The scar: the wind in the perfect storm-insights into the mysterious living tissue originating ventricular arrhythmias.

BACKGROUND: Arrhythmic death is very common among patients with structural heart disease, and it is estimated that in European countries, 1 per 1000 inhabitants yearly dies for sudden cardiac death (SCD), mainly as a result of ventricular arrhythmias (VA). The scar is the result of cardiac remodelling process that occurs in several cardiomyopathies, both ischemic and non-ischemic, and is considered the perfect substrate for re-entrant and non-re-entrant arrhythmias. METHODS: Our aim was to review published evidence on the histological and electrophysiological properties of myocardial scar and to review the central role of cardiac magnetic resonance (CMR) in assessing ventricular arrhythmias substrate and its potential implication in risk stratification of SCD. RESULTS: Scarring process affects both structural and electrical myocardial properties and paves the background for enhanced arrhythmogenicity. Non-uniform anisotropic conduction, gap junctions remodelling, source to sink mismatch and refractoriness dispersion are some of the underlining mechanisms contributing to arrhythmic potential of the scar. All these mechanisms lead to the initiation and maintenance of VA. CMR has a crucial role in the evaluation of patients suffering from VA, as it is considered the gold standard imaging test for scar characterization. Mounting evidences support the use of CMR not only for the definition of gross scar features, as size, localization and transmurality, but also for the identification of possible conducting channels suitable of discrete ablation. Moreover, several studies call out the CMR-based scar characterization as a stratification tool useful in selecting patients at risk of SCD and amenable to implantable cardioverter-defibrillator (ICD) implantation. CONCLUSIONS: Scar represents the substrate of ventricular arrhythmias. CMR, defining scar presence and its features, may be a useful tool for guiding ablation procedures and for identifying patients at risk of SCD amenable to ICD therapy.

A new echo-Doppler method to measure interatrial conduction time. Validation and clinical usefulness.

AIM: The aim of this study was to assess if knowing the interatrial conduction time is useful to better program atrioventricular delay in sequential pacing. The study proposes a new echo-Doppler method to measure interatrial conduction time, correlating it with electrophysiological measures. METHODS: The new method was tested in 30 subjects who underwent electrophysiological study. Interatrial conduction time by new method was taken during atrial pacing as the interval between the artefact of electrocardiogram pacing, shown on screen echo, and the onset of the A wave of the echo-Doppler mitral inflow. The electrophysiological measures were obtained, in the same subjects and at the same time, by a decapolar catheter in coronary sinus as intervals between the artefact of atrial pacing and the first positive left atrial deflection at proximal (C7C8) and distal (C1C2) electrodes. RESULTS: Echo-Doppler mean time was 114±12 ms, electrophysiological time was 107±14 ms at C7C8 and 124±11 ms at C1C2. Statistical analysis showed a good correlation (r=0.92, P<0.001) and accord (mean difference=6.6 ms) between the two methods. CONCLUSION: The new method to measure interatrial conduction time is consistent with the electrophysiological method; it could be particularly useful in programming the best AV delay in sequential and biventricular pacing, to avoid atrial contraction against a closed mitral valve.

Postcardioversion atrial electrophysiologic changes induced by oral verapamil in patients with persistent atrial fibrillation.

OBJECTIVES: The aim of our study was to verify the effect of oral administration of verapamil on atrial electrophysiologic characteristics after cardioversion of persistent atrial fibrillation (AF) in humans. BACKGROUND: Discordant findings have been reported regarding the efficacy of verapamil in preventing the electrical remodeling induced by AF. METHODS: We determined the effective refractory periods (ERPs) at five pacing cycle lengths (300 to 700 ms) and in five right atrial sites after internal cardioversion of persistent AF (mean duration 238.1+/-305.9 days) in 19 patients. Nine patients received oral verapamil (240 mg/day) starting four weeks before the electrophysiologic study, whereas the other 10 patients were in pharmacologic washout. RESULTS: The mean ERPs were 202.0+/-22.7 ms in the washout group and 189.3+/-18.5 ms in the verapamil group (p < 0.0001). The degree of adaptation of refractoriness to rate was similar in the two groups (mean slope value in the washout group and verapamil group: 0.07+/-0.03 and 0.08+/-0.05, respectively), showing a normal or nearly normal adaptation to rate in the majority of the paced sites in both groups. The mean ERP was slightly longer in the septum than in the lateral wall and in the roof, both in the washout and verapamil groups. CONCLUSIONS: In patients with persistent AF, long-term administration of verapamil before internal cardioversion resulted in 1) shortening of atrial ERPs; 2) no change in refractoriness dispersion within the right atrium; and 3) no change in atrial ERP adaptation to rate.

