Shocks in VF zone

Basic concepts




Defibrillators were originally developed to terminate life-threatening ventricular arrhythmias with an electrical shock. Cardioversion, which consists of delivering a low-energy shock synchronized to the upstroke of the R wave of an EGM, is distinguished from defibrillation, which consists of delivering a non-synchronized, high-energy shock. In the VF zone, synchronization of the shock may not be possible because of instability of the ventricular electrograms. In practice, defibrillators from the different manufacturers attempt to synchronize the shock to the R wave, including in the VF zone.

The effects of an electrical shock vary as a function of the energy delivered. Low energies, on the order of 1 J, delivered in the vulnerable period may induce an arrhythmia. The upper limit of vulnerability, a value correlated with the defibrillation threshold, is the lowest energy delivered in the ventricular vulnerable period that does not trigger VF. The probability of arrhythmia termination increases thereafter along an exponential curve as a function of the amplitude of the shock delivered, when synchronized with the R wave. Above a certain value, the risk of re-inducing an arrhythmia increases as well, thereby limiting the chances of therapeutic success. A shock of excessive amplitude may injure the myocardium. 



The stored energy subsequently delivered by a defibrillator is expressed by the formula:

stored energy = ½ CV; where C = capacitance and V = voltage;

Various characteristics of the shock waveform, shock vector, shock amplitude and number of shocks delivered determine the success of defibrillation and may or may not be programmable.

Several variables pertaining to the shock waveform, its polarity, vector and amplitude, and the number of shocks delivered are programmable according to the manufacturers.

Shock waveform

After a long history of monophasic configuration, a biphasic shock waveform, which lowered the defibrillation threshold, was introduced. The first phase of a biphasic shock is equivalent to that of a monophasic shock, though with a lower critical mass; the second phase brings the membrane potential back to as near to zero as possible, in order to prevent the re-induction of VT or VF. The shock waveform can be programmed in some devices to be mono- or biphasic; however, the nominal waveform is always biphasic and should not be changed.



Shock polarity

 2 polarities are possible; an anodal shock corresponds at a shock with the right ventricular electrode as the anode in the first phase of a biphasic shock and as the cathode in the second phase; in contrast, a cathodal shock corresponds at a shock with the right ventricular electrode as the cathode in the first phase of a biphasic shock and as the anode in the second phase. The nominal polarity for the shocks of Medtronic, Abbott and Microport CRM-Sorin devices is anodal. In contrast, it is cathodal in Biotronik and Boston Science devices. The superiority of a given polarity (anodal versus cathodal) is still under debate, the probabilistic nature of defibrillation makes clinical studies comparing these 2 polarities very difficult to perform. Litterature suggests that the defibrillation thresholds may be lower with a right ventricular electrode used as an anode for the first phase of a biphasic shock, particularly when the DFT is high. According to specific manufacturer, the polarity may or may not be reversed during the delivery of a series of shocks.

Shock vector

The programming of this parameter depends on the number of shocking electrodes available. The defibrillation shock is delivered via a dedicated lead, which may be a single coil (one defibrillation electrode or coil placed inside the right ventricle), or a dual coil (one distal defibrillation electrode placed inside the right ventricle, and one more proximal defibrillation electrode placed in the superior vena cava) lead. Single coil shocks are delivered between the distal coil of the right ventricular lead and the can, while dual coil shocks are delivered between 1) the distal coil, 2) the proximal coil and 3) the pulse generator. With a dual coil electrode, the shock vector can be changed by including or excluding the proximal electrode in the superior vena cava or by excluding the pulse generator (cold can). The greater defibrillation efficacy contributed by a dual coil lead is currently debated. The orientation of the shock vector must cover the left ventricle uniformly; this vector depends on the position of the defibrillation coil(s) and of the pulse generator with respect to the heart. The distal right ventricular coil must be entirely contained inside the ventricular cavity, while the proximal coil of a dual-coil electrode must be placed high enough to prevent the dissipation of current at the level of the right atrial cavity. Placing the proximal coil inside the superior vena cava may be challenging. If it is floating inside the right atrium, it might be preferable to use the lead as a single coil.

Shock amplitude

In the VF zone, the strength of the first and subsequent shocks is usually programmed at the highest value the device is able to deliver. Programming of the defibrillation shock amplitude can be guided by the defibrillation threshold, defined as the least amount of energy that converts VF to sinus rhythm.  According to the specific manufacturer, the amplitude corresponds to the delivered or the stored amount of energy.





Shock waveform

the shocks delivered by Abbott ICD can be programmed to be mono- or biphasic (the nominal waveform is biphasic).

Shock polarity

the nominal polarity of Abbott ICD is anodal. In any given patient, the shock polarity is programmed either anodal or cathodal; the polarity cannot be changed during the delivery of a series of shocks.

