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. 2020 Jul;36(4):308–317. doi: 10.6515/ACS.202007_36(4).20191201A

Sensing and Detection Functions in Implantable Cardioverter Defibrillators: The Good, the Bad and the Ugly

Antoine Kossaify 1
PMCID: PMC7355121  PMID: 32675922

Abstract

Implantable cardioverter defibrillators are small devices that have been proven to be beneficial by preventing sudden cardiac death, whether in primary or secondary prevention. Appropriate functioning of implantable cardioverter defibrillators is mainly dependent on the "good" sensing of ventricular electrogram waves, allowing for the adequate detection of ventricular arrhythmias in order to deliver appropriate therapy of either antitachycardia pacing or by delivering a shock according to the detected rhythm. Basic sensing function in defibrillators is based on detection rate and detection duration; additional parameters that are involved in the process of adequate detection include ventricular electrogram sensing, auto-adjusting sensitivity, supraventricular arrhythmia discrimination criteria, noise detection, and various dedicated algorithms. Defective sensing may result in the delivery of inappropriate therapy (oversensing) or inappropriate withholding of therapy (undersensing); the latter of which may lead to sudden cardiac death. This paper describes different clinical scenarios and programming tips to avoid abnormal or critical clinical situations.

Keywords: Algorithm, Cardioverter, Detection, Lead, Oversensing, Programming, Sensing, Undersensing, Ventricular arrhythmia, Ventricular electrogram

INTRODUCTION

The basic function of implantable cardioverter defibrillators (ICDs) is to monitor (sense) the patient’s intrinsic heart activity and to treat malignant ventricular arrhythmias (MVAs), either by antitachycardia pacing or by delivering a shock according to the detected rhythm. In this paper, MVA refers to ventricular tachycardia (VT) or ventricular fibrillation (VF). ICDs are highly effective in reducing mortality due to MVA, whether in primary or in secondary prevention.1-3 However, there is considerable morbidity associated with ICD use, including lead and/or device dysfunction.4,5 In addition, the occurrence of ICD shocks, even when appropriate, is associated with adverse effects such as psychological distress and compromised quality of life.6-8

The sensing function in an ICD represents the ability of the device to detect a biological or physical signal, whereas detection pertains to the interpretation of that signal by the device.9,10 In this respect, appropriate therapy requires both good sensing and appropriate detection, and both are mandatory for the delivery of appropriate therapy. Inappropriate therapy (IT) is considered to be when an ICD delivers therapy for a rhythm other than VT or VF, and therefore appropriate therapy is considered to be that delivered for a MVA through device algorithms and programmed features.11,12

"Good" sensing and/or detection functions in an ICD are considered to be such when the monitoring of normal rhythm and detection of abnormal ventricular rhythm are ensured. However, sensing and/or detection functions are not always optimal, and in cases of dysfunction, there may be oversensing ("bad") or undersensing ("ugly") and serious consequences may occur, such as the delivery of IT, or even worse, withholding therapy in cases of MVA.7,9,10

Of note, in the medical literature and in clinical practice, the use of the terms "sensing" and "detection" can often be confusing, and the use of "oversensing" or "undersensing" sometimes reflects "over-" or "under-detection", respectively. For instance, IT delivered for supraventricular tachycardia is not caused by oversensing, but by over-detection; IT, in this case, is due to a misinterpretation of ventricular rhythm and ventricular electrogram (VEGM), which are appropriately sensed but inappropriately detected (over-detection) as MVA. However, in clinical practice, the term "oversensing" is commonly used in such clinical scenarios.7,11,12

METHODS

A comprehensive search in PubMed/MEDLINE was conducted starting from 1990 using the following keywords as search criteria: "ICD, defibrillator, implantable, inappropriate therapy, sensing, detection, algorithm, programming"; then the terms "sensing" and "detection" were specifically searched in titles and abstracts, and therefore used as additional selection filters. Fifty-eight relevant articles were retained.

