Oversensing and use of an electrical scalpel

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Tracing N° 2
Boston Scientific Devices: ICD Field: Sensing
Patient

This 76-year-old man underwent implantation of a Boston Science Cognis triple chamber defibrillator in the context of ischemic cardiomyopathy with left bundle branch block. He underwent mitral valve surgery.

Summary

Episode corresponding to the beginning of the surgical intervention, diagnosed in the VT zone and treated by a salvo of ATP.

Graph and trace

Tracing

  1. atrial and biventricular stimulation;
  2. sensing of rapid ventricular signals of variable amplitude and morphology in the VF or VT zones; oversensing of disorganized signals by the atrial channel;
  3. interruption of oversensing;
  4. brief further oversensing;
  5. longer period of oversensing at the level of the atrial and RV channels;
  6. the 8 out of 10 VT counter is fulfilled (V-Epsd);
  7. persistence of oversensing for the Duration;
  8. detection of a sustained episode in the VT zone; the V>A is in favor of a ventricular origin, prompting a decision to treat;
  9. burst of 10 stimuli corresponding to the first treatment in the VT zone;
  10. oversensing becomes intermittent; the redetection criterion is never satisfied; the operating surgeon called a cardiologist to deprogram the defibrillator after having observed the burst of ATP on the oscilloscope.

Second summary

An episode of VF was detected by the device; however, no treatment was delivered as the Tachy mode was no longer in monitoring + treatment.

Second tracing

  1. recurrent oversensing visible on the A, RV and shock channels;
  2. detection of another episode (V-Epsd);
  3. at the end of Duration, sustained VF was diagnosed, though no therapy was delivered.
Comments

This defibrillator recipient underwent cardiac surgery. To prevent the delivery of inappropriate therapies due to the use of an electrical scalpel by the surgeon, 2 choices were available: 1) place a magnet over the pulse generator; the response to the magnet can be programmed in the a) OFF position (the magnet has no effect), in which case the electrograms (EGM) are recorded, though no therapy is delivered, or b) Tachy mode (no therapy and no EGM recording), or 2) change the programming; the Tachy mode can be programmed on monitor only or on OFF. An Electrocautery Protection Mode is also available when an electrical scalpel is in use. It prevents the delivery of inappropriate therapies (Tachy mode in OFF position) though also enables asynchronous pacing (AOO, VOO or DOO) which might be an advantage in pacemaker-dependent patients.

Oscilloscopic monitoring must continue throughout the procedure, as the patient is no longer protected by the defibrillator. The application of a magnet should not preclude a complete postoperative interrogation of the device.

Theoritical features / Basic concepts

Sensing and detecting episodes Detection zones SENSING AND DETECTING EPISODES Auto-adjusting sensitivity The effective operation of an Implantable Cardioverter Defibrillator (ICD) relies on its ability to detect both fast ventricular signals and low amplitude ventricular fibrillation signals without detecting T waves or extra-cardiac signals when the patient is in sinus rhythm. In order to ensure appropriate sensing, the device should be programmed with a high sensitivity (low detection threshold) and short refractory periods. Unlike a pacemaker the detection threshold is not programmed to a fixed value. The detection threshold automatically adjusts based on the amplitude of the preceding R-wave. Following a sensed or paced ventricular event the detection circuit starts a blanking period...