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ECG: From Basics to Clinical Emergencies

PART 1 - THE ELECTRICAL FOUNDATION

How the Heart's Electricity Works

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PART 2 - ECG PAPER AND LEADS

ECG Paper - Reading the Grid

• Small box (1 mm) = 0.04 seconds (40 ms) in time | 0.1 mV in voltage
• Large box (5 mm) = 0.2 seconds (200 ms) | 0.5 mV
• Standard speed: 25 mm/sec
• Standard gain: 10 mm = 1 mV

The 12 Leads - What They "See"

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PART 3 - THE WAVES, INTERVALS, AND SEGMENTS

P Wave

• Represents: Atrial depolarization (atria squeezing)
• Normal: Upright in lead II, biphasic in V1, duration < 0.12 sec, amplitude < 2.5 mm
• Abnormal:
  • Absent P waves = atrial fibrillation, junctional rhythm, severe hyperkalemia
  • Tall peaked P (> 2.5 mm in II) = P pulmonale (right atrial enlargement)
  • Broad notched P = P mitrale (left atrial enlargement)

PR Interval

• Represents: Time from atrial depolarization to start of ventricular depolarization (includes AV node delay)
• Normal: 0.12-0.20 sec (3-5 small boxes)
• Short PR (< 0.12 sec): Pre-excitation (WPW syndrome) - accessory pathway bypasses AV node
• Long PR (> 0.20 sec): First-degree heart block - AV node conduction slowed

QRS Complex

• Represents: Ventricular depolarization
• Normal duration: 0.06-0.10 sec (< 3 small boxes)
• Normal morphology: Q wave < 0.04 sec wide and < 25% of R height
• Wide QRS (> 0.12 sec): Bundle branch block, ventricular rhythm, hyperkalemia, drug toxicity

ST Segment

• Represents: Early ventricular repolarization (ventricles in "plateau" phase)
• Normal: Isoelectric (flat, at baseline)
• ST Elevation: Injury (STEMI, pericarditis, Brugada, early repolarization)
• ST Depression: Ischemia (NSTEMI, demand ischemia), reciprocal changes

T Wave

• Represents: Ventricular repolarization
• Normal: Upright in most leads (I, II, V3-V6), may be inverted in aVR, V1
• Tall/peaked T: Hyperkalemia, hyperacute STEMI
• Inverted T: Ischemia, right heart strain (PE), bundle branch block, LVH

QT Interval

• Represents: Total ventricular depolarization + repolarization
• Normal corrected (QTc): < 440 ms (men), < 460 ms (women)
• Prolonged QTc: Risk of torsades de pointes (dangerous VT)
  • Causes: Drugs (amiodarone, haloperidol, azithromycin, quinolones), hypokalemia, hypomagnesemia, congenital long QT

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PART 4 - A SYSTEMATIC APPROACH (DO THIS EVERY TIME)

1. RATE       → Count R-R intervals. 300/large boxes between R waves.
                 Normal: 60-100. <60 = bradycardia. >100 = tachycardia.

2. RHYTHM     → Regular or irregular? Is every QRS preceded by P wave?
                 Irregular = AF, multifocal AT, sinus arrhythmia

3. AXIS       → Look at leads I and aVF:
                 Both upright = Normal axis
                 I up, aVF down = Left axis deviation (LAD, LBBB, inferior MI)
                 I down, aVF up = Right axis deviation (RVH, PE, lateral MI)

4. P WAVE     → Present? Morphology normal? One P before each QRS?

5. PR INTERVAL → 0.12-0.20 sec? Consistent?

6. QRS        → Width < 0.12 sec? Q waves? R progression in V1-V6?

7. ST SEGMENT → Elevation or depression? Which leads? How many mm?

8. T WAVE     → Upright? Peaked? Inverted? Biphasic?

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PART 5 - LIFE-THREATENING ECG EMERGENCIES

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EMERGENCY 1 - STEMI (ST-Elevation Myocardial Infarction)

ECG Changes (in order of evolution):

STEMI Diagnosis Criteria:

• ST elevation in 2 or more contiguous leads:
  • ≥ 2 mm in V1-V4 (anterior leads)
  • ≥ 1 mm in limb leads (inferior or lateral)

Localization (which artery is blocked):

Clinical Presentation:

• Crushing, pressure-like chest pain radiating to left arm/jaw (may be absent in diabetics, elderly, women)
• Diaphoresis (cold sweating)
• Nausea, vomiting
• Dyspnea, sense of impending doom
• In women: atypical - fatigue, jaw pain, nausea without chest pain

What to Do - STEMI Protocol:

1. Give aspirin 325 mg immediately (chewed)
2. Call for primary PCI (cath lab activation) - target door-to-balloon < 90 minutes
3. If PCI unavailable within 120 min: give thrombolytics (tPA, streptokinase)
4. Dual antiplatelet (add P2Y12 inhibitor: clopidogrel, ticagrelor, prasugrel)
5. Anticoagulation (heparin)
6. Beta-blocker + nitrates for pain (avoid nitrates if inferior STEMI + hypotension - could be RV infarct)

STEMI Equivalent alert: The De Winter pattern (upsloping ST depression V1-V6 + tall symmetric T + ST elevation in aVR) = proximal LAD occlusion WITHOUT classic ST elevation. Treat as STEMI.

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EMERGENCY 2 - Ventricular Fibrillation (VF) / Pulseless Ventricular Tachycardia (pVT)

• VF: Completely chaotic, irregular, varying amplitude waveforms - no recognizable QRS complexes whatsoever
• VT (Ventricular Tachycardia): Wide QRS (> 0.12 sec), rate 100-250 bpm, regular, often with AV dissociation (P waves "march through" independently)

Clinical Presentation:

• Sudden collapse, unresponsive
• No pulse (check femoral or carotid)
• No breathing or gasping only
• Can be preceded by: palpitations, dizziness, syncope, chest pain

What to Do - VF/pVT Protocol:

1. CALL CODE, start CPR immediately
2. Defibrillate as fast as possible - every minute without defibrillation reduces survival by 7-10%
3. Continue CPR immediately after shock (2 min cycles)
4. IV/IO access - Adrenaline (epinephrine) 1 mg IV every 3-5 min
5. Amiodarone 300 mg IV for refractory VF (after 2nd or 3rd shock)
6. Address reversible causes (4 H's and 4 T's)

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EMERGENCY 3 - Complete (Third-Degree) Heart Block

• P waves present at normal rate (~75 bpm)
• QRS complexes present but at slow rate (~30-40 bpm)
• No relationship between P waves and QRS - they "march through" each other
• QRS may be wide (ventricular escape) or narrow (junctional escape)

Clinical Presentation:

• Syncope (Stokes-Adams attacks)
• Severe bradycardia
• Dizziness, pre-syncope
• Hypotension, heart failure
• Can present with cardiac arrest

What to Do:

1. Transcutaneous pacing immediately if hemodynamically unstable
2. Atropine 0.5-1 mg IV (may not work in complete block - try anyway)
3. Dopamine or adrenaline infusion as bridge
4. Definitive: Permanent pacemaker insertion

Second-degree blocks:

• Mobitz I (Wenckebach): Progressive PR lengthening until a beat is dropped. Usually benign, nodal level.
• Mobitz II: Constant PR, sudden dropped beat without warning. More dangerous - can progress to complete block. Often needs pacing.

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EMERGENCY 4 - Hyperkalemia ECG Changes

ECG Changes in Order of Severity (Harrison's Principles):

Clinical Presentation:

• Weakness, fatigue, muscle cramps
• Paralysis (ascending)
• Palpitations
• Often in: renal failure, diabetic ketoacidosis, Addison's disease, ACE inhibitor use, massive tissue destruction (rhabdomyolysis)

What to Do - Emergency Hyperkalemia:

1. Calcium gluconate 10 mL of 10% IV (or calcium chloride 3-5 mL) - membrane stabilization, works in minutes
2. Insulin 10 units + 50 mL 50% glucose IV - drives K into cells, peak effect ~1 hour
3. Sodium bicarbonate 50 mL of 8.4% - drives K into cells (if acidotic)
4. Salbutamol (albuterol) nebulizer - beta agonist drives K into cells
5. Furosemide IV - eliminates K (if renal function present)
6. Dialysis - definitive treatment in renal failure

Caution: Do NOT give calcium if patient is on digoxin - calcium potentiates digoxin toxicity and can trigger VF.