Changes in intracardiac atrial cardioversion threshold at rest and during exercise.

OBJECTIVES: We sought to analyze in patients with chronic atrial fibrillation (AF) the change in the intracardiac atrial defibrillation threshold (ADT) at rest and during exercise, to quantify the effective risk of low energy endocavitary cardioversion during the effort and to compare the ADT of chronic and reinduced AF. BACKGROUND: Low energy endocavitary cardioversion is a new alternative to transthoracic shock in patients with chronic AF. Nevertheless, patient discomfort and possible induction of ventricular arrhythmias should be further evaluated. METHODS: Sixteen patients with chronic AF were included in the study. Two 6F custom-made catheters (Electro-Catheter, Inc.) were used for shock delivery and one tetrapolar lead for ventricular synchronization. Without sedation and in a random order, patients underwent two sequences of shocks to determine the ADT at rest and during exercise. Exercise was performed isometrically by the superior limbs. Atrial fibrillation was reinduced by atrial pacing. After each shock, the patients were requested to grade their discomfort with a score from 1 to 5. The power of the study was > 90% in detecting a 25% difference in the ADT between groups. RESULTS: Patients were classified into two groups: Nine patients (group A) underwent the first cardioversion during exercise; seven patients (group B) underwent the first cardioversion at rest. In total, the mean (+/-SD) ADT was 6.70 +/- 1.54 J during exercise and 7.02 +/- 1.82 J at rest (p = 0.59). A significantly lower ADT was observed in the second shock sequence than the first one (6.32 +/- 2.09 J vs. 7.40 +/- 0.87 J, p < 0.05). The discomfort score was 3.25 +/- 0.86 at rest and 2.94 +/- 0.77 during exercise (p = 0.09). No complications occurred. CONCLUSIONS: Low energy endocavitary cardioversion is a safe and effective procedure in patients with chronic AF. Discomfort is not generally severe enough to result in procedure termination. The ADT is not influenced by exercise and is higher in chronic than in reinduced AF.

Indications for dual-chamber (DDD) pacing in implantable cardioverter-defibrillator patients.

New technologic development of implantable cardioverter-defibrillators (ICDs) keeps up with the exponential increase of their use for primary and secondary prevention of sudden cardiac death. The first-generation ICD with limited shock capability alone could be considered adequate in most cardiac arrest victims, but it was not suitable for sudden death prevention in all high-risk patients with cardiac disease. The second-generation ICD was comprised of hybrid pacemaker-defibrillator systems that provided on-demand ventricular antibradycardia pacing. The third-generation devices include additional functions, such as antitachycardia pacing for ventricular tachycardia (VT) reversion and low-energy ventricular cardioversion, in addition to ventricular defibrillation and single-chamber ventricular demand pacing. In the near future, advanced dual-chamber atrioventricular (AV) pacing and defibrillating systems will also be available. The dual chamber ICD will allow atrial inhibited/dual-chamber (AAI/DDD) rate-responsive pacing, simultaneous atrial and ventricular sensing to optimize the arrhythmia identification, and ICD shock delivery in the proper arrhythmia-related chamber. Clinical benefits of these devices compared with their cost and complexity will require careful evaluation.

Combining antiarrhythmic drugs and implantable devices therapy: benefits and outcome.

At least 50% of patients who received an ICD have been treated with antiarrhythmic drugs (AAD). The potential indications for combining antiarrhythmic drugs and ICD are generally the following: reduction of the number of episodes of ventricular tachycardia or ventricular fibrillation and therefore of the number of shocks, improving patient's quality of life and extending the battery life of the ICD, prevention of supraventricular arrhythmias and/or control of their rate, lengthening of the tachycardia cycle length to allow ventricular tachycardia conversion by antitachycardia pacing and reduction of the number of episodes of syncope. Although previous papers reported conflicting results about pharmacologic therapy in reducing the frequency of iCD shocks, some recent randomized prospective studies showed the efficacy of pharmacologic therapy in reducing the frequency of ICD shocks. The use of antiarrhythmic drugs can have also adverse effect: an increase in the defibrillation threshold, an increase in the pacing threshold and an increase in the VT cycle length leading to detection failure. We have also to consider that some advantages derived from antiarrhythmic drugs can be reached by the new devices with atrial sensing and pacing and/or the possibility of atrial defibrillation or by using catheter ablation as adjunctive therapy to ICD. For these reasons, the concomitant use of antiarrhythmic drugs and ICD should be evaluated in each patient in relation to specific clinical and electrophysiologic features including: the frequency, the rate and the clinical presentation of the ventricular arrhythmia, the effect of the selected drug on the defibrillation threshold, the defibrillation threshold at the implant, the effect of the selected drug on the ventricular function and the likelihood of proarrhythmic events.

