Unsuccessful defibrillation
Tracing
Manufacturer Boston Scientific
Device ICD
Field Therapy
N° 3
Patient
This 45-year-old man presenting with hypertrophic cardiomyopathy and a history of aborted sudden cardiac death underwent implantation of a Boston Science Teligen defibrillator. He was seen after a syncopal episode occurring during exercise, associated with the delivery of an electrical shock.
Summary
Episode diagnosed in the VF zone. A first shock of maximum strength was delivered, followed by diversion of the charge, followed by a second shock of maximum strength. The shock impedance was normal for a double coil lead.
Graph and trace
Tracing
- relatively rapid spontaneous rhythm (VS), as the patient was exercising;
- very rapid, spontaneous, polymorphous ventricular tachyarrhythmia, with cycles alternating between VT and VF zones;
- detection of the episode (V-Epsd; 8 out of 10 criterion fulfilled for the VT zone); onset of Duration for the VT zone;
- 8 out of 10 criterion fulfilled for the VF zone; onset of Duration for the VF zone (no visible marker on the tracing);
- persistence of the arrhythmia; end of 1 sec Duration in the VF zone (V-Detect);
- onset of the charge;
- end of the charge, which lasted >8 sec as the shock delivered was the strongest the device was capable of;
- at the end of the charge, the first ventricular cycle [VS] is between brackets as it fell in the refractory period following the end of the charge; the first ventricular cycle following that refractory period was not counted (--); the next 2 cycles are short (VF), the charge diversion window had ended and the 2 out of 3 criterion was fulfilled;
- 41-J electrical shock delivered on the second cycle;
- unsuccessful shock and persistence of the arrhythmia;
- redetection of the ventricular arrhythmia (V-Detect); start of the charge of the capacitors;
- diversion of the charge (Chrg Dvrt); the 6 out of 10 criterion was no longer fulfilled because 5 out of the last 10 cycles were classified VS due to prominent undersensing;
- further detection (8 short out of 10 cycles followed by 1-sec Duration); onset of another charge;
- a 500-ms diversion window follows the end of the charge, however without reconfirmation; therefore, the shock must be delivered, because a charge cannot be diverted twice during a same episode. This tracing illustrates the potentially serious consequences of major ventricular undersensing;
- successful electrical shock and termination of the arrhythmia.
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As on the previous tracing, the tachyarrhythmia was immediately very rapid, polymorphous and disorganized. The EGM illustrate the micro-voltage and noisy appearance of the tracing, underscoring the challenge represented by the detection of this type of arrhythmia. After the first shock, ventricular undersensing caused an erroneous diagnosis of return to sinus rhythm and diversion of the charge. It was, however, not possible to divert the charge twice during a same episode, which, in this case, was salutary, considering the marked degradation of the detection of VF observed near the end of the charge. This type of arrhythmia is a challenge in both its detection and its termination. It required 2 electrical shocks of maximum strength to terminate VF. Some, though not all the characteristics of the shock waveform can be optimized by programming. For example, the waveform is necessarily biphasic and the tilt is fixed and cannot be modified. Likewise, the duration of the 2 phases of the shock is the consequence of the tilt and impedance and, therefore, is not programmable. With a double coil lead, the vector can be modified (with a double coil, the shock is delivered between the RV coil, the pulse generator and the superior vena cava coil, with a single coil, the shock is delivered between the RV coil and the pulse generator, or the cold can, where the shock is delivered between the 2 coils). The shocks delivered were of a double coil type. The superior vena cava coil was properly placed in the innominate trunk ending at the junction between the superior vena cava and the right atrium. There was no waste of energy in the atrial cavity to explain a rise in the defibrillation threshold. A change of vector would have been unlikely to increase the likelihood of arrhythmia termination significantly. The shock polarity can also be modified. The nominal polarity of the first shocks is cathodal (initial = RV coil as cathode during the first phase, reversed during the second). It appears, however, that the thresholds are slightly lower for an anodal (reversed configuration) than for a cathodal shock, particularly when the defibrillation threshold is high. It was, therefore, tempting, in this patient, to reverse the polarity of the first shocks. It is noteworthy that the polarity of the last shock of a series is necessarily the reverse of the 7 previous shocks.