ECG~Reading

 Electrocardiogram (ECG/ EKG) records the heart’s electrical activity. Learning to read it helps identify rhythm issues, heart attacks, electrolyte imbalance, and more.

Here is a step-by-step approach used by clinicians.

1. Understand the Basic ECG Components

• P wave: Atrial depolarization (atria contracting)

• QRS complex: Ventricular depolarization (ventricles contracting)

• T wave: Ventricular repolarization (ventricles relaxing)

• PR interval: Time from atrial to ventricular activation

• ST segment: Early ventricular repolarization—very important in heart attacks

2. Follow the 6-Step ECG Interpretation Method

Step 1: Check Patient Details & Calibration

• Standard paper speed: 25 mm/sec

• Standard amplitude: 10 mm = 1 mV
  If calibration is wrong, all measurements will be inaccurate.

Step 2: Determine Heart Rate

You can calculate heart rate using:

Method A: Large Box Method

• Count the number of large squares between two R waves.

• Heart Rate = 300 ÷ number of large squares

Method B: Small Box Method (more precise)

• Heart Rate = 1500 ÷ number of small squares between two R waves

Method C: 6-Second Method

• Count the number of R waves in 6 seconds and multiply by 10.
  Useful for irregular rhythms.

Step 3: Evaluate Heart Rhythm

Look for:

• Regular or irregular rhythm

• Presence of P wave before every QRS

• Consistent PR interval

A regular rhythm with normal P → QRS → T pattern is called Normal Sinus Rhythm (NSR).

Step 4: Measure Key Intervals

• PR interval: 0.12–0.20 sec

  • Short PR → pre-excitation (e.g., WPW)

  • Long PR → 1° AV block

• QRS duration: < 0.12 sec

  • Wide QRS → bundle branch block or ventricular rhythm

• QT interval: Corrected QT (QTc) < 440 ms (men), < 460 ms (women)

• Long QT → risk of dangerous arrhythmias

Step 5: Check Cardiac Axis

Cardiac axis shows the direction of electrical activity.

• Normal axis: –30° to +90°

• Left axis deviation: Often in LVH, left anterior fascicular block

• Right axis deviation: RVH, pulmonary conditions

Quick method:

• If Lead I positive and aVF positive → Normal Axis

Step 6: Look for Abnormal Patterns

Common abnormalities include:

A. Ischemia & Myocardial Infarction (Heart Attack)

• ST elevation → Acute MI

• ST depression → Ischemia

• T-wave inversion → Ischemia or previous MI

• Pathological Q waves → Old MI

B. Arrhythmias

• Atrial fibrillation: No P waves, irregularly irregular rhythm

• Atrial flutter: Sawtooth F-waves

• Ventricular tachycardia: Wide QRS, fast rate

• Bradycardia: Slow heart rate (<60 bpm)

C. Chamber Enlargement

• Left ventricular hypertrophy (LVH): Tall R waves in V5–V6

• Right ventricular hypertrophy (RVH): Tall R in V1, right axis deviation

D. Electrolyte Imbalances

• Hyperkalemia: Peaked T-waves

• Hypokalemia: U-waves

• Hypocalcemia: Prolonged QT

12-Lead ECG Lead Placement Overview

• Limb leads: I, II, III, aVR, aVL, aVF

• Chest (precordial) leads: V1–V6
  Correct placement ensures accurate interpretation.

• Quick ECG Interpretation Checklist

1. Rate

2. Rhythm

3. P wave

4. PR interval

5. QRS width

6. QT interval

7. ST segment & T waves

8. Axis

9. Compare with previous ECG (if available)

• Conclusion

  Reading an ECG becomes easier with a structured approach.
  Start with the basics—Rate → Rhythm → Intervals → Axis → ST/T changes—and with practice, you will quickly identify normal and abnormal patterns.