II, negative in aVR, each followed by a QRS, rate 60–100 bpm.

Rhythm clue	Interpretation
No P waves, irregularly irregular	Atrial fibrillation
Sawtooth flutter waves ~300/min	Atrial flutter (usually 2:1 or 4:1 block)
P waves after QRS	Junctional rhythm
Wide QRS, no P, rate 20–40	Ventricular escape rhythm

Step 3 — Heart Rate

Method 1 (regular rhythm): Count large boxes between two R waves → Rate = 300 ÷ number of large boxes

Large boxes (R-R)	Heart rate
1	300
2	150
3	100
4	75
5	60
6	50

Method 2 (irregular rhythm): Count QRS complexes in a 6-second strip × 10.

Normal: 60–100 bpm | Bradycardia: <60 | Tachycardia: >100

Step 4 — PR Interval / AV Conduction

Normal: 0.12–0.20 s (3–5 small boxes)
Measured from start of P to start of QRS

Finding	Meaning
PR > 0.20 s (fixed)	1st-degree AV block
PR progressively lengthens → dropped QRS	2nd-degree block, Mobitz I (Wenckebach)
Fixed PR + sudden dropped QRS	2nd-degree block, Mobitz II
No relationship between P and QRS	3rd-degree (complete) AV block
PR < 0.12 s + delta wave	Pre-excitation (WPW)

Step 5 — QRS Interval

Normal: < 0.12 s (< 3 small boxes)
Measured from start to end of QRS

QRS Duration	Interpretation
< 0.12 s	Normal (narrow complex)
0.12–0.14 s	Incomplete bundle branch block
≥ 0.12 s (≥ 3 boxes)	Complete bundle branch block or ventricular rhythm

RBBB pattern: rSR' (M-shape) in V1, wide S in I and V6
LBBB pattern: Broad monophasic R in I, aVL, V5–V6; deep QS in V1; ST interpretation unreliable

Step 6 — QT / QTc Interval

Measured from start of QRS to end of T wave
Rate-dependent → use corrected QT (QTc)
Bazett formula: QTc = QT ÷ √(R-R interval in seconds)
Normal QTc: < 440 ms (men), < 460 ms (women)

QTc	Clinical significance
> 500 ms	High risk of torsades de pointes
Prolonged	Hypokalemia, hypomagnesemia, hypocalcemia, drugs (quinidine, sotalol, haloperidol, erythromycin), congenital LQTS
Short QT	Hypercalcemia, digitalis effect

Step 7 — Mean QRS Axis (Frontal Plane)

Normal axis: −30° to +90°

Quick method using leads I and aVF:

Lead I	Lead aVF	Axis
Positive	Positive	Normal (0° to +90°)
Positive	Negative	Left axis deviation (LAD)
Negative	Positive	Right axis deviation (RAD)
Negative	Negative	Extreme axis ("no man's land")

Left axis deviation (LAD, more negative than −30°): LBBB, left anterior fascicular block (LAFB), inferior MI, LVH
Right axis deviation (RAD, > +90°): RVH, RBBB, LPFB, lateral MI, Wolff-Parkinson-White, dextrocardia

Step 8 — P Waves

Normal: Upright in I, II, aVF; inverted in aVR; duration < 0.12 s; amplitude < 2.5 mm
Right atrial enlargement (P pulmonale): Tall, peaked P > 2.5 mm in II or V1 (positive component)
Left atrial abnormality (P mitrale): Broad, notched P in limb leads (> 0.12 s); biphasic P in V1 with prominent negative terminal component (> 1 mm × 1 mm)

Step 9 — QRS Voltages

Low voltage: QRS amplitude < 5 mm in all limb leads OR < 10 mm in all precordial leads
→ Causes: pericardial effusion, obesity, COPD/emphysema, hypothyroidism, infiltrative cardiomyopathy

Left ventricular hypertrophy (LVH):

SV1 + RV5 (or RV6) > 35 mm (Sokolow-Lyon)
Or R in aVL > 11 mm
Often accompanied by ST depression + T-wave inversion in lateral leads ("strain pattern")

Right ventricular hypertrophy (RVH):

R > S in V1 with right axis deviation
Dominant R in V1 ≥ 7 mm
ST-T inversion in V1–V3/V4

Step 10 — Precordial R-Wave Progression

R wave should grow progressively from V1 (small r) to V5–V6 (tall R)
Transition zone (R = S) normally at V3–V4
Poor R-wave progression (PRWP): R wave ≤ 3 mm through V1–V3
- Causes: anterior MI, LBBB, LVH, COPD, lead misplacement

Step 11 — Abnormal Q Waves

Pathological Q waves: Width ≥ 0.04 s (1 small box) AND depth ≥ 25% of the following R wave, in ≥ 2 contiguous leads
Indicate prior transmural (Q-wave) myocardial infarction

Localization by lead:

Leads with Q waves	Territory	Artery
V1–V4	Anterior	LAD
I, aVL, V5–V6	Lateral	LCx
II, III, aVF	Inferior	RCA (or LCx)
V1–V2	Posterior (tall R in V1–V2)	RCA/LCx

(Note: Small q waves in I, aVL, V5, V6 are normal septal q waves)

Step 12 — ST Segments

Should be isoelectric (on the baseline)
Measured at the J point (junction of QRS end and ST start)

ST Elevation:

≥ 1 mm in ≥ 2 contiguous limb leads OR ≥ 2 mm in ≥ 2 contiguous precordial leads = STEMI until proven otherwise
Also seen in: acute pericarditis (diffuse, saddle-shaped, PR depression), Brugada syndrome (coved type in V1–V3), LBBB, LVH, early repolarization (concave upward, notched J point)

ST Depression:

Subendocardial ischemia / NSTEMI
Digoxin effect (scooped/reverse tick)
LVH strain
Reciprocal changes in STEMI

Step 13 — T Waves

Normally upright in I, II, V3–V6; inverted in aVR; variable in III, aVL, V1
T-wave inversion: Ischemia, ventricular hypertrophy (strain), bundle branch block, CNS events (massive T-wave inversions in subarachnoid hemorrhage)
Peaked (hyperacute) T waves: Very early STEMI, hyperkalemia
Flattened T waves: Ischemia, hypokalemia, hypothyroidism

Step 14 — U Waves

Small deflection after the T wave, same polarity as T wave
Best seen in V2–V3
Prominent U waves: Hypokalemia, bradycardia, drugs (quinidine, amiodarone)
Inverted U waves: LVH, ischemia

Normal 12-Lead ECG for Reference

Normal 12-lead ECG showing all leads. The calibration pulse (1 mV, 200 ms) is visible at the left of each row. — Medical Physiology (Boron & Boulpaep)

Quick Reference: Normal Values

Parameter	Normal range
Heart rate	60–100 bpm
PR interval	0.12–0.20 s
QRS duration	< 0.12 s
QTc	< 440 ms (men), < 460 ms (women)
QRS axis	−30° to +90°
P wave duration	< 0.12 s
P wave amplitude	< 2.5 mm
R-wave transition	V3–V4

Key Principle

"Many mistakes in ECG interpretation are errors of omission. Therefore, a systematic approach is essential." — Harrison's Principles of Internal Medicine, 22nd ed.

Always:

Compare with previous ECGs
Correlate with the clinical context (age, symptoms, medications)
Do not rely solely on computer interpretation — always perform your own review

Sources: Harrison's Principles of Internal Medicine 22E (2025, McGraw-Hill); Medical Physiology, Boron & Boulpaep; Ganong's Review of Medical Physiology, 26th ed.

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