Arrhythmia discrimination using hemoglobin spectroscopy in humans.

BACKGROUND: Inappropriate therapies are frequently delivered by implantable cardioverter-defibrillators (ICDs). We have investigated muscle perfusion as a means of augmenting arrhythmia discrimination by using implanted near-infrared spectroscopy. OBJECTIVE: To evaluate hemodynamic stability by monitoring muscle perfusion from within the ICD pocket, in fresh tissue and inside the scar capsule on preexisting ICD generators, during induced cardiac arrhythmias, in humans. METHODS: The sensor was implanted on or under the pectoral muscle, during ICD defibrillation threshold testing. A microvascular oxygenation trend indicator (O2 Index) was computed during 74 induced ventricular fibrillation and 34 normal sinus rhythm episodes in 34 patients and also during 28 atrial and 90 ventricular overdrive pacing episodes as simulations of supraventricular and ventricular tachycardias, respectively. RESULTS: On average, the change in oxygenation, based on the O2 Index, was statistically significant (P <.003) from baseline within 3 seconds following cardiac arrest. An optimized O2 Index, used for detecting the hemodynamic trend, exhibited a decreasing trend during ventricular fibrillation (P <.0001) and was different from that during normal sinus rhythm (P <.0001). The sensitivity for the detection of ventricular fibrillation was 100%, and the specificity for the rejection of normal sinus rhythm was 82% in the presence of scar tissue on the optical sensor. For a 35-mm Hg drop in the mean arterial pressure as the threshold for hemodynamic instability, the specificity for the rejection of hemodynamically stable atrial and ventricular pacing episodes was 93% and 71%, respectively. CONCLUSION: An implantable near-infrared spectroscopic sensor may be useful for hemodynamic monitoring during cardiac arrhythmias to prevent inappropriate therapy.

Predictors of reduction in mitral regurgitation in patients undergoing cardiac resynchronisation treatment.

BACKGROUND: CRT causes reduction in MR due to left ventricular (LV) remodelling, but determinants of clinically meaningful MR reduction acutely after CRT have not been evaluated. OBJECTIVES: We evaluated echocardiographic predictors of significant reduction in functional mitral regurgitation (MR) by cardiac resynchronisation treatment (CRT). METHODS: 35 patients with >or= moderate to severe MR underwent CRT for presence of electrical and/or mechanical dyssynchrony. Significant reduction in MR post-CRT was defined as reduction to less than moderate MR (MR jet area/left atrial area <25%, group 1) on follow-up echocardiogram at 1.7 (SD 2.8) months post-CRT. RESULTS: Significant MR reduction of 62% (28%) from baseline MR occurred in 18 patients vs 22% (16%) in the remaining patients (group 2), p<0.01). Follow-up left ventricular ejection fraction (LVEF) was 0.43 (0.09) in group 1 patients vs 0.29% (0.1%) in group 2 patients (p<0.001). On multivariate analysis, time to peak strain in the mid inferior segment was the only significant predictor of MR reduction post-CRT (p = 0.008, OR = 1.023 (CI 1.006 to 1.041). The sensitivity and specificity of the combined variable of time to peak strain of >400 ms in the mid inferior segment and peak negative strain of >or=9% and 8% in the basal and mid posterior segments, respectively, to predict follow-up MR was 88% and 93% respectively and positive and negative predictive value was 94% and 87%. CONCLUSION: In patients with cardiomyopathy and significant MR, the presence of delayed longitudinal strain in the mid inferior LV segment along with preserved negative systolic strain in the basal and mid posterior segments predicts substantial reduction in MR post-CRT.

Worldwide clinical experience with a new dual-chamber implantable cardioverter defibrillator system.

