On the mechanism of the probabilistic nature of ventricular defibrillation threshold

The “Defibrillation Testing, Why Not?” survey. Testing of subcutaneous and transvenous defibrillators in the Italian clinical practice

Background

Defibrillation testing (DT) can be omitted in patients undergoing transvenous implantable cardioverter–defibrillator (T-ICD) implantation, but it is still recommended for patients at risk for a high defibrillation threshold and for ICD generator changes. Moreover, DT is still recommended on implantation of subcutaneous ICD (S-ICD). The aim of the present survey was to analyze the current practice of DT during T-ICD and S-ICD implantations.

Methods

In March 2021, an ad hoc questionnaire on the current performance of DT and the standard practice adopted during testing was completed at 72 Italian centers implanting S-ICD and T-ICD.

Results

48 (67%) operators reported never performing DT during de-novo T-ICD implantations, while no operators perform it systematically. The remaining respondents perform it for patients at risk for a high defibrillation threshold. DT is never performed at T-ICD generator change. At the time of de-novo S-ICD implantation, DT is never performed by 9 (13%) operators and performed systematically by 48 (66%). The remaining operators frequently omit DT in patients with more severe systolic dysfunction. DT is not performed at S-ICD generator change by 92% of operators. DT is conducted by delivering a first shock energy of 65 J by 60% of operators, while the remaining 40% test lower energy values.

Conclusions

In current clinical practice, most operators omit DT at T-ICD implantation, even when still recommended in the guidelines. DT is also frequently omitted at S-ICD implantation, and a wide variability exists among operators in the procedures followed during DT.

Subcutaneous implantable cardioverter-defibrillator and defibrillation testing: A propensity-matched pilot study

BACKGROUND

To date, only a few comparisons between subcutaneous implantable cardioverter-defibrillator (S-ICD) patients undergoing and those not undergoing defibrillation testing (DT) at implantation (DT+ vs DT-) have been reported.

OBJECTIVE

The purpose of this study was to compare long-term clinical outcomes of 2 propensity-matched cohorts of DT+ and DT- patients.

METHODS

Among consecutive S-ICD patients implanted across 17 centers from January 2015 to October 2020, DT- patients were 1:1 propensity-matched for baseline characteristics with DT+ patients. The primary outcome was a composite of ineffective shocks and cardiovascular mortality. Appropriate and inappropriate shock rates were deemed secondary outcomes.

RESULTS

Among 1290 patients, a total of 566 propensity-matched patients (283 DT+; 283 DT-) served as study population. Over median follow-up of 25.3 months, no significant differences in primary outcome event rates were found (10 DT+ vs 14 DT-; P = .404) as well as for ineffective shocks (5 DT- vs 3 DT+; P = .725). At multivariable Cox regression analysis, DT performance was associated with a reduction of neither the primary combined outcome nor ineffective shocks at follow-up. A high PRAETORIAN score was positively associated with both the primary outcome (hazard ratio 3.976; confidence interval 1.339-11.802; P = .013) and ineffective shocks alone at follow-up (hazard ratio 19.030; confidence interval 4.752-76.203; P = .003).

CONCLUSION

In 2 cohorts of strictly propensity-matched patients, DT performance was not associated with significant differences in cardiovascular mortality and ineffective shocks. The PRAETORIAN score is capable of correctly identifying a large percentage of patients at risk for ineffective shock conversion in both cohorts.

Spatiotemporal correlation uncovers characteristic lengths in cardiac tissue

Complex spatiotemporal patterns of action potential duration have been shown to occur in many mammalian hearts due to period-doubling bifurcations that develop with increasing frequency of stimulation. Here, through high-resolution optical mapping experiments and mathematical modeling, we introduce a characteristic spatial length of cardiac activity in canine ventricular wedges via a spatiotemporal correlation analysis, at different stimulation frequencies and during fibrillation. We show that the characteristic length ranges from 40 to 20 cm during one-to-one responses and it decreases to a specific value of about 3 cm at the transition from period-doubling bifurcation to fibrillation. We further show that during fibrillation, the characteristic length is about 1 cm. Another significant outcome of our analysis is the finding of a constitutive phenomenological law obtained from a nonlinear fitting of experimental data which relates the conduction velocity restitution curve with the characteristic length of the system. The fractional exponent of 3/2 in our phenomenological law is in agreement with the domain size remapping required to reproduce experimental fibrillation dynamics within a realistic cardiac domain via accurate mathematical models.

