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Arterial Blood Gas (ABG) - A Complete Beginner's Guide

Sources: Roberts & Hedges' Clinical Procedures in Emergency Medicine - Tintinalli's Emergency Medicine - Symptom to Diagnosis: An Evidence Based Guide - Frameworks for Internal Medicine

1. What Is an ABG?

An Arterial Blood Gas (ABG) is a blood test drawn from an artery (usually the radial artery) that directly measures how well the lungs are exchanging oxygen and carbon dioxide, and gives a complete picture of the body's acid-base status.

It reports:

pH - how acidic or alkaline the blood is
PaCO₂ - partial pressure of carbon dioxide (respiratory parameter)
PaO₂ - partial pressure of oxygen (oxygenation)
HCO₃⁻ - bicarbonate (metabolic/renal parameter, calculated from pH and PaCO₂)
SaO₂ - oxygen saturation of hemoglobin

2. Normal ABG Values (Memorize These)

Parameter	Normal Range	Significance
pH	7.35 - 7.45	Acid-base balance
PaCO₂	35 - 45 mmHg	Respiratory component
HCO₃⁻	22 - 26 mEq/L	Metabolic component
PaO₂	80 - 100 mmHg	Oxygenation
SaO₂	95 - 100%	Hemoglobin saturation

Simple rule: pH 7.40, PaCO₂ 40, HCO₃⁻ 24 are your anchor values.

3. Understanding pH - The Foundation

pH < 7.35 = Acidosis (too much acid)
pH > 7.45 = Alkalosis (too much base)
pH 7.35-7.45 = Normal (but a disorder can still exist with compensation!)

The pH is governed by the Henderson-Hasselbalch relationship:

pH depends on the ratio of HCO₃⁻ (kidneys) to PaCO₂ (lungs)

Think of it this way:

Lungs control CO₂ (respiratory component) - fast compensation (minutes to hours)
Kidneys control HCO₃⁻ (metabolic component) - slow compensation (3-5 days)

4. The Four Primary Acid-Base Disorders

Disorder	pH	Primary Change	Cause
Respiratory Acidosis	Low (<7.35)	PaCO₂ high (>45)	Hypoventilation - lung disease, sedation, neuromuscular
Respiratory Alkalosis	High (>7.45)	PaCO₂ low (<35)	Hyperventilation - anxiety, pain, PE, liver disease, altitude
Metabolic Acidosis	Low (<7.35)	HCO₃⁻ low (<22)	Diarrhea, DKA, lactic acidosis, renal failure
Metabolic Alkalosis	High (>7.45)	HCO₃⁻ high (>26)	Vomiting, diuretics, hypokalemia

5. Compensation - The Body's Auto-Correction

When a primary disorder occurs, the other system tries to compensate to pull pH back toward normal. Compensation is never complete - it does not fully normalize pH.

Primary Disorder	Compensatory Response
Metabolic acidosis	Increased ventilation (blows off CO₂, lowers PaCO₂)
Metabolic alkalosis	Decreased ventilation (retains CO₂, raises PaCO₂)
Respiratory acidosis	Kidneys retain HCO₃⁻ and excrete H⁺
Respiratory alkalosis	Kidneys excrete HCO₃⁻ and retain H⁺

Compensation Formulas (Expected Compensation)

Primary Disorder	Expected Compensation
Metabolic acidosis	ΔPaCO₂ = 1.3 × ΔHCO₃⁻ (Winter's formula)
Metabolic alkalosis	ΔPaCO₂ = 0.6 × ΔHCO₃⁻
Acute resp. acidosis	HCO₃⁻ rises 1 mEq/L per 10 mmHg ↑ PaCO₂
Chronic resp. acidosis	HCO₃⁻ rises 4 mEq/L per 10 mmHg ↑ PaCO₂
Acute resp. alkalosis	HCO₃⁻ falls 2 mEq/L per 10 mmHg ↓ PaCO₂
Chronic resp. alkalosis	HCO₃⁻ falls 5 mEq/L per 10 mmHg ↓ PaCO₂

Key point: If actual compensation does NOT match expected, a mixed disorder is present.

