# Clinical Cases: Electrocardiography Basics

## Case 1: Acute Anterior STEMI

### Patient Presentation
**Demographics:** 62-year-old male

**Chief Complaint:** Severe chest pain and shortness of breath for 45 minutes

**History of Present Illness:**
A 62-year-old male with history of diabetes, hyperlipidemia, and 40 pack-year smoking history presents with severe substernal chest pressure radiating to his left arm. The pain began suddenly 45 minutes ago while eating breakfast. He describes it as "an elephant sitting on my chest." He is also experiencing shortness of breath, nausea, and diaphoresis. He took aspirin at home before calling 911.

**Physical Examination:**
- Vital Signs: BP 145/95 mmHg, HR 105 bpm, RR 24/min, SpO2 93% on room air
- General: Diaphoretic, pale, anxious, clutching chest
- Cardiovascular: Tachycardic, regular rhythm, S4 gallop, no murmurs
- Lungs: Bibasilar crackles
- Extremities: Cool, slightly mottled

### Workup
- **ECG Findings:**
  - ST-segment elevation in V1-V4 (≥2 mm) - anterior leads
  - ST-segment elevation in I, aVL - high lateral leads
  - Reciprocal ST depression in II, III, aVF
  - Q waves beginning to form in V1-V3
  - Hyperacute T waves in V2-V4

- **ECG Interpretation:**
  - Rate: 105 bpm
  - Rhythm: Sinus tachycardia
  - Axis: Normal (approximately +30°)
  - Intervals: PR 160 ms, QRS 100 ms, QTc 440 ms
  - **Conclusion: Acute anterolateral STEMI**

- **Labs:** Troponin I: 12.5 ng/mL (markedly elevated), BNP: 450 pg/mL

### Diagnosis
**Acute anterolateral ST-elevation myocardial infarction (STEMI) - likely LAD occlusion**

*ECG Correlation:* The ECG leads reflect different anatomical regions:
- **Anterior leads (V1-V4):** Left anterior descending artery territory
- **Lateral leads (I, aVL, V5-V6):** Circumflex or diagonal branches
- **Inferior leads (II, III, aVF):** Right coronary artery territory

ST elevation in V1-V4 with lateral involvement indicates proximal LAD occlusion. Reciprocal changes (ST depression) in inferior leads support acute STEMI. The ST-segment represents the plateau phase (phase 2) of the action potential; ischemia causes abnormal repolarization creating the current of injury seen as ST elevation.

### Treatment
1. Immediate activation of cardiac catheterization lab
2. DAPT: Aspirin 325 mg (already given) + P2Y12 inhibitor (ticagrelor 180 mg or prasugrel 60 mg)
3. Anticoagulation: Heparin bolus
4. Primary PCI within 90 minutes (door-to-balloon time)
5. Oxygen for SpO2 <90%
6. Pain management (morphine if needed)
7. Beta-blocker (if no signs of heart failure or cardiogenic shock)

### Clinical Image
![Anterior STEMI ECG](case_01_image.jpg)

**Image Description:** 12-lead ECG demonstrating acute anterior STEMI with marked ST-segment elevation in precordial leads V1-V4. Note the convex (tombstone) ST elevation, hyperacute T waves, and reciprocal ST depression in inferior leads.

**Source:** Wikimedia Commons - Acute anterior myocardial infarction ECG
**License:** CC BY-SA 4.0
**URL:** https://commons.wikimedia.org/wiki/File:AMI_anterior_V1-V4.png

---

## Case 2: Atrial Fibrillation with Rapid Ventricular Response

### Patient Presentation
**Demographics:** 68-year-old female

**Chief Complaint:** Palpitations and fatigue for 3 days

**History of Present Illness:**
A 68-year-old female with history of hypertension and obesity presents with a 3-day history of palpitations and progressive fatigue. She describes her heart as "fluttering" and feels short of breath with minimal exertion. She denies chest pain or syncope. She has never experienced this before.

**Physical Examination:**
- Vital Signs: BP 135/88 mmHg, HR 142 bpm (irregularly irregular), RR 20/min, SpO2 96% on room air
- General: Mild distress, appears fatigued
- Cardiovascular: Irregularly irregular rhythm, variable S1 intensity, no murmurs
- Lungs: Clear bilaterally
- Extremities: Trace bilateral ankle edema

### Workup
- **ECG Findings:**
  - Absent P waves - replaced by fibrillatory waves (f waves)
  - Irregularly irregular R-R intervals
  - Ventricular rate 130-150 bpm
  - Narrow QRS complexes (88 ms)
  - No ST changes

- **ECG Interpretation:**
  - Rate: ~142 bpm (variable)
  - Rhythm: Atrial fibrillation with rapid ventricular response
  - Axis: Normal (+45°)
  - Intervals: QRS 88 ms, QTc 410 ms (no PR as no P waves)
  - **Conclusion: Atrial fibrillation with RVR**

- **Labs:** TSH: 0.1 mIU/L (suppressed), Free T4: 3.2 ng/dL (elevated)
- **Echocardiogram:** Normal LV function, dilated left atrium

### Diagnosis
**New-onset atrial fibrillation with rapid ventricular response, secondary to hyperthyroidism**

*ECG Correlation:* Atrial fibrillation is characterized by:
- **No organized P waves:** Multiple chaotic atrial impulses (350-600/min) create fibrillatory baseline
- **Irregular R-R intervals:** Variable conduction through AV node
- **Variable ventricular rate:** Depends on AV node conduction properties

The AV node acts as a "filter," protecting the ventricles from the rapid atrial rates. The irregularly irregular ventricular response is pathognomonic. Hyperthyroidism increases adrenergic tone and can trigger or exacerbate atrial fibrillation.