INTRODUCTION: Management of atrial tachyarrhythmias represents a significant challenge in patients with implantable cardioverter defibrillators (ICDs). Drug therapy of these arrhythmias is limited by moderate efficacy, ventricular proarrhythmia, and drug-device interactions. This study tested the safety and efficacy of a new dual-chamber ICD to detect and treat atrial as well as ventricular tachyarrhythmias. METHODS AND RESULTS: A dual-chamber ICD (Medtronic 7250 Jewel AF) was implanted in 293 of 303 patients at 49 centers in Europe, Canada, and North America. Specific data were collected at implant and during a mean follow-up period of 7.9+/-4.7 months. There were no clinically evident failures to detect and treat ventricular arrhythmias. In patients with at least one of the dual-chamber detection criteria activated, 1,056 of 1,192 episodes of ventricular tachycardia or fibrillation detected were judged to be appropriate (89% positive predictive accuracy). Therapy efficacy was 100% in the ventricular fibrillation zone and 98% in the ventricular tachycardia zone. Positive predictive accuracy for detection of atrial episodes was 95% (1,052/1,107). For episodes classified as atrial tachycardia by the device, the efficacy of atrial antitachycardia pacing and high-frequency (50-Hz) burst pacing was 55% and 17%, respectively. High-frequency burst pacing terminated 16.8% of episodes classified as atrial fibrillation, and atrial defibrillation had an estimated efficacy of 76%. The actuarial estimates of 6-month complication-free survival and total survival were 88% and 94%, respectively. CONCLUSION: This novel dual-chamber ICD is capable of safely and effectively discriminating atrial from ventricular tachyarrhythmias and of treating atrial tachyarrhythmias without compromising detection and treatment of ventricular tachyarrhythmias.

Supraventricular tachycardia-ventricular tachycardia discrimination algorithms in implantable cardioverter defibrillators: state-of-the-art review.

To reduce inappropriate therapy of supraventricular tachycardia (SVT), implantable cardioverter defibrillators (ICDs) include algorithms to discriminate ventricular tachycardia (VT) from SVT. Dual-chamber algorithms analyze atrial and ventricular rates or AV relationship. They provide advantages over single-chamber algorithms, but introduce new ways to detect SVT as VT inappropriately and to underdetect VT. Unlike pacemakers, dual-chamber ICDs require accurate atrial sensing during high ventricular rates. A postventricular atrial blanking period prevents oversensing of far-field R waves as atrial electrograms, but causes underdetection of atrial fibrillation during high ventricular rates. Tachycardias with 1:1 AV relationship and VT during atrial tachyarrhythmias present specific SVT-VT discrimination problems. The first dual-chamber algorithms performed comparably to single-chamber algorithms. Present dual-chamber algorithms correct some limitations of earlier versions.

Implantation of cardioverter defibrillators without induction of ventricular fibrillation.

BACKGROUND: The upper limit of vulnerability (ULV) is the weakest shock at which ventricular fibrillation (VF) is not induced by a T-wave shock. This study tested the hypothesis that a vulnerability safety margin based on the ULV can be used as an implantable cardioverter-defibrillator implantation criterion. METHODS AND RESULTS: Implantable cardioverter-defibrillators were implanted in 80 patients if T-wave shocks did not induce VF and the baseline-rhythm R wave was >/=7 mV. The T-wave shock was 10 J in the first 45 patients (group A) and 15 J in the last 35 patients (group B). After inductionless implantations, the first VF shock was programmed to the T-wave shock plus 5 J. If T-wave shocks induced VF, the ULV was measured and the first shock was programmed to the ULV+5 J. Inductionless implantations were performed in 58 patients (72%), 28 in group A (62%) and 30 in group B (86%; P=0.04). If T-wave scanning had been done at 15 J in group A patients, inductionless implantations could have been performed in 84% of them. At 3 months, VF was induced twice during electrophysiological study in 75 patients (94%). All VFs were detected in 80% of implantable cardioverter-defibrillator recipients using a vulnerability safety margin based on a T-wave scan at 15.

Prepulses reduce the pain of cutaneous electrical shocks.

OBJECTIVE: Both the startle reflex elicited by an intense acoustic or tactile stimulus and the perceived intensity of that stimulus can be diminished by a weak "prepulse" that precedes the startling stimulus. The present study examined whether prepulses can also diminish the pain produced by an intense electrical stimulus similar to that used to treat life-threatening cardiac arrhythmias in conscious patients with implantable cardioverter/defibrillators or transcutaneous pacemakers. METHODS: Perceptual and pain thresholds for electrical shocks to the arm were determined in 20 adults. Participants then rated the painfulness of 25 electrical shocks that were 1.5 times the pain threshold (mean shock intensity, approximately 160 V) and either presented alone or preceded (at 40-60 ms) by weak electrical prepulses equal to or 25% above the perceptual threshold. RESULTS: Prepulses significantly reduced the pain produced by the intense shocks. Individuals with the lowest pain thresholds experienced the greatest pain reduction with prepulses. In these more sensitive individuals, the most effective prepulses reduced perceived pain by 26% across the entire test session and by 54% in the initial block of five shocks. CONCLUSIONS: Prepulses may be useful in diminishing the pain associated with the therapeutic electrical shocks used to treat cardiac arrhythmias.

