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ECG in the ICU - Key Patterns with Annotated Images

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1. ECG Basics - Normal Waveform (Know this first!)

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2. STEMI Equivalents - Critical ICU Patterns

What to look for (key patterns):

ICU pearl: De Winter and posterior MI are "STEMI equivalents" - they need the same urgent reperfusion but have NO ST elevation! Missing these is a common fatal error.

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3. Pulmonary Embolism ECG Changes

PE ECG findings (by mechanism):

ICU pearl: Negative T waves in both lead III and V1 = 88% sensitive, 99% specific for PE vs ACS. This single finding is your best ECG discriminator between the two.

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4. Hyperkalemia vs Hypokalemia - Electrolyte Emergencies

Hyperkalemia progression (K+ rising):

Hypokalemia signs:

• T wave inversion + ST depression + Prominent U wave (U > T in same lead = dangerous)

ICU pearl: Peaked T waves from hyperkalemia can mimic hyperacute T waves of STEMI. Always check your potassium before activating the cath lab in an ICU patient!

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5. Quick ICU ECG Checklist

1. Rate       → Tachycardia? Bradycardia?
2. Rhythm     → Regular? P waves present? P:QRS relationship?
3. Axis       → Left (LAD)? Right (RAD)? 
4. PR interval → Prolonged = AV block? Short = WPW/pre-excitation?
5. QRS width  → Wide = BBB, hyperkalemia, toxin, pacemaker?
6. ST segment → Elevation (STEMI)? Depression (ischemia, PE, digoxin)?
7. T waves    → Peaked (hyperK)? Inverted (ischemia, PE, strain)?
8. QT interval → Prolonged = torsades risk (drugs, hypoMg, hypoK, hypoCA)?
9. U waves    → Prominent = hypokalemia?

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6. Dangerous Arrhythmias to Spot Immediately in ICU

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Summary: The "Can't Miss" ICU ECG Diagnoses

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📋 ECG Basics - A Complete Beginner's Guide

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🔴 STEP 1 - What IS an ECG?

SA Node → Internodal pathways → AV Node → Bundle of His → Left & Right Bundle Branches → Purkinje Fibers

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🔴 STEP 2 - Understanding the ECG Paper

The Grid - memorize this:

🔴 Key rule: At 25 mm/sec standard speed, 5 large boxes = 1 second

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🔴 STEP 3 - The ECG Waveform Explained

Every wave explained:

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🔴 P WAVE

• What it is: Atrial depolarization (atria contracting)
• Normal: Small rounded hump before the QRS
• Duration: 0.06 - 0.11 sec (< 3 small boxes)
• Height: < 2.5 mm
• Should be: Upright in leads I, II, aVF; inverted in aVR
• ❗ If absent = atrial fibrillation or junctional rhythm

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🔴 PR INTERVAL

• What it is: Time from start of P wave to start of QRS (AV node delay)
• Normal: 0.12 - 0.20 sec (3 to 5 small boxes)
• ❗ Prolonged PR (>0.20 s) = AV block (1st degree)
• ❗ Short PR (<0.12 s) = WPW syndrome or junctional rhythm

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🔴 QRS COMPLEX

• What it is: Ventricular depolarization (ventricles contracting - the main pumping action)
• Q wave: Small initial downward deflection
• R wave: Main upward spike - the tallest part
• S wave: Downward deflection after R
• Normal width: < 0.12 sec (< 3 small boxes)
• ❗ Wide QRS (>0.12 s) = Bundle branch block, hyperkalemia, ventricular rhythm

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🔴 ST SEGMENT

• What it is: Period between ventricular depolarization and repolarization (ventricles are contracted, no electrical activity)
• Normal: Flat, at the isoelectric line (same level as baseline)
• ❗ ST elevation = STEMI (heart attack), pericarditis
• ❗ ST depression = NSTEMI, ischemia, digoxin effect

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🔴 T WAVE

• What it is: Ventricular repolarization (ventricles relaxing)
• Normal: Rounded, upright in most leads; same direction as QRS
• Height: < 5 mm in limb leads, < 10 mm in precordial leads
• ❗ Peaked T waves = Hyperkalemia (earliest sign!)
• ❗ Inverted T waves = Ischemia, LVH, PE, RBBB

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🔴 QT INTERVAL

• What it is: Total ventricular activity (depolarization + repolarization)
• Normal: < 0.44 sec (< 11 small boxes) - corrected for rate (QTc)
• ❗ Long QTc = Risk of Torsades de Pointes (dangerous arrhythmia)
• Causes of long QT: drugs (amiodarone, haloperidol, methadone), hypoKalemia, hypoMagnesemia, hypoCa

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🔴 U WAVE

• Small positive deflection after the T wave
• Normal in some patients
• ❗ Prominent U wave = Hypokalemia

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🔴 STEP 4 - How to Calculate Heart Rate

Method 1: Large Box Method (for regular rhythms - quickest!)

🔴 Mnemonic: 300 - 150 - 100 - 75 - 60 - 50

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Method 2: Small Box Method (more precise)

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Method 3: 6-Second Method (for irregular rhythms like AF)

• Count the number of QRS complexes in a 10-second strip
• Multiply by 6 = heart rate per minute

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🔴 STEP 5 - Electrode Placement (How to Put On the Leads)

Chest (Precordial) Leads:

Limb Leads:

🔴 Memory trick for limb leads: "Ride Your Green Bike" = Right arm Red, Left arm Yellow (ride a bike), Green Left leg, Black Right leg

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🔴 STEP 6 - ECG Axis (Quick Method)

The Quick 2-Lead Axis Method (Use Lead I + aVF):

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🔴 STEP 7 - The Systematic Approach (Never Miss Anything!)

┌─────────────────────────────────────────────────┐
│  ECG READING CHECKLIST (for freshers)           │
├─────────────────────────────────────────────────┤
│  1. RATE     → Count using 300 / large box rule │
│               Normal = 60-100 bpm               │
│  2. RHYTHM   → Regular or irregular?            │
│               P wave before every QRS?          │
│  3. AXIS     → Check Lead I and aVF             │
│  4. P WAVE   → Present? Upright in II?          │
│               Duration < 0.11s? Height <2.5mm?  │
│  5. PR INT.  → 0.12-0.20 sec? (3-5 boxes)       │
│  6. QRS      → Width < 0.12s? (< 3 boxes)       │
│               Look for Q waves (pathological?)  │
│  7. ST SEG.  → Elevated? Depressed? Flat?       │
│  8. T WAVE   → Upright? Inverted? Peaked?       │
│  9. QT INT.  → < 0.44 sec? (< 11 small boxes)  │
│ 10. U WAVE   → Prominent?                       │
└─────────────────────────────────────────────────┘

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🔴 STEP 8 - What Leads "See" (Which Part of the Heart)

🔴 Golden rule: ST changes in 2 or more contiguous leads from the same territory = localized pathology (e.g., STEMI)

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🔴 Quick Summary Card for Freshers

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Tip for freshers: Print this checklist and physically go through each step on every ECG you read. After ~50 ECGs it becomes automatic!