ECG

MUBS – MODERN UNIVERSITY FOR BUSINESS & SCIENCE

ECG BASICs, Arrhythmias and Their Management

WHAT IS AN ECG?

  • The heart functions as a pump, driven by intrinsic electrical impulses that cause it to beat.
  • An Electrocardiogram (ECG) is a paper-based recording of the electrical activity of the heart.
  • The ECG captures where the electrical impulses initiate and the path they take through the heart.

ELECTRICAL IMPULSES IN THE HEART

Sinus Node

  • The electrical impulse begins at the Sinus Node, which spreads across both atria.
  • This is represented in the ECG as the P Wave, indicating atrial depolarization.

Atrioventricular (AV) Node

  • After the impulse reaches the AV Node, it has three primary functions:
    1. Delay Impulse: This allows blood to move from the Atria to the Ventricles.
    2. Secondary Pacemaker: Functions at a rate of 45-60 beats per minute (bpm).
    3. Electrical Filter: Prevents rapid impulses from reaching the ventricles.

His Bundle and Purkinje Fibers

  • The His Bundle and Purkinje Fibers provide a rapid conduction pathway with automaticity at rates of 40 bpm (His Bundle) and 15-30 bpm (Purkinje Fibers).
  • The impulse that travels through these structures generates the QRS Complex.

QRS and T Waves

  • The QRS is followed by an isoelectric segment known as the ST Segment.
  • The ventricles then repolarize, resulting in the T Wave.

ECG LEADS

12-Lead ECG Overview

  • A 12-lead ECG employs 10 electrodes:
    • 4 Limb Electrodes
    • 6 Precordial (chest) Electrodes
  • This method detects 12 leads, which consist of:
    • 6 Precordial leads (V1 to V6)
    • 6 Limb leads (I, II, III, aVR, aVL, aVF)
  • Understanding these leads helps identify the direction of electrical impulses and the corresponding heart locations.

Lead Configuration

Limb Leads
  • Bipolar Limb Leads:
    • Lead I: Left Arm (LA) - Right Arm (RA)
    • Lead II: Left Leg (LL) - Right Arm (RA)
    • Lead III: Left Leg (LL) - Left Arm (LA)
  • Augmented Unipolar Limb Leads:
    • aVR, aVL, and aVF
Precordial (Chest) Leads
  • Lead mapping of V1 through V6 allows for the detection of different wall segments of the heart.

ECG PAPER

Measurements

  • Horizontal Measurement:
    • One small square = 0.04 milliseconds (mSec)
    • One large square = 0.2 mSec
  • Vertical Measurement:
    • One large square = 0.5 mV
  • The standard calibration for ECG paper is 25 mm/sec (0.1 mV/mm).

NORMAL ECG

  • Components of a normal ECG include:
    • P Wave
    • QRS Complex
    • T Wave
    • U Wave (if present)
    • PR interval, QRS interval, and QT interval measurements assist in analyzing rhythm and heart function.

HEART RATE ASSESSMENT

Questions to Evaluate

  1. What is the heart rate?
  2. Is the rhythm regular or irregular?
  3. Is there evidence of atrial activity?
  4. Are all P waves occurring in the correct location?
  5. Are all P waves of the correct shape?
  6. Is the QRS complex broad or narrow?

P WAVE

  • Normal Characteristics:
    • Width: 2 mm
    • Height: 2.5 mm
  • Potential abnormalities:
    • Tall P Waves: Indicate right atrial enlargement.
    • Wide P Waves: Indicate left atrial enlargement.
    • Abnormalities may also suggest conditions like atrial fibrillation, flutter, or other ectopic rhythms.

PR INTERVAL

Normal & Abnormal Values

  • Normal: 3 to 5 mm wide (0.12 - 0.20 seconds).
  • Causes of Prolongation:
    • First-degree AV block, hypercalcemia, drug effects.
  • Causes of Shortening:
    • Normal variant, can indicate Mobitz type conditions.

QRS COMPLEX

  • Normal Width: < 3 mm (0.12 seconds).
  • Abnormalities Indicating:
    • Size increases can indicate left ventricular hypertrophy (LVH), while invalid QRS may indicate aberrant conduction or ventricular rhythms.

QT INTERVAL

Measurement and Implications

  • Normal Duration: 0.35 – 0.45 seconds (8 to 12 small squares).
  • Prolongation Causes:
    • Drug effects, hypothyroidism, long QT syndrome.
  • Contraction Issues:
    • If prolonged, interventions may be warranted including medication adjustments.

ST SEGMENT

Assessment

  • Elevation: Indicates conditions like STEMI, left bundle branch block (LBBB), and Prinzmetal's angina.
  • Depression: Indicate non-STEMI or various drug effects.
  • Specific measurements:
    • 4 mm elevation equals 4 small boxes above baseline.