Prospective randomized comparison of 50%/50% versus 65%/65% tilt biphasic waveform on defibrillation in humans.

It is unknown if there is a single optimal biphasic waveform for defibrillation. Biphasic waveform tilt may be an important determinant of defibrillation efficacy. The purpose of this study was to compare acute defibrillation success with a three-electrode configuration in humans using 50%/50% versus 65%/65% tilt truncated exponential, biphasic waveforms delivered through a 110-microF capacitor. Acute DFTs for biphasic waveforms with 50%/50% versus 65%/65% tilt were measured in random order in 60 patients using a binary search method. The electrode configuration consisted of a RV coil as the cathode, and a SVC coil plus a pectoral active can emulator (CAN) as the anode. The waveforms were derived from an external voltage source with 110-microF capacitance, and the leading edge voltage of phase 2 was equal to the trailing edge voltage of phase 1. Stored energy DFT (9.2 +/- 5.7 [50%/50%] vs 10.8 +/- 6.4 [65%/65%] J, P = 0.007), current DFT (10.9 +/- 4.0 [50%/50%] vs 12.0 +/- 4.4 [65%/65%] A, P = 0.002) and voltage DFT (391 +/- 118 [50%/50%] vs 424 +/- 128 [65%/65%] V, P = 0.004) were significantly lower for the 50%/50% tilt waveform versus the 65%/65% tilt waveform using this three-electrode configuration and a 110-microF capacitor. For an RV(-)/SVC plus CAN(+) electrode configuration and a 110-microF capacitor, a 50%/50% tilt biphasic waveform results in a 15% reduction in energy DFT, 9% reduction in current DFT, and 8% reduction in voltage DFT versus a 65%/65% tilt biphasic waveform.

Multicenter study of principles-based waveforms for external defibrillation.

STUDY OBJECTIVE: The efficacy of a shock waveform for external defibrillation depends on the waveform characteristics. Recently, design principles based on cardiac electrophysiology have been developed to determine optimal waveform characteristics. The objective of this clinical trial was to evaluate the efficacy of principles-based monophasic and biphasic waveforms for external defibrillation. METHODS: A prospective, randomized, blinded, multicenter study of 118 patients undergoing electrophysiologic testing or receiving an implantable defibrillator was conducted. Ventricular fibrillation was induced, and defibrillation was attempted in each patient with a biphasic and a monophasic waveform. Patients were randomly placed into 2 groups: group 1 received shocks of escalating energy, and group 2 received only high-energy shocks. RESULTS: The biphasic waveform achieved a first-shock success rate of 100% in group 1 (95% confidence interval [CI] 95.1% to 100%) and group 2 (95% CI 94.6% to 100%), with average delivered energies of 201+/-17 J and 295+/-28 J, respectively. The monophasic waveform demonstrated a 96.7% (95% CI 89.1% to 100%) first-shock success rate and average delivered energy of 215+/-12 J for group 1 and a 98.2% (95% CI 91.7% to 100%) first-shock success rate and average delivered energy of 352+/-13 J for group 2. CONCLUSION: Using principles of electrophysiology, it is possible to design both biphasic and monophasic waveforms for external defibrillation that achieve a high first-shock efficacy.

Implantable cardioverter defribrillator shocks: a troubleshooting guide.

Implantable cardioverter defibrillators deliver shocks in response to electrical signals that satisfy programmed criteria for detection of VT or VF. The first step in diagnosis of inappropriate shocks in patients with ICDs is to determine if the shock was delivered in response to a true tachyarrhythmia by inspecting data stored in the ICD. Shocks occur in the absence of tachyarrhythmias because nonarrhythmic physiologic or nonphysiologic signals are oversensed by the ICD and detected as arrhythmias. Diagnosis and causes of oversensing are reviewed. The second step in diagnosis is to determine if the tachyarrhythmia stored in the VT/VF episode log is VT/VF or SVT by analyzing stored electrograms. Frequent or repetitive shocks constitute an electrophysiologic emergency. The approach to this problem is reviewed.