Tilt and duration of the phases of a biphasic shock

At a fixed tilt (nominally 65% for each of the 2 phases), the shock is interrupted when the residual capacitor voltage has reached a fixed percentage. The measured pulse duration is a function of the impedance, while the energy delivered is fixed. The tilt of both phases is the same. The optimal duration of the second phase depends on a) the duration of the first phase, b) the defibrillation impedance and c) a membrane time constant. In presence of a high defibrillation threshold, it is not advised to change the tilt (50% for each phase, for example). However, if the defibrillation threshold and impedance are both high, one can optimize the duration of both phases and reprogram a fixed pulse duration. A high impedance indicates an impediment in the transmission of current. Therefore, to deliver a same amount of energy, a long pulse duration is needed, incurring a risk of hyperpolarisation and loss of energy. Consequently, it is preferable to limit the pulse duration. Depending on the measured impedance, the ICD suggests a series of optimal durations in 3 colors: blue corresponds to a typical time constant, green to a more rapid constant, and yellow to a slower time constant.

It is noteworthy that defibrillation impedance can be measured at the time of electrical shock delivery for treatment of a ventricular arrhythmia, or during low-voltage shocks, as the latter measurements are close to the actual values (<10% of variation).

Number of shocks

In the VF zone, the highest number of consecutive shocks is fixed at 6.

VT and VF zones

The programming of shock polarity and waveform in the VT versus VF zones cannot be different.

Shock confirmation

For a shock to be delivered 1) the charge must have ended, 2) the arrhythmia must have been re-confirmed by ≥6 short cycles, usually sensed during the charge unless the latter is very short, 3) the event to which the shock is synchronized cannot be the event immediately following the end of the charge, explaining why the shock is often synchronized to the next cycle, and 4) the mean and instantaneous sensed cycles to which the shock is synchronized cannot be sinus (no shock delivered after a long cycle).

Latest generation of defibrillators

  • Shock amplitude: the Unify™ and Fortify™ models increase the delivered energy to a maximum of 40 J (890 V, or 45 stored J). This amount of energy is available for the second shock only, in order to limit the charge time of the first therapy.
  • Charge times: the charge times remain constant throughout these devices’ longevity (30 J <6 sec; 36 J <9 sec; 40 J <11 sec).
  • DeFT Response™: this function manages and lowers a high defibrillation threshold non-invasively by optimizing the duration of shock phases as a function of the electrodes and the defibrillation impedance. A fixed tilt or an optimized duration of a phase can be programmed as a function of the defibrillation impedance.



Shock wave configuration, tilt and duration of the phases of a biphasic shock

The initially monophasic waveforms became biphasic in more recent defibrillators. In Biotronik devices, the voltage-controlled biphasic shock is a 60/50 fixed-tilt shock. The tilt of the first phase is 40% meaning that 60% of the initial voltage is delivered during the first phase (fixed tilt at 60). The cut-off voltage of the second phase is 20%, meaning that 50% of the remaining voltage (40%/2) is delivered during the second phase (tilt at 50). The delivered voltage is constant, and the duration of each phase varies depending on the shock impedance, i.e. increasing as the impedance increases.

A second shock wave can be programmed (biphasic II, voltage/controlled pulse duration). The voltage load is 100% and the tilt of the first phase is 40%. The cut-off of the second phase occurs after a fixed, 2-ms pulse duration. This can be programmed in patients whose defibrillation threshold is high, particularly when treated with amiodarone, which may increase the choronic defibrillation threshold.

Polarity and shock vector

The shock polarity can be set on normal, reversed or alternating. With a single coil lead, a normal polarity for Biotronik defibrillators means that the shock is delivered between the can, as the anode, and the right ventricular coil as the cathode. With a reversal of polarity, the right ventricular coil becomes the anode, which inverses the 2 phases of a biphasic shock (negative first phase and positive second phase). With a dual coil lead and a normal polarity, the shock is delivered between the can and the distal, right ventricular apical coil and between the proximal shock coil in the superior vena cava and the distal right ventricular apical coil. With reversed polarity, the current direction is opposite. When the polarity is set on alternating, the first shocks’ polarity is normal, then alternates between normal and reversed after the delivery of a first shock of maximum energy. 



Shocks amplitude

The energy of first shock delivered in the various zones is programmable between 2 J and maximum, while the energy of the second shock is programmable between 4 J and maximum; the energy of the second shock must be greater than that of the first. The subsequent shocks are delivered at the highest energy.

Number of shocks

In the VF zone, the number of consecutive shocks is programmable, up to a maximum of 8. While the amplitude of the first 2 shocks is programmable, the next 6 shocks are delivered at the maximum energy of 40 J.

