• ECG Waveform Amplitude
• Electrical Alternans
• Mechanism of Electrical Alternans
• Repolarization Alternans
• T Wave Alternans
• U Wave Alternans
• ST Segment Alternans
• Conduction Alternans
• Motion Alternans

ECG Waveform Amplitude

• Action Potential during depolarization and repolarization of the myocardium
• Creates an electrical vector, which is displayed by individual ECG leads
• The amplitude of the ECG waveform depends on
• The size of the electrical vector
• The direction of the electrical vector
• Each wave and deflection has its own amplitude

Mechanism of Electrical Alternans

• During the cardiac cycle (action potential), from beat to beat, the changes
• Size or direction of the electrical vector
  
  
• There are 3 main causes of electrical alternans
  
  
• Repolarization alternans
• Changes in repolarization during the action potential
• Changes in the repolarization part of the ECG waveform (S wave, T wave, U wave)
• Conduction Alternans
• Changes in the impulse conduction through the conduction system or through the myocardium
• Changes the size and direction of the vector
• Changes in the amplitude on the ECG (P wave, QRS complex)
• Motion Alternans
• Is the most common cause of electrical alternans
• Occurs in pericardial effusion
• The heart "swims" in the effusion
• The heart partially rotates from beat to beat
• Changes the direction of the main electrical vector
• On the ECG, the amplitude of QRS complexes changes

T Wave Alternans

• Most commonly occurs with a prolonged QT interval
• T wave alternans with a prolonged QT interval
• Is a risk factor for the development of Torsades de Pointes
• T wave alternans is a risk factor for the development of:
• Ventricular tachycardia
• Ventricular fibrillation
• Sudden cardiac death

Repolarization Alternans of the T Wave

• On the ECG, there is a prolonged QT interval
• QT interval: 0.6s
• T wave alternans in the context of a prolonged QT interval
• Increases the risk of malignant arrhythmia such as Torsades de Pointes

ST Segment Alternans

• Ischemia (myocardial infarction) is seen on the ECG as changes in the ST segment
• ST segment alternans during ischemia (infarction)
• Increases the risk of malignant arrhythmia:
• Ventricular tachycardia
• Ventricular fibrillation

Repolarization Alternans of the ST Segment

• On the ECG there is an acute STEMI infarction
• Pardee's wave
• Huge ST elevations merging with R waves and T waves
• Typical for acute STEMI infarction
• ST segment alternans (in ischemia/infarction)
• Increases the risk of malignant ventricular arrhythmia

QRS Alternans

• On the ECG, orthodromic AV reentry tachycardia is observed
• In ECG with sinus rhythm, the patient had a delta wave (WPW syndrome)
• QRS alternans (amplitude of the QRS complexes varies)
• It is very common in orthodromic AVRT

Motion Alternans

• It is the most common cause of electrical alternans
• Occurs in pericardial effusion
• The heart "floats" in the effusion
• The heart partially rotates from beat to beat
• Changes the direction of the main electrical vector
• On the ECG, the amplitude of QRS complexes changes
  
  
• For pericardial tamponade, the characteristic triad includes:
• QRS alternans
• Sinus tachycardia
• Low QRS voltage