Ventricular Fibrillation Induction Using Nonsynchronized Low Energy External Shock During Rapid Ventricular Pacing: Method of Induction When Fibrillation Mode of ICD Fails

Third-generation implantable cardioverter defibrillators (ICD) are frequently implanted with non-thoracotomy systems and provide noninvasive methods for electrical stimulation and ventricular fibrillation induction. These modalities facilitate postoperative testing of the ICD. Rapid right ventricular burst pacing via the defibrillator is commonly used for initiation of ventricular tachyarrhythmias. However, with the available third-generation devices, ventricular fibrillation (VF) induction may be impossible in up to 19% of the patients. In these cases, transvenous placement of a right ventricular catheter has been required to generate VF and appropriately evaluate the device. We report a new technique of noninvasive induction of VF using a low energy external nonsynchronized shock delivered during ICD fibrillation induction pacing. In three patients, after all efforts to induce VF by the Ventritex Cadence V-100 had failed, a 20 J nonsynchronized shock was delivered during rapid RV pacing. This resulted in VF on the first attempt in all patients. This noninvasive technique of VF initiation may provide a useful clinical approach to ICD testing that eliminates the costs and risks of an invasive procedure.     

Late Retesting of System Performance in ICD Patients Without Spontaneous Shocks

One hundred five implantable cardioverter defibrillator (ICD) patients (71 ± 9 years of age, 83% men) without spontaneous ICD discharges for ≥ 12 months were tested to assess high voltage (HV) circuit integrity and the system's ability to recognize and terminate ventricular fibrillation (VF). Indications for ICD implantation were sustained ventricular tachycardia (VT) (35%), cardiac arrest (27%), and inducible VT (38%). Eighty-two percent of the patients had coronary artery disease (CAD), and the mean left ventricular ejection fraction (LVEF) was 36%± 13%. Results: One hundred patients had inducible VF and five did not. Testing led to ICD reprogramming in 50 (49%) patients. Two (1.9%) patients required ICD replacement: (1) a 45-year-old patient with a Ventritex 110 ICD implanted for 13 months interfaced with a CPI 0062 lead implanted for 46 months could not be defibrillated internally (impedance nonmeasurable); (2) an 82-year-old patient with a 23-month-old Medtronic 7219 ICD interfaced with 6936 and 6933 leads whose defibrillation threshold (DFT) had doubled since implantation (24 J from 12 J). Lead fractures were found in both cases (proximal coil of the 0062, and subcutaneously in the 6933). Based on DFT determinations, the first shock output was programmed lower in 37 patients and higher in 10 patients. Shock pulse width was changed in one patient and the ventricular refractory period in another. No programming changes were made in 54 (51%) patients. Conclusions: (1) Late testing of HV circuit integrity in ICD patients without an ICD shock in ≥ 12 months identifies previously unsuspected HV lead fractures; (2) chronic DFT testing resulted in HV output reprogramming in one-half of the patients.     

Transcutaneous T Wave Shock: A Universal Method for Ventricular Fibrillation Induction

Our objective was to develop a universal noninvasive method for VF induction. ICD implantation requires VF induction. Conventional rapid ventricular stimulation may fail to induce VF. Some ICDs can deliver low energy shocks on the T wave to induce VF. We hypothesized that an external dual chamber pacemaker and an external defibrillator could be configured to allow reliable VF induction with any ICD system. A surface ECC signal was delivered to the atrial channel of an external dual chamber DDD pacemaker. The 'AV' delay was adjusted so that the ventricular output of the pacemaker was delivered to an external defibrillator synchronized to deliver 5–50 J. Twenty-six patients at ICD implant or follow-up had VF induced in native rhythm (sinus rhythm or atrial fibrillation), or during a ventricular pacing train (3–8 beats at cycle length 500–880 ms). VF was successfully induced in 14 of 25 (56%) patients in native rhythm; and in 16 of 17 (94%) patients during pacing (P = 0.013). VF induction success rate was 36% in native rhythm (31/86 attempts) and 88% during pacing (69/78 attempts) (P < 0.001). The 'R' to shock interval was 269 ± 31 ms in native rhythm and 257 ± 48 ms during pacing. Energy delivered from the external defibrillator was 19 ± 3 J in native rhythm and 21 ± 6 J during pacing. We concluded that VF induction by synchronizing a small external shock to the T wave is a fast, effective way to reliably ensure arrhythmia induction with any ICD at implant or follow-up. This method is more successful during pacing than in sinus rhythm.     

