Understanding electrocardiography fundamentals and its interpretation

Mercy

Non-Critical Ward Nursing Incharge, Kauvery Heartcity, Trichy, Tamil Nadu

Background

Definition

Electrocardiography (ECG or EKG) is a non-invasive medical test used to measure the cardiac action potential from body surface. It records the electrical impulses including rate, rhythm as they travel from upper chamber to lower chamber of the heart muscle.

Purpose

  • Detect cardiac abnormalities.
  • Monitor heart health (arrhythmias, ischemia, and rhythm of electrolyte abnormalities).
  • Guide treatment plans for cardiac diseases.

Basic Anatomy of the Heart

Heart Structure Overview

  • Atria: (2) Upper chambers.
  • Ventricles: (2) Lower chambers.
  • Valves: (2) AV valves
  • Pulmonary Valve and Aortic valve
  • Vessels: Aortic and Pulmonary

Electrical System

Electrical Pathway

  • SA Node (Intrinsic pacemaker)
  • AV Node
  • Bundle of His
  • Right and Left Bundle Branches
  • Purkinje Fibers (Ventricular wall)

ECG Basics

ECG Waveform

  • P Wave: Atrial depolarization.
  • QRS Complex: Ventricular depolarization.
  • T Wave: Ventricular repolarization.
  • U Wave: (Rare) Repolarization of the Purkinje fibers.

ECG Intervals

  • PR Interval (0.12 – 0.20sec): Time from the onset of atrial depolarization to the onset of ventricular depolarization.
  • QT Interval (0.36 – 0.44sec): Duration of ventricular depolarization and repolarization.
  • RR Interval: Time between successive R-waves (used to calculate heart rate).

ECG Lead System

Standard 12-Lead ECG

10 electrodes placed on the body (4 limb electrodes and 6 chest electrodes) to create 12 different views (leads) of the heart’s electrical activity.

Leads and Views

  • Limb Leads: I, II, III, aVR, aVL, aVF (frontal plane).
  • Chest Leads: V1, V2, V3, V4, V5, V6 (horizontal plane).

Lead Placements

  • V1 – Junction of the 4th ICS, Right sternal border.
  • V2 – Junction of the 4th ICS, Left sternal border.
  • V3 – Midway between V2 and V4.
  • V4 – Junction of 5th ICS, Mid clavicle.
  • V5 – Anterior aspect of axilla, same line as V4.
  • V6 – Mid axilla, same line a V4.
  • 4 limb leads (for grounding etc)

Lead Placements

ECG Paper and Time Measurement

ECG Paper Grid

  • One small box = 0.04 sec
  • One large box = 0.2 sec
  • 5 large boxes = 1 sec

Measuring Heart Rate

  • Method 1: Count the number of R-R intervals in 6 seconds, then multiply by 10.
  • Method 2: Count the number of large squares between two consecutive R-waves and divide by 300.

Common ECG Findings

Normal Sinus Rhythm

  • Regular rhythm with heart rate between 60-80 bpm.
  • Consistent P waves, QRS complexes, and T waves.

Arrhythmias

  • Atrial Fibrillation (AF): Irregular rhythm with no clear P waves.
  • Ventricular Tachycardia (VT): Wide, fast QRS complexes.
  • Heart Block: Delayed or absent conduction between atria and ventricles.

Ischemia and Infarction

  • ST Elevation: Indicator of myocardial infarction (heart attack).
  • ST Depression: Suggestive of ischemia.

Interpretation Tips

Systematic Approach

  • Heart Rate: Calculate the rate
  • Rhythm: Identify whether regular or irregular and sinus or non sinus.
  • P Waves: Are they present and followed by QRS complexes?
  • PR Interval: Is it within the normal range (0.12-0.20 seconds)?
  • QRS Complex: Narrow or wide? Is it normal?
  • ST Segment: Look for elevation or depression.
  • T Waves: Are they inverted or peaked?
  • QT Interval: Is it shortened or prolonged?

Normal ECG

Conclusion

ECG is essential tool for diagnosing cardiac problems. Understanding ECG interpretation is a systematic process. A solid foundation in recognizing normal vs. abnormal patterns helps healthcare providers make timely diagnosis and interventions. Ongoing practice and case study reviews are key to mastering ECG interpretation.

Nightingale Journals

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