Shock confirmation

If a) the shock confirmation is programmed ON, and b) the defibrillator detects 3 cycles in the sinus or bradycardia zones out of 4 cycles during charging, the charge is interrupted and a phase of redetection/termination of episode begins. In absence of detection of 3 slow cycles out of 4, the charge goes on uninterrupted and, at the end of the charge, the device delivers a shock 30 ms after a short cycle. That short cycle at the end of the charge is indispensable. If, at the end of the charge, 3 long cycles are detected, the charge is abandoned and the capacitors progressively lose their charge over a period of up to 10 minutes. During that period, if another episode is detected, the charge is shorter, using the residual energy already charged up. After a shock has been delivered, the next shock undergoes confirmation (uncommitted). However, after a charge has been interrupted, the next shock does not undergo confirmation (committed). Therefore, two consecutive charges cannot be interrupted, which might be problematic in case of VF undersensing. If the shock confirmation is OFF, an on-going charge cannot be interrupted. At the end of the charge, an attempt is made by the device to synchronize the shock; however in absence of a detectable R wave, a non-synchronised shock is delivered 2 sec after the end of the charge. At the end of the charge, a 50 ms ventricular blanking period prevents the occurrence of any detection.

Post-shock pacing

A 1-sec blanking period without detection or pacing follows the delivery of all shocks. After that blanking period, post-shock pacing begins for a duration that is programmable between OFF and 10 min, with a default of 10 sec. Post-shock pacing is in DDI for the DDD(R), DDI(R), or AAI(R) modes, in VVI for the VVI(R) mode, and in VDI for the VDD(R) or VDI(R) modes.



Polarity and shock vector

The Active Can/SVC Coil parameter and the Pathway parameter specify the electrodes and direction of current flow for defibrillation and cardioversion pulses.

The Active Can/SVC Coil parameter has the following settings:

The Can+SVC On setting connects the Active Can and the SVC Coil. Current flows between these electrodes and the RV Coil. The Can Off setting disables the Active Can feature. In this case, an SVC lead must be implanted. Current flows between the SVC Coil and the RV Coil. The SVC Off setting ensures that the SVC lead, if implanted, is not used. Current flows between the Active Can and the RV Coil.

The settings for the Pathway parameter are AX>B and B>AX. AX refers to the Active Can and SVC Coil electrodes, which may be used individually or in combination. B refers to the RV Coil electrode. The Pathway setting defines direction of current flow during the initial segment of the biphasic waveform. If the parameter is set to AX>B, current flows from the Active Can and SVC Coil to the RV Coil. If the parameter is set to B>AX, this current flow is reversed.

Number of shocks

In the VF zone, up to a maximum of 6 consecutive shocks can be programmed.

Shock confirmation

 The device attempts to synchronize it to a nonrefractory ventricular event that meets one of the following conditions:

  • The event is the second tachyarrhythmic ventricular event after charging, and it is outside the atrial vulnerable period (window extending from 150 ms to 400 ms after a sensed atrial event. A defibrillation therapy is withheld during this period to avoid inducing an atrial tachyarrhythmia).
  • The event is the third tachyarrhythmic ventricular event.

Synchronizing the initial defibrillation therapy: the device attempts to synchronize the defibrillation therapy to the second ventricular tachyarrhythmic event that occurs after charging ends, provided that it is outside the ventricular refractory period and the atrial vulnerable period.

Synchronizing subsequent defibrillation therapies: if the first defibrillation therapy fails to terminate a VF episode, the device starts a 900 ms synchronization window and attempts to synchronize each subsequent defibrillation therapy to a sensed ventricular event. If synchronization is not possible, the device delivers the defibrillation therapy asynchronously after 900 ms.



Shock waveform

The shock waveform is necessarily biphasic and is not programmable with the latest Boston Scientific defibrillators.

Shock vector

The programming of the shock vector depends on the number of high-voltage electrodes available. Single-coil shocks can only be delivered between the distal coil of the RV lead and the pulse generator. Double-coil shocks can be delivered among the distal coil, the proximal coil and the pulse generator.

The following programmable configurations are available with a double-coil lead:

  • RV coil to right atrial coil and can: this vector is also known as the V-TRIAD (double-coil). The pulse generator is an active electrode (hot can) combined with the double-coil defibrillation lead. The energy is simultaneously delivered from the distal to the proximal coil and from the distal coil to the can.
  • RV coil to can: this vector also uses the pulse generator as an active electrode (hot can; single-coil). The energy is only delivered between the distal coil and the pulse generator.
  • RV coil to right atrial coil: this vector, also known as “cold can”, removes the can as an active electrode. The energy is delivered between the distal and the proximal coils.

RV to can is the only working single-coil lead configuration. The 2 others, albeit programmable, should not be used. With the RV to right atrial coils configuration, no shock would be delivered. Therefore, this vector should never be used with a single-coil lead. When programmed, an alert screen appears that asks for verification that the lead is indeed a double coil.

