ECG

Electrocardiogram (ECG) Overview

  • Definition: An electrocardiogram is a record of the electrical signals generated by the heart through repeated cardiac cycles.

Components of an Electrocardiogram

Major Waves

  • P wave: Represents atrial depolarization.
    • Significance: A larger P wave than normal may indicate an enlarged atrium.
  • QRS complex: Represents ventricular depolarization.
    • Significance: An enlarged Q wave can indicate myocardial infarction; a larger R wave can indicate enlarged ventricles.
  • T wave: Represents ventricular repolarization.
    • Characteristics: A flatter T wave may occur when there is insufficient oxygen, whereas an elevated T wave can indicate hyperkalemia.

Key Intervals and Segments

  • P-Q interval: Reflects the depolarization of the AV node.
    • Associated Conditions: Can be affected by coronary artery disease (CAD) and rheumatic fever.
  • S-T segment: Represents the plateau phase of depolarization.
    • Significance: Elevated during acute myocardial infarction, and depressed in ischemic heart disease.
  • Q-T interval: Measures from the beginning of depolarization to the end of repolarization.
    • Implications: Lengthened with myocardial damage, myocardial ischemia, or conduction issues.

ECG Patterns and Cardiac Cycles

  • Cardiac Electrical Events: Recorded during one cardiac cycle, featuring magnitude and direction of electrical forces.

  • SA Node Activity:

    • The SA node propagates signals throughout the atrial muscle and down to the AV node in approximately 0.03 seconds.
    • Following depolarization of atrial contractile fibers, the P wave appears in the ECG.

  • Atrial Systole: Occurs concurrently with the P wave. Action potentials slow at the AV node, creating a 0.1-second delay allowing atrial contraction and increasing ventricular blood volume.

  • Propagation through AV Bundle: Roughly 0.2 seconds after the P wave onset, the action potential spreads through the bundle branches and Purkinje fibers across the entire ventricular myocardium, resulting in the QRS complex.

    • Atrial repolarization occurs simultaneously but is masked by QRS complex due to its larger amplitude.

  • Ventricular Systole: Begins shortly after the QRS complex and is evident during the S-T segment; blood is expelled towards semilunar valves.

  • Repolarization: Begins at the apex of the heart and spreads throughout the ventricular myocardium, generating the T wave approximately 0.4 seconds after the onset of the P wave.

  • Ventricular Diastole: Ventricles begin to relax shortly after the T wave; by 0.6 seconds, ventricular repolarization is complete.

    • The ECG cycle repeats at 0.8 seconds with the next P wave, initiating another atrial contraction.

Principles of ECG

Waves, Segments, and Intervals

  1. Waves: Represent deflection of the needle.
  2. Segments: Indicate periods with no electrical activity.
    • Notable segments include the P-R and S-T segments.
  3. Intervals: Measure the duration of various electrical activities:
    • P-R interval: Spread of depolarization in the atrium and conduction through AV node.
    • QRS interval: Time it takes for depolarization to spread over the ventricles.
    • Q-T interval: Represents the time of systole.

Timescale and Voltage in ECG

X-axis (Time)

  • 1 minute: Equivalent to 300 big squares.
  • 1 big square: Equal to 0.20 seconds.
    • Calculation: 60 seconds divided by 300 results in 25 mm.*
  • Little square: Represents 0.04 seconds (5 little squares = 1 big square).

Y-axis (Voltage)

  • 1 small square: Corresponds to 0.1 mVolts.

Normal Values in ECG

  • Heart Rate: 72 beats/min.
  • P wave duration: 0.10 seconds (2.5 little squares).
  • PR segment duration: also 0.10 seconds (2.5 little squares; can range between 0.12-0.16 seconds).
  • QRS duration: 0.10 seconds (2.5 little squares).
  • QT interval: 0.4 seconds (10 little squares).
  • T wave duration: approximately 3 little squares.

ECG Leads and Arrangement

  • Lead Configuration:
    • Lead I: (+) (-)
    • aVL: (+) (-)
    • aVR: (+) (+)
    • aVF: Lead II (+) (-) (-) (-)
    • Ground reference:
    • Chest leads labeled as: V1, V2, V3, V4, V5, V6.

Cardiac Arrhythmias

Definitions

  • Tachycardia: Refers to a rapid heart rate (> 100 beats/min).
  • Bradycardia: Refers to a slow heart rate (< 60 beats/min).
  • Fibrillation: Describes rapid uncoordinated heartbeats.

Types of Arrhythmias

  • Normal Sinus Rhythm:
    • Characterized by:
    1. Heart rate between 100-60 beats/min.
    2. Originating at the SA node.
    3. Passing through the normal Purkinje system with normal velocity.
Examples of Arrhythmias:
  1. Second-degree AV Block: More P waves than QRS complexes are visible.
  2. Junctional Rhythm: Characterized by electrical activity in the atria.
  3. Ventricular Fibrillation: Absence of detectable P waves.

Determination of Heart Rate

  • Calculation Methods:
    • Divide 300 by the number of large boxes between consecutive R waves.
    • Use 1500 divided by the number of small boxes for a more precise measure.

Tachycardia: Detailed Analysis

  • Definition: Rapid heart rate > 100 beats/min. Normal ECG may show intervals greater than these limits.
  • Physiological Causes:
    • Increased temperature leads to approximately 10 beats/min increase for each degree rise in Fahrenheit.
    • Sympathetic stimulation (e.g., following severe blood loss).
    • Toxic conditions of the heart.
    • Classification based on rate:
    • 100-150 beats/min: Simple tachycardia.
    • 150-250 beats/min: Paroxysmal tachycardia.
    • 250-350 beats/min: Flutter.
    • Above 350 beats/min: Fibrillations (atrial or ventricular).

Bradycardia: Detailed Analysis

  • Definition: Slow heart rate < 60 beats/min.
  • Severity Classification:
    • 60-40 beats/min: Mild bradycardia.
    • 40-20 beats/min: Moderate bradycardia.
    • 20-0 beats/min: Severe bradycardia.
  • Physiological Adaptations:
    • Athletes may experience lower heart rates due to larger and stronger hearts increasing stroke volume.
    • Impact of vagal stimulation on heart rate regulation.
    • Carotid sinus syndrome due to sensitive baroreceptors in the carotid arteries.

Physiological (Respiratory) Sinus Arrhythmia

  • The SA node exhibits slight variability; heart rate is not perfectly regular.
  • Heart Rate Variability (HRV): Affected by aging, disease, medications, and other factors.
  • Autonomics Influence:
    • Vagal activity fluctuates with respiration:
    • During inspiration: Inhibited vagal activity leads to an increased heart rate.
    • During expiration: Enhanced vagal activity leads to a decreased heart rate.
  • Complications: Lack of HR fluctuation during respiratory cycles in patients with autonomic issues (e.g., diabetic autonomic neuropathy or after heart transplantation).