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EMERGENCY 5 - Pulmonary Embolism (PE)

ECG Pattern (classic but not always present):

• S1Q3T3: Deep S wave in lead I, Q wave and T inversion in lead III
• Sinus tachycardia (most common finding - present in ~90%)
• Right bundle branch block (RBBB): rSR' pattern in V1, S wave in V5-V6
• Right axis deviation
• T wave inversions in V1-V4 (right heart strain pattern)
• P pulmonale (tall peaked P in lead II)

Clinical Presentation:

• Sudden onset dyspnea (most common)
• Pleuritic chest pain
• Hemoptysis
• Tachycardia (HR > 100)
• Hypoxia (O2 sat drops)
• Syncope in massive PE
• Signs of DVT: unilateral leg swelling, calf tenderness
• Risk factors: immobility, recent surgery, cancer, OCP, prior DVT/PE

What to Do:

1. High-flow O2
2. IV heparin bolus + infusion (anticoagulation)
3. If massive PE with hemodynamic collapse: systemic thrombolysis (tPA 100 mg over 2 hours) or surgical embolectomy
4. CT pulmonary angiography (CTPA) to confirm - do not wait for imaging if patient crashing

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EMERGENCY 6 - Atrial Fibrillation (AF) with Rapid Ventricular Rate

ECG Pattern:

• Absent P waves (replaced by chaotic fibrillatory baseline, especially visible in V1)
• Irregularly irregular QRS rhythm (no two R-R intervals are the same)
• Narrow QRS (unless aberrant conduction or bundle branch block)
• Variable rate depending on AV node conduction

Clinical Presentation:

• Palpitations ("fluttering" or "irregular" heartbeat)
• Dyspnea
• Fatigue
• Dizziness, presyncope
• Risk of stroke (clot forms in left atrial appendage during AF)
• In rapid AF: hemodynamic instability, chest pain

What to Do:

1. If unstable (hypotension, chest pain, altered consciousness): Synchronized DC cardioversion immediately (100-200 J)
2. If stable: Rate control first - metoprolol IV, diltiazem IV, or digoxin (in heart failure)
3. Anticoagulation (DOAC or warfarin) if AF > 48 hours to prevent stroke
4. Rhythm control options: amiodarone, flecainide, electrical cardioversion

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EMERGENCY 7 - Long QT / Torsades de Pointes

ECG Pattern of Torsades:

• Polymorphic VT (QRS complexes twist around the baseline - "twisting of the points")
• Preceded by prolonged QTc on baseline ECG
• Often triggered by a "short-long-short" sequence (PVC followed by pause)

Common Causes of Long QT:

• Drugs: Amiodarone, sotalol, haloperidol, methadone, quinolones, macrolides, antifungals
• Electrolytes: Hypokalemia, hypomagnesemia, hypocalcemia
• Congenital: Romano-Ward syndrome, Jervell and Lange-Nielsen syndrome
• Cardiac: Bradycardia, myocarditis, hypothyroidism

What to Do:

1. Magnesium sulfate 2g IV over 5 minutes (first-line, even if Mg level is normal)
2. Overdrive pacing (to keep rate > 100 and shorten QT)
3. Stop all QT-prolonging drugs
4. Correct electrolytes (K > 4.5 mEq/L, Mg > 2 mEq/L)
5. If degenerates to VF: defibrillate

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PART 6 - QUICK REFERENCE EMERGENCY RECOGNITION TABLE

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PART 7 - ECG CHANGES IN COMMON CONDITIONS (SUMMARY)

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PART 8 - CLINICAL PEARLS TO REMEMBER

1. "Time is muscle" in STEMI - every 30-minute delay in PCI = 7.5% more mortality.
2. Normal ECG does not rule out ACS - up to 5% of STEMI patients have a normal initial ECG (posterior STEMI, early hyperacute changes).
3. Always check a 12-lead, not just a monitor rhythm - rhythm strips miss ST changes.
4. LBBB new or presumed new = STEMI equivalent - activate cath lab.
5. The most common ECG finding in PE is sinus tachycardia, not S1Q3T3.
6. VT vs. SVT with aberrancy: When in doubt, treat wide complex tachycardia as VT. Never give verapamil for wide complex tachycardia - it can precipitate VF.
7. Reciprocal ST depression confirms STEMI (inferior STEMI → ST depression in I, aVL; anterior STEMI → ST depression in inferior leads).
8. Peaked T in V1-V4 without chest pain - think hyperkalemia first, then hyperacute STEMI.
9. In complete heart block, atropine may not work - move straight to pacing.
10. QT interval must be rate-corrected (QTc) - use Bazett's formula: QTc = QT / √(RR in seconds).

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