Here is the classic Acid-Base Map - it visually plots pH vs. PaCO₂ with HCO₃⁻ isopleths to identify simple and mixed disorders:

Acid-Base Map showing pH vs PaCO₂ with zones for all acid-base disorders

The "N" in the center is the normal point. Zone 1 = mixed resp+metabolic acidosis, Zone 2 = mixed resp+metabolic alkalosis, Zone 3 = metabolic alkalosis + resp acidosis, Zone 4 = metabolic acidosis + resp alkalosis.

6. Step-by-Step ABG Interpretation (The 5-Step Method)

Step 1: Is the pH acidotic or alkalotic?

pH < 7.35 → Acidosis
pH > 7.45 → Alkalosis
pH 7.35-7.45 → Normal (but check if compensation is masking a disorder)

Step 2: Identify the primary cause - respiratory or metabolic?

Look at PaCO₂ and HCO₃⁻
If pH is low (acidosis):
- PaCO₂ high → Respiratory acidosis (CO₂ is acid)
- HCO₃⁻ low → Metabolic acidosis
If pH is high (alkalosis):
- PaCO₂ low → Respiratory alkalosis
- HCO₃⁻ high → Metabolic alkalosis
The parameter that matches the pH direction is the primary disorder

Step 3: Is compensation appropriate?

Use the formulas above. If the actual value differs from expected:

Less compensation than expected → Second primary disorder adding to the problem
More compensation than expected → Second primary disorder working against the first

Step 4: If metabolic acidosis - calculate the Anion Gap (AG)

Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻)

Normal AG = 12 ± 4 mEq/L
AG > 16 → Anion Gap Metabolic Acidosis (MUDPILES causes)
AG normal → Non-Anion Gap Metabolic Acidosis (HARDUPS causes)

MUDPILES (high anion gap causes):

Methanol, Uremia, DKA, Propylene glycol/Paraldehyde, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates

HARDUPS (normal anion gap causes):

Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureteral diversion, Pancreatic fistula, Saline infusion

Step 5: If AG metabolic acidosis - calculate the Delta-Delta (ΔΔ)

ΔAG = calculated AG - 12

In pure AG metabolic acidosis, ΔAG ≈ decrease in HCO₃⁻ (ratio ~1:1).

If HCO₃⁻ is lower than expected from the ΔAG → also a non-AG metabolic acidosis
If HCO₃⁻ is higher than expected from the ΔAG → also a metabolic alkalosis hidden underneath

7. Oxygenation Assessment

The ABG also tells you about oxygenation (separate from acid-base):

PaO₂ normal: 80-100 mmHg
PaO₂ 60-80 mmHg: Mild hypoxemia
PaO₂ 40-60 mmHg: Moderate hypoxemia
PaO₂ < 40 mmHg: Severe hypoxemia

The A-a Gradient (Alveolar-arterial oxygen gradient)

This tells you whether low O₂ is due to the lungs failing or something outside the lungs (hypoventilation only).

PAO₂ = FiO₂ × (Patm - PH₂O) - PaCO₂/0.8

On room air (FiO₂ = 0.21): PAO₂ ≈ 150 - (PaCO₂ / 0.8)
A-a gradient = PAO₂ - PaO₂
Normal A-a gradient ≈ age/4 + 4 (increases with age)

A-a Gradient	Interpretation
Normal (<15)	Hypoventilation or low FiO₂ - lungs are fine
Elevated (>15)	Lung pathology (pneumonia, PE, ARDS, pulmonary edema)

8. Arterial vs. Venous Blood Gas

Feature	Arterial (ABG)	Venous (VBG)
pH	Reference standard	~0.05 lower than ABG
PaCO₂	Reference standard	Up to 20 mmHg higher
PaO₂	Reference standard	Cannot use for oxygenation
Lactate	Yes (standard)	Correlates well with ABG for normal/high values
Clinical use	Gold standard	Useful quick screen; abnormal values need ABG confirmation

(Tintinalli's Emergency Medicine, p. 121)

