Invariably, NTLs have a high-energy coil located near the distal end and lying within the right ventricular cavity. Manufacturers have released NTLs with similar construction, although some details differ (Table 115-1). The physics of DC flow, however, requires at least one other electrode to complete the shocking circuit. The development of smaller ICD generators has allowed for pectoral implantation, which has enabled the use of the generator casing as the second electrode (i.e., “hot can”). An animal study compared the defibrillation efficacy of a hot can ICD system placed in the left pectoral or subaxillary location with a right pectoral location, and left or right abdominal locations. The left pectoral and axillary subcutaneous positions were superior to all other locations. The right pectoral location was superior to either abdominal location. These results imply that alternative ICD implantation sites are feasible in the event of an inability to implant a left prepectoral device: left subclavian venous occlusion, history of left mastectomy or radiation, left sided arteriovenous fistula, or other reasons to avoid the left prepectoral area.

Table 115-1

Summary of Characteristics of Currently Available Nonthoracotomy Leads

DBS, Drawn brazed strand cable; MP35N, cobalt-chrome-nickel alloy; MP35N-Ag, cobalt-chrome-nickel alloy with silver core; Pt-Ir, platinum iridium alloy; Pt-Ir-Fr, platinum iridium fractal coated; TN-Pt-Ir, titanium nitride coated iridium alloy.

Tachyarrhythmia Detection

The recognition of tachyarrhythmias by an ICD is a complex interaction of several dependent variables; however, the task of the ICD is more complicated because it must also recognize the lack of tachyarrhythmias. This central insight is crucial, because the patient will spend all but a small fraction of his or her life in a nontachycardia rhythm. Therefore, assuming the efficacy of the pacing and shock therapies, rhythm recognition arbitrates between quality and length of life.

Not all of the factors required for accurate rhythm recognition are potentially affected by the ICD technology. Most notably, the rate and mechanism of the arrhythmia and the programming of the ICD are major determinants of rhythm recognition and are factors that are almost completely independent of the technologic solution. However, by understanding the nature of the signals presented to the ICD, allowing the ICD to adapt to these signals, and limiting programming options, appropriate ICD function is frequently achieved.

Sensing

All current ICDs use ventricular heart rate as the cornerstone variable in tachycardia recognition. To determine heart rate, the interval between each depolarization of the ventricle must be measured. It is not interval recognition but the detection of individual electrogram events that becomes the basic building block in this process. The process begins with the placement of the sensing lead. The sensed electrogram depends on the health of the myocardium in close proximity to the lead, the far-field structures of the diaphragm, anterior chest wall and right atrium, and other electrical devices such as pacemakers, cellular phones, and other sources of electromagnetic interference. Detection of the electrogram events is completely dependent on the quality of the signal, and the quality of the signal is determined primarilly at the time of the lead placement. Additional aspects that are potentially dependent on the position of the lead are measures of electrogram morphology, such as electrogram event width.

In dual-chamber devices, the accurate recognition of arrhythmias adds another layer of complexity and hopefully specificity with the inclusion of atrial lead data input into the generator. The intracardiac location of the atrial lead (far away from the annulus to minimize the far field right ventricular signal) and a short interelectrode distance between the distal and proximal electrodes improve the signal-to-noise ratio of the sensed atrial signal and can thus improve the accuracy of the data used for tachcyardia discrimination.

Band-Pass Filtering

The sense amplifier processes signals presented to the pulse generator by the sensing electrodes and allows signals of certain frequencies to be presented to the detection logic, whereas others are filtered out. This band-pass filter consists of a high-frequency cuttoff to filter out myopotentials signals and a low-frequency cutoff to filter out repolarization T wave signals. The mid range is intended to represent a band of frequency containing true signal events. The intent is to prevent extraneous signals from fooling the device into falsely detecting tachyarrhythmias. Unfortunately, there is some frequency overlap between repolarization and depolarization waves, atrial and ventricular events, postpacing polarization and depolarization of the ventricles, myopotentials and cardiac depolarizations, and environmental signals and cardiac events.