A Grouped Up-and-Down Method Used for Efficacy Comparison Between Two Different Defibrillation Waveforms

Electrical defibrillation, which consists of delivering a therapeutic dose of the electrical current to the fibrillating heart with the aid of a defibrillator, is still the only effective way to treat life-threatening ventricular fibrillation (VF). However, the efficacy of electrical therapy for terminating VF is highly dependent on the waveform applied. When new defibrillation waveforms or techniques are developed, their efficacy needs to be accurately evaluated and compared to those in use. A common method for the comparison of defibrillation efficacy is to estimate and compare the individual defibrillation threshold (DFT) by constructing dose response curves or using an up-and-down method. Since DFT is calculated by repetitive and sequential shocks, there will be variability for each measurement and for each individual. This creates a considerable uncertainty for paired comparison. In this paper, a novel grouped up-and-down method is developed for the comparison of defibrillation efficacy between two different defibrillation waveforms or techniques. The efficacy of two commonly used biphasic defibrillation waveforms was compared in a porcine model of cardiac arrest using the developed method. Experimental results demonstrate that the proposed method is more sensitive for efficacy comparison and requires less defibrillation attempts compared with traditional DFT methods.

Defibrillation testing during implantable cardioverter-defibrillator implantation in Italian current practice: the Assessment of Long-term Induction clinical ValuE (ALIVE) project

BACKGROUND

Clinical practice with regard to defibrillation threshold (DFT) testing during implantable cardioverter-defibrillator (ICD) implantation varies considerably, even among experienced implanting centers. International guidelines do not as yet mandate DFT testing.

OBJECTIVE

The objective of this project is to assess current clinical decision making regarding DFT testing during ICD implantation.

METHODS

The ALIVE project collected data on DFT testing from a multicenter network of Italian clinicians sharing a common system for the collection, management, analysis, and reporting of clinical and diagnostic data from patients with Medtronic (Minneapolis, MN) implantable devices.

RESULTS

Data on 2,082 consecutive patients implanted with a Medtronic ICD in 111 Italian centers, over the period 2007 to 2010, were analyzed. Defibrillation threshold testing was performed in 33% of cases (678/2,082). The main reasons for performing the test were physician's clinical practice ("I always perform DFT") (80%) and secondary prevention implantation (12%). The main reasons for not performing DFT testing were centers' practice (44%), primary prevention (31%), and device replacement (15%). In 22 patients, ventricular fibrillation induction was not achieved; 656 patients completed DFT testing: 633 patients (96%) performed a single test, 19 patients (3%) performed a second induction test, and 4 patients (0.6%) underwent an additional induction test.

CONCLUSIONS

The preliminary results of the ALIVE project show that a great number of implant procedures are performed without DFT testing in the common practice of the participating centers. We also measured an inhomogeneous, center-dependent DFT testing behavior, which suggests the importance of defining a common guideline for ICD implant testing. Follow-up data on our patients will provide more information on the clinical value of the test.