9. Worked Examples

Example A - Metabolic Acidosis with Compensation

Values: Na⁺ 133, Cl⁻ 118, pH 7.26, PaCO₂ 13, HCO₃⁻ 5

pH 7.26 → Acidosis
HCO₃⁻ 5 (low) + PaCO₂ 13 (low) → Primary = metabolic acidosis (low HCO₃⁻ drives pH down; low PaCO₂ is compensation, not the cause)
Expected PaCO₂ = 40 - (1.3 × (25-5)) = 40 - 26 = 14 mmHg → Actual is 13, almost exactly matching → compensation is appropriate, single disorder
AG = 133 - (118 + 5) = 10 → Normal AG → Non-anion gap metabolic acidosis
Cause: diarrhea (classic non-AG acidosis from HCO₃⁻ loss)

(Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 1734)

Example B - Lactic Acidosis in Sepsis

Values: Na⁺ 138, Cl⁻ 102, pH 7.29, PaCO₂ 30, HCO₃⁻ 14

pH 7.29 → Acidosis
HCO₃⁻ 14 (low) → Primary = metabolic acidosis
AG = 138 - (102 + 14) = 22 → Elevated → Anion Gap metabolic acidosis
Cause in context (hypotension + infection): lactic acidosis from septic shock

(Symptom to Diagnosis, p. 72)

10. Quick Reference Summary Table

pH	PaCO₂	HCO₃⁻	Disorder
↓	↑	Normal (or ↑ if compensating)	Respiratory Acidosis
↑	↓	Normal (or ↓ if compensating)	Respiratory Alkalosis
↓	Normal (or ↓ if compensating)	↓	Metabolic Acidosis
↑	Normal (or ↑ if compensating)	↑	Metabolic Alkalosis

Summary of the Learning Path

Master normal values - pH 7.40, PaCO₂ 40, HCO₃⁻ 24
Identify acidosis vs. alkalosis from pH
Find the primary disorder - which parameter matches the pH change
Check compensation using formulas - unmatch = mixed disorder
Calculate anion gap in metabolic acidosis
Calculate delta-delta to find hidden second disorder
Assess oxygenation via PaO₂ and A-a gradient separately

Once you're comfortable with simple disorders, practice with clinical cases involving mixed disorders - that's where ABG interpretation truly shines.

Would you like to go deeper into any specific area - such as:

Mixed acid-base disorders with practice cases?
Causes and management of each disorder?
Oxygenation failures and the A-a gradient?
A practice quiz with ABG values to interpret?

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ECG Abnormalities - A Comprehensive Guide

Sources: Rosen's Emergency Medicine - Tintinalli's Emergency Medicine - Frameworks for Internal Medicine - Goldman-Cecil Medicine - Miller's Anesthesia

Since you already know P, QRS, and T waves, we'll build directly on that to cover all major ECG abnormalities systematically.

Overview: Categories of ECG Abnormalities

There are 6 major categories to master:

Rate & Rhythm abnormalities (arrhythmias)
Conduction blocks (AV blocks, bundle branch blocks)
Ischemia & Infarction (ST changes, Q waves)
Chamber enlargement (hypertrophy, dilation)
Interval abnormalities (PR, QRS, QT)
Miscellaneous (electrolyte effects, pericarditis, WPW)

Category 1: Rate & Rhythm Abnormalities

Normal Sinus Rhythm (your baseline reference)

Rate: 60-100 bpm
Every P wave followed by a QRS
Regular P-P and R-R intervals
PR interval: 120-200 ms

1A. Sinus Tachycardia

Rate > 100 bpm
Normal P before every QRS, normal morphology
Causes: fever, pain, hypovolemia, anxiety, PE, hyperthyroidism
Not a primary rhythm problem - treat the cause

1B. Sinus Bradycardia

Rate < 60 bpm
Normal P before every QRS
Causes: athletes, vagal tone, beta-blockers, hypothyroidism, inferior MI
Treat only if symptomatic

1C. Atrial Fibrillation (AF)

The most common sustained arrhythmia

ECG features:

No visible P waves - replaced by chaotic, irregular fibrillatory (f) waves
Irregularly irregular R-R intervals (classic)
Narrow QRS usually (unless aberrant conduction)
Rate usually 100-160 bpm if uncontrolled

Memory trick: "No P, Irregular, Irregular"

Causes: Hypertension, valvular disease, heart failure, alcohol (holiday heart), hyperthyroidism, ischemia