Single shock endocavitary low energy intracardiac cardioversion of chronic atrial fibrillation.

BACKGROUND: Discomfort related to low-energy internal cardioversion (LEIC) represents a real problem in patients (pts) with atrial fibrillation (AF). The aim of our study was to verify if a single shock could restore sinus rhythm (SR) with a lower discomfort for the pt. METHODS: Thirty pts with chronic AF were randomized to receive a single 350 V shock (15 pts) or multiple shocks of increasing energy (15 pts). Three leads were positioned, respectively, in the coronary sinus and in the lateral right wall for shock delivery, and in the right ventricular apex for R wave synchronization. Truncated, biphasic shocks were used. In the first group a single 350 V shock was directly delivered and a second 400 V shock was given only if SR has not been restored. In the second group, beginning at 50 volts the voltage was increased in steps of 50 volts until SR restoration. No patient was sedated. After each shock the pts were asked to rate their discomfort on a scale of 1 to 5 (1 = not perceived, 5 = severe discomfort). RESULTS: SR was restored in all the subjects. In group 1 SR was obtained in 12/15 (80%) pts with the first 350 V (8.1 +/- 0.8 joules) shock, while the remaining 3 patients required the second 400 V (10.2 +/- 0.3 joules) shock. In group 2 the mean atrial defibrillation threshold was 346.7 +/- 1029.7 volts (8.0 +/- 101.5 joules). Then discomfort score was 2.5 +/- 0.6 in group 1 and 3.3 +/- 10.6 in group 2 (p < 0.01). CONCLUSIONS: A single shock of 350 V restores SR in the majority of pts with chronic AF; by use this new approach, LEIC is tolerated better than the multiple shocks step-up protocol.

Electrophysiology of atrial fibrillation: evolving insights.

In the last few years many studies have been performed to better understand the pathophysiological nature of atrial fibrillation (AF). These recent observations provide new insights into the initiation and perpetuation of AF, underlying the importance of the pulmonary veins as major sources of atrial triggers and introducing new concepts such as atrial electrical remodeling and spatial heterogeneity of the electrophysiologic characteristics of this arrhythmia. The purpose of this review was to provide current knowledge about AF electrophysiology in an effort to unite old models and new concepts.

Facilitation of atrioventricular reentrant tachycardia by iatrogenic right bundle branch block.

The present case report describes the diagnosis of a concealed bypass tract in the right lateral wall revealed by electrophysiologic evaluation performed in a patient with rare palpitations. A iatrogenic right bundle branch block (RBBB) caused the occurrence of an incessant atrioventricular reentrant tachycardia. The disappearance of the RBBB determined a very difficult induction of the tachycardia that, when induced, showed a shorter cycle length and ventriculoatrial interval than those observed during RBBB tachycardia. The presence of a RBBB ipsilateral to the right free wall accessory pathway provided a critical delay within the circuit thus allowing the bypass tract to recover excitability. This relevant delay also allows the sinus beat to initiate and stabilize the tachycardia thus rendering it incessant.

Repolarization and refractoriness in patients with persistent atrial fibrillation.

The use of monophasic action potential (MAP) recordings has been of important value during atrial fibrillation to understand the possibility of local pacing capture during the arrhythmia, while MAP and refractoriness determination after sinus rhythm restoration have highlighted the issue of electrophysiological remodeling owing to rate. Moreover the contemporary recording of MAP and refractoriness at the same atrial sites permitted to better understand the behavior of the ERP/MAP ratio in these patients. Local atrial pacing capture has been demonstrated in humans with chronic atrial fibrillation and suggests the presence of re-entrant circuits with large excitable gaps. The studies about atrial remodeling have shown a shortening of atrial ERP or monophasic action potential duration after cardioversion of persistent AF, while discordant results have been observed for what it concerns refractoriness adaptation to rate. Finally, the recording of a mean ERP/MAP90 ratio < 1 at all the pacing cycle lengths, indicates that no post-repolarization refractoriness was present after cardioversion of persistent atrial fibrillation.