Mechanisms of ventricular fibrillation induction by 60-Hz alternating current in isolated swine right ventricle.

BACKGROUND: The mechanisms by which 60-Hz alternating current (AC) can induce ventricular fibrillation (VF) are unknown. METHODS AND RESULTS: We studied 7 isolated perfused swine right ventricles in vitro. The action potential duration restitution curve was determined. Optical mapping techniques were used to determine the patterns of activation on the epicardium during 5-second 60-Hz AC stimulation (10 to 999 microA). AC captured the right ventricles at 100+/-65 microA, which is significantly lower than the direct current pacing threshold (0.77+/-0.45 mA, P:<0.05). AC induced ventricular tachycardia or VF at 477+/-266 microA, when the stimulated responses to AC had (1) short activation CLs (128+/-14 ms), (2) short diastolic intervals (16+/-9 ms), and (3) short diastolic intervals associated with a steep action potential duration restitution curve. Optical mapping studies showed that during rapid ventricular stimulation by AC, a wave front might encounter the refractory tail of an earlier wave front, resulting in the formation of a wave break and VF. Computer simulations reproduced these results. CONCLUSIONS: AC at strengths less than the regular pacing threshold can capture the ventricle at fast rates. Accidental AC leak to the ventricles could precipitate VF and sudden death if AC results in a fast ventricular rate coupled with a steep restitution curve and a nonuniform recovery of excitability of the myocardium.

Accessory atrioventricular node with properties of a typical accessory pathway: anatomic-electrophysiologic correlation.

We report an accessory AV node producing ventricular preexcitation and comprising the retrograde limb of AV reentrant tachycardia (AVRT). A 66-year-old man presented with an anteroseptal myocardial infarction and thereafter developed recurrent, drug-refractory AVRT requiring multiple cardioversions. Electrophysiologic findings were typical for a concealed anteroseptal accessory pathway 0.5 cm anterior to the His bundle. The patient died of intractable heart failure after endocardial resection for a left ventricular aneurysm and coronary bypass grafting. Pathologic examination revealed a para-Hisian accessory AV node connecting the right atrium to ventricular myocardium immediately anterior to the His bundle at a depth of 4 mm from the endocardium. No typical AV accessory pathway was found. This is the first report of an accessory AV node that participated in AVRT. It was deeper than typical radiofrequency catheter ablation lesions.

Detection of atrial fibrillation and flutter by a dual-chamber implantable cardioverter-defibrillator. For the Worldwide Jewel AF Investigators.

BACKGROUND: To distinguish prolonged episodes of atrial fibrillation (AF) that require cardioversion from self-terminating episodes that do not, an atrial implantable cardioverter-defibrillator (ICD) must be able to detect AF continuously for extended periods. The ICD should discriminate between atrial tachycardia/flutter (AT), which may be terminated by antitachycardia pacing, and AF, which requires cardioversion. METHODS AND RESULTS: We studied 80 patients with AT/AF and ventricular arrhythmias who were treated with a new atrial/dual-chamber ICD. During a follow-up period lasting 6+/-2 months, we validated spontaneous, device-defined AT/AF episodes by stored electrograms in all patients. In 58 patients, we performed 80 Holter recordings with telemetered atrial electrograms, both to validate the continuous detection of AT/AF and to determine the sensitivity of the detection of AT/AF. Detection was appropriate in 98% of 132 AF episodes and 88% of 190 AT episodes (98% of 128 AT episodes with an atrial cycle length <300 ms). Intermittent sensing of far-field R waves during sinus tachycardia caused 27 inappropriate AT/AF detections; these detections lasted 2.6+/-2.0 minutes. AT/AF was detected continuously in 27 of 28 patients who had spontaneous episodes of AT/AF (96%). The device memory recorded 90 appropriate AT/AF episodes lasting >1 hour, for a total of 2697 hours of continuous detection of AT/AF. During Holter monitoring, the sensitivity of the detection of AT/AF (116 hours) was 100%; the specificity of the detection of non-AT/AF rhythms (1290 hours) was 99.99%. Of 166 appropriate episodes detected as AT, 45% were terminated by antitachycardia pacing. CONCLUSIONS: A new ICD detects AT/AF accurately and continuously. Therapy may be programmed for long-duration AT/AF, with a low risk of underdetection. Discrimination of AT from AF permits successful pacing therapy for a significant fraction of AT.

