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ECG (Electrocardiogram) - Complete Guide

1. What is an ECG?

The ECG (electrocardiogram, also written EKG) records tiny electrical potential differences on the body surface that reflect the heart's electrical activity. These potentials arise because the myocardium does not depolarize all at once - atria before ventricles, with specific sequences in each - creating measurable voltage differences detectable by skin electrodes.

First introduced by Einthoven in the early 1900s, the standard ECG records 10 seconds of cardiac activity from 12 vantage points (leads) derived from 10 electrodes.

Goldman-Cecil Medicine, p. 421
Costanzo Physiology 7th Edition, p. 148

2. The Cardiac Conduction System (Foundation)

Before reading an ECG, you must understand what generates each waveform:

Cardiac conduction system showing SA node, AV node, Bundle of His, right and left bundle branches, and Purkinje fibers

The conduction system has four components:

Structure	Location	Function
SA node	High right atrium near SVC junction	Primary pacemaker - fires spontaneously at highest rate
AV node	Near coronary sinus opening, AV septum	Physiologic conduction delay (protects ventricles from rapid atrial rates)
Bundle of His + Bundle Branches	Interventricular septum	Rapid conduction to both ventricles
Purkinje fibers	Subendocardial network	Final delivery of impulse to ventricular myocardium

The AV node is an area of relatively slow conduction - this delay between atrial and ventricular contraction is normal and reflected in the PR interval. Conduction velocity in the His-Purkinje system is much faster than in atrial tissue, which is why the QRS duration is similar to P wave duration despite the far larger ventricular mass.

Gray's Anatomy for Students, p. 247
Goldman-Cecil Medicine, p. 421

3. ECG Paper and Calibration

Standard ECG paper has two grid sizes:

	Small box (1 mm)	Big box (5 mm)
Time (X-axis)	0.04 sec (40 ms)	0.2 sec (200 ms)
Voltage (Y-axis)	0.1 mV	0.5 mV

Standard recording speed: 25 mm/sec Standard calibration: 10 mm = 1 mV

A standard ECG = 10 seconds total, with leads grouped: I/II/III → aVR/aVL/aVF → V1/V2/V3 → V4/V5/V6 (each group 2.5 seconds), plus a full 10-second rhythm strip.

Goldman-Cecil Medicine, p. 422

4. The ECG Waveforms

P Wave

Represents atrial depolarization
Duration reflects conduction time through the atria
Atrial repolarization is not visible (buried within the QRS complex)
Normal duration: < 120 ms (3 small boxes)

PR Interval

From onset of P wave to onset of QRS complex
Represents total conduction time: atria + AV node + His-Purkinje
The PR segment (flat portion) = AV node conduction delay
Normal: 120-200 ms (3-5 small boxes)
Prolonged PR (>200 ms) = 1st-degree AV block (more accurately: delayed AV conduction)
Short PR = preexcitation (WPW), junctional rhythm, or enhanced AV conduction

QRS Complex

Represents ventricular depolarization
Q = initial negative deflection
R = positive deflection
S = negative deflection following a positive deflection
Capital letters (Q, R, S) = amplitude ≥5 mm; lowercase (q, r, s) = <5 mm
Normal duration: 75-110 ms (just under 3 small boxes)
Prolonged QRS (>120 ms) = bundle branch block or intraventricular conduction delay

ST Segment

From end of QRS to beginning of T wave (at the J point)
Represents the plateau phase of ventricular action potential
Normally isoelectric (flat)
Elevation or depression = ischemia, infarction, pericarditis, other

T Wave

Represents ventricular repolarization
Normally same direction as QRS in most leads
Normally upright in I, II, V3-V6; inverted in aVR

QT Interval

From onset of QRS to end of T wave
Represents total ventricular electrical activity (depolarization + repolarization)
Must be rate-corrected (QTc) using Bazett's formula:

QTc = QT / √RR (both in seconds)

Normal QTc: men ≤450 ms; women ≤460 ms
Prolonged QTc → risk of torsades de pointes

U Wave

Small deflection after T wave, same polarity
Not always visible; prominent U waves suggest hypokalemia
Costanzo Physiology 7th Edition, p. 149
Medical Physiology (Boron & Boulpaep), p. 733
Goldman-Cecil Medicine, p. 422-423

5. Normal Intervals - Quick Reference

Parameter	Normal Range
Heart rate	50-100 bpm
P wave duration	< 120 ms
PR interval	120-200 ms
QRS duration	75-110 ms
QTc (males)	390-450 ms
QTc (females)	390-460 ms
QRS axis	-30° to +90°

Goldman-Cecil Medicine (Table 42-1)

6. The 12 Leads - Where They Look

The 12 leads provide 12 different "camera angles" on the heart's electrical activity.