[Latent long QT syndrome: discription of a clinical case]. / Sindrome del QT lungo latente: descrizione di un caso clinico.

Latent forms of long QT syndrome have been already reported. We describe one case of a 27 years old female patient who experienced an episode of cardiac arrest after several puffs of salbutamole. The malignant arrhythmia causing the cardiac arrest was torsade de pointes degenerated into ventricular fibrillation. The patient ECG showed a normal QTc basal interval and the correct diagnosis was made by contemporary recording of the ECG and MAP during orciprenalina infusion. After drug infusion, we have recorded a MAP lengthening and a dispersion of MAP duration between the right ventricular apex and the right ventricular outflow tract. These modifications were concomitant with the appearance of "humps" (probably related to the presence of early afterdepolarizations), a QT interval lengthening and morphologic changes of the T and U waves.

Influence of digital and analogue cellular telephones on implanted pacemakers.

UNLABELLED: The aim of this study was to find out whether digital and analogue cellular 'phones affect patients with pacemakers. The study comprised continuous ECG monitoring of 200 pacemaker patients. During the monitoring certain conditions caused by interference created by the telephone were looked for: temporary or prolonged pacemaker inhibition; a shift to asynchronous mode caused by electromagnetic interference; an increase in ventricular pacing in dual chamber pacemakers, up to the programmed upper rate. The Global System for Mobile Communications system interfered with pacing 97 times in 43 patients (21.5%). During tests on Total Access of Communication System telephones, there were 60 cases of pacing interference in 35 patients (17.5%). There were 131 interference episodes during ringing vs 26 during the on/off phase; (P < 0.0001); 106 at maximum sensitivity level vs 51 at the 'base' value; P < 0.0001). Prolonged pacing inhibition (> 4 s) was seen at the pacemaker 'base' sensing value in six patients using the Global system but in only one patient using Total Access. CONCLUSION: Cellular 'phones may be dangerous for pacemaker patients. However, they can be used safely if patients do not carry the 'phone close to the pacemaker, which is the only place where high risk interference has been observed.

[Ventricular nodal reentry tachycardia without ventricular dilatation: diagnosis and treatment by radiofrequency catheter ablation]. / Tachicardia ventricolare da rientro branca-branca in assenza di dilatazione ventricolare: diagnosi e trattamento mediante ablazione transcatetere a radiofrequenza.

Bundle branch reentry tachycardia has been reported in patients with left ventricular dilatation, especially in those with dilated cardiomyopathy and aortic regurgitation. These patients show aspecific intraventricular conduction delay on the ECG and a prolonged H-V interval at the electrophysiologic study. We report 2 cases of bundle branch reentry tachycardia in patients without left ventricular enlargement to help the correct diagnosis of the arrhythmia. A correct diagnosis is very important because bundle branch reentry tachycardia is easily and safely treated by right bundle transcatheter radiofrequency ablation. We also report electrophysiologic characteristics we found during the study and the ablation: -contrary to the data reported in literature, at ventricular tachycardia starting, modifications of the V-V interval are not always preceded by similar variations in the H-H interval. -during right bundle radiofrequency ablation, a QRS narrowing may precede right bundle branch block appearance. This QRS normalisation should induce to continue and not to stop energy delivering.

Variable cerebral dysfunction during tilt induced vasovagal syncope.

Electroencephalographic (EEG) monitoring was performed during head-up tilt testing (HUT) in a group of 63 consecutive patients (27 males, 36 females, mean age 41.5 years) with a history of recurrent syncope of unknown origin despite extensive clinical and laboratory evaluation. Syncope occurred in 27/63 patients (42.8%) during HUT and was cardioinhibitory in 11/27 (40.7%) and vasodepressor in 16/27 (59.3%). All patients with a negative response to HUT had no significant EEG modifications. In patients with vasodepressor syncope a generalized high amplitude 4-5 Hz (theta range) slowing of EEG activity appeared at the onset of syncope, followed by an increase in brain wave amplitude with a reduction of frequency at 1.5-3 Hz (delta range). The return to the supine position was associated with brain wave amplitude reduction and frequency increase to 4-5 Hz, followed by restoration of a normal EEG pattern and arousal (mean total duration of syncope 23.2 s). In patients with cardioinhibitory syncope, a generalized high amplitude EEG slowing in the theta range was noted at the onset of syncope, followed by a brain wave amplitude increase and slowing in the delta range. A sudden reduction of brain wave amplitude ensued leading to the disappearance of electroencephalographic activity ("flat" EEG). The return to the supine position was not followed by immediate resolution of EEG abnormalities or consciousness recovery, both occurring after a longer time interval (mean total duration of syncope 41.4 s). EEG monitoring during HUT allowed the recording and systematic description of electroencephalographic abnormalities developing in the course of tilt induced vasovagal syncope.