1D. Atrial Flutter

ECG features:

Sawtooth flutter (F) waves at ~300/min, best seen in II, III, aVF
Regular atrial rate ~300; ventricular rate depends on AV block ratio (usually 2:1 = 150 bpm, or 4:1 = 75 bpm)
Regular R-R intervals (unlike AF)

Memory trick: "Sawtooth = Flutter" - look for the sawtooth in the inferior leads

1E. Supraventricular Tachycardia (SVT)

ECG features:

Sudden onset narrow-complex tachycardia, rate 150-250 bpm
P waves often buried in or just after the QRS (retrograde P)
Regular R-R intervals
Abrupt start and stop ("paroxysmal")

Types:

AVNRT (most common) - retrograde P just after QRS, "pseudo-R'" in V1
AVRT (WPW pathway) - delta wave at baseline
Atrial tachycardia - abnormal P wave morphology before each QRS

1F. Ventricular Tachycardia (VT)

ECG features:

Wide QRS (>120 ms), rate >100 bpm (usually 150-250 bpm)
Regular or slightly irregular
AV dissociation (P waves marching independently, no relationship to QRS) - pathognomonic
Capture beats (narrow QRS amid wide complexes) - confirms VT
Fusion beats (partially conducted beat) - confirms VT
Concordance: all QRS in precordial leads pointing same direction (positive or negative)

Key rule: Wide-complex tachycardia = VT until proven otherwise (never assume SVT with aberrancy in an unstable patient)

1G. Ventricular Fibrillation (VF)

Completely chaotic, irregular, no organized complexes
No QRS, no P, no T visible
Immediately life-threatening - start CPR

Category 2: Conduction Blocks

2A. AV Blocks (Heart Block)

The AV node controls the gate between atria and ventricles. Damage to this gate = heart block.

Heart block classification showing spectrum from first-degree to third-degree

First-Degree AV Block

PR interval > 200 ms (>5 small squares) on ECG
Every P wave still conducts to QRS - no dropped beats
Usually benign, no treatment needed
Prevalence ~1% in people under 60

Each P (marked) is followed by a QRS, but the PR interval is longer than normal.

Second-Degree AV Block - Type I (Wenckebach / Mobitz I)

PR interval progressively lengthens beat by beat until one P wave is blocked (dropped QRS)
Pattern then resets and repeats
Block is at the AV node level
Usually benign, often asymptomatic, may resolve
Common in inferior MI (RCA supplies AV node in 80%)

Memory trick: "Longer, longer, longer, DROP - then you have a Wenckebach block"

Second-Degree AV Block - Type II (Mobitz II)

PR interval is constant but suddenly a P wave is not followed by QRS (dropped beat)
No warning - the drop is sudden
Block is below the AV node (in His-Purkinje system)
More dangerous - can progress to complete heart block
Requires pacemaker

Memory trick: "Mobitz II = Suddenly Missing" (the QRS drops without warning)

Third-Degree AV Block (Complete Heart Block)

Complete AV dissociation - P waves and QRS complexes march independently
P rate ≠ QRS rate (P rate usually faster, ~70-80; escape QRS rate ~30-40 if ventricular)
Wide escape QRS (idioventricular) if block is below His; narrow if junctional escape
Life-threatening - causes syncope, hemodynamic collapse
Requires emergency pacing

ECG clue: Count P waves and QRS separately - they are unrelated to each other.

(Miller's Anesthesia, 10e, p. 12222)

2B. Bundle Branch Blocks (BBB)

The bundle branches carry impulses to the left and right ventricles. When blocked, one ventricle depolarizes late, causing a wide QRS (>120 ms).

Right Bundle Branch Block (RBBB)

ECG features:

Wide QRS > 120 ms
rSR' pattern in V1 ("M-shape" or "rabbit ears")
Wide, slurred S wave in leads I, V5, V6
ST depression and T-wave inversion in right precordial leads (V1-V3) - secondary changes, not ischemia

Memory trick for BBB: "WiLLiaM MaRRoW"

WiLLiaM: In LBBB - W pattern in V1, M pattern in V5/V6
MaRRoW: In RBBB - M pattern in V1, W pattern in V5/V6

RBBB ECG - note the rSR' (M-shape) in V1 and wide S waves in lateral leads

RBBB: Wide QRS with rSR' pattern in V1 and wide terminal S in V5/V6.