Time to First Shock in Implantable Cardioverter Defibrillator (ICD) Patients with Chagas Cardiomyopathy

The time to first ICD shock has been extensively studied in patients with coronary artery disease (CAD). However, there are no published data on ICD shocks in patients with Chagas cardiomyopathy (ChC). The occurrence of the first appropriate ICD shock during the first 6 months of follow-up in 20 patients with ChC (group 1) and 35 CAD patients (group 2) was analyzed retrospectively. All patients had received a third-generation pectoral ICD for ventricular tachycardia or fibrillation (VT/VF). Indications for ICD implantation were refractoriness to drug therapy or noninducibility of VT/VF at EPS in cardiac arrest survivors. Results: The mean age, left ventricular ejection fraction (LVEF), and sex in groups I and II were 57.4 ± 7 years versus 64 ± 9 (P < 0.01), 30.9%± 10% versus 32.9%± 10% (P = NS), and 10 men versus 31 women (P < 0.005), respectively. Six months after ICD implantation, 85% (17/20) group I patients received appropriate ICD shocks versus 51 % (18/35) in group 2, a statistically significant difference (P < 0.02, RR: 1.65, OR: 5.35). Conclusions: The incidence of appropriate ICD shocks within the first 6 months postimplantation was significantly higher in ChC patients than in CAD patients. ChC patients were younger and more often women than CAD patients.     

The Value of Electrophysiology Study and Prophylactic Implantation of Cardioverter Defibrillator in Patients with Hypertrophic Cardiomyopathy

Fifty-three consecutive patients with hypertrophic cardiomyopathy (HCM) and no history of sudden death underwent electrophysiology (EP) study. Sustained polymorphic ventricular tachycardia (VT) or ventricular fibrillation (VF) was induced in 19 patients (35%). Patients with prior syncope or near syncope had a higher incidence of VT/VF inducibility. An implantable cardioverter defibrillator (ICD) was placed in 14 of the 19 patients. Of the remaining 5 patients with inducible VT/VF, three refused ICD implantation, while two underwent septal myectomy and VT/VF was no longer inducible afier the operation. None of the patients received antiarrhythmic drugs. During a mean follow-up period of 47 ± 31 (2–117) months, no events occurred in the 34 patients with negative EP study. Three events occurred among the 19 patients with inducible VT/VF. One patient died suddenly, one developed wide complex tachycardia which required resuscitation, and one patient received an appropriate ICD shock. In conclusion, sustained polymorphic VT/VF was inducible in about one-third of patients with HCM. Noninducibility of VT/VF appeared to predict a favorable prognosis. Although the overall event rate was low in patients with inducible VT/VF, prophylactic ICD implantation in patients with multiple risk factors may be appropriate.     

Ventricular Arrhythmia Inducibility Predicts Subsequent ICD Activation in Nonischemic Cardiomyopathy Patients: A DEFINITE Substudy

Objectives: We evaluated whether electrophysiologic (EP) inducibility predicts the subsequent occurrence of spontaneous ventricular tachycardia (VT) or ventricular fibrillation (VF) in the Defibrillators in Nonischemic Cardiomyopathy Treatment Evaluation (DEFINITE) trial.
Background: Inducibility of ventricular arrhythmias has been widely used as a risk marker to select implantable cardioverter defibrillator (ICD) candidates, but is believed not to be predictive in nonischemic cardiomyopathy patients.
Methods: In DEFINITE, patients randomized to the ICD arm, but not the conventional arm, underwent noninvasive EP testing via the ICD shortly after ICD implantation using up to three extrastimuli at three cycle lengths plus burst pacing. Inducibility was defined as monomorphic or polymorphic VT or VF lasting 15 seconds. Patients were followed for a median of 29 ± 14 months (interquartile range = 2–41). An independent committee, blinded to inducibility status, characterized the rhythm triggering ICD shocks.
Results: Inducibility, found in 29 of 204 patients (VT in 13, VF in 16), was associated with diabetes (41.4% vs 20.6%, P = 0.014) and a slightly higher ejection fraction (23.2 ± 5.9 vs 20.5 ± 5.7, P = 0.021). In follow-up, 34.5% of the inducible group (10 of 29) experienced ICD therapy for VT or VF or arrhythmic death versus 12.0% (21 of 175) noninducible patients (hazard ratio = 2.60, P = 0.014).
Conclusions: In DEFINITE patients, inducibility of either VT or VF was associated with an increased likelihood of subsequent ICD therapy for VT or VF, and should be one factor considered in risk stratifying nonischemic cardiomyopathy patients.     