Role of maximum rate of depolarization in predicting action potential duration during ventricular fibrillation

During ventricular fibrillation (VF) only 39% of the variation in action potential duration (APD) is accounted for by the previous diastolic interval [DI((n-1))], i.e., restitution, and the previous APD [APD((n-1))], i.e., memory. We tested the hypothesis that a characteristic of the AP upstroke, the maximum rate of depolarization (V(max)), also helps account for its APD. A floating microelectrode was used to make transmembrane recordings at 16,000 samples/s from the anterior left ventricular wall during four 20-s episodes of VF in each of six pigs. V(max), time from V(max) to 60% repolarization (APD(60)), and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD(60) (APD(60n)) was predicted by V(max) of that AP, the four previous DIs (n-1, n - 2, n - 3, n - 4), and the three previous APD(60)s (n-1, n - 2, n - 3). V(max) entered in the regression equation significantly more often (86% of VF episodes) than either APD((n-1)) (47% of episodes) or DI((n-1)) (58% of episodes). When these three variables entered first or second, their coefficients were almost always positive, consistent with a longer APD associated with 1) a larger V(max), 2) a longer APD((n-1)), and 3) a longer DI((n-1)). R(2) of the regression for all entered variables was 0.51 +/- 0.01 (mean +/- SD). During the first 20 s of VF in swine, V(max) is a more important determinant of APD than the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF.

Preshock phase singularity and the outcome of ventricular defibrillation

BACKGROUND

Phase singularity (PS) is a topological defect that serves as a source of ventricular fibrillation (VF). Whether or not the quantity of preshock PS determines defibrillation outcome is unclear.

OBJECTIVE

The purpose of this study was to test the hypothesis that the number of PSs at the time of shock is an important factor that determines the shock outcome.

METHODS

Isolated, perfused rabbit hearts (n = 7) were optically mapped with a potentiometric dye (di-4-ANNEPS). Shocks were delivered during short (10 seconds) and long (1 minute) VF, and the outcome was classified as successful type A (immediate termination), type B (postshock repetitive responses before termination), and unsuccessful.

RESULTS

When shock strengths of 50% probability of successful defibrillation (DFT50) +/- 50 V were given in short VF, the types A and B and unsuccessful shocks were associated with a preshock PS number of 0.3 +/- 0.4, 1.4 +/- 0.3, and 1.5 +/- 0.4 (P <.01 by analysis of variance) and shock strengths of 205 +/- 77, 207 +/- 65, and 173 +/- 74 V (P <.01), respectively. When the same shocks were applied during long VF, the PS numbers were 1.7 +/- 0.5, 3.0 +/- 0.5, and 3.5 +/- 0.6, respectively (P <.01), and the shock strengths were 282 +/- 100, 283 +/- 135, and 256 +/- 126 V, respectively (P <.01). If we only analyze shocks with strength at DFT(50), the preshock PS number was still significantly different for short VF (0.6 +/- 0.5, 1.6 +/- 0.9, and 1.5 +/- 0.8; P <.05) and for long VF (1.4 +/- 0.5, 2.7 +/- 0.6, and 2.7+/-1.3; P <.05), respectively. All preshock PSs were eliminated by shocks. However, rapid repetitive activity was then reinitiated in unsuccessful and type B successful shocks but not in type A successful shocks.

CONCLUSIONS

A low number or an absence of preshock PS was associated with type A successful defibrillation. There was no difference in preshock PS numbers between unsuccessful and type B successful defibrillation.

Inductionless or Limited Shock Testing Is Possible in Most Patients With Implantable Cardioverter- Defibrillators/Cardiac Resynchronization Therapy Defibrillators

Background— Implantable cardioverter-defibrillators and cardiac resynchronization therapy defibrillators have relied on multiple ventricular fibrillation (VF) induction/defibrillation tests at implantation to ensure that the device can reliably sense, detect, and convert VF. The ASSURE Study (Arrhythmia Single Shock Defibrillation Threshold Testing Versus Upper Limit of Vulnerability: Risk Reduction Evaluation With Implantable Cardioverter-Defibrillator Implantations) is the first large, multicenter, prospective trial comparing vulnerability safety margin testing versus defibrillation safety margin testing with a single VF induction/defibrillation.

