Overview
This lecture introduces a step-by-step approach to reading electrocardiograms (ECGs), covering key components, systematic analysis, and common abnormalities.
ECG Basics
- An ECG records the heart's electrical activity during the cardiac cycle.
- The P wave represents atrial depolarization.
- A standard 12-lead ECG uses 4 limb and 6 chest electrodes for multiple heart views.
- Leads 1, aVL, V5, V6 = lateral heart; leads 2, 3, aVF = inferior heart; V1-V4 = septal/anterior views.
Initial Checks
- Confirm patient details, date/time, and machine calibration (25 mm/sec, 10 mm/mV) before interpreting.
- Calibration errors can mimic or hide abnormalities.
Axis Determination
- The cardiac axis is the overall direction of electrical activity, normally -30° to +90°.
- Use the quadrant method with leads 1 and aVF to quickly determine axis type.
Rate Calculation
- Each small square on ECG paper = 40 ms; 5 small squares = 1 large square (200 ms).
- Regular rhythm: 300 ÷ number of large squares between QRS complexes = heart rate.
- Irregular rhythm: Count QRS in 10 seconds, multiply by 6.
- Normal heart rate is 60-100 bpm; bradycardia <60, tachycardia >100 bpm.
Rhythm Assessment
- Check regularity with R-R intervals.
- Irregularly irregular rhythm suggests atrial fibrillation; regularly irregular may indicate heart block.
- Evaluate relationship between P waves and QRS complexes.
Key ECG Components
- P Wave: Normal if positive in lead 2, <120 ms duration, <2.5 mm amplitude; abnormal with atrial enlargement or absent in atrial fibrillation.
- PR Interval: Normal 120-200 ms; prolonged = AV block, short = accessory pathway (e.g., WPW).
- QRS Complex: Narrow = supraventricular origin, wide = ventricular origin or bundle branch block.
- Q Waves: Pathological if >25% of QRS or >40 ms wide, indicating previous ischemia.
- ST Segment: Elevation suggests myocardial infarction or pericarditis; depression indicates ischemia.
- T Waves: Normally positive; inversion can be normal in some leads or indicate ischemia/hyperkalemia.
- QT Interval: Corrected for heart rate; normal <440 ms (males), <460 ms (females); prolonged QT can cause arrhythmias.
- U Waves: May appear with electrolyte imbalance or hypothermia.
Categorizing Arrhythmias
- Tachycardias: Broad vs. narrow complex; regular vs. irregular.
- Bradycardias: Presence/absence of P waves and P-QRS relationship.
Clinical Context and Limitations
- ECG is a snapshot; correlate with clinical history and symptoms.
- Serial ECGs or longer-term monitoring may be needed for intermittent arrhythmias.
Key Terms & Definitions
- Depolarization — Process of electrical activation in heart muscle.
- Axis — The direction of the heart's electrical activity.
- Bradycardia — Heart rate below 60 bpm.
- Tachycardia — Heart rate above 100 bpm.
- Bundle Branch Block — A delay/block in electrical conduction through right or left bundle branches.
Action Items / Next Steps
- Practice reading ECGs using recommended online resources.
- Review arrhythmia patterns and apply the systematic approach.
- Ensure familiarity with normal ECG intervals and morphologies.