Safe performance of magnetic resonance imaging on a patient with an ICD

This is a report on a patient with an implanted cardioverter defibrillator (ICD) who intentionally underwent magnetic resonance imaging (MRI) of a malignant brain tumor. To avoid inadequate detection of ventricular tachycardia (VT) or ventricular fibrillation (VF), the ICD was inactivated by programming the VT-detection and VT/VF-therapy status off. The patient came through the protocol safely and without any difficulty or discomfort. There was no arrhythmic event. MRI affected neither programmed data nor the function of the ICD system.     

Failure of transvenous ICD to terminate ventricular fibrillation in a patient with left ventricular noncompaction and polycystic kidneys

Implantable cardioverter defibrillator (ICD) testing in patients with left ventricular noncompaction (LVNC) at the time of implantation and potential difficulties with ventricular fibrillation (VF) induction/termination in LVNC patients are often not stated in the literature. This report describes the failure of transvenous implantation of an ICD in a 40-year-old patient with LVNC and polycystic kidneys. A high defibrillation threshold (DFT) prevented termination of ICD-induced VF. This case suggests that DFT testing should be considered in any LVNC patient during ICD implantation. The association of LVNC and polycystic kidneys is also discussed.     

Prevention of inappropriate ICD shocks in patients with Brugada syndrome

Background  

In Brugada syndrome implantable cardioverter defibrillator (ICD) therapy is associated with a high rate of inappropriate therapies, mainly due to supraventricular tachyarrhythmias (SVT) (2.7–14.1%/year). Aim of the present study was to evaluate a single ventricular fibrillation (VF) detection zone with a high cut-off rate with respect to prevention of inappropriate ICD shock due to SVT and safety of this programming.     

Isoelectric PVCs Diagnosed with a Monitor with Simultaneous ECG, and ICD Intracardiac Electrogram and Marker Channels

A 78-year-old woman with a remote history of chronic atrial fibrillation and catheter ablation of the atrioventricular node with a permanent ventricular pacemaker sustained a syncopal episode. Interrogation of her pacemaker revealed electrograms of ventricular fibrillation with a duration of 26 seconds. The patient underwent upgrade to an implantable cardioverter defibrillator (ICD). Postimplant, on telemetry, pauses were seen without pacing artifact, suspicious for oversensing of something by the ICD. Interrogation and threshold testing of the ICD established that both sensing (of her junctional escape rhythm) and pacing thresholds were excellent. A Holter monitor was utilized to evaluate the patient's rhythm and device function. It incorporated simultaneous tracings from multiple surface electrocardiogram leads and both intracardiac electrograms and marker channel recordings from the ICD. The causes for the pauses were found to be isoelectric premature ventricular complexes not seen on the telemetry tracings.     

The dilemma of ICD implant testing

Ventricular fibrillation (VF) has been induced at implantable cardioverter defibrillator (ICD) implant to ensure reliable sensing, detection, and defibrillation. Despite its risks, the value was self-evident for early ICDs: failure of defibrillation was common, recipients had a high risk of ventricular tachycardia (VT) or VF, and the only therapy for rapid VT or VF was a shock. Today, failure of defibrillation is rare, the risk of VT/VF is lower in some recipients, antitachycardia pacing is applied for fast VT, and vulnerability testing permits assessment of defibrillation efficacy without inducing VF in most patients. This review reappraises ICD implant testing. At implant, defibrillation success is influenced by both predictable and unpredictable factors, including those related to the patient, ICD system, drugs, and complications. For left pectoral implants of high-output ICDs, the probability of passing a 10 J safety margin is approximately 95%, the probability that a maximum output shock will defibrillate is approximately 99%, and the incidence of system revision based on testing is < or = 5%. Bayes' Theorem predicts that implant testing identifies < or = 50% of patients at high risk for unsuccessful defibrillation. Most patients who fail implant criteria have false negative tests and may undergo unnecessary revision of their ICD systems. The first-shock success rate for spontaneous VT/VF ranges from 83% to 93%, lower than that for induced VF. Thus, shocks for spontaneous VT/VF fail for reasons that are not evaluated at implant. Whether system revision based on implant testing improves this success rate is unknown. The risks of implant testing include those related to VF and those related to shocks alone. The former may be due to circulatory arrest alone or the combination of circulatory arrest and shocks. Vulnerability testing reduces risks related to VF, but not those related to shocks. Mortality from implant testing probably is 0.1-0.2%. Overall, VF should be induced to assess sensing in approximately 5% of ICD recipients. Defibrillation or vulnerability testing is indicated in 20-40% of recipients who can be identified as having a higher-than-usual probability of an inadequate defibrillation safety margin based on patient-specific factors. However, implant testing is too risky in approximately 5% of recipients and may not be worth the risks in 10-30%. In 25-50% of ICD recipients, testing cannot be identified as either critical or contraindicated.     