Left Bundle Branch Block (LBBB)

ECG features:

Wide QRS > 120 ms
Broad, notched (M-shaped) R wave in I, aVL, V5, V6 (lateral leads)
Small r and deep S in V1-V3 (right precordial leads)
No septal Q waves in lateral leads (septum depolarizes right to left - reversed)
T waves and ST segments discordant (opposite direction) to QRS

Clinical importance: New LBBB with chest pain is treated as a STEMI equivalent. LBBB makes interpreting ischemia very difficult.

LBBB ECG (E) - broad notched R in lateral leads, deep S in V1-V3, no septal Q waves

Top (D): Left posterior fascicular block. Bottom (E): LBBB - note the broad, notched R waves in lateral leads and deep S waves in V1-V3.

(Goldman-Cecil Medicine, p. 426-427)

Category 3: Ischemia & Myocardial Infarction

This is the most clinically urgent category. The ECG changes in MI follow a time sequence.

Timeline of MI Changes

Time	ECG Change
Minutes (earliest)	Hyperacute T waves (tall, broad, peaked)
Hours	ST elevation (convex/tombstone)
Hours-Days	Q wave formation (pathological)
Days-Weeks	T-wave inversion
Weeks-permanent	Persistent Q waves (scar)

Hyperacute T Waves (Earliest Sign)

Tall, broad, peaked T waves in the affected territory
Often missed - this is the "pre-STEMI" phase

Hyperacute T waves (A) and early STEMI development (B) - same patient 30 min apart

Tracing A: Broad, tall hyperacute T waves in V3-V4. Tracing B: Same patient 30 minutes later - frank ST elevation now visible in V1-V4.

ST Elevation - STEMI Localisation

Criteria for STEMI: ≥1 mm ST elevation in 2 contiguous leads (≥2 mm in V1-V3 in men)

Territory	Elevated Leads	Artery Involved
Anteroseptal	V1, V2, (V3)	LAD (proximal)
Anterior	V1-V4	LAD
Anterolateral	V1-V6, I, aVL	LAD (large)
Lateral	I, aVL	LCx or diagonal
Inferior	II, III, aVF	RCA (80%) or LCx
Posterior	Tall R in V1-V2, ST depression V1-V2	RCA or LCx
Right ventricular	V3R-V6R (right-sided leads)	RCA proximal

Reciprocal changes: ST depression in leads opposite the infarct - confirms STEMI and indicates larger territory at risk.

(Tintinalli's Emergency Medicine, Table 49-4)

Pathological Q Waves (Old MI Marker)

Width ≥ 40 ms (1 small square) OR depth ≥ 25% of the following R wave
Indicate transmural necrosis (irreversible)
May persist for life as a "scar marker"

ST Depression (Subendocardial Ischemia / NSTEMI)

Horizontal or downsloping ST depression ≥ 0.5-1 mm
Represents subendocardial ischemia (NSTEMI or unstable angina)
Diffuse ST depression with ST elevation in aVR = left main or proximal LAD occlusion

ST Elevation Mimics (Not Always MI!)

Condition	ECG Clue
Pericarditis	Diffuse ST elevation (all leads except aVR), concave shape, PR depression
Benign Early Repolarization	Concave ST elevation in precordial leads, J-point notching, young males, stable
LBBB	Wide QRS, discordant ST changes, no Q waves
LV Aneurysm	Persistent ST elevation weeks after MI, no symptoms
Hypertrophic cardiomyopathy	Strain pattern + voltage criteria
Brugada pattern	ST elevation in V1-V2 with RBBB morphology

(Rosen's Emergency Medicine, pp. 1000-1007)

Category 4: Chamber Enlargement

Left Ventricular Hypertrophy (LVH)

Sokolow-Lyon criterion: S in V1 + R in V5 or V6 > 35 mm
Cornell criterion: R in aVL + S in V3 > 28 mm (men) or > 20 mm (women)
Associated "strain pattern": ST depression + T-wave inversion in lateral leads (I, aVL, V5-V6)
Sensitivity only 30-50%, but specificity 85-95%