T WAVE

Normal and Abnormal Characteristics

  • Normal: Less than 2/3 the height of R wave.
  • Abnormalities:
    • Excessive height can signal hyperkalemia, myocardial infarction (MI), while inverted T waves might indicate ischemic conditions.

HEART RATE EVALUATION TECHNIQUES

Methods

Rule of 300
  • Count the number of big squares between two QRS complexes and divide 300 by that number to determine the heart rate.
Rule of 10 Seconds
  • Count the number of beats within 10 seconds (50 squares) then multiply by 6.

CONDUCTION PROBLEMS

Pathways in Heart Conduction

  • Impulses travel from the Sinoatrial (SA) node through the Atrioventricular (AV) node, His Bundle, and subsequently through the bundle branches.

AV Node and His Bundle Issues

First Degree Heart Block
  • Characterized by prolonged PR interval; causes include coronary artery disease (CAD), digoxin toxicity, etc.
Second Degree Heart Block
  • Sometimes results in failure of conduction through the AV node; can be Mobitz type I or II, with Mobitz I being more benign.

THIRD DEGREE HEART BLOCK

Characteristics and Management

  • Complete AV Block: No impulses conducted to the ventricles; can result in escape rhythms.
  • Causes include inferior MI or electrolyte imbalances.
  • Patients at high risk for sudden cardiac death; often require pacemaker insertion for management.

BUNDLE BRANCH BLOCK

Normal vs Abnormal QRS Duration

  • Normal QRS duration is less than 120 ms.
  • A delay in conduction within the bundles can lead to wider QRS complexes.
  • Right Bundle Branch Block (RBBB) leads to characteristic patterns in electrocardiography.

RBBB characteristics

  • Often presents with an RSR pattern in V1 and a widened S wave in V6.
  • Causes include right ventricular hypertrophy, pulmonary embolism, and other conditions.
  • Usually requires no specific treatment.

LEFT BUNDLE BRANCH BLOCK

  • Can be identified with a broad QRS complex and is associated with conditions like aortic stenosis and ischemic heart disease.
  • A complete cardiac evaluation is recommended for treatment.

ARRHYTHMIAS

Division of Abnormal Rhythms

  • Abnormal rhythms categorized as:
    • Bradycardias: Slow and sustained
    • Tachycardias: Fast and sustained
    • Fibrillation: Disorganized activation

SINUS BRADYCARDIA

  • Defined as a resting heart rate of < 60 bpm.
  • Common causes include sleep, vagal tone, myocardial infarction, and others.

SINUS TACHYCARDIA

  • Heart rates exceeding 100 bpm in adults; can be caused by exercise, anxiety, or various pharmacological agents.

EXTRASYSTOLES

  • Occur due to depolarization from abnormal locations, leading to premature contractions.

JUNCTIONAL TACHYCARDIA

  • Characterized by absence of P waves and regular QRS complexes; treated with vagal maneuvers and medications like adenosine.

VENTRICULAR TACHYCARDIA

Characteristics

  • Monomorphic or polymorphic presentation; QRS complexes become wide and potentially irregular.
  • Requires immediate attention if unstable.
Management of V-Tach
  • For stable patients, anti-arrhythmic medications (e.g., Amiodarone) may be administered.
  • For unstable patients or those with no pulse, CPR, defibrillation, and administration of medications are crucial steps.

ATRIAL FIBRILLATION

Diagnostic Criteria

  • Absence of identifiable P waves; presence of irregular QRS complexes with a varying ventricular response.

Treatment Principles

  • ABC Treatment:
    • Avoid Stroke: Use anticoagulants such as Warfarin and NOACs.
    • Better Symptoms: Control heart rate with beta-blockers or digoxin.
    • Heart Rhythm Control: Cardioversion or antiarrhythmics may be necessary.

VENTRICULAR FIBRILLATION

Characteristics and Management

  • Identified by a total lack of organized electrical activity, leading to loss of consciousness.
  • Requires immediate defibrillation, early CPR, and correction of underlying causes.

CARDIOVERSION AND DEFIBRILLATION

Definitions

  • Cardioversion: Synchronized shocks for stabilization of irregular heartbeats.
  • Defibrillation: Used for patients without a pulse, delivering higher energy shocks.
Energy Levels Used
  • Cardioversion typically necessitates 100-120 joules, while defibrillation ranges from 200-320 joules (monophasic) to 120-200 joules (biphasic).

CARDIAC RESYNCHRONIZATION THERAPY (CRT)

Overview of CRT

  • Consists of a pulse generator and leads to synchronize ventricular contractions for better efficiency in heart function.
  • CRT-P used for slow rhythms, while CRT-D can correct fast arrhythmias with defibrillation capability.

CONCLUSION

  • Proper analysis and management of various ECG patterns and heart rhythms are paramount for patient care and outcomes. Understanding the principles of electrical conduction in the heart aids in diagnosis, management, and prevention of serious cardiovascular conditions.