• Action Potential and ECG
• Cardiac Memory
• Cardiac Memory and ECG
• ECG and Memory T Waves

Action Potential and ECG

• Action Potential (impulse) is a rapid change in the electrical voltage of cardiomyocytes
• It arises from changes in the concentration of intracellular and extracellular ions
• The action potential propagates
• Quickly through the conduction system
• Slowly through the working myocardium

ECG (Lead V6) and Action Potential

• QRS Complex represents ventricular depolarization
• During the formation of the QRS complex, a depolarization wave spreads through the ventricles
• ST Segment is the isoelectric line
• During the second phase of the action potential, the entire myocardium is depolarized (-), and no electrical vector is generated
• T Wave represents ventricular repolarization
• The electrical vector during repolarization has the same direction as during depolarization
• Repolarization, however, lasts longer, so the T wave is wider than the QRS complex
• QT Interval corresponds to one cardiac cycle:
• Depolarization + repolarization of the ventricles
• P Wave represents atrial depolarization
• The action potential of the P wave is not visible in the image
• Atrial repolarization is hidden within the QRS complex

Cardiac Memory and ECG

Wide QRS Complex

• During abnormal ventricular depolarization
• For example, in ventricular tachycardia
• A cardiac memory may develop
• Specifically in repolarization (T waves)

Sinus Rhythm and Memory T Waves

• After a change from abnormal depolarization
• Typically to a sinus rhythm
• The heart remembers the repolarization during the wide QRS
• Memory inverted T waves develop

WPW Syndrome (Type B)

• WPW Syndrome Type B
• Shortened PQ interval < 0.12s
• Delta wave and widened QRS approximately 0.15s
• Negative QRS (V1)
• This indicates that the Kent bundle is in the right atrium
• Widened QRS complex
• This represents abnormal ventricular depolarization
• (through the AV junction and also through the accessory pathway)
• Secondarily, ventricular repolarization is also abnormal
• (which the heart often remembers)

Memory T Waves

• The patient has a pacemaker implanted for sick sinus syndrome
• However, on this ECG, ventricular pacing is turned off
• With sinus rhythm, memory T waves are present
• Positive T wave (aVL)
• Negative T waves (V1-V6)
• Maximum negative T wave in precordial leads (V1-V6)
• It is larger than the negative T wave (III)