Right Ventricular Hypertrophy (RVH)

Tall R wave in V1 (R > S in V1)
Right axis deviation (> +90°)
ST depression + T inversion in V1-V3 (strain pattern)
Causes: pulmonary hypertension, COPD, congenital heart disease

Left Atrial Enlargement (P mitrale)

Broad, notched P wave in lead II ("M-shaped P") - duration > 120 ms
Biphasic P in V1 with wide, deep terminal negative component
Cause: mitral stenosis/regurgitation, LV failure

Right Atrial Enlargement (P pulmonale)

Tall, peaked P wave > 2.5 mm in lead II
Cause: COPD, pulmonary hypertension, tricuspid disease

Category 5: Interval Abnormalities

Prolonged QT Interval

Corrected QT (QTc) = QT / √RR (Bazett's formula)
Normal QTc: < 440 ms (men), < 460 ms (women)
QTc > 500 ms = high risk of Torsades de Pointes (a dangerous VT)

Causes of long QT:

Drugs: Amiodarone, sotalol, haloperidol, erythromycin, methadone, chloroquine
Electrolytes: Hypokalemia, hypomagnesemia, hypocalcemia
Congenital: Long QT syndrome (Romano-Ward, Jervell-Lange-Nielsen)
Cardiac: Myocarditis, bradycardia

Short QT

QTc < 360 ms
Associated with hypercalcemia, digoxin, short QT syndrome
Risk of VF

Prolonged PR (First-degree block - covered above)

Wide QRS (>120 ms)

RBBB, LBBB, ventricular paced rhythm, hyperkalemia, toxicity (Na channel blockers)

Category 6: Miscellaneous Important Patterns

6A. Wolff-Parkinson-White (WPW)

ECG features:

Short PR < 120 ms (pre-excitation via accessory pathway)
Delta wave (slurred upstroke of QRS)
Wide QRS
Risk: can conduct AF rapidly at 300+ bpm → VF

6B. Hyperkalemia (Peaked T waves → Sine Wave → VF)

Progressive ECG changes with rising K+:

Tall, peaked, narrow ("tented") T waves (K > 5.5)
Prolonged PR, widened QRS (K > 6.5)
Loss of P waves (K > 7)
Sine wave pattern (K > 8)
VF / asystole

6C. Hypokalemia

Flattened T waves
Prominent U waves (positive deflection after T wave, best in V2-V3)
ST depression
Prolonged QU interval (often mistaken for long QT)

6D. Pericarditis

Diffuse saddle-shaped (concave) ST elevation in most leads except aVR (ST depression in aVR)
PR depression (most specific, best in II and V6)
Evolves through 4 stages over days-weeks
No reciprocal changes (unlike STEMI)

6E. Pulmonary Embolism (PE)

Most common: Sinus tachycardia
Classic (but uncommon, <20%): S1Q3T3 pattern - deep S in lead I, Q wave and T inversion in lead III
Right heart strain: right axis deviation, RBBB, T-wave inversions in V1-V4
Atrial fibrillation in 10-35% of cases

Quick Reference Summary Table

Abnormality	Key ECG Finding
Atrial fibrillation	No P waves, irregularly irregular
Atrial flutter	Sawtooth F waves ~300/min
SVT	Narrow QRS tachycardia, P buried in QRS
VT	Wide QRS >100 bpm, AV dissociation
1st degree AV block	PR > 200 ms
2nd degree Mobitz I	Progressive PR lengthening → dropped QRS
2nd degree Mobitz II	Constant PR → sudden dropped QRS
3rd degree (complete)	P and QRS completely independent
RBBB	rSR' in V1, wide S in I/V5/V6
LBBB	Broad notched R in I/V5/V6, deep S in V1
STEMI inferior	ST elevation in II, III, aVF
STEMI anterior	ST elevation in V1-V4
Old MI	Pathological Q waves
Pericarditis	Diffuse concave ST elevation, PR depression
LVH	S(V1) + R(V5) > 35 mm
Hyperkalemia	Peaked T → wide QRS → sine wave
Long QT	QTc > 440 ms, risk of Torsades
WPW	Short PR, delta wave

Systematic Approach to Reading ANY ECG

Use this order every time so you never miss anything:

Rate - count R-R intervals (300/number of big squares between beats)
Rhythm - regular or irregular? P before every QRS?
Axis - normal (-30° to +90°)?
P wave - present, normal shape, duration?
PR interval - 120-200 ms?
QRS - narrow (<120 ms) or wide? Morphology?
ST segment - elevation, depression, flat?
T wave - upright, inverted, peaked?
QT interval - calculate QTc
Look for extras - U waves, delta waves, Q waves?

