🫀 How to Read an ECG Like a Critical Care Pro

🫀 How to Read an ECG Like a Critical Care Pro

(And Not Panic When the Examiner Hands You One)

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Reading an ECG in critical care isn't just about spotting a STEMI or counting how fast the heart is racing—it’s about reading between the lines, literally. In the ICU or exam hall, your ECG game needs to be systematic, reproducible, and razor-sharp.

Here’s your step-by-step playbook—tested in battle (and many exams)—to help you decode those 12 slippery leads with confidence.

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✅ Step-by-Step ECG Interpretation

(Critical Care-Focused and Viva-Proof)

🔹 Step 0: Calibration Check

Before you even look at the squiggles:

• Paper speed: 25 mm/sec

• Voltage: 10 mm = 1 mV

• Each small box: 0.04 sec (horizontal), 0.1 mV (vertical)

🧠 Always check calibration—an uncalibrated ECG is like examining a blurry chest X-ray.

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🔹 Step 1: Patient Details

• Is this your patient? Is this today’s ECG?

• Don’t interpret the wrong tracing under pressure.

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🔹 Step 2: Heart Rate

• Count the QRS complexes in a 10-second strip × 6

• Or use the 300 Rule: 300 ÷ number of large boxes between two R waves
  (Only works for regular rhythms)

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🔹 Step 3: Rhythm

• Regular or irregular?

• P wave before every QRS?

• QRS after every P wave?

• Look closely at Lead II and V1: ideal for assessing atrial activity.

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🔹 Step 4: Axis Determination

Use leads I and aVF:

• ➕ in both = Normal Axis

• ➕ in I, ➖ in aVF = Left Axis Deviation

• ➖ in I, ➕ in aVF = Right Axis Deviation

(Axis tells you more than direction—it’s a reflection of underlying pathology like PE or LVH.)

• Left Axis Deviation (LAD) (I positive, aVF negative) can show up in:

  • Left anterior fascicular block

  • Inferior wall MI

  • Left ventricular hypertrophy (LVH)

  • Advanced aging, and sometimes in obesity or high diaphragm positions (think post-op abdominal patients)

• Right Axis Deviation (RAD) (I negative, aVF positive) is a red flag for:

  • Pulmonary embolism (PE)

  • Right ventricular hypertrophy (RVH) (seen in chronic lung diseases like COPD)

  • Lateral wall MI

  • Congenital heart diseases (e.g., ASD)

  • Normal in young, thin individuals or athletes (so always correlate clinically!)

🔹 Step 5: P Waves (Atrial Activity)

• Upright in I, II, aVF?

• Peaked in II = Right Atrial Enlargement

• M-shaped in II or biphasic in V1 = Left Atrial Enlargement

• No P waves? Consider AF or junctional rhythm.

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🔹 Step 6: PR Interval (0.12–0.20 s)

• Prolonged → AV Block

• Short → Pre-excitation (think WPW)

• Variable → Consider Mobitz or complete heart block

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🔹 Step 7: QRS Complex

• Normal width = < 0.12 s

• Wide = Bundle branch block or ventricular rhythm

  • RBBB: RSR' in V1

  • LBBB: Broad R in I and V6, deep S in V1

• Look for pathologic Q waves (MI), delta waves (WPW), or fragmented QRS

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🔹 Step 8: ST Segment

• Elevation or depression?

• Localize the infarct:

Area	Leads
Anterior	V1–V4
Lateral	I, aVL, V5–V6
Inferior	II, III, aVF
Posterior	V1–V2 (ST ↓, tall R)

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🔹 Step 9: T Waves

• Tall and peaked? → Hyperkalemia

• Inverted? → Ischemia, strain

• Flat or biphasic? → Nonspecific but look for context

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🔹 Step 10: QT Interval (Corrected)

• QTc = QT / √RR

• Normal: < 440 ms (men), < 460 ms (women)