Cardiovascular collapse caused by electrocardiographically silent 60-Hz intracardiac leakage current. Implications for electrical safety.

BACKGROUND: The national standard for safe 60-Hz intracardiac leakage current under a single-fault condition is 50 microA. This standard is intended to protect patients from alternating current (AC) at levels below the threshold for sensation, but the minimum unsafe level for AC in closed-chest humans is not known. To determine this value, we studied 40 patients at testing of implantable cardioverter-defibrillators using a programmable source of 60-Hz AC. METHODS AND RESULTS: We applied AC for 5-second test periods in increasing strengths until ventricular fibrillation (VF) was induced or 1 mA was reached. Two current paths were tested: bipolar, between tip and ring electrodes of a right ventricular pacing catheter, and unipolar, from tip to a remote electrode. We observed a characteristic sequence of 3 responses as AC was increased: (1) intermittent ventricular capture with QRS morphology identical to pacing through the electrodes (minimum value, 20 microA); (2) continuous capture at cycle length 282+/-88 ms (minimum value, 32 microA); and (3) VF persisting after AC termination (minimum value, 49 microA). Continuous capture caused loss of pulsatile arterial pressure and cardiovascular collapse (mean arterial pressure, 32+/-8 mm Hg) for the duration of AC with no ECG evidence of AC stimulation. Thus, the clinical picture was that of hypotensive ventricular tachycardia (VT). The continuous-capture threshold was /=5 seconds should be

Current concepts of ventricular defibrillation.

The aim of this article is to review the current concepts of ventricular defibrillation. We studied the interaction between strong electrical stimulus and cardiac responses in both animal models and in humans. We found that a premature stimulus (S2) of appropriate strength results in figure-eight reentry in vitro by inducing propagated graded responses. The same stimulation protocol induces figure-eight reentry and ventricular fibrillation (VF) in vivo. When the S2 strength and the magnitude of graded responses increase beyond a critical level, the increase in refractoriness at the site of the stimulus becomes so long that the unidirectional block becomes bidirectional block, preventing the formation of reentry (upper limit of vulnerability [ULV]). In other studies, we found that the effects of an electrical stimulation on reentry is in part determined by the timing of the stimulus. A protective zone is present after the induction of VF and after an unsuccessful defibrillation shock during which an electrical stimulus can terminate reentry and protect the heart from VF. These results indicate that the effects of a defibrillation shock is dependent on both the strength and the timing of the shock. Timing is not important in areas where the shock field strength is > or = ULV because the shock terminates all reentry but cannot reinitiate new ones. However, in areas where shock field strength is < ULV, the effects of the shock are determined by the timing of the shock relative to local VF activations. This ULV hypothesis of defibrillation explains the probabilistic nature of ventricular defibrillation. It also indicates that, to achieve a high probability of successful defibrillation, a shock must result in a shock field strength of > or = ULV throughout the ventricles.

Effect of shock waveform on relationship between upper limit of vulnerability and defibrillation threshold.