Frequency and Amplitude

Both large signal amplitude and high-frequency content improves the chances of the signal being detected. In addition, there is a strong relation between the amplitude of the signal presented to the band pass filter and the frequencies contained within that signal. Larger signals usually improve the specificity of electrogram event detection by placing the frequency content into the center frequencies of the band-pass filter. However, a signal that is relatively small, such as 4 to 6 mV, but has good frequency content as represented by the slew rate measured on the pacing system analyzer of more than 1 V/s will probably do better than a larger signal such as 8 to 10 mV, with much poorer frequency content of less than 0.1 V/s.

Autogain and Autothreshold

One of the larger challenges presented to ICD detection algorithms is the marked variability in the amplitudes of the signals presented to the ventricular lead: naturally conducted beats through the His-Purkinje system of 5 to 25 mV, paced amplitudes of 5000 mV with polarization voltage after each pacer spike, premature ventricular contractions and ventricular tachycardia amplitudes from 2 to 25 mV, asystolic electrogram amplitudes of 0 to 0.15 mV, and ventricular fibrillation amplitudes from 0.2 to 20 mV. The transitions between each of the rhythms must be identified accurately.

The most difficult problems are distinguishing frequent excursions from low to high amplitude and back to low-amplitude electrogram events. The two approaches are the use of autogain and autothreshold. The autogain technique uses fixed amplitude voltage thresholds and amplifies the signal at continually varying gains according to various algorithms to single count both small and large signals. The autothreshold technique uses one amplifier gain and a continuously varying threshold to accomplish the same end. Usually the threshold will vary within a single cycle, decaying after each sensed event after an adjustment proportional to the amplitude of the signal. Sometimes the floor or minimal voltage limit can be programmed to distinguish between noise on the signal and low-amplitude VF signals. After a paced beat, the autogain is set to maximum and the autothreshold is set to minumum.

Estimation of Adequate Amplitude

The ventricular sensing electrode is placed during a nontachycardia rhythm. Therefore, the adequacy of that placement must be determined before the VF is induced for defibrillation adequacy testing. Although there is marked variability in the average electrogram event amplitude between VF episodes and within a single VF episode, there is a relation between the amplitude of the sinus rhythm electrogram amplitude and the average, minimum, and maximum VF electrogram events of subsequently induced arrhythmias. As an estimate of adequacy, sinus rhythm electrograms of at least 5 mV reliably predict that less than 10% of the electrogram events will be undersensed and that failure of tachyarrhythmia detection is eliminated.

Detection Algorithms

Once an electrogram event is marked or detected, the detection algorithm interprets the pattern of the intervals between the events. The algorithms attempt to develop a hierarchy of detected rhythms that require different therapies such as bradycardia, ventricular tachycardia requiring antitachycardia pacing, ventricular tachycardia requiring low-energy shocks, and VF requiring higher-energy shocks. The exclusion of sinus tachycardia and atrial fibrillation with overlapping rates with ventricular arrhythmias is particularly difficult. Additional criteria to distinguish these rhythms include the ventricular rate criterion the duration of the arrhythmia, the acuteness of the increase in rate, and the beat-to-beat variation of the cycle length. Derivatives of the electrogram morphology including templates, duration, and vector correlations, and with the addition of an atrial electrode, the pattern of the relation of the atrial and ventricular events have more recently improved the specificity of arrhythmia detection.

Sensitivity Versus Specificity: Discriminating Supraventricular from Ventricular Arrhythmias

Life-threatening arrhythmias must always be detected and eliminated; however, the arrhythmia’s mechanism, rate, or both do not always correlate with its hemodynamic significance. Atrial flutter with 1 : 1 atrioventricular (AV) conduction or monomorphic ventricular tachycardia at 110 beats/min could result in hemodynamic collapse. The lower the rate cutoff, the greater the sensitivity, but the poorer the specificity for truly life-threatening situations. The challenge is to use the algorithms to avoid the delivery of inappropriate therapy so that the ICD is better tolerated and contributes to an improved quality of life.