Would you like to go deeper into any specific section? I can do:

Practice cases with real ECG patterns to interpret
Deep dive into MI localisation and reciprocal changes
Arrhythmia algorithms (when to cardiovert, when to use drugs)
Torsades de Pointes and long QT management

Search for ECG images of each abnormality

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Here is the complete visual ECG atlas from the medical library. All images are from authoritative textbooks (Tintinalli's Emergency Medicine, Rosen's Emergency Medicine, Braunwald's Heart Disease, Goldman-Cecil Medicine, Morgan & Mikhail's Anesthesiology).

ECG Abnormalities - Visual Atlas

1. Atrial Fibrillation (AF)

What to look for: No P waves, chaotic baseline, irregularly irregular R-R intervals.

Three examples of Atrial Fibrillation - note the absent P waves and completely irregular R-R intervals in all three strips

Three real examples of AF. Strip A (top): fast ventricular rate ~150 bpm. Strip B (middle): controlled rate ~80 bpm. Strip C (bottom): slower rate. Key: no organized P waves anywhere, beat-to-beat intervals are always different.

2. Ventricular Tachycardia (VT) - Monomorphic

What to look for: Wide QRS (>120 ms), fast regular rate >100 bpm, no P waves associated with QRS.

Three examples of Monomorphic VT - A at 270 bpm, B at 220 bpm, C at 180 bpm

A: VT at 270 bpm - very wide complexes. B: VT at 220 bpm - classic monomorphic pattern. C: VT at 180 bpm - slightly slower but still wide and regular. All have consistent beat-to-beat QRS morphology.

3. VT vs SVT with Aberrancy - Key Distinguishing Features

What to look for: AV dissociation (C), fusion beats and capture beats (D) = diagnostic of VT.

Four panels showing VT differentiation - A and B show uniform wide-complex tachycardia, C shows P waves independent of QRS (AV dissociation = VT), D shows fusion and capture beats

Panel C (arrows): P waves marching completely independently of QRS - this is AV dissociation, pathognomonic for VT. Panel D (arrows): capture beat (narrow QRS) and fusion beat (intermediate morphology) amid wide complexes - confirms VT.

4. Second-Degree AV Block - Mobitz I (Wenckebach)

What to look for: PR interval gets progressively longer until one QRS drops (missing beat), then resets.

Wenckebach / Mobitz I AV block - arrows point to nonconducted P waves (dropped QRS)

Arrows point to the P waves that are blocked (no QRS follows). Before each blocked P, the PR interval was getting longer and longer - classic Wenckebach pattern.

5. Second-Degree AV Block - Mobitz II

What to look for: Constant PR interval, then suddenly a QRS is missing with NO prior PR prolongation.

Three examples of Mobitz II AV block - A: narrow QRS with dropped beats; B: wide QRS (infranodal block); C: high-grade AV block with 2+ consecutive P waves not conducted

A: Narrow QRS - sudden dropped beat, PR stays the same before and after. B: Wide QRS - infranodal block. C: High-grade AV block with 2 consecutive P waves blocked (arrows) - very dangerous, nearly complete block.

6. Third-Degree AV Block (Complete Heart Block)

What to look for: P waves and QRS complexes completely independent (marching to their own rates). P rate > QRS rate.

Complete (3rd degree) AV block - both strips show P waves (atrial rate ~83) with escape ventricular rhythm (rate ~50) with NO relationship between P waves and QRS

Top strip (A): See the slow, wide escape QRS complexes with P waves randomly placed throughout - no P consistently precedes any QRS. Bottom strip (B): Arrows point to visible P waves marching at their own rate, completely dissociated from the escape rhythm.

