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ECG: Basics to Advanced - Exam Preparation Guide

Sources: Guyton & Hall Medical Physiology, Goldman-Cecil Medicine, Harrison's Principles of Internal Medicine, Costanzo Physiology, Harrison's 22nd Edition

PART 1: WHAT IS AN ECG?

An electrocardiogram (ECG or EKG) records the tiny electrical potentials that appear on the body surface as the cardiac impulse travels through the heart. When a cardiac impulse passes through the heart, electrical current spreads into the adjacent tissues and to the skin surface. Electrodes placed on the skin capture these potential differences and display them as a tracing over time.

Standard ECG: 12 leads (10 electrodes - 4 limb, 6 chest + right leg ground)
Paper speed: 25 mm/second (standard)
Calibration: 1 mV = 10 small squares (10 mm) vertically

PART 2: THE CONDUCTION SYSTEM

Understanding the conduction system is the foundation of ECG interpretation.

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

The pathway of electrical activation (as described in Goldman-Cecil Medicine):

SA node (sinoatrial node) - high lateral right atrium near the superior vena cava; the dominant pacemaker, fires spontaneously at the highest rate (60-100 bpm intrinsic)
Bachmann's bundle - speeds depolarization to the left atrium
AV node - the ONLY electrical connection between atria and ventricles; physiologic conduction delay occurs here
Bundle of His - exits the AV node; capable of rapid conduction
Right and left bundle branches - the left divides into left anterior and left posterior fascicles
Purkinje fibers - finest ramifications; distribute depolarization to ventricular myocardium
Ventricular myocardium - contracts after electrical depolarization

Backup pacemaker rates (important for exams):

Pacemaker site	Intrinsic rate
SA node	60-100 bpm
AV node / junctional	40-60 bpm
Ventricle (idioventricular)	20-40 bpm

PART 3: THE ECG PAPER

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

Grid squares:

Small box = 1 mm = 0.04 sec (horizontally), 0.1 mV (vertically)
Large box = 5 mm = 0.20 sec (horizontally), 0.5 mV (vertically)

PART 4: ECG WAVES, INTERVALS AND SEGMENTS

This section is the most tested area for beginners.

The Waves

Wave	What it represents	Notes
P wave	Atrial depolarization	Normal: <0.12 sec, <2.5 mm tall; positive in leads I, II, aVF
QRS complex	Ventricular depolarization	Q = first negative; R = positive; S = negative after R
T wave	Ventricular repolarization	Normally same direction as QRS; broad and slightly asymmetric
U wave	Late repolarization (Purkinje / mid-myocardial M-cells)	Small, after T wave; more prominent with hypokalemia and bradycardia

Key concept (Guyton & Hall): Atrial repolarization also occurs but is buried in the QRS complex and never seen on a normal ECG because the ventricular mass is far larger.

The Intervals

Interval	Measured from...	Normal value	What it includes
PR interval	Start of P to start of QRS	0.12 - 0.20 sec (120-200 ms)	Atrial depolarization + AV node delay
QRS duration	Start to end of QRS	<0.10 sec; abnormal >0.12 sec	Ventricular depolarization
QT interval	Start of QRS to end of T	~0.35-0.40 sec (rate-dependent)	Total ventricular electrical activity
QTc (corrected)	QT / √RR (Bazett's formula)	Men: <450 ms; Women: <460 ms	Heart-rate-corrected QT

The ST segment (from J-point to start of T wave): corresponds to the plateau of the ventricular action potential. Normally isoelectric (flat). Any deviation is clinically significant.

How to Calculate Heart Rate From ECG

Method 1 (regular rhythm):

Heart rate = 300 ÷ number of large boxes between R waves

Method 2 (exact):

Heart rate = 1500 ÷ number of small boxes between R waves

Example: RR interval = 5 large boxes → HR = 300/5 = 60 bpm

PART 5: THE 12 LEADS - WHERE THEY "LOOK"

Limb Leads (Frontal Plane)

Lead	View	Positive electrode
I	Lateral	Left arm
II	Inferior-lateral	Left leg
III	Inferior	Left leg
aVR	Right shoulder (cavity)	Right arm
aVL	High lateral	Left arm
aVF	Inferior	Left foot

Precordial (Chest) Leads - V1 to V6 (Horizontal Plane)

Lead	Anatomical territory
V1-V2	Right ventricle / septal
V3-V4	Anterior wall
V5-V6	Lateral wall
II, III, aVF	Inferior wall
I, aVL, V5-V6	Lateral wall

Lead placement (chest leads):

V1: 4th intercostal space, right sternal border
V2: 4th intercostal space, left sternal border
V3: Between V2 and V4
V4: 5th intercostal space, midclavicular line
V5: Anterior axillary line (same horizontal level as V4)
V6: Midaxillary line (same horizontal level as V4)

PART 6: CARDIAC AXIS

The QRS axis represents the average direction of ventricular depolarization in the frontal plane.