Intra-atrial defibrillation of human atrial fibrillation.

Low-energy intra-atrial defibrillation is a new therapeutic option for restoring sinus rhythm in patients with atrial fibrillation (AF). The success rate is quite high when right atrium-coronary sinus or right atrium-pulmonary artery electrode configurations are used, although the former is associated with a slightly lower defibrillation threshold. Several issues regarding the safety and tolerability of the procedure remain unresolved. Our experience and that of others indicate that low-energy intra-atrial cardioversion is safe even during exercise, provided the shock is well synchronized with the QRS and the preshock RR interval is > 500 msec. Reported defibrillation thresholds in patients with persistent AF range from 4 to 8 J, and shocks of this type inevitably are associated with some degree of discomfort. Measures aimed at lowering the defibrillation threshold (e.g., use of biphasic waveforms, antiarrhythmic pretreatment, and use of a single effective shock, as opposed to the multiple shocks delivered in research settings) can be expected to increase patient tolerance and extend the indications for low-energy intra-atrial cardioversion. The procedure currently is indicated for patients with persistent AF who are resistant to external defibrillation unable or unwilling to undergo general anesthesia. The procedure could be expanded to patients affected by obesity in whom the efficacy of external cardioversion is lower.

Head-up tilt testing potentiated with low-dose sublingual isosorbide dinitrate: a simplified time-saving approach for the evaluation of unexplained syncope.

BACKGROUND: Head-up tilt testing is widely used in the clinical assessment of patients with unexplained syncope. However, the lack of a standard methodology and the conflicting results concerning sensitivity and specificity of the procedure have prompted further studies to define a more cost-effective approach for tilt testing. OBJECTIVES: Our clinical investigation was undertaken to assess the diagnostic value in unexplained syncope of a simple and time-saving protocol for head-up tilt testing, including low-dose sublingual isosorbide dinitrate administration. PATIENTS AND METHODS: A group of 73 consecutive patients (43 women and 30 men, mean age 39.6+/-21.8 years) with unexplained syncope despite conventional clinical cardiovascular and neurologic assessment and 10 asymptomatic control subjects underwent head-up tilt testing with isosorbide dinitrate challenge. Participants were tilted at 60 degrees for 30 minutes without medication; if no symptoms occurred, 1.25 mg of isosorbide dinitrate was administered sublingually and tilting was continued for an additional 15 minutes. RESULTS: During the drug-free phase of the test 14 (19.2%) patients had syncope. After isosorbide dinitrate administration syncope occurred in another 28 patients (38.3%); minor symptoms in association with hypotension developed in 10 (13.7%) patients. The test result was negative in all control subjects. The positive rate and specificity of head-up tilt testing with isosorbide dinitrate provocation were 57.5% and 100%, respectively. CONCLUSIONS: This new practical diagnostic procedure was found to be fairly sensitive and clearly specific in inducing a vasovagal reflex in patients with syncope of uncertain origin. Consequently, such approach could give a significant contribution in the diagnostic workup of these patients.

Transoesophageal low-energy cardioversion of atrial fibrillation. Results with the oesophageal-right atrial lead configuration.