7. WPW (Wolff-Parkinson-White Syndrome)

What to look for: Short PR (<120 ms), delta wave (slurred upstroke of QRS), wide QRS at baseline.

WPW syndrome - 12-lead ECG showing classic short PR interval and delta waves (arrows) indicating pre-excitation via accessory pathway

Thick arrows (V1, V2): The delta wave inflection point where slow accessory pathway conduction merges with rapid His-Purkinje conduction. Horizontal arrows (V2, V3): the transition point. Note the PR interval is clearly shortened across all leads.

8. Hyperkalemia - Progressive ECG Changes

What to look for: Peaked T → wide QRS → loss of P → sine wave → VF.

Hyperkalemia ECG changes in 3 stages - Left: normal; Middle: peaked T waves and widened QRS (moderate hyperkalemia); Right: sine wave pattern (severe, pre-arrest)

Left panel: Normal baseline (P, QRS, T labeled). Middle panel: Moderately elevated K⁺ - note the tall, peaked, narrow T wave dwarfing the R wave, with some QRS widening and PR prolongation. Right panel: Severely elevated K⁺ - classic sine wave, P wave gone, QRS and T wave merge - immediately pre-VF.

9. First-Degree AV Block

What to look for: Every P conducts, but PR > 200 ms (> 1 big square). Regular, just slow conduction.

First-degree AV block from Rosen's - regular rhythm with clearly prolonged PR interval on every beat

Every P wave is followed by a QRS, but the PR interval is visibly prolonged - you can measure it is >200 ms. No dropped beats.

10. Second-Degree AV Block Mobitz I (Wenckebach) - Classic Strip

What to look for: Grouped beats (clusters of QRS), PR gets longer within each group, last beat drops.

Classic Wenckebach strip - PR progressively prolongs then QRS drops, pattern repeats

You can see the PR interval getting slightly longer across each group of beats, then a pause (dropped QRS), then the cycle resets. Classic "grouped beating" of Wenckebach.

11. STEMI - Hyperacute T waves → ST Elevation (Same Patient, 30 min apart)

What to look for: Early: tall, broad T waves in affected leads. Later: frank ST elevation.

Hyperacute STEMI evolution - Panel A shows tall broad T waves in V3-V4 (hyperacute phase). Panel B is the same patient 30 minutes later with frank ST elevation V1-V4

A: Chest pain but ST not yet elevated - look at V3-V4, huge broad peaked T waves = hyperacute T waves (earliest STEMI sign, often missed!). B: Same patient 30 min later - clear ST elevation in V1-V4, now obvious anterior STEMI.

12. Bundle Branch Blocks (RBBB and LBBB)

(These were shared in the previous session but reproduced here for completeness)

RBBB - rSR' (M-shape) in V1, wide S in lateral leads:

RBBB 12-lead ECG - M-shaped rSR' in V1, wide terminal S waves in V5/V6

LBBB - broad notched R in lateral leads, deep S in V1-V3:

LBBB 12-lead ECG (E, lower panel) - broad notched R in I, aVL, V5, V6; deep S in V1-V3

Quick Visual Recognition Table

Abnormality	Single Most Important Visual Feature
AF	No P waves, irregular beats
Atrial Flutter	Sawtooth F waves in II/III/aVF
Monomorphic VT	Wide QRS, regular, fast, no P-QRS link
Wenckebach	PR gets longer → dropped QRS → repeats
Mobitz II	Constant PR → sudden dropped QRS
Complete heart block	P and QRS march independently
RBBB	rSR' (M shape) in V1
LBBB	Broad notched R in I/V5/V6
WPW	Short PR + delta wave
Hyperkalemia	Peaked T → sine wave
Early STEMI	Hyperacute T waves in affected territory
Established STEMI	Convex ST elevation in territory
Wenckebach (first-degree)	Long PR, every P conducts

These are all sourced directly from Tintinalli's Emergency Medicine, Rosen's Emergency Medicine, Braunwald's Heart Disease, Goldman-Cecil Medicine, and Morgan & Mikhail's Anesthesiology.

Would you like to practice interpreting ECGs with a quiz, or go deeper into any specific abnormality?

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