Axis	Degrees	Normal?
Normal	-30° to +90°	Yes
Left axis deviation (LAD)	More negative than -30°	Abnormal
Right axis deviation (RAD)	More positive than +90°	Abnormal
Extreme (northwest)	-90° to ±180°	Abnormal

Quick method to estimate axis:

Look at leads I and aVF
If QRS is positive in I and positive in aVF: Normal axis
If QRS is positive in I and negative in aVF: Left axis deviation
If QRS is negative in I and positive in aVF: Right axis deviation
If QRS is negative in both: Extreme axis

Causes of LAD: Left anterior fascicular block (most common in adults), inferior MI, WPW, LBBB

Causes of RAD: Right ventricular hypertrophy, left posterior fascicular block, pulmonary embolism, lateral MI, WPW, RBBB, normal variant (young/thin individuals)

PART 7: SYSTEMATIC APPROACH TO READING AN ECG

Always use a systematic approach to avoid missing findings. The Goldman-Cecil Medicine approach:

Rate - Calculate heart rate
Rhythm - Is it regular? Are P waves present? Is every P followed by a QRS?
Axis - Normal, LAD, or RAD?
PR interval - Short/normal/prolonged?
QRS duration - Narrow or wide? Any bundle branch block?
ST segments - Elevation, depression, or normal?
T waves - Normal direction? Peaked, inverted, flat?
QT interval - Prolonged?
Examine by regions - Septal (V3-V4), anterior (V2-V4), lateral (I, aVL, V5-V6), inferior (II, III, aVF)

PART 8: HEART BLOCKS (CONDUCTION DELAYS)

AV Blocks

Block	ECG finding	Clinical significance
1st degree AV block	PR interval >200 ms; every P conducts	Usually benign
2nd degree - Mobitz I (Wenckebach)	Progressive PR lengthening until a P wave is dropped (no QRS); then resets	AV node disease; often reversible
2nd degree - Mobitz II	Fixed PR interval, sudden non-conducted P wave	Below AV node; can progress to complete block
3rd degree (Complete)	P waves and QRS are independent; AV dissociation	Medical emergency; needs pacemaker

Memory tip: Mobitz I = "longer, longer, longer, DROP (then have a shorter break)" = Wenckebach

Bundle Branch Blocks (QRS >0.12 sec)

RBBB and LBBB ECG patterns compared to normal in leads V1 and V6

Right Bundle Branch Block (RBBB):

QRS ≥0.12 sec
rSR' pattern in V1 ("rabbit ears" or "M" pattern)
Wide, slurred S wave in I, V5, V6
T-wave inversion in V1-V3 (secondary change)
Mnemonic: MaRRoW (RBBB: M in V1, W in V6)

Left Bundle Branch Block (LBBB):

QRS ≥0.12 sec
Broad monophasic R (no Q) in I, V5, V6
QS or rS in V1
T-wave inversion in lateral leads (secondary change - concordance with QRS)
Mnemonic: WiLLiaM (LBBB: W in V1, M in V6)

Clinical pearl: New LBBB with chest pain = STEMI equivalent until proven otherwise. LBBB is a marker of significant cardiac disease (coronary artery disease, hypertensive heart disease, cardiomyopathy, aortic valve disease).

Fascicular blocks (hemiblocks): Affect a fascicle of the left bundle; QRS <0.12 sec but axis shifts

Left anterior fascicular block: LAD (axis more negative than -45°)
Left posterior fascicular block: RAD (axis more positive than +110°); rare; must exclude other causes

PART 9: CHAMBER ENLARGEMENT

Finding	ECG criteria
Right atrial enlargement (P pulmonale)	P wave >2.5 mm tall in II, III, aVF; peaked
Left atrial abnormality (P mitrale)	P wave >0.12 sec wide; notched (bifid) in II; biphasic in V1 with negative terminal portion >1 mm
Left ventricular hypertrophy (LVH)	SV1 + RV5 or RV6 >35 mm (Sokolow-Lyon); may have ST depression + T-wave inversion ("strain") in lateral leads
Right ventricular hypertrophy (RVH)	R/S ratio >1 in V1; RAD; ST depression + T-wave inversion in V1-V3; dominant R in V1