Methods and Results— A total of 426 patients receiving an implantable cardioverter-defibrillator or cardiac resynchronization therapy defibrillator underwent vulnerability safety margin or defibrillation safety margin screening at 14 J in a randomized order. After this, patients underwent confirmatory testing, which required 2 VF conversions without failure at ≤21 J. Patients who passed their first 14-J and confirmatory tests, irrespective of the results of their second 14-J test, had their devices programmed to a 21-J shock for ventricular tachycardia (VT) or VF ≥200 bpm and were followed up for 1 year. Of 420 patients who underwent 14-J vulnerability safety margin screening, 322 (76.7%) passed. Of these, 317 (98.4%) also passed 21-J confirmatory tests. Of 416 patients who underwent 14-J defibrillation safety margin screening, 343 (82.5%) passed, and 338 (98.5%) also passed 21-J confirmatory tests. Most clinical VT/VF episodes (32 of 37, or 86%) were terminated by the first shock, with no difference in first shock success. In all observed cases in which the first shock was unsuccessful, subsequent shocks terminated VT/VF without complication.

Conclusions— Although spontaneous episodes of fast VT/VF were limited, there was no difference in the odds of first shock efficacy between groups. Screening with vulnerability safety margin or defibrillation safety margin may allow for inductionless or limited shock testing in most patients.

Restitution Properties During Ventricular Fibrillation in the In Situ Swine Heart

Background— Although restitution has been hypothesized to determine action potential duration (APD) during ventricular fibrillation (VF), cardiac memory may also be important.

Methods and Results— Transmembrane recordings were made with a floating microelectrode from the anterior right ventricular wall in 6 pigs during up to 60 seconds of VF. The recordings were divided into 5-second intervals, and APD 60 and the diastolic interval (DI) were calculated for each activation cycle throughout each interval. Stepwise linear regression was used to determine how well each APD 60 [APD 60 (n)] was predicted by the 4 previous DIs (n−1, n−2, n−3, n−4) and the 3 previous APD 60 s (n−1, n−2, n−3). A mean±SD of 3±1.5 of the variables entered the regression equation. DI(n−1) (70% of intervals) and APD 60 (n−1) (71% of intervals) appeared most frequently in the regression equations and were the first or second variables entered during the stepwise regression in 87% and 76% of the intervals in which they were present, respectively. The coefficients of DI(n−1) and APD 60 (n−1) were positive 89% and 98% of the time, respectively. R 2 of the regression for all entered variables during all intervals was 0.39±0.05.

Conclusions— The high incidence and positive coefficient of DI(n−1) indicate that restitution is important in determining APD during VF, whereas the similarly high incidence and positive coefficient of APD(n−1) indicate that cardiac memory is equally important. The finding that the regression equation accounts for only 39% of the variability of APD indicates that factors other than restitution and memory are also important in determining APD during VF.

Addition of a Defibrillation Electrode in the Low Right Atrium to a Right Ventricular Lead Does Not Reduce Ventricular Defibrillation Thresholds

Transvenous unipolar active can defibrillation systems have proven to be effective in treating ventricular tachyarrhythmias. However, a further reduction of ventricular defibrillation thresholds (V-DFT) would increase the longevity, reduce the size of pulse generators, and help to avoid additional leads in patients with inacceptable high V-DFTs. In a finite difference computer model, the extension of the right ventricular (RV) defibrillation coil into the low right atrium led to a 40% reduction of unipolar V-DFT. To evaluate this finding, we conducted a prospective, randomized study in 11 patients receiving an ICD. Extension of the RV electrode was simulated by adding a second coil placed in the low right atrium with the same polarity. Using a binary search protocol, V-DFT was determined with and without the additional electrode in each patient. Total shock impedance was significantly lower in the two coil (low RA) configuration, compared to the single coil (RV) configuration. Corresponding values were 49.9 +/- 6.7 Ohm and 61.1 +/- 9.3 Ohm, respectively (P < 0.01, paired t-test). However, there was no reduction, but even a nonsignificant increase in V-DFTs. Mean V-DFT in the RV configuration was 12.0 +/- 5.6 J and 16.3 +/- 7.8 J in the low RA configuration (P = 0.09, paired t-test). Despite a reduction in total impedance, the addition of a defibrillation coil in the low right atrium does not reduce ventricular defibrillation thresholds.