Rate and Duration

The primary characteristic of ventricular arrhythmias requiring treatment is the rate and duration of the arrhythmia. Slow or nonsustained arrhythmias do not require therapy; however, the negative consequence of waiting to see whether an arrhythmia is going to end without an intervention from the ICD increases with rapid rates. The possibility that failure to detect low-amplitude electrogram events will cause a delay or prevent detection is also a serious issue. Consequently, the faster the rhythm, the shorter the duration that should be required for detection. Arrhtyhmia rates are measured by measuring the beat-to-beat interval and are then classified as ventricular tachycardia or ventricular fibrillation. The duration of the arrhythmia is then measured by the number of interval to detect (NID) or the length of time to detect (seconds). For ventricular tachycardia detection, consecutive intervals faster than a programmed rate provide an effective algorithm. However, for VF, a probabilistic NID counter where X short cycle length intervals out of a larger number of Y consecutive intervals is more sensitive because of the marked variability of electrogram amplitudes and intervals during VF.

Sudden Onset

Reentrant ventricular tachycardia and some not clearly reentrant arrhythmias start at a rapid rate, which is usually distinct from the preceding nontachycardia rhythm. The sudden shortening of the cycle length is a factor that is somewhat useful in differentiating sinus tachycardia from slow ventricular tachycardia. The best criterion for preventing ventricular tachycardia detection during sinus tachycardia is programming a more rapid ventricular tachycardia detection rate; however, when that is not possible there is some, but limited, value in using sudden onset. Unfortunately, using this approach increases the risk that the arrhythmia will not be treated if the ventricular tachycardia arises in the setting of sinus tachycardia or exercise. In addition, this discriminator might inhibit therapies for ventricular tachycardia (VT) that started below the programmed detection rate, even when it accelerates beyond the programmed detection rate.

Cycle Length Stability

Most ventricular tachycardias, in distinction from the ventricular response in atrial fibrillation, have a stable cycle length with little beat-to-beat variation (<40 ms) in the interval. Unfortunately, this is not always the case, but when the ventricular response is rapid during relatively infrequent episodes of atrial fibrillation, this can be an effective tool. If there is continuous atrial fibrillation or frequent paroxysmal episodes, especially in the setting of a slow ventricular tachycardia that also needs treatment, then AV junctional ablation with a rate-responsive pacemaker mode is a better choice.

Morphology and QRS Width

Electrogram width measurements, morphology criteria, and most recently morphology template matching and vector correlation techniques have been used to enhance the quickness and specificity of tachycardia detection with some success. The digital image of every tachycardia electrogram is compared to a presaved template of the baseline rhythm. Although helpful, this discriminator is limited by multiple scenarios, including physiological rate-dependent changes in electrogram morphology and truncation and allignment errors in the digitalization process.

Atrial Lead Criteria

With placement of an atrial lead, the relation of atrial activity to the ventricular activity is available for analysis. If the data are accurate, with the caveats mentioned earlier concerning far-field signals, then AV dissociation during a rapid ventricular rhythm makes the diagnosis of ventricular tachycardia certain. If there is a 1 : 1 relation, the chance that the rhythm is ventricular tachycardia or supraventricular tachycardia is dependent on the ratio of the durations of the AV to the VA interval. Unfortunately, it is still possible to have atrial tachyarrhythmia and ventricular tachycardia at the same time. Atrial leads reduce the incidence but will not prevent inappropriate therapy because all algorithms are biased so that VF is rarely, if ever, missed. In one study, the addition of an atrial lead significantly decrease the rate of inappropriate supraventricular tachycardia (SVT) detection (as VT) and the rate of inappropriate therapy delivery, but it did not decrease the rate of ICD shock; however, this is not a consistent finding.³