EFFICACY OF LOWER-ENERGY BIPHASIC SHOCKS FOR TRANSTHORACIC DEFIBRILLATION: A FOLLOW-UP CLINICAL STUDY

Objective. This clinical study prospectively evaluated the first-shock defibrillation efficacy of 150-joule impedance-compensated, 200-µF biphasic truncated exponential (BTE) shocks in patients with electrically-induced ventricular fibrillation (VF), and compared it with a historical control group treated with 200-J monophasic damped sine (MDS) shocks. Methods. Ventricular tachyarrhythmias were induced in patients undergoing electrophysiologic (EP) testing for ventricular arrhythmias or testing of an implantable cardioverter-defibrillator (ICD). A 150-J shock was delivered as the primary therapy to terminate induced arrhythmias in the EP group, and as a “rescue” shock when a single ICD shock failed to terminate the arrhythmias in the ICD group. Results. Ninety-six patients received study shocks. The preshock rhythm was classified as VF in 77 patients and as ventricular tachycardia (VT) in 19 patients. First-shock success rates for VF and VT were 75 out of 77 (97.4%) and 19 out of 19 (100%) for the 150-J BTE compared with the historical control rates of 61 out of 68 (89.7%) and 29 out of 31 (94%) for 200-J MDS. The first-shock success rate for VF treated with 150-J BTE was technically equivalent to that of 200-J MDS (p = 0.001). The transthoracic impedance did not vary between groups, yet the peak current delivered by the 150-J BTE shock was about 50% lower. Conclusions. This study demonstrated that 150-J shocks of this impedance-compensated, 200-µF BTE waveform provided very high efficacy for defibrillation of short duration, electrically-induced VF. These lower-energy biphasic shocks had a success rate equivalent to that of 200-J MDS shocks, and they provided this efficacy while exposing patients to much less current than the monophasic shocks.     

New ICD-Technologies: First Clinical Experience with Dual-Chamber Sensing for Differentiation of Supraventricular Tachyarrhythmias

Inappropriate ICD therapy for supraventricular arrhythmias remains an unsolved problem and may lead to serious clinical situations. Current algorithms for differentiation of supraventricular and ventricular arrhythmias are based on ventricular sensing solely and, therefore, lack semitivity and specificity. This preliminary analysis from a multicenter trial comprises data from the first 26 patients who received a Res-Q? Micron active-can ICD (Stdzer Intermedics) with a ventricular defibrillation lead and an additional bipolar lead for atrial sensing. Digitized atrial and ventricular waveform storage as well as interval charts from 102 induced and 30 spontaneous arrhythmia episodes were prospectively collected and analyzed with regard to appropriateness of ICD therapy. From all 132 arrhythmia episodes, high-quality stored dual-chamber intracardiac electrograms (JFXJM) could be retrieved for further analysis: in 40 (30%) episodes, atrial fibrillation (AF with rapid ventricular response 22, AF with VT9, AF with VF 9) was identified as the underlying intrinsic rhythm, and inappropriate ICD therapy was delivered in 4/22 (18%) episodes of AF with rapid ventricular response. In the remaining 92 (70%) episodes, sinus rhythm was the underlying atrial rhythm (SR with VT 13, SR with VF 79), and no inappropriate therapy was observed. Three of 22 (15%) high-energy shocks delivered for ventricular arrhythmias (VT 9, VF 9, rapid AF 4) terminated AF at the same time. In total, there were 3 complications (2 atrial lead dislodgments, I revision for bleeding). Both atrial lead dislodgments occurred in the 2 patients with passive-fixation leads compared to none in the 24 patients with active-fixation leads (p - 0.003). In conclusion, dual-chamber sensing and waveform storage of the new Res-Q? Micron offer very helpful diagnostic tools for the detection of inappropriate ICD-therapy. Placement of an additional atrial lead is safe and does not interfere with proper ICD function. However, for avoidance of atrial lead dislodgment, active fixation leads are recommended With the tested active-can lead configuration, the efficacy of successful atrial cardioversion by high-energy shocks delivered for ventricular arrhythmias seems to be low.     