PART 10: ISCHEMIA AND INFARCTION

Phases of STEMI (ST Elevation MI)

Phase	Time from onset	ECG changes
Hyperacute	Minutes	Tall, broad "hyperacute" T waves
Acute	Hours	ST elevation (tombstone); then T-wave inversion begins
Subacute	Days	ST elevation settling; deep Q waves forming; T-wave inversions
Old / chronic	Weeks-months	Pathological Q waves; ST may normalize

Definition of pathological Q wave: >0.04 sec (1 small box) wide AND >25% of R wave height in that lead.

Localization of MI by Leads

Territory	Leads with changes	Artery involved
Anterior	V1-V4	LAD (left anterior descending)
Anteroseptal	V1-V3	LAD (septal branches)
Lateral	I, aVL, V5-V6	Circumflex (LCx)
Anterolateral	V1-V6 + I, aVL	LAD or LCx
Inferior	II, III, aVF	RCA (right coronary artery); rarely LCx
Posterior	Reciprocal ST depression in V1-V3; tall R in V1	RCA or LCx
Right ventricular	ST elevation in V3R, V4R (right chest leads)	Proximal RCA

Key rule from Harrison's: ST elevations in a regional distribution (matching a coronary territory) = STEMI. Non-regional ST changes may represent Brugada, pericarditis, hypokalemia, etc.

Pericarditis vs. STEMI:

Feature	STEMI	Pericarditis
ST elevation	Regional (one territory)	Diffuse (multiple territories)
Shape	Convex upward ("tombstone")	Concave upward ("saddle-shaped")
Reciprocal changes	Present	Absent (except aVR)
PR depression	Absent	Present (classic)
Q waves	Develop	Do not develop

NSTEMI / Unstable Angina

No ST elevation
ST depression or T-wave inversions in the territory of ischemia
Subendocardial ischemia: ST vector shifts toward the endocardium; overlying leads show ST depression

PART 11: ARRHYTHMIAS

Tachyarrhythmias (HR >100 bpm)

Step 1: Is the QRS narrow (<0.12 sec) or wide (≥0.12 sec)?

Narrow Complex Tachycardias (SVT - Supraventricular)

Rhythm	Rate	P waves	Regularity
Sinus tachycardia	100-180	Normal, before each QRS	Regular
Atrial fibrillation (AF)	Ventricular 100-180	None; fibrillatory baseline	Irregularly irregular
Atrial flutter	Atrial 300; ventricular 150 (2:1)	"Sawtooth" waves in II, III, aVF	Regular or regular-irregular
AVNRT	150-250	Retrograde P (buried in QRS or just after)	Regular
AVRT (WPW)	150-250	Retrograde P after QRS	Regular
Multifocal atrial tachycardia (MAT)	100-150	≥3 different P morphologies	Irregular

Atrial Fibrillation is the most common sustained cardiac arrhythmia. Key features:

Absent P waves; replaced by irregular fibrillatory baseline (f waves)
Irregularly irregular ventricular response
Narrow QRS (unless aberrant conduction or WPW)

Atrial Flutter:

"Sawtooth" pattern at 300 bpm in inferior leads (II, III, aVF)
Usually 2:1 AV block → ventricular rate ~150 bpm
Regular ventricular response

Wide Complex Tachycardias

Ventricular tachycardia (VT): QRS >0.12 sec; AV dissociation; fusion beats; capture beats; QRS axis often extreme
SVT with aberrant conduction (BBB): Wide QRS but SVT origin
Pre-excited tachycardia (WPW): Delta waves, short PR

Brugada criteria to distinguish VT from SVT with BBB:

Absence of RS complex in all precordial leads → VT
RS interval >100 ms in any precordial lead → VT
AV dissociation → VT
Morphology criteria for VT (if none of above) → SVT with BBB

In doubt: always treat wide complex tachycardia as VT until proven otherwise

Special Patterns

WPW (Wolff-Parkinson-White):

Short PR (<0.12 sec)
Delta wave (slurred upstroke at start of QRS)
Wide QRS
ST-T changes discordant from QRS
Accessory pathway (Bundle of Kent) bypasses the AV node

Long QT Syndrome:

QTc >450 ms (men), >460 ms (women)
Predisposes to Torsades de Pointes (TdP) - a polymorphic VT
Causes: drugs (antiarrhythmics, antibiotics, antipsychotics), hypokalemia, hypomagnesemia, congenital LQTS
Never give QT-prolonging drugs without checking baseline QTc