Redetection and Reconfirmation

Detection of tachycardia, like any other test, follows bayesian statistics. The probability of a truly positive result is directly proportional to the prevalence of the condition in the population being treated. Once a rhythm has been detected with rigorous criteria and treated, the likelihood is that a rhythm that meets the rate criteria will need to be treated again, and quickly, to prevent syncope. Redetection is the process with which the ICD examines whether the programmed therapy was effective in terminating the presenting arrhyhtmia. In most ICDs, the SVT-VT discriminators are not applied during this process, and redetection is based solely on the programmed rate cut off. The quickness and sensitivity of arrhythmias algorithms are enhanced during redetection by reducing the number of intervals required and by eliminating additional criteria. Conversely, it takes time to charge the ICD capacitors, during which time the arrhythmia may have terminated. For this reason, before the delivery of therapy, reconfirmation of the tachycardia is required to prevent shocks when treating patients with nonsustained tachycardia during sinus rhythm. The danger is that the autogain or autothreshold algorithm has failed to detect low-amplitude VF during progressive hemodynamic collapse. Therefore, reconfirmation of rhythms in the VF zone should be used with care. Some devices permit the allocation of committed and noncommitted shocks in the VF or in the ventricular tachycardia and VF rate zones.

Tachyarrhythmia Therapy

Pacing Therapy

All transvenous ICDs provide for bradycardia support with single-, dual-, or triple-chamber stimulation; however, the subcutaneous defibrillator only provides temporary postshock cardiac stimulation. The pacing mode, rate, and stimulation output is often separately programmable after a high-energy shock for a period to provide overdrive suppression of arrhythmias. Although many patients with ICDs receive dual-chambered rate-adaptive pacemakers, a small minority of these patients have an indication or need for pacing. The Dual Chamber and VVI Implantable Defibrillator (DAVID) trial examined the effect of pacing in the DDDR mode compared with ventricular backup pacing in patients who had ICD but no pacing indications. In these patients, DDDR pacing increased the combined end-point of hospitalization for congestive heart failure (CHF) or death.⁴ In the DAVID-II trial, atrial support pacing at AAI-70 was tested and showed no significant improvement in quality-of-life outcomes compared with VVI-40.⁵ As a result, patients without clear indications for antibradycardia pacing should be set to ventricular backup pacing. In patients with wide QRS duration and New York Heart Association functional class III or intravenous CHF, biventricular stimulation is appropriate. This therapy could also be applied in patients with less severe CHF or need for antibradycardia pacing.⁶

In addition, antitachycardia pacing is available in all the current transvnous ICDs, but not in the subcutaneous defibrillator. This therapy consists of the delivery of a short burst of ventricular pacing, at a programmable rate faster than the detected ventricular tachycardia rate. A variety of antitachycardia pacing techniques are available, including simple bursts at a fixed rate and ramp pacing, in which each subsequent paced interval is incrementally shorter (i.e., faster) than the preceding interval. Effectiveness has been demonstrated to be 90% and 72% in terminating slower and fast ventricular tachycardia using antitachycardia pacing programmed empirically. Its effectiveness was 95% when the testing of the antitachycardia pacing modality was performed before discharge. Antitachycardia pacing was also effective in terminating supraventricular tachcyardias for which patients would have received a shock.

For some patients, a drawback to antitachycardia pacing is the potential for acceleration of the tachycardia to greater rates or to VF, so that high-energy shocks are required. Although 4% of patients experience acceleration of the tachycardia by antitachycardia pacing, this was not associated with any increase in mortality or incidence of syncopal events.7,8 Compared to shock therapy, antitachycardia pacing is delivered with no delay from the detection of the tachycardia (no capacitor to charge), with less morbidity (pain and ventricular stunning) and less battery usage. Once antitachycardia pacing therapy fails to terminate the tachyarrhythmia, shock therapy is delivered. In fact, in the EMPIRIC trial, standardized prespecified ICD programming with antitachycardia pacing during slow and fast ventricular tachycardia was at least as effective as physician-tailored programming without an increase in mortality or shock-related morbidity.⁹