INTRODUCTION: The upper limit of vulnerability (ULV) correlates with the defibrillation threshold (DFT). The ULV can be determined with a single episode of ventricular fibrillation and is more reproducible than the single-point DFT. The critical-point hypothesis of defibrillation predicts that the relation between the ULV and the DFT is independent of shock waveform. The principal goal of this study was to test this prediction. METHODS AND RESULTS: We studied 45 patients at implants of pectoral cardioverter defibrillators. In the monophasic-biphasic group (n = 15), DFT and ULV were determined for monophasic and biphasic pulses from a 120-microF capacitor. In the 60- to 110-microF group (n = 30), DFT and ULV were compared for a clinically used 110-microF waveform and a novel 60-microF waveform with 70% phase 1 tilt and 7-msec phase 2 duration. In the monophasic-biphasic group, all measures of ULV and DFT were greater for monophasic than biphasic waveforms (P < 0.0001). In the 60- to 110-microF group, the current and voltage at the ULV and DFT were higher for the 60-microF waveform (P < 0.0001), but stored energy was lower (ULV 17%, P < 0.0001; DFT 19%, P = 0.03). There was a close correlation between ULV and DFT for both the monophasic-biphasic group (monophasic r2 = 0.75, P < 0.001; biphasic r2 = 0.82, P < 0.001) and the 60- to 110-microF group (60 microF r2 = 0.81 P < 0.001; 110 microF r2 = 0.75, P < 0.001). The ratio of ULV to DFT was not significantly different for monophasic versus biphasic pulses (1.17 +/- 0.12 vs 1.14 +/- 0.19, P = 0.19) or 60-microF versus 110-microF pulses (1.15 +/- 0.16 vs 1.11 +/- 0.14, P = 0.82). The slopes of the ULV versus DFT regression lines also were not significantly different (monophasic vs biphasic pulses, P = 0.46; 60-microF vs 110-microF pulses, P = 0.99). The sample sizes required to detect the observed differences between experimental conditions (P < 0.05) were 4 for ULV versus 6 for DFT in the monophasic-biphasic group (95% power) and 11 for ULV versus 31 for DFT in the 60- to 110-microF group (75% power). CONCLUSION: The relation between ULV and DFT is independent of shock waveform. Fewer patients are required to detect a moderate difference in efficacy of defibrillation waveforms by ULV than by DFT. A small-capacitor biphasic waveform with a long second phase defibrillates with lower stored energy than a clinically used waveform.

Application of models of defibrillation to human defibrillation data: implications for optimizing implantable defibrillator capacitance.

BACKGROUND: Theoretical models predict that optimal capacitance for implantable cardioverter-defibrillators (ICDs) is proportional to the time-dependent parameter of the strength-duration relationship. The hyperbolic model gives this relationship for average current in terms of the chronaxie (t(c)). The exponential model gives the relationship for leading-edge current in terms of the membrane time constant (tau(m)). We hypothesized that these models predict results of clinical studies of ICD capacitance if human time constants are used. METHODS AND RESULTS: We studied 12 patients with epicardial ICDs and 15 patients with transvenous ICDs. Defibrillation threshold (DFT) was determined for 120-microF monophasic capacitive-discharge pulses at pulse widths of 1.5, 3.0, 7.5, and 15 ms. To compare the predictions of the average-current versus leading-edge-current methods, we derived a new exponential average-current model. We then calculated individual patient time parameters for each model. Model predictions were validated by retrospective comparison with clinical crossover studies of small-capacitor and standard-capacitor waveforms. All three models provided a good fit to the data (r2=.88 to .97, P<.001). Time constants were lower for transvenous pathways (53+/-7 omega) than epicardial pathways (36+/-6 omega) (t(c), P<.001; average-current tau(m), P=.002; leading-edge-current tau(m), P<.06). For epicardial pathways, optimal capacitance was greater for either average-current model than for the leading-edge-current model (P<.001). For transvenous pathways, optimal capacitance differed for all three models (P<.001). All models provided a good correlation with the effect of capacitance on DFT in previous clinical studies: r2=.75 to .84, P<.003. For 90-microF, 120-microF, and 150-microF capacitors, predicted stored-energy DFTs were 3% to 8%, 8% to 16%, and 14% to 26% above that for the optimal capacitance. CONCLUSIONS: Model predictions based on measured human cardiac-muscle time parameter have a good correlation with clinical studies of ICD capacitance. Most of the predicted reduction in DFT can be achieved with approximately 90-microF capacitors.

Atypical atrioventricular node reciprocating tachycardia masquerading as tachycardia using a left-sided accessory pathway.

OBJECTIVES: The study was performed to document that atrioventricular node reciprocating tachycardia (AVNRT) can be associated with eccentric retrograde left-sided activation, masquerading as tachycardia using a left accessory pathway. BACKGROUND: The eccentric retrograde left-sided activation during tachycardia is thought to be diagnostic of the presence of a left free wall accessory pathway. However, it is not known whether AVNRT can occur with eccentric retrograde left-sided activation. METHODS: We studied 356 patients with AVNRT who underwent catheter ablation. Retrograde atrial activation during tachycardia and ventricular pacing were determined by intracardiac recordings, including the use of a decapolar coronary sinus catheter. RESULTS: The retrograde atrial activation was eccentric in 20 patients (6%). Eight of these patients had the earliest retrograde atrial activation recorded in the lateral coronary sinus leads, and 12 had the earliest retrograde atrial activation recorded in the posterior coronary sinus leads, with the most proximal coronary sinus electrode pair straddling the coronary sinus orifice. These tachycardias were either the fast-slow or the slow-slow form of AVNRT. The slow-fast form of AVNRT was also inducible in 17 of the 20 patients. Successful ablation of the slow pathway in the right atrial septum near the coronary sinus ostium prevented the induction and clinical recurrence of reciprocating tachycardia in all patients. CONCLUSIONS: Atypical AVNRT with eccentric retrograde left-sided activation was demonstrated in 6% of all patients with AVNRT masquerading as tachycardia using a left-sided accessory pathway. Ablation of the slow pathway at the posterior aspects of the right atrial septum resulted in a cure in these patients.

