Electrocardiogram (ECG) Interpretation

Introduction

• ECG or EKG is a crucial skill in medicine, often overlooked.
• Represents electrical activity in the heart corresponding to the cardiac cycle.
• Basics:
  • P wave: atrial depolarization.
  • Normal 12-lead ECG: 4 limb electrodes, 6 chest electrodes.
  • Leads provide different views of the heart.

Pre-Interpretation Steps

1. Correct Patient Information
   • Ensure correct patient data, date, and calibration.
2. Calibration Settings
   • Default: 25 mm/second, 10 mm/millivolt.

Axis Interpretation

• Axis: overall direction of electrical activity.
• Normal range: -30 to 90 degrees.
• Methods:
  • Quadrant Method: use Lead 1 and avf.
  • Axis deviations (right, left, extreme) indicate different conditions.

Rate and Rhythm

1. Rate
   • Each small square: 1 mm = 40 ms, large squares = 200 ms.
   • Determine rate with 300 divided by large squares or count QRS in 10 seconds.
   • Normal: 60-100 bpm; below: bradycardia, above: tachycardia.
2. Rhythm
   • Regular: consistent RR interval.
   • Irregular: irregularly irregular (e.g., atrial fibrillation), regularly irregular (e.g., certain second-degree heart blocks).

Components of ECG

1. P Wave
   • Represents atrial depolarization.
   • Normal: positive in Lead 2, < 120 ms, < 2.5 mm.
   • Abnormalities: increased duration/amplitude, absence.
2. PR Interval
   • Time between atrial and ventricular depolarization.
   • Normal: 120-200 ms.
   • Abnormalities: prolongation (first-degree AV block), shortening (accessory pathway), variable (other AV blocks).

Morphology of Remaining Components

1. QRS Complex
   • Represents ventricular depolarization.
   • Narrow: supraventricular origin; Wide: ventricular origin or conduction block.
   • Morphology: distinguish left and right bundle branch blocks.
2. ST Segment
   • Interval between ventricular depolarization and repolarization.
   • Elevation: myocardial infarction, pericarditis.
   • Reciprocal changes indicate ischemia.
3. T Waves
   • Represent ventricular repolarization.
   • Descriptors: tall, flat, inverted, biphasic.
   • Abnormalities: hyperkalemia, ischemia, hypokalemia.
4. QT Interval
   • From Q wave start to T wave end.
   • Normal: 360-440 ms (males), 360-460 ms (females).
   • Prolongation: predisposes to arrhythmias.
5. U Waves
   • Occasionally after T waves due to electrolyte imbalance, hypothermia.

Categorization & Importance

• Divide tachycardia into broad/narrow, regular/irregular.
• Bradycardia: presence/absence of P waves.
• ECG: snapshot of heart's activity.
• Correlate with patient history and presentation.
• Monitoring changes over time essential for conditions like ischemia.

Resources

• Recommended for further study: Life in the Fast Lane website.