Shock Therapy

The electrical shocks delivered by ICDs arise from the discharge of the capacitors through the heart via the high-voltage electrodes. Capacitor discharge follows an exponential decay (Figure 115-2), dependent on two variables: the capacitance, and the impedance of the electrode/patient body system. As either of these variables is increased, the voltage and current discharge decayed more slowly, producing a flatter waveform. One to three capacitors are commonly used in each generator, each rated at 106 to 480 µF capacitance and capable of a maximum voltage of 350 to 850 V. These capacitors are usually charged in parallel, to a programmable voltage up to the maximum. When discharged, however, these capacitors are configured in series, so that the total voltage is doubled or tripled depending on the number of capacitors to a maximum voltage of 750 to 850 V. Although the voltage is multiplied in the series configuration, the capacitance is reduced by the number of capacitors used; therefore, the resultant system capacitance is 120 to 150 µF for most currently available clinical devices. Lowering the capacitance of the system will have favorable clinical outcome by decreasing the discharge time. Because most lead impedance values are between 30 and 70 Ω, voltage decreases by 60% to 90% in 20 ms. The shock waveform (discussed later) is generated by the discharge of the capacitors through the electrode system to the patient. Unlike with antitachycardia pacing, ICD shocks can be associated with increased mortality and morbidity. In the Primary Prevention Parameters Evaluation trial, programming strategies to decrease ICD shocks by avoiding therapy for slow and nonsuatained ventricular tachcyardia and applying antitachycardia even to faster VT were associated with less morbidity including mortality.¹⁰

Figure 115-2 The monophasic waveform is created by truncation of a capacitor discharge and is the simplest implantable cardioverter defibrillator (ICD) waveform. The initial voltage (V_I) is dependent on the charge placed on the capacitor. The waveform is truncated after a variable duration (depending on the manufacturer) when a certain final voltage (V_f) is reached. The rate of voltage decay (tilt) is inversely proportional to both the impedance of the lead system and the total capacitance in the ICD. Tilt is calculated according to the formula given such that waveform A demonstrates a lower tilt than waveform B.

Defibrillation Threshold and Safety Margin

The main determinant of success of defibrillation is the magnitude of the electric field generated across the heart, which is proportional to the spatial derivative of the stored voltage. However, because device companies report their device outputs in energy units (Joules), it has been a trend to refer to effective defibrillation energy in the Joule unit of measure. The simplest measure of defibrillation effectiveness is usually termed the DFT, defined as the lowest delivered shock strength required for defibrillation; however, the clinical predictive value of defibrillation testing is limited by the probabilistic nature of defibrillation. The DFT is often determined in a step-down manner, in which progressively lower-intensity shocks are delivered after VF is induced, and the lowest successful shock strength is defined as the DFT. Alternatively, shocks can be delivered in a step-up fashion, beginning at low energies. However, the step-up method will have more failed shocks for patients with high energy requirements. Actually, the time of circulatory arrest is shorter for most patients with the step-up method because the charge times are much shorter with lower energy, but longer for the few patients with high energy requirements. A third method of estimating defibrillation efficacy has been validated, in which test shocks are delivered to the vulnerable period of the cardiac cycle, near the peak of the surface T wave. Theoretically, low-energy shocks delivered during this phase of repolarization will produce VF caused by the resultant dispersion of repolarization times. If a test shock delivered during this period fails to result in VF, it is deemed to be above the DFT; subsequently, lower shock intensities can then be tested until VF occurs. This concept is termed the upper limit of vulnerability and has the advantage of inducing VF only once during the testing period. The shock of the lowest energy that does not induce VF is considered a similar indicator of defibrillation efficacy. The disadvantage of this method is that the ability of the ICD to detect VF correctly and the subsequent decision-making algorithm to delivery VF therapy will not have been tested.

