Defibrillators were originally developed to terminate life-threatening ventricular arrhythmias with an electrical shock. One might theoretically distinguish cardioversion, corresponding to the delivery of a low-energy shock synchronized to the ascending phase of the R wave of the electrogram (EGM), from defibrillation, which corresponds to the delivery of a high-energy, non-synchronized shock. In the ventricular fibrillation (VF) zone, synchronization may be impossible because of the instability of the ventricular EGMs. In practice, a defibrillator manufactured by Saint Jude Medical attempts to systematically 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. A low-energy shock, on the order of 1 Joule (J), delivered in the vulnerable period, can induce an arrhythmia. The upper limit of vulnerability corresponds to the lowest shock energy, which, when delivered during the ventricular vulnerable period, does not induce VF, a value that has been correlated with the defibrillation threshold. The probability of arrhythmia termination increases thereafter, along an exponential curve, as the shock (synchronized with the R wave) amplitude increases. Beyond a certain strength, however, the risk of re-inducing an arrhythmia increases as well, limiting the success of the therapy. An excessively strong shock may also cause myocardial injury.
The energy stored, then delivered by a defibrillator is expressed as:
1/2 CV2 where C = capacitor and V = voltage.
The energy delivered can be increased by increasing the capacity or by increasing the voltage. With the devices manufactured by Saint Jude Medical, a choice was made to use an average strength capacitor (90/110 μF) and a high voltage (830 V).
Several variables pertaining to the shock waveform, its vector and amplitude, and the number of shocks delivered are programmable.
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 ventricular tachycardia (VT) or VF.
The shocks delivered by Saint Jude Medical ICD can be programmed to be mono- or biphasic; the nominal waveform is biphasic and should preferably not be changed, even in presence of a high defibrillation threshold, since a) the latter is likely to be even higher with a monophasic shock, and b) more importantly, the risk of arrhythmia re-induction is increased.
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 coil, while dual coil shocks are delivered between 1) the distal coil, 2) the proximal coil and 3) the pulse generator. The anode is usually included between the 2 shocking electrodes and the cathode is the pulse generator. With a dual coil electrode, the shock vector can be changed by including or excluding (single coil shock) the proximal electrode in the superior vena cava. In presence of a high defibrillation threshold, the superior vena cava electrode can be excluded in case it is floating in the low right atrium, where some of the delivered energy is wasted.
The nominal polarity of Saint Jude Medical 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. A high defibrillation threshold might be lowered by programming the right ventricular electrode as the anode in the first phase of a biphasic shock. Therefore, in presence of a high defibrillation threshold with an anodal polarity, the programming of a cathodal polarity is discouraged.
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 tests, as the latter measurements are close to the actual values (<10% of variation).
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. In the VT zone, the first shock can be programmed either empirically between 5 and 10 J, sparing the battery and shortening the capacitor’s charge time, or at a higher amplitude with a view to optimize the likelihood of successful treatment of the arrhythmia.
In the VF zone, the highest number of consecutive shocks is fixed at 6, limiting the risk of delivering an endless string of inappropriate shocks.
The programming of shock polarity and waveform in the VT versus VF zones cannot be different.
For a shock to be delivered
- the charge must have ended,
- the arrhythmia must have been re-confirmed by ≥6 short cycles, usually sensed during the charge unless the latter is very short,
- 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
- the mean and instantaneous sensed cycles to which the shock is synchronized cannot be sinus (no shock delivered after a long cycle).
To guarantee the highest likelihood of a successful second shock, 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.
The charge times remain constant throughout these devices’ longevity (30 J <6 sec; 36 J <9 sec; 40 J <11 sec).
This function manages and lowers a high defibrillation threshold non-invasively by optimising 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.
One of the priorities, when programming a defibrillator, is to lower the rate of shock delivery without compromising the patient’s safety, using the least aggressive and painful method to terminate the arrhythmia. Antitachycardia pacing (ATP) captures and interrupts an organized VT by penetrating its circuit. The ventricle must, therefore, be paced at a rate faster than that of the tachycardia. Since ATP is painless, lowers the energy consumption and spares the batteries, it must be favored for the treatment of organized ventricular tachyarrhythmia, even when rapid. The efficacy of this kind of therapy has been confirmed over a wide range of VT rates, up to 240 bpm. It is now customary to program a burst of ATP in the fast VT or even the VF zone, either during or before the charge of the capacitors, unless it has been previously found ineffective or proarrhythmic.
Several parameters must be programmed to optimise the efficacy of ATP:
Burst or ramp ATP
In a burst, the VVO pacing cycle length of a sequence remains fixed, and the first stimulus is synchronized with the last sensed QRS.
In a ramp, the cycle length shortens from one cycle to the next by a programmable decrement.
Number of ATP attempts
The number of programmed ATP attempts varies depending on the rate of the tachycardia. In a slow VT zone, multiple sequences can be programmed in order to delay as much as possible the delivery of a shock for a tachycardia that is not immediately life-threatening. One might also withhold the programming of a shock altogether in this slow VT zone. For tachycardias between 150 and 200 bpm, 3 to 6 consecutive sequences of ATP are usually programmed. For more rapid VT, a single sequence is programmed, in order to not delay the delivery of shocks for a tachycardia that compromises the patient’s hemodynamic function and prognosis.
Number of pulses per sequence
On average, 5 to 15 consecutives pulses are programmed in each salvo. An insufficient number of pulses might fail to penetrate the tachycardia circuit, leaving the salvo ineffective. On the other hand, too many pulses might terminate and then re-induce the tachycardia. An additional stimulus can be systematically added from one sequence to the next.
Length of ATP cycles
The length of ATP cycles is expressed as a percentage of the tachycardia cycle length, and is usually programmed between 80 and 90% of the mean of the last 4 cycles preceding the diagnosis. The greater the pacing cycle decrement, the faster the pacing rate and the greater the risk of tachycardia acceleration.
Re-calculation between each salvo
The initial coupling interval is recalculated as a function of the VT cycle.
Each initial coupling interval is shortened by Scan step between each burst. The maximum shortening is fixed by the Max step.
Shortest coupling interval
A rate limit is programmable beyond which, regardless of programming, the ATP cycle will not shorten further. When, during a ramp for example, the shortest coupling interval has been reached, additional ATP attempts will be made at that minimum cycle, without further decrement.
The initial programming is empiric, though must be adapted thereafter as a function of:
- the various arrhythmias recorded by the device and analyzed during the patient follow-up;
- the efficacy (termination of the episode) / adverse effect (acceleration of the tachyarrhythmia) ratio associated with an ATP sequence.
ATP can terminate rapid ventricular tachyarrhythmias. The latest ICD enable the programming of a salvo in the VF zone, before or during the charge of the capacitors.
ATP during the charge enables painless VT therapy without delaying the shock, when necessary. The characteristics of ATP during the charge are the same as those of the first ATP in the fastest VT zone. Once the diagnosis of VF is confirmed, the charge begins and ATP is delivered simultaneously. If, thereafter, VF is re-confirmed, a shock is delivered. However, if sinus rhythm has returned, the shock is withheld. This allows a painless management of the tachycardia when ATP is successful, without delaying the shock when unsuccessful. However, successful ATP saves little energy, as >80% of the charge takes place during ATP.
ATP before the charge might save energy. Once the diagnosis of VF is confirmed, ATP is delivered. If VF is re-confirmed after ATP, the capacitor begins charging. If the arrhythmia has been terminated, the charge is withheld, which saves energy. However, unsuccessful ATP delays the shock delivery by a few seconds.