THE FUNDAMENTALS FOR SENSING AND DETECTION IN ICD

Unlike electrocardiographic analysis methods that rely on the post analysis of recorded signals, ICDs must make decisions on the spot, and rhythm analysis is a "live" process, which is mandatory for a timely intervention when required. However, stored intracardiac VEGM, episode diagnostics, and available marker channels allow for a post analysis that contributes significantly to the understanding of the sensing-detection process; moreover, alerts and remote monitoring allow for the close monitoring of an ICD’s function, namely sensing capability.9,10,12

The successful accomplishment of the chain process of sensing-detection-therapy requires an optimal status and function of all ICD components. Sensing threshold depends on the programmed sensitivity, and most ICD brands also have an auto-adjusting sensitivity allowing to keep a safe margin for R wave detection.11 The fundamental sensing function in ICDs is based on detection rate and detection duration. However, sensing in ICDs is optimized using amplifiers, bandpass filters, rectifiers, and auto-adjusting sensitivity.10,13 Amplifiers are useful to provide amplitude gain when signals are low, bandpass filters allow for the acceptance of signals with frequency compatible with QRS and therefore to reject P or T signals, and rectifiers are useful to cancel out the effects of polarity.10 These features are necessary to avoid undersensing of low-amplitude cardiac activity in cases of MVA, and to avoid oversensing of cardiac and noncardiac activity.14

Sensing is dependent on R wave acquisition, a process necessary for determining the amplitude and timing of individual cardiac depolarizations. Correspondingly, detection is the process by which an ICD algorithm classifies a sequence of sensed signals to determine the nature of cardiac rhythm, and thus sensing typically precedes detection.11 While the sensing function is limited to the acquisition of a signal, the detection function is a more complex function that involves complex algorithms with multiple interacting parameters, such as ventricular rate, QRS morphology (wavelet), onset and stability.9,10

Adequate programming is an essential step for appropriate sensing and detection, and parameters should be adjusted according to the clinical context, and on a case-by-case tailored basis, in order to enhance optimal detection and to avoid IT.15,16 Of note, higher rate cutoffs, prolonged ICD detection times, and multiple antitachycardia pacing sequences along with active supraventricular tachycardia (SVT) discriminators have been reported to result in a significant decrease in the percentage of IT.15,17 Discrimination criteria allow filtering in order to rule out confounding conditions that may give rise to a high ventricular rate.10 The chain of sensing-detection-therapy is triggered according to dedicated algorithms and to pre-programmed features. The basic features for MVA detection are based on rate zone, and the number of intervals detected (NID). Enhancement criteria for MVA detection are based on stability (regularity), onset, and wavelet, and on the atrioventricular relationship in dual-chamber ICDs.9,10,12

Lead integrity plays a critical role in adequate sensing, and lead dysfunction (i.e., lead dislodgment, insulation defect, and broken wires) may result in over or undersensing.4,18 An initial suitable lead positioning to acquire an R wave ≥ 5 mV with a slew rate ≥ 0.75 V/s is essential for sustainable optimal sensing, and acquiring a P wave ≥ 2 mV with a slew rate ≥ 0.5 V/s is required to ensure adequate P wave sensing, especially in cases of SVT in order to avoid IT.13,14 Of note, leads may deteriorate over time, and the annual failure rate increases progressively and may reach up to 20% after 10 years of implantation.19 Common causes of chronic lead failure include insulation defects and lead fractures, whereas acute lead failure is usually related to connection defects.19 Figure 1 illustrates the most prominent features involved in the process of sensing and detection.

Figure 1.

Figure 1

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Image showing the sequence and main parameters involved in sensing, detection and re-detection processes. NID, number of intervals detected; SVT, supraventricular tachycardia.