The concept of the DFT is overly simplistic, because electrical defibrillation is best described as a sigmoid-shaped probability function (Figure 115-3). Nonetheless, the step-down DFT is a convenient clinical measurement to obtain during implantation and generally correlates with a probability of success of approximately 70% to 80%. The probability of success with this approach has significant clinical relevance, because appropriate implantation of ICDs requires a high confidence that the first shock will be successful. Historically, using epicardial leads and monophasic waveforms, a safety margin (a margin between the DFT and the maximal output of the ICD) of 10 J was considered minimal implantation criteria. More recent studies have suggested that, with biphasic shocks and NTLs, a margin of 1.9-fold the DFT energy provides a 95% probability of success. It is important to note that DFT testing is device- and lead-specific because the delivered energy is not the only parameter required to ensure successful defibrillation. Other parameters include lead/body impedance, waveform tilt, truncation, pulse duration, and positive and negative-phase peak voltage; these can all affect defibrillation success and are usually device- or lead-specific.^11,12

Figure 115-3 This graph shows the relationship between the probability of successful defibrillation and the intensity of the shock—that is, the greater the shock intensity, the higher the probability of success. The ED₅₀ and ED₉₀ refer to the shock intensities that result in successful defibrillation 50% and 90% of the time, respectively. In the examples given, curves A and B represent waveforms A and B with different defibrillation efficacy. Waveform A shows a greater efficacy (lower ED₅₀ and ED₉₀) than waveform B.

Monophasic Waveforms

The simplest defibrillator waveform is the monophasic waveform, in which the capacitor discharge is truncated before complete discharge of the capacitor, because the terminal voltage tail may prove proarrhythmic. Monophasic waveforms were used in early ICD generators and were effective in epicardial systems. The magnitude of a monophasic (or biphasic) shock is generally described by both the amplitude (peak voltage or current) and tilt of the waveform (see Figure 115-2). For example, a waveform whose amplitude is reduced to half the initial value has a 50% tilt.

When the NTL was first developed and monophasic waveforms were used, the right ventricular coil electrode was configured as the cathode (negative), whereas the superior vena cava coil or subcutaneous patch electrode was configured as the anode (positive). When subsequent animal and clinical studies were performed comparing the right ventricular cathodal configuration with the right ventricular anodal configuration, the anodal right ventricular electrode resulted in a significantly lower DFT in patients and a lower need for supplementary electrodes.

Biphasic Waveforms

The most beneficial ICD innovation, from a viewpoint of defibrillation efficacy, was the development of the biphasic defibrillation waveform (Figure 115-4). This waveform consists of the capacitor discharge divided into two phases of opposite polarity. The first phase is identical to a monophasic waveform (although usually of a shorter duration) before the capacitor discharge is truncated. The electrical connection within the generator is then quickly reversed (usually within 2 to 3 µs), and the second phase is discharged in the opposite polarity for an additional period (usually 3 to 6 ms). Table 115-2 lists the biphasic waveform characteristics currently available in clinical ICDs.

Table 115-2

Summary of Biphasic Waveform Characteristics in Currently Available Commercial Implantable Cardioverter Defibrillators

PW, Pulse width.

^*Capacitance values may vary between models within a given manufacturer and older models may vary from these specifications. In models in which phase parameters are programmable, the value given is the default value programmed in the factory. When a range of values is given, it refers to different models offered by the same manufacturer.

^†Applies only to the atrial shock tilt, the ventricular shocks are not tilt programmable.

Figure 115-4 The biphasic waveform currently used in all current implantable defibrillators is composed of a capacitor discharge divided into two phases of opposite polarity. The duration of each phase is dependent on the manufacturer and is programmable in some devices. The initial voltage of the second phase is equivalent to the remaining voltage on the capacitor after truncation of phase I in most cases.

The basic physiology of defibrillation and its relationship to best or optimized biphasic waveform is well studied. Biphasic waveform characteristics in animals identified two factors that appear to maximize defibrillation efficacy as follows: (1) the first phase should be longer than the second phase; and (2) the polarity of the first phase appears to be important only at phase 1 durations greater than 10 ms or when the second phase duration is greater than the first phase.