Brugada Syndrome:

ST elevation in V1-V3 with RBBB-like pattern (Type 1: "coved" pattern with ≥2 mm J-point elevation)
Normal coronary arteries
Risk of sudden cardiac death from VF
Triggered by fever, sodium channel blockers

Bradyarrhythmias (HR <60 bpm)

Rhythm	Rate	ECG finding
Sinus bradycardia	<60	Normal P, QRS, T but slow
Junctional rhythm	40-60	No P wave or retrograde P (negative in II)
Idioventricular rhythm	20-40	Wide QRS, no P, very slow
Complete heart block	<40 (escape)	P waves and QRS completely independent

PART 12: ELECTROLYTE AND DRUG EFFECTS

Hyperkalemia (High K+) - Progressive Changes

Level	ECG change
5.5-6.5 mEq/L	Tall peaked (tented) T waves in precordial leads
6.5-7.5 mEq/L	PR prolongation; P wave flattening
7.5-8.0 mEq/L	Wide QRS (>0.12 sec); loss of P waves
>8.0 mEq/L	Sine wave pattern; VF; asystole

Hypokalemia (Low K+)

Flattened/inverted T waves
Prominent U wave (may merge with T, appearing as prolonged QT)
ST depression
QTc prolongation

Hypercalcemia

Short QT interval (most specific finding)

Hypocalcemia

Long QT interval (prolonged ST segment)

Digoxin

"Salvador Dali mustache" or "reverse tick" ST depression (scooped/concave) in lateral leads
PR prolongation, bradycardia
AV block at toxic levels
Bidirectional VT at very toxic levels

Tricyclic Antidepressants (Overdose)

Sinus tachycardia (most common)
QRS prolongation >100 ms (sodium channel blockade - most dangerous finding)
Prolonged QT
Right axis deviation of terminal 40 ms
RBBB pattern
Risk of VF at QRS >160 ms

PART 13: COMMON EXAM HIGH-YIELD PATTERNS

Pattern	Diagnosis to think of
ST elevation V1-V3 + RBBB morphology (coved)	Brugada syndrome
Short PR + delta wave	WPW
Sawtooth at 300 bpm	Atrial flutter
Irregularly irregular, no P waves	Atrial fibrillation
Progressive PR lengthening → dropped beat	Mobitz I (Wenckebach)
Fixed PR → sudden dropped beat	Mobitz II
P and QRS completely independent	Complete heart block
RSR' in V1 + wide S in V5/V6	RBBB
Wide R (no Q) in V5/V6 + QS in V1	LBBB
Peaked T waves + wide QRS	Hyperkalemia
Prominent U wave	Hypokalemia
Short QT	Hypercalcemia
Long QT	Hypocalcemia, hypokalemia, drugs
Diffuse concave ST + PR depression	Pericarditis
Regional ST elevation + reciprocal changes	STEMI
S1Q3T3	Pulmonary embolism
Tall R in V1 + RAD + RVH strain	Pulmonary hypertension

PART 14: NORMAL VALUES SUMMARY

Parameter	Normal range
Heart rate	60-100 bpm
PR interval	0.12-0.20 sec
QRS duration	<0.10 sec (borderline 0.10-0.12; abnormal >0.12)
QT interval	~0.35-0.40 sec (rate dependent)
QTc (Bazett)	Men <450 ms; Women <460 ms
QRS axis	-30° to +90°
P wave	<0.12 sec, <2.5 mm

PART 15: MEMORY AIDS AND MNEMONICS

"WiLLiaM MaRRoW" → LBBB = W in V1, M in V6; RBBB = M in V1, W in V6
"DIRT" → ST Depression = Ischemia, RBBB, Tachycardia (reciprocal)
"ELEVATION" → ST Elevation in STEMI
Rate rule of 300: 300-150-100-75-60-50 (1,2,3,4,5,6 large boxes between R waves)
AV blocks: "Longer, longer, longer, DROP, shorter RP" = Wenckebach/Mobitz I
Axis: "Thumbs up = normal; Left thumb up, right down = LAD; Left down, right up = RAD"

This guide draws from:

Guyton and Hall Textbook of Medical Physiology, Chapters 11-12
Goldman-Cecil Medicine, 27th Edition, Chapter 42
Harrison's Principles of Internal Medicine, 22nd Edition, Chapter 247
Costanzo Physiology, 7th Edition, Chapter 4

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