APPROPRIATE SENSING: “GOOD” SENSING

A correct sensing function in ICDs is based on appropriately sensing normal rhythm as well as accurate detection of MVA, without oversensing or undersensing. For this purpose, dedicated algorithms to detect MVA are implemented, and their function is mainly based on analysis of timing and morphology of sensed events.13,20 Detection of MVA occurs when the programmed minimal NID is satisfied, and when the ventricular rate is above the arrhythmia detection rate, as programmed. In addition, confounding conditions causing a high ventricular rate or pseudo ventricular arrhythmia [i.e., SVT, T wave oversensing (TWOS), lead noise, and electromagnetic interference, etc.] are considered to be inhibitors by dedicated algorithms, and therefore may result in delayed or withheld therapy according to the programming settings.10,12 A time-out safety parameter that is programmable can be set up as safety criteria in order to overrule these "therapy inhibitors" in cases of persistent detection of a high ventricular rate, allowing for the delivery of therapy once the programmed time has elapsed.13

Tailored programming must be applied in each patient according to the clinical setting in order to optimize the detection and therapy of MVA. Importantly, lead positioning is essential for an adequate R wave sensing. The operator must find an adequate lead position to acquire an optimal R wave (≥ 5 mV; a slew rate ≥ 0.75 V/s), then ventricular sensitivity can be tuned with a sufficient margin to enable adequate sensing, even in cases when MVA may generate low-amplitude cardiac potentials.15,21 Lead integrity and a secure connection are essential for adequate transmission of VEGM. A connection defect usually appears early after implantation, resulting in an extremely high (above normal range, i.e., > 2000 ohms) lead impedance, whereas core lead dysfunction (insulation or wire continuity default) usually occurs in the long-term course.4

Device algorithms are activated in order to timely detect and classify any abnormal rhythm, whether or not it is MVA. Algorithm function may vary from one ICD brand to another, and it may also vary if it is a single or dual chamber ICD.10 However, the fundamentals of sensing and detection, in most ICDs, are based on ventricular rate and NID along with additional parameters (wavelet, P/R relationship, stability, onset, etc.) acting as facilitators or inhibitors.10,12 Redetection is triggered soon after delivery of the first therapy, and for safety reasons, in most ICD brands, algorithms of redetection are blind to SVT discrimination criteria.

Another safety feature called "zone merging" is activated during redetection. In cases of three rate zones for detection [VT, fast VT (FVT) and VF], zone merging means that VT is treated as FVT and FVT as VF, leading to more aggressive therapy in order to provide maximal efficacy and safety.22

Of note, if a first therapy (shock) for VF is unsuccessful and the device redetects a VF episode, the second charge begins and the next therapy will be a "committed shock" delivered at the end of the charge even if spontaneous termination of arrhythmia occurs. The probability of a spontaneous termination of VF during charging is very low after a first unsuccessful shock, and the committed shock aims to provide maximal safety in cases of unsuccessful first therapy. Similarly, when the MVA rate fluctuates between two zones and for safety reasons, ICDs implement "combined counting" (CNID, combined number of intervals detected). The MVA is then classified as the one within the higher rate zone (FVT or VF), and therapy is delivered accordingly.9,22

Many advanced algorithms for MVA detection have been developed for each ICD brand (i.e., PR Logic/Wavelet, MedtronicTM; SMART detection, biotronikTM; Rhythm ID, Boston ScientificTM; SecureSense, AbbottTM).22-26 The function of these algorithms is essentially based on a virtually similar principle, mainly the A/V relationship (in dual-chamber ICDs), QRS morphology (wavelet), onset, and regularity (stability). Additional algorithm features are useful to avoid TWOS and RV lead noise sensing, and these algorithms intervene as facilitators or inhibitors in order to ensure appropriate therapy.9,22,27

In summary, optimal sensing/detection requires adequate lead positioning and lead integrity, advanced device features with dedicated algorithms, along with tailored programming and regular device follow-up in order to adapt programming parameters to disease progression.