Additional studies have examined the effectiveness of other phase characteristics. One interesting modification involves doubling the second phase initial voltage by switching the capacitance from series to parallel between the first and second phases. Doing so also reduces the capacitance of the second phase by half, resulting in an increased tilt and a more rapid decay of voltage; animal studies have suggested that this improves the defibrillation efficacy. Multiple recent studies have reported improved defibrillation thresholds with modification of the tilt percentages. The results of these studies were then used to modify the preset tilts in the current available devices. Some devices also have the capacity to specify the duration of the first and second phase of the biphasic shock, which determines the tilt. Although this specificity can make a difference in a few patients, in general it is not required.

System Malfunction

ICDs and their lead systems are complex medical device systems. System integrity is challenged by several factors, including high temperature, high osmolarity, high salt environment, and continuous mechanical stress from the beating heart and body movement. Despite these challenges, the “failure prediction” is small—less than 1% over 5 years. ICD safety alerts or recalls are usually generated in response to a higher than normal projected risk of failure or if the failure would subject the patient to a serious adverse consequence. System failure could occur in the device battery, device electronics, or lead design and interact with the implantation techniques and patient characteristics.

Fortunately, primary battery failure is extremely rare. More often, battery depletion is caused by excessive power drain from pacing, capacitor charge, and electrogram storage. High pacing thresholds and lead insulation failure are additional causes of excessive current drain. On the other hand, repetitive capacitor charges owing to nonsustained ventricular tachycardia or temporary ventricular oversensing can cause important reductions in battery longevity. In 2012, Medtronic identified a rare (0.04% to 0.15%) dysfunction resulting in a greater-than-expected drop in the Entrust ICD battery voltage. This drop resulted in shortened predicted longevity of the ICD and quicker transition from ERI to end of life. Usually these high-current drain issues can be managed with increased frequency of battery assessments.

In 2005, the U.S. Food and Drug Administration (FDA) issued a class I recall (reasonable probability that the use of these devices will cause serious adverse health consequences or death with failure) regarding the use of Guidant PRIZM 2 DR CONTAK Renewal and CONTAK Renewal 2 devices because of failure deliver a life-saving therapy as a result of short circuiting that occurred across the space between the anode (+) backfill tube and the cathode (−) DF feedthrough wire. In 2005, there was also a voluntary class II recall (risk of adverse health consequences or death is remote) for the Medtronic Marquis regarding a battery short circuit.

Failure of transvenous high-voltage ICD leads could be the result of a combination of multiple factors: patient-related, operator-related, and engineering design–related factors. Patient-related factors include thoracic inlet syndrome, Twiddler syndrome, and certain activities like bench pressing. Operator methods that have contributed to lead failure include subclavian or second rib approach, venous access, poorly applied suture sleeves, excessively tight lead coiling within the pocket, over-torquing of the lead, and iatrogenic damage to the leads during implantation. Lead-related factors that can contribute to lead failure include lead engineering and suture design flaws.

Sprint Fidelis ICD leads (Medtronic) were the subject of class I recall because of higher-than-expected conductor fracture rate. There was also evidence of increased risk of high-voltage conductor fracture if a pace-sense conductor fracture has previously occurred; therefore, it is recommended to implant a new high-voltage lead instead of a pace/sense lead if a lead fracture of any type has occurred. Use of a lead integrity algorithm is recommended: extending arrhythmia detection, activating audible alarms, and using quick alert notification to monitor fully functional leads. A lead integrity algorithm could provide most patients with a 3-day warning before inappropriate shocks.

St. Jude Medical Riata and Riata ST leads were the subjects of class I recall because of higher-than-expected erosion in the silicone coating around its conductors. Insulation breaks seen in these leads could be caused by an inside-out or an outside-in abrasion (see Figure 115-5). The latter often occur close to the pocket and are often the result of lead contact with another part of the lead or the pulse generator. Although inside-out externalizations of the inner cables are not uncommon and can be visualized easily using fluoroscopy, they do not seem to place patients at significant risk of electrical dysfunction. However, outside-in abrasions are difficult to be identified using fluoroscopy and can result in shorting of the high voltage; this is not unique to the Riata lead family, nor is it a new failure mechanism.

