An ECG (Electrocardiogram) is a non-invasive test that records the electrical activity of the heart over time. It helps diagnose heart rhythm abnormalities, ischemia (lack of blood flow), heart attacks, and structural heart problems.
How an ECG Works
- Electrodes (10–12) are placed on the chest, arms, and legs.
- The machine detects electrical impulses generated by the heart.
- Results are displayed as waveforms (P, QRS, T, U waves)on graph paper.
ECG Leads & Views
| Lead Type |
Views |
Key Uses |
| Limb Leads (I, II, III, aVR, aVL, aVF) |
Frontal plane |
Rhythm, inferior/posterior MI |
| Chest Leads (V1–V6) |
Horizontal plane |
Anterior/lateral MI, ventricular hypertrophy |
| 12-Lead ECG |
Combines limb + chest leads |
Full heart assessment |
Normal ECG Waveform Components
| Wave |
Duration |
Amplitude |
Meaning |
| P wave |
≤0.12 sec |
≤2.5 mm (0.25 mV) |
Atrial depolarization |
| PR interval |
0.12–0.20 sec |
– |
AV node conduction delay |
| QRS complex |
≤0.10 sec |
5–30 mm (ventricular depolarization) |
Ventricular contraction |
| ST segment |
– |
Isoelectric (flat) |
Ventricular repolarization begins |
| T wave |
– |
1–5 mm |
Ventricular repolarization |
| QT interval |
<0.44 sec (HR-dependent) |
– |
Total ventricular activity |
ECG Interpretation: Step-by-Step
1. Check Heart Rate (HR)
- Normal:60–100 bpm
- Bradycardia:<60 bpm
- Tachycardia:>100 bpm
- Calculate HR:
- Regular rhythm:HR = 300 / # of big boxes between R waves
- Irregular rhythm:Count QRS in 6 sec × 10
2. Assess Rhythm (Regular or Irregular?)
- Sinus rhythm:Normal P waves before every QRS.
- Atrial fibrillation (AFib):No P waves, irregularly irregular.
- Ventricular tachycardia (VT):Wide QRS, no P waves.
3. Analyze P Waves
- Absent P waves?→ Atrial fibrillation/flutter, junctional rhythm.
- Inverted P in II?→ Junctional or ectopic atrial rhythm.
4. Measure PR Interval
- Normal:12–0.20 sec
- Short PR (<0.12 sec):WPW syndrome (delta wave).
- Long PR (>0.20 sec):1st-degree AV block.
5. Examine QRS Complex
- Normal:≤0.10 sec (narrow)
- Wide QRS (>0.12 sec):Bundle branch block, VT, hyperkalemia.
- Pathological Q waves (>1 mm deep, >0.04 sec):Old MI.
6. Check ST Segment
- Elevation (>1 mm in 2+ leads):STEMI (heart attack).
- Depression (>0.5 mm):Ischemia, NSTEMI.
7. Evaluate T Waves
- Peaked T waves:
- Inverted T waves:Ischemia, strain, infarction.
8. Measure QT Interval
- Normal:<0.44 sec (corrected for HR: QTc = QT/√RR)
- Prolonged QT (>0.47 sec):Risk of Torsades de Pointes.
Common ECG Abnormalities
| Condition |
ECG Findings |
Clinical Significance |
| Atrial Fibrillation (AFib) |
No P waves, irregularly irregular QRS |
Stroke risk, requires anticoagulation |
| Ventricular Tachycardia (VT) |
Wide QRS (>0.12 sec), no P waves |
Life-threatening, needs urgent cardioversion |
| STEMI (Heart Attack) |
ST elevation in ≥2 contiguous leads |
Requires emergency PCI/stenting |
| Left Bundle Branch Block (LBBB) |
Wide QRS, “M-shaped” in V6 |
Masks STEMI; consider if new-onset |
| Hyperkalemia |
Peaked T waves, wide QRS, sine wave (critical) |
Needs urgent calcium gluconate |
| Hypokalemia |
ST depression, U waves, flattened T waves |
Risk of arrhythmias |
ECG vs. ECHO vs. Stress Test
| Test |
What It Shows |
When Used |
| ECG |
Electrical activity |
Arrhythmias, ischemia, heart attack |
| Echocardiogram (ECHO) |
Heart structure & function |
Valve disease, heart failure |
| Stress Test (TMT) |
Heart under exertion |
Coronary artery disease screening |
Key Takeaways
✔ ECG is the first-line test for heart rhythm & ischemia.
✔ ST elevation = STEMI (emergency!)
✔ Wide QRS = VT or bundle branch block
✔ Prolonged QT = Risk of deadly arrhythmias