OVERSENSING: “BAD” SENSING

Oversensing is the main cause of IT delivered by ICDs. In addition to the painful and terrible sensation of shocks, unnecessary shocks are associated with increased mortality.28,29 In addition, IT has the disadvantages of causing premature battery depletion and in rare cases, inappropriate shocks may trigger a true MVA.30,31 Nearly one-fifth of patients with ICDs experience IT, caused mainly by SVT with a rapid ventricular response, followed by oversensing of the artifact and noise, electromagnetic or myopotential interference, TWOS and cross-chamber sensing.32-34

Oversensing due to SVT is essentially managed by adequate programming of SVT discrimination criteria. Most ICDs have inherent algorithms based on stability, onset, and QRS morphology (wavelet), however, only dual-chamber ICDs provide the A/V relationship feature.10 To maintain safety and to avoid withholding therapy when a true MVA is suspected, these algorithms are overridden by other parameters based on "high rate timeout" and "SVT limit".12,13

The "high rate timeout" feature allows for the delivery of therapy if a high ventricular rate persists beyond the preset time delay, even if the arrhythmia is classified as being SVT. The "SVT limit" allows the ICD to deliver therapy if the ventricular rate surpasses a preset rate threshold, even if the arrhythmia is classified as being SVT.10,12

Oversensing due to cross-chamber sensing is due to atrial sensing via the ventricular lead and may lead to double R counting. This issue can be suppressed or reduced by decreasing ventricular sensitivity while maintaining a reasonable safety limit, by switching ventricular sensing and atrial pacing to bipolar mode, by reducing atrial pulse width and amplitude, and by increasing ventricular blanking after A-pace.30

TWOS is another cause of double counting and may lead to IT.35 TWOS occurs when the T wave amplitude is marked, especially during exercise and during other situations that may lead to sinus tachycardia. TWOS is dealt with by decreasing ventricular sensitivity while maintaining a safety limit, by increasing ventricular blanking after V-sense, increasing NID, and by avoiding conditions that may increase T wave amplitude (i.e. hyperkalemia). Of note, some ICD devices (i.e., BiotronikTM) have intrinsic dedicated algorithms that prevent TWOS, and these algorithms are based on lower and upper limit sensing, along with wave frequency analysis.22 When all of the above approaches fail, lead repositioning may be required as an ultimate solution for TWOS. Figure 2 illustrates an example of TWOS.

Figure 2.

Figure 2

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T wave oversensing (TWOS): example of TWOS where ample T waves are sensed as R waves and ventricular rate is therefore detected in the VF zone. Upper strip, atrial electrogram; mid-strip, ventricular electrogram; lower strip: markers; Ab, atrial blanking; AR, atrial refractory; FS, VF sensed; VS, ventricle sensed.

Oversensing due to noise detection is mainly due to lead dysfunction such as insulant defects and electrical continuity issues (wire breaks or connection trouble).36,37 Other sources of noise and electromagnetic interference include domestic or medical equipment, such as magnetic resonance imaging, therapeutic diathermy, microwave diathermy, therapeutic ionizing radiation, electrocautery, high-voltage power transmission equipment, intense electromagnetic fields, induction heaters, ultrasonic high energy, lithotripsy equipment, high frequency surgical equipment, and security equipment (i.e., antitheft, etc.).36 Patient awareness regarding sources of electromagnetic interference is essential, and activation of dedicated algorithms [sensing integrity counter (SIC)] that detect noise and interference (i.e. very short ventricular intervals < or = 130 ms) allow for the differentiation of lead failure from true MVA and therefore to withhold IT without compromising detection of MVA.37

Electrical storm is a particular situation where the device may deliver successive repetitive therapies, whether or not they are appropriate, and this situation represents a medical emergency. Common causes of rhythmic storm are SVT, lead dysfunction, far-field atrial electrograms, TWOS, and diaphragmatic potentials; a case-by-case tailored approach is necessary to manage electrical storm.38-41

Importantly, tailored and judicious programming along with regular patient follow-up are essential to reduce IT, as unnecessary shocks are associated with increased morbidity and mortality.29,42,43 Table 1 summarizes the conditions leading to oversensing and essential management steps.

Table 1. Common features in oversensing: causes and management.