Figure 115-5 Inside-out externalizations of the inner cables of Riata leads visualized using fluoroscopy (red arrows).

Not all recalled elements needs to be replaced. Multiple issues should be addressed before replacement, including the risk of keeping the recalled unit, risk of replacement (infection and extraction), risk of failure of the new element to be replaced, and the age of the unit to be replaced.

Recent and Future Directions

Remote Monitoring

The development of the device monitoring systems with or without wandless communication has permitted the early diagnosis of arrhythmias and system malfunction from the patient’s home and with few limits to geographic location or distance from the monitoring physician. It has the potential to safely decrease clinic visits by 40% as seen in the TRUST trial.¹³ The CONNECT trial showed that remote monitoring with automatic clinician alerts can reduce the time between clinical events and clinical decisions, as well as the length of hospitalization stay. The ECOST trial showed that remote monitoring allows for early and reliable dectection of ICD lead failure and can reduce inappropriate ICD shocks and save battery life. However, other trials like the Tele-HF and EVATEL, did not show any significant benefit of remote monitoring on hospital readmission, all-cause mortality, and major clinical events. Additional studies are needed to evaluate whether monitoring will result in improvements in outcomes or reduction in heath care expendature.

Long-Distance Telemetric Communication

Long-distance telemetric communication (wandless telemetry) in these devices is sometimes accompished through Medical Implant Communications Service, which is an ultra-low-power mobile radio frequency service designed for transmitting diagnostic and therapeutic information from implanted medical devices. Its band operates between 402 and 405 MHz. This bandwidth does not need any licensing and is shared by different manufacturers (currently Medtronic, St. Jude Medical, and Biotronik, Portland, Oregon). Each manufacturer has applied mechanisms to avoid interference and cross communication with other devices. There is another frequency spectrum in use for long-range telemetry by Boston Scientific (St. Paul, MN), the Industrial, Scientific, and Medical band (902 to 928 MHz). A frequency of 914 MHz is used for communication from the ICD to the home monitor, which is also coupled with Bluetooth communication with a weight scale and blood pressure cuff.

Investigational Shock Waveform

Multiple triphasic waveform patterns were tested in animals and failed to improve shock efficacy in terminating ventricular fibrillation over biphasic waves.¹⁴ However, multiple serial monophasic shocks were shown to terminate monomorphic ventricular tachycardia with less total energy requirement and peak voltage compared to a single monophasic or biphasic shock in a canine myocardial infarction model.¹⁵ If these findings could be replicated in humans, it could translate into prolonged battery life, less pain, and less myocardial stunning and necrosis from the ICD shocks.

Subcutaneous Implantable Cardioverter Defibrillators

Subcutaneous ICDs were developed to address some of the problems and limitations of transvenous lead system ICDs (see Figure 115-6). These systems include but are not limited to: myocardial perforation and tamponade, vascular thrombosis and dissection, pulmonary embolism, brachial plexus injury, and percutaneous lead extraction. The Q-TRAK subcutaneous ICD lead (Boston Scientific) has two sensing electrodes separated by an 8-cm defibrillation coil producing three sensing vectors when connected to the defibrillation can. The lead is tunneled subcutaneously parallel to the left sternal border and is extended to connect to the subcutaneous ICD (SQ-RX Pulse Generator, Boston Scientific) that is placed in the anterior or midaxillary region around the fifth or sixth intercostal spaces. The ICD has a lithium-manganese oxide battery that can deliver an 80-J shock with a fixed 50% tilt. Cardiac signal sensing is automatically selected from the three available sensing vectors (distal-proximal electrodes, distal electrode-can, and proximal electrode-can) based on the analysis of the signal amplitude and signal-to-noise ratio. Arrhythmia detection algorithm uses morphology, beat-to-beat morphology, and QRS width analysis to define treatable arrhythmias. This algorithm is highly sensitive for the detection of ventricular arrhythmias similar to algorithms found in transvenous ICDs.¹⁶