Limb Leads (Frontal Plane)

Bipolar leads measure voltage difference between two limb electrodes:

Lead I: Right arm (-) vs. Left arm (+) → looks left
Lead II: Right arm (-) vs. Left leg (+) → looks down-left
Lead III: Left arm (-) vs. Left leg (+) → looks down-right

Augmented unipolar leads compare one limb to a combined reference:

aVR: Right arm → looks toward right shoulder (normally negative P and QRS)
aVL: Left arm → looks toward left shoulder
aVF: Left foot → looks inferiorly (normally positive P and QRS)

Precordial Leads (Horizontal Plane)

Placed on specific chest wall positions:

Precordial lead positioning and R-wave progression from V1 to V6

Lead	Position	Looks at
V1	4th intercostal space, right sternal border	Right ventricle / septum
V2	4th intercostal space, left sternal border	Right ventricle / septum
V3	Between V2 and V4	Anterior wall
V4	5th intercostal space, midclavicular line	Anterior wall
V5	Anterior axillary line, same level as V4	Lateral wall
V6	Midaxillary line, same level as V4	Lateral wall

Regional groupings for pathology:

Inferior: II, III, aVF (RCA territory)
Anterior: V1-V4 (LAD territory)
Lateral: I, aVL, V5-V6 (LCx territory)
Septal: V1-V2

Normal Precordial R-Wave Progression

V1: small r, deep S
Transition (R = S) at V3 or V4
V5-V6: large R, small s (septal q waves are normal here)
Goldman-Cecil Medicine, p. 423-424

7. Calculating Heart Rate

Regular rhythm: Count the number of big boxes between two consecutive R waves, then use:

RR interval (big boxes)	Heart rate
1 big box	300 bpm
2 big boxes	150 bpm
3 big boxes	100 bpm
4 big boxes	75 bpm
5 big boxes	60 bpm
6 big boxes	50 bpm

Irregular rhythm (e.g., AF): Count beats in 10-second strip × 6 = bpm

Formula: HR (bpm) = 60,000 / RR interval (ms)

8. QRS Axis

The QRS axis describes the mean direction of ventricular depolarization in the frontal plane.

Axis	Range	Interpretation
Normal	-30° to +90°	Normal
Left axis deviation (LAD)	-30° to -90°	Left anterior fascicular block, inferior MI, etc.
Right axis deviation (RAD)	+90° to +180°	Right ventricular hypertrophy, left posterior fascicular block, etc.
Extreme RAD	-90° to ±180°	Severe pathology

Quick estimation method:

Find the lead where QRS is most isoelectric (equal positive and negative) - the axis is perpendicular to this lead
Confirm direction using the adjacent leads

Shortcut:

If QRS is positive in both Lead I and aVF → normal axis
Positive I, negative aVF → possible LAD
Negative I, positive aVF → RAD
Goldman-Cecil Medicine, p. 424-425

9. A Systematic Approach to ECG Interpretation

Harrison's Principles of Internal Medicine lists 14 parameters to assess in every ECG:

Standardization & technical quality - correct calibration (1 mV = 10 mm), lead placement, artifacts
Rhythm - regular vs. irregular; sinus vs. non-sinus
Heart rate - use RR interval method above
PR interval / AV conduction - normal 120-200 ms
QRS duration - normal 75-110 ms
QT / QTc interval - calculate and correct for rate
QRS axis - normal, LAD, or RAD
P waves - morphology, duration, axis
QRS voltages - high (hypertrophy) or low
Precordial R-wave progression - normal transition V3-V4
Abnormal Q waves - pathological Q = width >40 ms or depth >25% R wave
ST segments - elevation or depression
T waves - direction, morphology, inversions
U waves - presence, size, polarity

Comparison with previous ECGs is invaluable.

Harrison's Principles of Internal Medicine 22E, p. 1592

10. A Normal 12-Lead ECG - What to Expect

Normal 12-lead ECG showing all leads I, II, III, aVR, aVL, aVF, V1-V6

In a normal ECG:

Sinus rhythm (P before every QRS, consistent PR interval)
P waves upright in I, II, aVF; negative in aVR
PR interval 120-200 ms
QRS < 110 ms
Axis between -30° and +90°
R-wave transition at V3-V4
Septal q waves in V5-V6 (normal - from early septal depolarization)
T waves upright in I, II, V3-V6

11. Key Abnormal Patterns (Overview)

Finding	Significance
PR > 200 ms	1st-degree AV block
Progressively lengthening PR → dropped QRS	2nd-degree AV block (Mobitz I / Wenckebach)
Fixed PR + dropped QRS	2nd-degree AV block (Mobitz II)
No relationship between P and QRS	3rd-degree (complete) AV block
Wide QRS (≥120 ms) with RBBB pattern	Right bundle branch block
Wide QRS with LBBB pattern	Left bundle branch block
Short PR + delta wave	WPW syndrome (preexcitation)
ST elevation (regional)	STEMI (until proven otherwise)
ST depression	Ischemia, NSTEMI, digoxin effect
Pathological Q waves	Prior myocardial infarction
Peaked T waves	Hyperkalemia
Prolonged QTc	Drug effects, hypokalemia, hypomagnesemia, congenital LQTS
J wave (Osborn wave)	Hypothermia

Summary Flow for Reading Any ECG

Rate → Rhythm → Axis → Intervals (PR, QRS, QTc) 
→ P waves → QRS morphology → ST segments → T waves → U waves
→ Compare with previous → Clinical correlation

The ECG is a powerful tool, but errors are common when interpretation is rushed or unsystematic. As Harrison's notes: "Many mistakes in ECG interpretation are errors of omission."

Sources: Goldman-Cecil Medicine 2-Volume Set (Ganz & Link); Harrison's Principles of Internal Medicine 22E; Costanzo Physiology 7th Ed; Medical Physiology (Boron & Boulpaep); Gray's Anatomy for Students; Miller's Anesthesia 10th Ed

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