Oversensing  
Causes Management
SVT (discrimination criteria as facilitators) Medical therapy to avoid SVT.
  Enhance discrimination criteria (as inhibitors).13
Cross-chamber sensing Bipolar ventricular sensing, post-ventricular blanking, and refractory period adjustment, reduce ventricular sensitivity, reduce atrial output.30
Near-field sensing  
TWOS Post-ventricular blanking and refractory periods adjustment, increase NID, activate dedicated algorithm when available.22
Lead dysfunction, noise detection Chronic: perform regular follow up, monitor lead impedance trend, and activate lead integrity alerts.36
Far-field sensing, electromagnetic interference Avoid electromagnetic fields, enhance patient education, and perform temporary backup programming when exposed to electromagnetic fields if needed.36
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SVT, supraventricular tachycardia; TOWS, T wave oversensing.

UNDERSENSING: “UGLY” SENSING

Undersensing is the most serious and potentially lethal condition that may be encountered in ICDs and may result from device and/or lead dysfunction. A lack or insufficient acquisition of intrinsic ventricular activity (R wave) may lead to under-detection, which implies that the device cannot correctly detect and classify the nature of the underlying rhythm.10

Specifically, lead dysfunction results in undersensing, and device dysfunction results in under-detection.10 Acquiring an optimal R wave during device implantation by seeking an appropriate lead position is necessary, preferably ≥ 5 mV.44,45 Nevertheless, neither optimal R wave sensing during sinus rhythm nor repeated ICD testing can always predict undersensing of clinical VF episodes;45 however, undersensing VF is extremely rare when intrinsic R waves are ≥ 5 mV in sinus rhythm.45

While lead dysfunction leading to undersensing is commonly related to electrical wire discontinuity or electrical leakage due to insulation defects, device dysfunction leading to under-detection may be related to battery depletion, misprogramming or rarely, electrical internal circuit faults.9,10,12 Of note, remodeling at the lead-myocardium related to fibrosis may result in reduced transmission of the VEGM to the device, which constitutes a relatively common cause of chronic progressive undersensing in the long term.44

Appropriate VF detection implies that a high ventricular rate above the detection rate threshold results in ICD charging then VF therapy, provided that the NID is satisfied. Conversely, VT detection algorithms are often more intricate, based on rate and NID, along with discrimination criteria acting as facilitators or inhibitors in order to ensure appropriate therapy and to avoid IT. These criteria can withhold therapy (under-detection) if they are overactive; however, two parameters allow these inhibitors to be overridden in order to ensure backup safety therapy: "SVT rate limit" and "high-rate timeout".13,46

Electrical alternans of the VEGM is a rare etiology of VT undersensing, and is often associated with surface QRS alternans. Non-counting the smaller component of the VEGM during VT yielding a 2:1 counting is the mechanism of undersensing in electrical alternans, and management is often difficult, and mainly consists of increasing sensitivity along with management of the underlying cardiac condition.47

The usefulness of predischarge defibrillation testing has recently been called into question: at implantation, defibrillation success is influenced by many factors, including those related to the patient, lead configuration, and drugs.48 Moreover, the risks of implant testing include those related to VF and those related to shocks. Defibrillation testing may be performed on a case-by-case basis, selectively and with caution, namely in secondary prevention and for patients suspected of having undersensing.48,49

Prevention of undersensing is based on a comprehensive multifactorial approach: acquisition of adequate VEGM with R wave signal ≥ 5 mV during implantation, appropriate initial programming with auto-adjusting sensitivity, activation of algorithms allowing for appropriate analysis of VEGM, rejection of noise and interference, regular follow-up with analysis of diagnostic data (events, memory and markers), and activation of alerts enabling warning about potential lead integrity defects.9,50

Subcutaneous ICDs (S-ICDs) represent another challenge in terms of sensing and detection, given that sensing is a kind of far-field VEGM and therefore, may imply a potentially higher risk of overor undersensing.51,52 However, S-ICDs have demonstrated high efficacy for treating VT/VF, broadly similar to that encountered with transvenous ICDs.53 Moreover, many authors have reported that rates of inappropriate shocks with S-ICD were comparable to those observed intransvenous ICDs.29,54,55 Table 2 summarizes the common conditions leading to undersensing and management steps.