• Prolonged QT = Risk of Torsades de Pointes

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🔹 Step 11: U Waves

• Prominent in hypokalemia

• Also seen in bradycardia, digoxin toxicity
  (Often forgotten, but vital in ICU electrolyte derangements)

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🚨 Critical Diagnoses You Should Spot in the ICU

• STEMI/NSTEMI → localized ST changes

• Hyperkalemia → peaked T, wide QRS

• Pericarditis → global ST elevation, PR depression

• PE → S1Q3T3, RBBB, T wave inversions in V1–V3

• Tamponade → low voltage, electrical alternans

• Heart blocks → AV conduction issues

• Tachy/bradyarrhythmias → AF, SVT, VT, CHB

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🧠 Mnemonics to Live (and Pass) By

✔️ "Rate - Rhythm - Axis - PQRST"

Repeat it until it haunts your dreams.

✔️ "LII LIaVF V5/6"

Helps localize MI: Inferior, Lateral, Anterior

🩺 Common ECG Questions in Critical Care Exams (With Fast Clues)

⚡ 1. What are J Waves (Osborn Waves) and when do you see them?

Clue: Positive deflection at the J point
Seen in:

• Hypothermia (core temp <32°C) ❄️

• Hypercalcemia

• Brain injury (rare)

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⚡ 2. What is the ECG finding in Brugada Syndrome?

Clue: Coved ST elevation in V1–V3 + RBBB pattern
Seen in:

• Sudden cardiac death in young males

• Often unmasked by fever, sodium channel blockers

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⚡ 3. What are Delta Waves? What condition shows them?

Clue: Slurred upstroke of QRS
Seen in:

• Wolff-Parkinson-White (WPW) Syndrome

• Short PR + wide QRS + delta wave

• Risk: pre-excited AF → VF

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⚡ 4. What are ECG changes in Tricyclic Antidepressant (TCA) poisoning?

Clue: Wide QRS, R > 3 mm in aVR
Other findings:

• Prolonged PR, QRS, QT

• Terminal R wave in aVR

• Risk of ventricular arrhythmias

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⚡ 5. What is S1Q3T3 pattern and when is it seen?

Clue:

• Deep S in lead I

• Q wave and inverted T in lead III
  Seen in:

• Pulmonary Embolism

• Often with tachycardia, RBBB, RAD

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⚡ 6. What are ECG changes in Hyperkalemia?

Clue: Tall T → Wide QRS → Sine wave

• Peaked T waves (early)

• Loss of P wave

• Wide QRS

• Ventricular fibrillation risk

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⚡ 7. What are ECG changes in Hypokalemia?

Clue: U remember the U

• U waves

• Flattened or inverted T waves

• Prolonged QT → risk of Torsades

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⚡ 8. What’s the classical ECG in Pericarditis?

Clue:

• Diffuse ST elevation

• PR depression

• No reciprocal changes (unlike MI)

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⚡ 9. Which ECG findings suggest Digoxin toxicity?

Clue:

• “Scooped” ST depression (Salvador Dali sagging mustache sign)

• Arrhythmias: AV block, VT, bidirectional VT

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⚡ 10. ECG findings in Organophosphate poisoning?

Clue: Bradycardia, QT prolongation, AV blocks

• Sinus bradycardia most common

• Can progress to complete heart block

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📚 Must-Use Resources to Master ECGs

1. 🖥️ Life in the Fast Lane – ECG Library https://litfl.com/top-100/ecg/ 

• Over 100 annotated ECGs

• Covers core and advanced ICU topics

• Real-world case vignettes
  📌 If you only use one resource, let it be this.

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📘 Books:

• ECG Made Easy – John Hampton
  Simple, clean, and actually fun.

• ✍️ Final Thoughts

• An ECG in ICU isn't just a test. It’s your stethoscope for the heart’s electricity.

In exams, it’s a mirror of your clinical logic.
So make your approach habitual, your understanding physiologic, and your presentation crisp.

Read one ECG daily, every day.
You’ll soon go from hesitant to heroic—with every lead you read.