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Electric shock during a barbecue - Visia AF MRI XT VR

Patient


Male with ischaemic cardiomyopathy implanted with a single-chamber defibrillator (Visia AF MRI XT VR) who received an electric shock during a barbecue.

Trace


1- Does the interval plot suggest a ventricular rhythm disorder or a problem with oversensing?
The plot shows a sudden shortening of the ventricular cycle length which becomes very short, falling into the VF zone at the limit of the programmed blanking value (around 120 ms); the existence of these very short cycles makes it unlikely that this episode is physiological and related to a true episode of ventricular arrhythmia; the most likely diagnosis is therefore that of oversensing with an electric shock that interrupts oversensing.

2-What is your diagnosis?
The EGM shows saturation of the baseline on the 2 channels (bipolar channel and shock channel), the signals observed on the shock channel being of higher amplitude; this is characteristic of oversensing due to electromagnetic interference (50 Hz); these signals are detected as “FS” by the defibrillator; when the FS counter is reached (probabilistic counter 30/40), the device charges its capacitors and delivers a shock after confirming that at least 2/5 cycles are fast; after the shock sinus rhythm reappears.

Take home message

  • The risk of electromagnetic interference with an implantable defibrillator has been frequently described in the hospital environment, in the patient’s home or during professional activities; interference can occur by conduction if the patient is in direct contact with the emitting source or by radiation if the patient is located within an electromagnetic field.
  • The newest defibrillators are protected against most sources of interference that patients may encounter in their daily lives; interfering signals are typically filtered, with analysis restricted to a narrow bandwidth corresponding to physiological signals (high-pass and low-pass filters); however, the high adaptive sensitivity required in a defibrillator for correct detection of signals during ventricular fibrillation may favour detection of non-physiological signals falling within the same bandwidth.
  • Signals due to electromagnetic interference may not be filtered appropriately with consequences of varying severity, ranging from occurrence of inappropriate therapies, inhibition of pacing in a dependent patient, inappropriate fallback due to incorrect diagnosis of supraventricular arrhythmia, rapid ventricular pacing synchronised to atrial oversensing, suspension of therapy detection or fallback to an asynchronous mode; in exceptional cases, interference with a high-intensity field can cause permanent circuit damage.
  • The diagnosis of electromagnetic interference is based on confirmation of exposure to a source at the time of the episode and oversensing of characteristic signals (fast, regular and saturating the baseline).
  • If the oversensing is prolonged, a single electric shock is usually sufficient to interrupt the oversensing, as the patient usually stops the offending activity immediately.
  • Electromagnetic interference is more frequent with an integrated bipolar lead than with «true» bipolar detection, as the detection antenna is wider; the characteristic high-frequency signals, which are obviously non-physiological, are detected on available atrial and ventricular channels (possible diagnoses of bi-tachycardia or AF + VF) and are generally of greater amplitude on the shock channel than on the sensing channel.

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