### Treatment
1. **Rate control:** IV diltiazem bolus 20 mg, then infusion (avoid beta-blockers in thyrotoxicosis until controlled)
2. **Anticoagulation:** CHA2DS2-VASc score = 3 (female, age, HTN) → Start anticoagulation with DOAC
3. **Treat underlying cause:** Methimazole for hyperthyroidism
4. Consider cardioversion once rate controlled and anticoagulated ≥3 weeks or TEE shows no LA thrombus

### Clinical Image
![Atrial Fibrillation ECG](case_02_image.jpg)

**Image Description:** 12-lead ECG demonstrating atrial fibrillation with rapid ventricular response. Note the absence of P waves, irregularly irregular R-R intervals, and the fibrillatory baseline best seen in V1.

**Source:** Wikimedia Commons - Atrial fibrillation ECG
**License:** CC BY-SA 3.0
**URL:** https://commons.wikimedia.org/wiki/File:Atrial_fibrillation.svg

---

## Case 3: Left Bundle Branch Block

### Patient Presentation
**Demographics:** 75-year-old male

**Chief Complaint:** Routine pre-operative evaluation

**History of Present Illness:**
A 75-year-old male with history of hypertension and coronary artery disease (prior LAD stent 5 years ago) is undergoing pre-operative evaluation for elective hip replacement surgery. He denies chest pain, shortness of breath, palpitations, or syncope. He exercises regularly and can walk 2 miles without symptoms. His last stress test was 2 years ago and was normal.

**Physical Examination:**
- Vital Signs: BP 138/82 mmHg, HR 68 bpm, RR 14/min, SpO2 98% on room air
- General: Well-appearing, no acute distress
- Cardiovascular: Regular rate and rhythm, paradoxically split S2, no murmurs
- Lungs: Clear bilaterally

### Workup
- **ECG Findings:**
  - QRS duration: 160 ms (prolonged >120 ms)
  - Broad, notched R waves in lateral leads (I, aVL, V5-V6) - "M" pattern
  - Deep S waves in V1-V3
  - Absent septal Q waves in lateral leads
  - ST-segment and T-wave changes discordant to QRS (appropriate)
  - Left axis deviation

- **ECG Interpretation:**
  - Rate: 68 bpm
  - Rhythm: Normal sinus rhythm
  - Axis: Left axis deviation (-45°)
  - Intervals: PR 180 ms, QRS 160 ms
  - **Conclusion: Left bundle branch block (LBBB)**

- **Comparison:** New compared to ECG 2 years ago (which showed normal QRS duration)
- **Echocardiogram ordered:** LVEF 40%, septal dyskinesis, mild LV dilation

### Diagnosis
**New left bundle branch block with newly reduced ejection fraction - requires further evaluation for ischemia**

*ECG Correlation:* LBBB occurs when conduction through the left bundle is blocked:
- **Septal depolarization reversed:** Right-to-left instead of left-to-right (loss of septal Q waves)
- **LV depolarization delayed:** Impulse travels through myocardium, not Purkinje fibers
- **Prolonged QRS:** >120 ms due to slow muscle conduction
- **Secondary repolarization changes:** ST/T wave changes opposite to QRS direction (appropriate discordance)

Paradoxically split S2 occurs because delayed LV activation causes A2 to occur after P2. New LBBB can indicate underlying coronary artery disease, cardiomyopathy, or conduction system disease.

### Treatment
1. Cardiology consultation for new LBBB with reduced EF
2. Stress testing unreliable for ischemia detection in LBBB → proceed to coronary angiography
3. Initiate guideline-directed medical therapy for heart failure (ACE inhibitor, beta-blocker)
4. Consider cardiac resynchronization therapy (CRT) if EF remains ≤35% with LBBB on optimal medical therapy
5. Postpone elective surgery pending cardiac workup

### Clinical Image
![LBBB ECG](case_03_image.jpg)

**Image Description:** 12-lead ECG demonstrating left bundle branch block with characteristic findings: QRS >120 ms, broad notched R waves in I, aVL, V5-V6, deep S waves in V1-V3, and appropriate discordance of ST-T changes.

**Source:** Wikimedia Commons - Left bundle branch block ECG
**License:** CC BY-SA 3.0
**URL:** https://commons.wikimedia.org/wiki/File:LBBB_ECG.png