BACKGROUND: Low energy internal cardioversion is a safe and effective procedure to restore sinus rhythm in patients with atrial fibrillation refractory to external cardioversion. However the procedure is invasive and fluoroscopy is mandatory. Aim of the study To assess the efficacy, safety and tolerability of a new simplified procedure of low energy internal cardioversion. METHODS: Twenty-five consecutive patients (19 males and 6 females) with persistent atrial fibrillation were submitted to low energy internal cardioversion using a step-up protocol (in steps of 50 V, starting from 300 V). A large surface area lead (cathode) was positioned in the oesophagus, 45 cm from the nasal orifice. A second large surface area lead (anode) was positioned in the right atrium. A quadripolar lead was positioned at the right ventricular apex to achieve ventricular synchronization and back-up pacing. Oesophageal endoscopy was performed within 24 h of the end of the procedure and repeated after 48 h, if injury to the oesophageal mucosa had occurred. RESULTS: Sinus rhythm was restored in 23 patients (92%) with a mean delivered energy of 15.74 J (range 5-27) and a mean impedance of 48 Omega. In two patients, endoscopy revealed that small burns had occurred in the oesophageal mucosa. Such lesions spontaneously healed after 48 h. CONCLUSIONS: This new technique of performing low energy internal cardioversion is effective and safe and avoids the positioning of a lead in the coronary sinus or in the left pulmonary artery, thereby simplifying the procedure.

DDD pacing with rate drop response function versus DDI with rate hysteresis pacing for cardioinhibitory vasovagal syncope.

BACKGROUND: The effectiveness of cardiac pacing in preventing vasovagal syncope remains controversial. However, DDI pacing with rate hysteresis has been reported to prevent the recurrence of cardioinhibitory vasovagal syncope in up to 35% of affected subjects and to reduce the overall incidence of syncopal episodes in the others. Recently, DDD pacing with a new promising rate drop response function (Medtronic Thera-I model 7960) has become available in clinical practice. AIM OF THE STUDY: The aim of the present open trial was to test the effectiveness of this new pacing modality in patients with cardioinhibitory vasovagal syncope. STUDY POPULATION AND METHODS: The study population included 20 patients (12 males and 8 females; mean age 61.1 +/- 14 yrs) with recurrent syncope (mean number of prior episode = 6.8, range 5-11) and cardioinhibitory responses during two head-up tilt tests: the first diagnostic and the second during drug therapy with either beta-blockade or etilephrine. The study patients were randomized to receive either DDI pacing with rate hysteresis (8 patients) or DDD pacing with rate drop response function (11 patients). The head-up tilt test performed 1 month after pacemaker implantation was positive in 3 of 12 patients (25%) with DDD pacing with rate drop response function and in 5 of 8 patients (62.5%) with DDI pacing with rate hysteresis. The mean duration of follow-up was 17.7 +/- 7.4 months. During follow-up no patients with a DDD pacemaker with rate drop response function had syncope, while 3 of 8 patients with a DDI pacemaker with rate hysteresis had recurrence of syncope (P < 0.05). CONCLUSIONS: These data suggest that DDD pacing with rate drop response function is effective in cardioinhibitory vasovagal syncope and may be preferable to DDI pacing with rate hysteresis.

Transthoracic DC shock may represent a serious hazard in pacemaker dependent patients.

External defibrillation is widely used for the termination of various atrial and ventricular tachyarrhythmias, including pacemaker patients. Our study was intended to evaluate the effects of DC shocks in 36 patients with unipolar pacemakers implanted in the right pectoral region (25 DDD, 10 VVI, 3 AAI). The shocks were delivered with paddles on the anterior surface of the thorax, as far as possible away from the pacemaker. The pacing output was programmed at 0.5 msec and 5 V (25 patients), 4 V (1 patient), and 2.5 V (10 patients). Transient loss of capture occurred in 18 patients (50%). These patients, compared with those without capture failure, received higher peak and cumulative shock energies, respectively, 216 +/- 99 versus 123 +/- 50 joules (P < 0.002) and 352 +/- 62 versus 147 +/- 98 joules (P < 0.004) and had a lower pacemaker pulse amplitude (4.0 +/- 1.2 vs 4.6 +/- 1.0 V, P = 0.11). Failure to capture lasted from 5 seconds to 30 minutes (mean 157 sec). In 15 patients the ventricular stimulation threshold was measured before and serially after cardioversion. A six-fold threshold increase was observed 3 minutes after the shock (P < 0.004) with gradual recovery to nearly baseline values at 24 hours. Transient sensing failure occurred in 7 of the 17 patients in whom it could be evaluated (41%). Furthermore, three cases of shock induced pacemaker malfunctions were observed requiring replacement of the stimulator in two patients. In conclusion, the incidence of loss of capture in pacemaker patients subjected to electrical cardioversion/defibrillation is high.(ABSTRACT TRUNCATED AT 250 WORDS)