Using the upper limit of vulnerability to assess defibrillation efficacy at implantation of ICDs

The upper limit of vulnerability (ULV) is the weakest shock strength at or above which ventricular fibrillation (VF) is not induced when the shock is delivered during the vulnerable period. The ULV, a measurement made in regular rhythm, provides an estimate of the minimum shock strength required for reliable defibrillation that is as accurate or more accurate than the defibrillation threshold (DFT). The ULV hypothesis of defibrillation postulates a mechanistic relationship between the ULV-measured during regular rhythm-and the minimum shock strength that defibrillates reliably. Vulnerability testing can be applied at implantable cardioverter defibrillator (ICD) implant to confirm a clinically adequate defibrillation safety margin without inducing VF in 75%-95% of ICD recipients. Alternatively, the ULV provides an accurate patient-specific safety margin with a single fibrillation-defibrillation episode. Programming first ICD shocks based on patient-specific measurements of ULV rather than programming routinely to maximum output shortens charge time and may reduce the probability of syncope as ICDs age and charge times increase. Because the ULV is more reproducible than the DFT, it provides greater statistical power for clinical research with fewer episodes of VF. Limited evidence suggests that vulnerability testing is safer than conventional defibrillation testing.     

Automatic determination of timing intervals for upper limit of vulnerability using ICD electrograms

Background: Implantable cardioverter defibrillator (ICD) implant testing based on the upper limit of vulnerability, or vulnerability testing, permits assessment of defibrillation safety margins without inducing ventricular fibrillation (VF) in most patients. Vulnerability testing requires that T-wave shocks be timed at the most vulnerable intervals of the cardiac cycle, defined as intervals at which the strongest shock induces VF. Our goal was to develop and test an automated method to select these timing intervals using ICD intracardiac electrograms (EGMs).
Methods: At ICD implant in 22 patients, we determined the range of the most vulnerable intervals by scanning the T wave with shocks. Simultaneously, EGMs were recorded for 351 pacing sequences used for measurement of timing intervals or T-wave shocks. EGMs were analyzed off-line using a novel automated method to identify a stable point near the maximum slope of the T wave in the far-field (shock) EGM. Fiducial timing points based both on the EGM and on the electrocardiogram (ECG) were used to predict the most vulnerable intervals. We compared the predicted most vulnerable to the measured most vulnerable intervals determined by T-shock scans.
Results: Automatically determined timing points from EGMs and operator-determined timing points from the surface ECG had comparable accuracy in identifying the measured most vulnerable intervals (91% EGM vs 86% ECG, P = NS).
Conclusions: An automated method based on ICD EGMs identifies the most vulnerable intervals with accuracy comparable to the operator-performed, clinical method based on the surface ECG. This EGM method can be implemented efficiently in an ICD to automate vulnerability testing.     

Multicenter Experiences with a Single Lead Electrode for Dual Chamber ICD Systems

Monitoring of atrial signals improves the accuracy in identifying supraventricular tachyarrhythmias to prevent inappropriate therapies in patients with implantable ICDs. Since difficulties due to the additional atrial lead were found in dual chamber ICD systems with two leads, the authors designed a single pass VDD lead for use with dual chamber ICDs. After a successful animal study, the prototype VDD lead (single coil defibrillation lead with two additional fractally coated rings for bipolar sensing in the atrium) was temporarily used in 30 patients during a German multicenter study. Atrial and ventricular signals were recorded during sinus rhythm (SR), atrial flutter, AF, and VT or VF. The implantation of the lead was successful in 27 of 30 patients. Mean atrial pacing threshold was 2.5 +/- 0.9 V/0.5 ms, mean atrial impedance was 213 +/- 31 ohms. Atrial amplitudes were greater during SR (2.7 +/- 1.6 mV) than during atrial flutter (1.46 +/- 0.3 mV, P < 0.05) or AF (0.93 +/- 0.37 mV, P < 0.01). During VF atrial "sinus" signals had significantly (P < 0.01) lower amplitudes (1.4 +/- 0.52 mV) than during SR. The mean ventricular sensing was 13.3 +/- 7.9 mV and mean ventricular impedance was 577 +/- 64 ohms. Defibrillation was successful with a 20-J shock in all patients. In addition, 99.6% of P waves could be detected in SR and 84.4% of flutter waves during atrial flutter. During AF, 56.6% of atrial signals could be detected without modification of the signal amplifier. In conclusion, a new designed VDD dual chamber lead provides stable detection of atrial and ventricular signals during SR and atrial flutter. Reliable detection of atrial signals is possible without modification of the ICD amplifier.     

