• Limb Leads and the QRS Complex
• Precordial Leads and the QRS Complex
• QRS Complex
• QRS Complex Nomenclature
• Fragmentation of the QRS Complex
• QRS Complex Concordance
• QRS Complex Width
• Narrow QRS Complex (<110ms)
• Wide QRS Complex (>110ms)
• QRS Complex Amplitude
• Increased QRS Complex Amplitude
• Decreased QRS Complex Amplitude
• Electrical QRS Alternans

Limb Leads and the QRS Complex

• Limb leads (I, II, III, aVF, aVR, aVL)
• View the heart in the frontal plane
• If the direction of the electrical vector
• Toward the lead - creates a positive deflection
• Away from the lead - creates a negative deflection
• The ventricles are activated in the following order:
1. Ventricular septum (V_S - septal vector)
• Q wave
2. Left and right ventricles (V_M - main vector)
• R wave
3. Apex of the left ventricle (V_T - terminal vector)
• S wave
• The three ventricular vectors create sequentially
• the QRS complex
  
  
• Each lead views the same vector from a different angle
• The same vector appears differently in each lead

QRS Complex

• On an ECG, it shows ventricular depolarization
• It is sometimes referred to as the QRS interval
• Ventricular depolarization has 3 electrical vectors
• Each vector has a different direction
• Ventricular depolarization is always referred to as the QRS complex
• Even if the QRS has a different configuration (RS, qR, Rs, R, rSR)
• The ventricles have a more massive myocardium than the atria
• Therefore, the P wave is smaller (atrial depolarization)
• Ventricular depolarization (QRS complex) lasts 70ms - 110ms

Fragmentation of the QRS Complex

• The QRS complex represents electrical vectors during ventricular depolarization
• Electrical vectors (QRS complexes) most commonly change in:
• Disorders of the conduction system
• Hypertrophy of heart chambers
• Scar tissue from a myocardial infarction

Fragmentation of the QRS Complex

• Notch
• Is a notch in the QRS complex that does not cross the isoelectric line
• Notch can be caused by:
• Benign early repolarization
• Ventricular tachycardia
• Blockade of the left and right Tawar bundle branch
• P mitrale
• Slurred Wave
• The transition in a certain part of the QRS complex is rounded (slurred)
• Slurred wave can be caused by:
• Delta wave (WPW syndrome)
• Blockade of the left and right Tawar bundle branch
• Benign early repolarization
• Delta Wave
• Is a slurred beginning of the QRS complex (the base of the QRS complex is widened)
• Occurs due to activation of the ventricles via an accessory pathway (Kent bundle)
• Occurs in:
• WPW syndrome
• Osborn Wave (J Wave)
• Is a notch on the descending part of the QRS complex (resembles the letter h)
• Occurs in:
• Hypothermia
• Benign early repolarization
• Hypercalcemia
• Epsilon Wave
• Is a small positive wave at the end of the QRS complex
• Occurs in arrhythmogenic right ventricular dysplasia
• Due to delayed depolarization of the right ventricle

QRS Complex Width

• Has a width of 70ms - 110ms (max. 2.5 squares)
• Because the ventricles depolarize within 110ms



• The width of the QRS complex depends on the method of ventricular depolarization
• Narrow QRS complex (< 110ms)
• Wide QRS complex (>110ms)

Narrow QRS Complex (<110ms)

• The ventricles are activated
  • By a supraventricular impulse through an intact conduction system
  • The impulse propagates quickly through the conduction system
  • This allows the ventricles to depolarize within 110ms
    
    
• Most common causes of a narrow QRS complex
• Sinus QRS (normal P wave)
• Atrial QRS (abnormal P wave, fibrillation, atrial flutter)
• Junctional QRS (no P wave or abnormal P wave with PQ < 0.12s)
• Supraventricular tachycardia

Wide QRS Complex (>110ms)

• The ventricles are activated
  • Through a disruption in the conduction system in the ventricles
  • Depolarization of the ventricles thus takes longer (>110ms)
  • Mechanisms of a wide QRS complex
  • Supraventricular impulse with aberrant conduction
  • Supraventricular impulse with accessory pathway
  • Ventricular impulse from an ectopic ventricular focus
  • Most common causes of a wide QRS complex:
  • Right Bundle Branch Block
  • Has rsR' in V1 (right rabbit ear is larger) and deep S in V6
  • Left Bundle Branch Block
  • Has dominant S in V1, wide R with notch in V6, and absence of Q wave in lateral leads
  • Ventricular Tachycardia
  • Signs of ventricular tachycardia: AV dissociation, captured and fused beats, QRS concordance (V1-V6)
  • Ventricular Extrasystole
  • Has complete compensatory pause and discordance
  • Hyperkalemia
  • Has high peaked T waves
  • Tricyclic Antidepressants (intoxication)
  • Results in tachycardia and dominant R' (aVR)
  • Pacemaker (ventricular pacing)
  • There is a stimulus artifact before the QRS
  • Hypothermia
  • Has bradycardia, prolonged QT interval, and Osborn wave
  • WPW Syndrome (Pre-excitation Syndromes)
  • Has delta wave and shortened PQ interval (<0.12s)
  
  

QRS Complex Amplitude

• Amplitude (voltage) depends on
• Calibration of the ECG device
• Direction of the main electrical vector
• Myocardial thickness (the more cells, the larger the vector)

Decreased QRS Complex Amplitude

• Only one condition needs to be met:
• QRS amplitude in limb leads < 5mm
• QRS amplitude in chest leads < 10mm
  
  
• Causes of decreased amplitude (voltage) on EKG:
• Fluid
• Pericardial effusion (has a triad: low voltage, tachycardia, electrical alternans)
• Pleural effusion
• Fat
• Obesity
• Air
• Emphysema
• Pneumothorax
• Infiltrative diseases
• Myxedema
• Restrictive cardiomyopathy
• Constrictive pericarditis
• Loss of viable myocardium
• Previous myocardial infarction
• Dilated cardiomyopathy (advanced stage)

Electrical QRS Alternans

• Electrical alternans
• It is a change in electrical activity that regularly changes with each heartbeat
• QRS alternans is a regular change in QRS amplitude > 1mm
  
  
• Most common causes of QRS alternans:
• Pericardial effusion
• The heart "floats" in the effusion and the electrical heart axis changes regularly with each beat
• The direction of the vector to the ECG leads changes
• The mechanism is the mechanical rotation of the heart within the pericardium
• Ventricular tachycardia
• Supraventricular tachycardia (most commonly Orthodromic AVRT)