Therapies Abbott

Abbott Device: ICD Field: Therapy
  1.  ELECTRICAL SHOCKS

  2. SHOCK WAVEFORM

  3. SHOCK VECTOR

  4. SHOCK POLARITY

  5. TILT PULSE DURATION

  6. SHOCK AMPLITUDE

  7. NUMBER OF SHOCKS

  8. SHOCK CONFIRMATION

  9. LAST GENERATION DEFIBRILLATORS

  10. ANTITACHYCARDIA PACING

  11. ATP IN THE VF ZONE

 

 

Electrical shocks

 
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. 

 

Variable effects of electrical shocks as a function of the energy delivered
 
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. 
 

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 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.
 

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 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.
 

Shock polarity

 
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. 
 

Tilt pulse duration

 
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.
 

Number of shocks

 
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. 
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).

 

Last generation defibrillators

 
Shock amplitude
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.
 
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 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. 
 

Antitachycardia pacing

 
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.
 
Sweep
 
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 in the VF zone

 
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.

Theoritical features / Basic concepts

Shock therapie Anti-tachycardia Pacing   SHOCK THERAPIES Shock therapies ICD were originally invented to terminate malignant ventricular arrhythmias using DC shocks. Cardioversion is the delivery of a DC shock synchronized to the rising edge of the R wave of the electrocardiogram to terminate a supraventricular or a ventricular tachyarrhythmia, whereas defibrillation is the delivery of an unsynchronized DC shock to terminate VF. During VF, the instability and low voltage of the ventricular EGM sometimes precludes the synchronization of ICD shocks. Medtronic ICDs systematically seek to synchronize the shock delivery, including in the VF zone. Several shock characteristics are programmable, including vector, amplitude and number of shocks delivered. Shock waveform: early defibrillators...

Abbott paths associated with this specificity

This 64-year-old man presenting with ischemic cardiomyopathy and permanent atrial fibrillation (AF) underwent implantation of a St Jude Medical ICD for secondary prevention after an episode of aborted sudden death. He was seen in consultation after a presyncopal episode and an electrical shock.

This 64-year-old man presenting with ischemic cardiomyopathy and permanent atrial fibrillation (AF) underwent implantation of an ICD for secondary prevention after an episode of aborted sudden death. He was seen in consultation after a presyncopal episode and an electrical shock.

Main programmed parameters

This 69-year-old recipient of an Atlas single chamber ICD, implanted after an episode of aborted sudden cardiac death, was seen in consultation for evaluation of syncope followed by an electrical shock. The patient was in atrioventricular (AV) block and chronic AF.

Main programmed parameters

This 28-year-old man presenting with hypertrophic cardiomyopathy, a family history of sudden death, episodes of non-sustained ventricular tachycardia (VT) and a 33-mm thick septum, received an Atlas dual chamber ICD and complained of syncope and a perception of electrical shock.

Main programmed parameters

This 57-year-old, overweight man, suffering from ischemic cardiomyopathy with a 25% left ventricular ejection fraction underwent implantation of an Atlas dual chamber ICD for primary prevention. He was on a regimen of amiodarone for paroxysmal AF. He presented after suffering a 1-h episode of chest pain, followed by prolonged syncope and, according to his wife, several body jolts consistent with the delivery of electrical shocks.

This 63-year-old woman suffering from advanced dilated cardiomyopathy received an Atlas dual chamber ICD for primary prevention, and was seen in routine consultation.

Main programmed parameters

This 74-year-old man presenting with ischemic cardiomyopathy, 25% left ventricular ejection fraction, in New York Heart Association functional class III, and left bundle branch block, received a Promote triple chamber CRT-D and was seen on a routine visit.

Main programmed parameters

This 86-year-old man presenting with ischemic cardiomyopathy and 25% left ventricular ejection fraction, received a Current dual chamber ICD for secondary prevention (VT). He was seen in consultation for evaluation of palpitation followed by electrical shock.

Main programmed parameters

This 48-year-old man suffering from dilated cardiomyopathy, with a 25% left ventricular ejection fraction, 140 ms QRS duration and in New York Heart Association functional class III, received a Promote triple chamber CRT-D for secondary prevention, and was seen in consultation for electrical shock.

Main programmed parameters

This 74-year-old man received an AnalyST Accel single chamber ICD for secondary prevention in the context of ischemic cardiomyopathy with syncope. The patient, who was in chronic AF and had undergone His bundle ablation, was seen in routine consultation, complaining of palpitation.

Main programmed parameters

This 34-year-old man suffering from hypertrophic cardiomyopathy underwent implantation of an Atlas dual chamber ICD for VT complicated by syncope. He was seen after he suffered a syncopal event followed by a shock while exercising.

Main programmed parameters

This 60-year-old man presenting with ischemic cardiomyopathy, a 25% left ventricular ejection fraction and a wide QRS, received a Promote Accel triple chamber CRT-D for a primary prevention indication. He was seen for a routine ambulatory visit.

Main programmed parameters

This 76-year-old man with a history of myocardial infarction received an Atlas dual chamber ICD for sustained VT at 170 bpm.

Main programmed parameters

This 49-year-old man presenting with a very severe dilated cardiomyopathy and a 15% left ventricular ejection fraction, received an Atlas dual chamber ICD and was seen for a routine visit.

Main programmed parameters

This 35-year-old man presenting with arrhythmogenic right ventricular dysplasia underwent implantation of an Atlas single chamber ICD for VT complicated by syncope during beta-adrenergic blockade. He was seen after an episode of syncope and electrical shock during exercise.

Main programmed parameters

This 57-year-old man presenting with ischemic cardiomyopathy and episodes of sustained VT complicated by syncope received a Fortify single chamber ICD. He was seen in consultation after he experienced electrical shocks.