Oversensing of pectoral myopotentials was relatively common in patients implanted with an unipolar pacemaker and could result in inhibition of ventricular pacing and syncope in pacemaker dependent patients. In defibrillators, the can positioned in the pocket in the vicinity of pectoral muscles, is not part of the sensing circuit so the myopotentials generated by the pectoral muscles should not affect the sensing circuit.
The amplitude of pectoral myopotentials is usually more important when recorded on the high voltage channel (includes the can which is placed in the pocket at the vicinity of pectoral muscles). Shock EGM is not integrated in the count so, no therapy is usually delivered but oversensing may however alter the quality of the discrimination algorithm based on the morphology analyzed on this high voltage channel.
In contrast, in the presence of an insulation failure (typically an erosion leading to current leak) of the pocket portion of the RV lead, the sensing channel may detect pectoral myopotentials and may lead to inappropriate therapies or inhibition of pacing. Inspection of the EGMs stored in the memory reveals the presence of high-frequency non-physiological signals. In clinic, oversensing can be replicated by isometric left arm exercise or manipulation of the pocket. Suspicion of pectoral oversensing by the near field channel must lead to assess different aspects of lead function, including pacing and shock RV lead impedances, amplitude of the sensed R-wave and pacing threshold and to perform a chest X-ray. Association of pectoral myopotential oversensing and decreased pacing impedance raises the suspicion of lead insulation defect.
In very rare cases, pectoral oversensing can be observed in DF-1 systems when pins have been inadvertently reversed.