• Sensing
• Oversensing
• Myopotentials of Skeletal Muscles
• ECG and Musculoskeletal Oversensing

Sensing

• Sensing is the ability of a pacemaker to detect the heart's own electrical impulses (P waves, QRS complexes) through the electrode.
• Impulses from the chamber are input information for the pacemaker, which then
• evaluates their presence (as well as frequency and regularity)
• Based on sensing, the pacemaker initiates pacing.

Myopotentials of Skeletal Muscles

• Pacemaker electrodes
• Detect electrical activity (action potential)
• from the heart, but also from skeletal muscles
• Skeletal muscles generate electrical activity during movement
  
  
• Musculoskeletal oversensing
• Muscles generate an electrical impulse
• that exceeds the sensitivity threshold of the pacemaker
• resulting in musculoskeletal oversensing
  
  
• Deltopectoral muscles
  • Can cause oversensing in a unipolar pacemaker
• Diaphragmatic muscle
  • Can cause oversensing in a bipolar pacemaker

Musculoskeletal Oversensing and VVI Pacemaker

• The patient has an implanted VVI pacemaker
• Own QRS complexes (positive sensing) inhibit pacing
• The pacemaker is set to pace at a rate of 70/min.
• Pacing artifacts have a frequency of 70/min.
• In the middle of the ECG, we see electrical activity of the muscles
• The pacemaker mistakenly sensed it (oversensing) as own QRS
• Positive sensing, thus turning off ventricular pacing
• During muscle activity
• No pacing artifacts (and subsequently no QRS) are present
• The pacemaker "thinks" that the heart is generating its own QRS complexes