Table 2. Common features in undersensing: causes and management.

Undersensing  
Causes Management
VT mistaken for SVT (discrimination criteria as inhibitors) SVT rate limit,
  High rate timeout,
  Enhance discrimination criteria.46
Low or absent R wave acquisition (faulty lead position, faulty connection, fibrosis at lead myocardium interface) Increase ventricular sensing, change lead position, check connection, observe R wave trend, and perform defibrillation testing on a case-by-case decision.44
Under-detection (misprogramming: high sensing threshold, high rate detection threshold) Optimize VT/VF detection criteria.46
Lead dysfunction (electrical discontinuity, broken wire) Perform regular follow up, monitor lead impedance trend, and activate lead integrity alerts.36
Electrical alternans Optimize sensitivity and avoid and treat underlying conditions that trigger electrical alternans.47
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SVT, supraventricular tachycardia; VF, ventricular fibrillation; VT, ventricular tachycardia.

CLINICAL IMPLICATIONS

Heart failure remains nowadays a major cause of morbidity and mortality despite all medical progress, and heart failure with reduced ejection fraction is relatively a common condition leading to ICD implantation, often as primary prevention after medical therapy has been optimized.56,57

Inappropriate sensing, whether oversensing or undersensing, most commonly involves a multifactorial process, which is usually related to patient condition (underlying condition, therapy), device function (programmability, algorithms, etc.) and patient-device interaction (initial programming, programming optimization, follow-up).58 Current ICDs vary considerably in details regarding diagnostic data, programmability and therapy options; therefore, a comprehensive knowledge of each device is necessary to prevent, diagnose and treat any potential sensing disorders. Events, markers, and memory analysis allow for enhanced and tailored programmability for optimal sensing efficacy. In this regard, regular follow-up is essential for close monitoring, adjusting therapy, and device programming optimization according to clinical settings.

Good sensing is a process resulting from comprehensive and accurate monitoring and management of patients with ICDs, including the technique of implantation, medical therapy, initial programming, and subsequent device follow-up. Oversensing is a relatively common phenomenon and it is a frequent cause of IT. Accordingly, in patients with recurrent SVT, a dual chamber ICD must be considered in order to better implement SVT discrimination criteria. Undersensing is still the most serious condition encountered in patients with ICDs, given that it may lead to sudden cardiac death. Tailored programming and case-by-case defibrillation testing are essential in patients in whom undersensing is suspected. Moreover, defibrillation testing should be considered more frequently when dealing with ICDs implanted in the setting of secondary prevention.

S-ICDs have been shown to have efficacious sensing capabilities, however they still have some drawbacks (i.e. lack of antitachycardia pacing, they cannot be upgraded to biventricular), and these issues must be taken into consideration before S-ICD implantation.53,54 Remote monitoring is a feature that is still poorly developed worldwide owing to technical challenges, however it allows for the detection of early changes in device function, therefore reducing the risk of over- or undersensing.56

CONCLUSIONS

Optimal ICD function requires appropriate sensing and detection. Discrimination criteria act as facilitators or inhibitors during the detection phase in order to fine-tune the classification of arrhythmia and then to deliver or withhold therapy.

"Good" sensing enhances the appropriate detection of MVA in order to deliver therapy of either antitachycardia pacing or defibrillation. Oversensing ("bad") is the main cause of IT delivered by ICDs. Undersensing ("ugly") occurs when a true MVA is not detected, and depending on the clinical scenario, the consequence may be critical or even fatal. ICDs are valuable implantable devices, and tailored programming along with regular follow-up is essential to maintain "good" sensing, prevent "bad" oversensing, and importantly, to avoid "ugly" undersensing.

DECLARATION OF CONFLICTING INTERESTS

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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