A Rare type of Ventricular Oversensing in ICD Therapy—Inappropriate ICD Shock Delivery Due to Triple Counting

Irregular sensing by triple counting of wide QRS complexes resulted in inappropriate shocks in a patient with a biventricular implantable cardioverter defibrillator (ICD): A 66‐year‐old male patient with ischemic cardiomyopathy, left bundle branch block, and impaired left ventricular function received a biventricular ICD for optimal therapy of heart failure (CHF). Two years after implantation, the patient experienced recurrent unexpected ICD shocks without clinical symptoms of malignant tachyarrhythmia, or worsened CHF. The patient's condition rapidly worsened, with progressive cardiogenic shock and electrical–mechanical dissociation. After unsuccessful resuscitation of the patient the interrogation of the ICD showed an initial triple counting of extremely wide and fragmented QRS complexes with inappropriate shocks. (PACE 2010; 33:e17–e19)     

Detection of the defibrillation threshold using the upper limit of vulnerability following defibrillator implantation

OBJECTIVE: This study was designed to test defibrillation threshold (DFT) with the least number of fibrillation inductions using upper limit of vulnerability (ULV) and to describe the most practical set of ICD during DFT following implantation. BACKGROUND: Although the correlation between ULV and DFT has been well described, there has been no uniform DFT testing protocol taking the advantage of ULV after defibrillator (ICD) implantation. METHODS: A total of 26 patients undergoing a new ICD implantation had a DFT induced with scanned T wave shock. The hypothesis that ventricular fibrillation (VF) could be defibrillated with 5 J higher than the highest T wave shock needed to induce VF or with 10 J if the T wave shock needed to induce VF was less than 5 J, was tested and 20 patients fulfilled these criteria. The methodology is improved by detecting peak T wave with 12-lead ECG, applying biphasic T wave shock and scanning the T wave shock in a wider window. RESULTS: Five patients in the first group (n = 15) and one patient in the second group (n = 11) did not fulfill the above hypothesis. The common features of six patients who did not fulfill the hypothesis were that T wave shock needed to induce VF was either under 5 J (5 patients) or high (1 patient). CONCLUSION: This study revealed the importance of methodology in studies regarding ULV and DFT. Following ICD implantation, we propose the first biphasic T wave detected by 12-lead ECG and rescue shock set at 10 and 15 J, respectively. If any of the scanned T wave (40 ms before and 40 ms after the peak T wave with decrements and increments of 20 ms) shocks could not induce VF, then the T wave and the first rescue shock should be set at 5 and 10 J, respectively. If the induction of VF has been unsuccessful with T wave shock at 5 J, then a safe defibrillation with 10 J should be expected in majority.     

Potential Hazards of Fixed Gain Sensing and Arrhythmia Reconfirmation for Implantable Cardioverter Defibrillators

Appropriate sensing of ventricular tachycardia (VT) and ventricular fibrillation (VF)is of paramount importance for safety of patients with implanted cardioverter defibrillators (ICDs). Recently, the Guardian^R ATP 4210, a new third generation ICD that uses programmable but fixed sensing during sinus rhythm and doubles its sensitivity settings when VF is detected, to a maximum programmable sensitivity of 1 mV, has been tested in phase I and II clinical trials. A reconfirmation algorithm of this ICD confirms the presence of VT or VF prior to therapy. This case report describes undersensing of VF in a patient with the Guardian^R ATP 4210 at the maximum programmed sensitivity of 1 mV. Inappropriate episodes of asystole and prolonged bradycardias were also observed in this patient due to shortcomings in the reconfirmation algorithm design. Reoperation was required, with positioning of a new endocardial sensing lead to correct the undersensing of VF. This, however, did not correct asystolic pauses following antitachycardia pacing or spontaneous tachycardio termination prior to therapy. This case report highlights the hazards of fixed gain sensing for implantable ICDs and a potential limitation of a specific tachyarrhythmia reconfirmation algorithm used in this device.