Shock polarity

Polarity can be programmed as initial or reversed. With an initial polarity, the RV coil is negative for the first phase (cathodal), while the pulse generator, the superior vena cava coil, or both are positive (anodal). This initial polarity corresponds to a cathodal shock. With a reversed polarity, the RV coil is positive for the first phase of the shock (anodal), while the pulse generator, the superior vena cava coil, or both are negative (cathodal). This reversed polarity corresponds therefore to an anodal shock.

The selection of the shock pertains to all the shocks delivered by the device. In case of prior unsuccessful shocks in a zone, the last shock of that zone is automatically delivered with the reversed polarity from the preceding shock (initial or reversed)

For a double-coil lead, 6 choices of configurations are therefore available by changing the polarity and the shock vector.

Tilt and phase duration

The tilt of the Boston Scientific defibrillators is a non-programmable 80%. The first phase is truncated when the initial leading edge voltage has decreased by 60% (leaving a residual voltage of 100% - 60% = 40%). The second phase is truncated when the leading edge voltage (corresponding to the 40% residual voltage) has decreased by 50% (40/2 = 20%).

Shocks amplitude

The energy delivered is usually about 14% lower than the stored energy. The first two shocks in each ventricular zone can be programmed between 0.1 and 41 J in order to optimize the charge time, the pulse generator’s life expectancy and the safety margins. The energy of the second shock must be the same or higher lower than that of the first. The energy of the remaining shocks in each zone is fixed at a maximum of 41 J.

In the VT zone, the shocks can also be de-programmed, leaving the zone a) as a monitor (no therapy delivered), or b) limited to deliver bursts of ATP.

Number of shocks

The maximum number of consecutive shocks that can be delivered in the VF zone is fixed at 8, thereby limiting the risk of an endless series of inappropriate shocks. In the VT zone, the number is limited to 6 and in the VT-1 zone to 5 shocks.

Shock confirmation

Ventricular shock therapy can be programmed to be non-committed or committed. The objective of a re-confirmation is to not deliver an unnecessary shock in case of spontaneous termination of the arrhythmia. The device monitors the tachyarrhythmias during and immediately after the charge of the capacitors. During that phase, it examines whether the tachyarrhythmia has spontaneously ended, and determines whether a shock is needed.

If the Committed Shock feature is programmed ON, the shock is systematically delivered synchronously with the first sensed R-wave following a 500-ms delay after the capacitors are charged whether the arrhythmia is sustained or not. The 500-ms delay allows a minimum reaction time for a divert command to be issued from the programmer (the operator is able to cancel the shock delivery with the programmer). A forced 135-ms refractory period elapses at the end of the charge, and events occurring during the first 135 ms of the 500-ms delay are ignored. If no R wave is sensed during the 2 sec following the end of the charge, the shock is delivered asynchronously at the end of the 2 sec delay.

If the Committed Shock feature is programmed OFF, at the end of the charge, a reconfirmation phase takes place, when the device determines whether the arrhythmia has ended (no shock delivered) or not (shock delivered) as follows:

  1. During charging of the capacitors, the pulse generator continues to sense the arrhythmia. The sensed and paced cycles are evaluated. If 5 slow sensed or paced cycles are counted in a 10-cycle detection window (or 4 slow consecutive cycles after an unsuccessful QUICK CONVERT ATP attempt), the device stops the charge and considers it a Diverted-Reconfirm.
  2. If 5 out of 10 cycles are not diagnosed as slow (or fewer than 4 slow consecutive cycles are sensed after an unsuccessful QUICK CONVERT ATP attempt) and the charge has ended, a post-charge reconfirmation takes place of the end of the charge. After the post-charge refractory period and the first sensed event, the pulse generator measures up to 3 post-charge intervals, and compares them with the lowest rate threshold.
  • If 2 of the 3 intervals following the charge are faster than the lowest rate threshold (programming of the lowest tachycardia zone, i.e. the VF zone if a single zone is programmed, VT zone if 2 zones and VT-1 if 3 zones are programmed), the shock is delivered synchronously with the second fast event.
  • If 2 of the 3 intervals following the charge are slower than the lowest rate threshold, the shock is not delivered. If no cycle is sensed, pacing begins at the slowest programmed rate following a 2-sec period. If the shock is not delivered, or if the pacing pulses are delivered, it is also considered a Diverted-Reconfirm.

The reconfirmation algorithm does not allow 2 consecutive Diverted-Reconfirm cycles. If an arrhythmia is redetected after a Diverted-Reconfirm, the next shock of the episode is delivered as if Committed Shock were programmed ON. After a shock has been delivered, the reconfirmation algorithm can be re-applied.

A shock is followed by a non-programmable 500-ms refractory period.