Upper limit of vulnerability predicts chronic defibrillation threshold for transvenous implantable defibrillators.

INTRODUCTION: The upper limit of vulnerability (ULV) is the shock strength at or above which ventricular fibrillation cannot be induced when delivered in the vulnerable period. It correlates acutely with the acute defibrillation threshold (DFT) and can be determined with a single episode of fibrillation. The goal of this prospective study was to determine the relationship between the ULV and the chronic DFT. METHODS AND RESULTS: We studied 40 patients at, and 3 months after, implantation of transvenous cardioverter defibrillators. The ULV was defined as the weakest biphasic shock that failed to induce fibrillation when delivered 0, 20, and 40 msec before the peak of the T wave. patients were classified as clinically stable or unstable based on prospectively defined criteria. There were no significant differences between the group means for the acute and chronic determinations of ULV (13.5 +/- 5.3 J vs 12.4 +/- 6.8 J, P = 0.25) and DFT (10.1 +/- 5.0 J vs 9.9 +/- 5.7 J, P = 0.74). Five patients (15%) were classified as unstable. The strength of the correlation between acute ULV and acute DFT (r = 0.74, P < 0.001) was similar to that between the chronic ULV and chronic DFT (r = 0.82, P < 0.001). There was a correlation between the change in ULV from acute to chronic and the corresponding change in DFT (r = 0.67, P < 0.001). The chronic DFT was less than the acute ULV +3 J in all 35 stable patients, but it was greater in 2 of 5 unstable patients (P = 0.04). CONCLUSIONS: The strength of the correlation between the chronic ULV and the chronic DFT is comparable to that between the acute ULV and the acute DFT. Temporal changes in the ULV predict temporal changes in the DFT. In clinically stable patients, a defibrillation safety margin of 3 J above the acute ULV proved an adequate chronic safety margin.

Programming of implantable cardioverter-defibrillators on the basis of the upper limit of vulnerability.

BACKGROUND: A patient-specific measure of defibrillation efficacy that requires a minimum number of ventricular fibrillation (VF) episodes would be valuable for programming implantable cardioverter-defibrillators (ICDs). The upper limit of vulnerability (ULV) is the weakest shock strength at or above which VF is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It correlates with the defibrillation threshold (DFT) and can be determined with a single episode of VF. The objective of this study was to test the hypothesis that ICDs programmed on the basis of the ULV convert spontaneous ICD-detected VF reliably. METHODS AND RESULTS: We studied 100 consecutive patients at ICD implantation and during follow-up of 20 +/- 7 months. At implantation, the ULV and DFT were determined, and the ICD system was tested at a shock strength equal to the ULV + 3 J. During follow-up, the strength of the first shock was programmed to the ULV + 5 J for arrhythmias detected in the VF zone (cycle length < 292 +/- 17 ms). We reviewed stored detection intervals and electrograms from spontaneous episodes of ICD-detected VF to determine the success rate for appropriate first shocks. The programmed first-shock strength was 17.5 +/- 5.2 J. During follow-up, there were 120 appropriate first shocks in 37 patients. The arrhythmia was rapid monomorphic ventricular tachycardia (VT) in 70% of episodes (31 patients), VF in 11% (13 patients), polymorphic VT in 1%, and unclassified in 17% (15 patients). The first shock was successful in 119 of 120 episodes (99%; 95% CI, 93% to 100%). One unclassified episode required two shocks. No patient had syncope associated with an ICD shock or arrhythmic death. CONCLUSIONS: ICD shocks can be programmed on the basis of the ULV, a measurement made in regular rhythm, without a direct measure of defibrillation efficacy.