A Case of Cross Stimulation

Cross stimulation in a dual chamber pacing system, in which the atrial stimulus intermittently captured the right ventricle, occurred immediately after pacemaker implantation in a 71-year-old man. It was prevented temporarily by reducing the pacing rate so that P wave synchronous ventricular (VDD) pacing resulted and by reducing the output of the atrial circuit from 5 to 4 volts. Cross stimulation disappeared spontaneously 14 days after surgery.     

The Combined Transvenous Implantation of Cardioverter Defibrillators and Permanent Pacemakers

We developed criteria for implantation and programming of permanent endocardial pacemakers in patients with a nonthoracotomy ICD system. These criteria were prospectively used in 10 patients who recieved an ICD prior to (n = 5) or following (n = 5) implantation of a dual chamber (n = 6) or ventricular (n = 4) pacemaker with a unipolar (n = 4) or bipolar (n = 6) lead configuration. All patients were tested for interactions or malfunctions. Undersensing of ventricular fibrillation by the atrial sense amplifier and inadequate atrial pacing occurred in one patient with a unipolar dual chamber system programmed to AAIR but didn't impair ICD sensing. Transient or permanent loss of capture or sensing of the pacemaker was not observed after ICD shocks with the output programmed to double pulse width and voltage of stimulation threshold and the sensitivity to 50% of the detected R wave. One episode of transient reprogramming occurred without clinical consequences. One unipolar ventricular pacemaker lead had to be exchanged against a bipolar lead because of oversensing of the pacing artifact by the ICD. There was no failure of an ICD to detect ventricular arrhythmias due to inadequate pacemaker activity. During a follow-up period of 21 ± 11 months, a total of 78 ventricular arrhythmias were effectively treated in six patients. Thus, a combined use of transvenous ICD and pacemaker is possible despite the close vicinity of pacing and defibrillations leads. Optimized programming different to the common settings is required. As interactions occurred only in unipolar pacemaker leads bipolar systems should be used in these patients.     

European clinical experience with a dual chamber single pass sensing and pacing defibrillation lead

Dual chamber ICDs are increasingly implanted nowadays, mainly to improve discrimination between supraventricular and ventricular arrhythmias but also to maintain AV synchrony in patients with bradycardia. The aim of this study was to investigate a new single pass right ventricular defibrillation lead capable of true bipolar sensing and pacing in the right atrium and integrated bipolar sensing and pacing in the right ventricle. The performance of the lead was evaluated in 57 patients (age 61 +/- 12 years; New York Heart Association 1.9 +/- 0.6, left ventricular ejection fraction 0.38 +/- 0.15) at implant, at prehospital discharge, and during a 1-year follow-up. Sensing and pacing behavior of the lead was evaluated in six different body positions. In four patients, no stable position of the atrial electrode could intraoperatively be found. The intraoperative atrial sensing was 2.3 +/- 1.6 mV and the atrial pacing threshold 0.8 +/- 0.5 V at 0.5 ms. At follow-up, the atrial sensing ranged from 1.5 mV to 2.2 mV and the atrial pacing threshold product from 0.8 to 1.7 V/ms. In 11 patients, an intermittent atrial sensing problem and in 24 patients an atrial pacing dysfunction were observed in at least one body position. In 565 episodes, a sensitivity of 100% and a specificity of 96.5% were found for ventricular arrhythmias. In conclusion, this single pass defibrillation lead performed well as a VDD lead and for dual chamber arrhythmia discrimination. However, loss of atrial capture in 45% of patients preclude its use in patients depending on atrial pacing.     

Chronic Steroid-Eluting Lead Performance: A Comparison of Atrial and Ventricular Pacing

It is generally believed that atrial pacing leads have higher stimulation thresholds and long-term complication rates than ventricular leads, and this is one of the factors limiting the use of dual chamber pacing. A study was undertaken to compare atrial and ventricular bipolar tined steroid-eluting leads in two designs: the Medtronic CapSure SP and the Telectronics Encor Dec. There were 123 pairs of leads: 81 CapSure SP and 42 Encor Dec. Bipolar atrial and ventricular stimulation thresholds, electrograms. and pacing impedance were measured using the Telectronics META DDDR pulse generator immediately postimplantation, and at 1, 3, and 6 months for all leads and at 12, 18, and 24 months for the CapSure SP. The only major lead complication was a 2% atrial lead dislodgment rate. All leads demonstrated low stimulation thresholds, with the CapSure SP leads having lower values than comparable Encor Dec leads. All leads had a mean range of 0.53–0.89 V at all testing periods with P < 0.05 for atrial leads only. There were no differences in electrogram size between manufacturers and no instances of atrial and ventricular undersensing. Pacing impedance was about 100 Ω higher for the Encor Dec leads (P < 0.05, atrial leads only), suggesting that these leads will result in lower pacing energy losses provided the pulse generators are at identical settings. More than 90% of patients could be paced chronically in the atrium and ventricle at 2.5 V, but for chronic 1.6-V pacing, the CapSure SP leads were superior. In conclusion, atrial and ventricular steroid-eluting leads of both manufacturers gave excellent stimulation threshold results allowing low energy dual chamber pacing.     

Unexpected loss of bipolar lead function associated with lead connection design in dual chamber pacemakers

Unexpected loss of bipolar pacing function can occur in Guidant VIGOR dual chamber pacemakers with QC2 (Quick Connect) headers. In a retrospective review of 305 implanted patients, 23 exhibited unexpected loss of bipolar pacing lead function from < 1 to 51 months postimplant. Atrial lead problems were seen in 17 patients and ventricular lead problems in 6 (P < 0.05 atrial vs ventricular). Significant symptoms developed in six patients including two with syncope as a result of noncapture. No significant change in bipolar sensing function was seen. Reprogramming devices to the unipolar mode restored appropriate pacing thresholds and lead impedance. The changes in bipolar lead function are likely caused by failure of the QC2 header lead connection system over time.     

VDD Single Pass Lead Pacing; Sustained Pacemaker Mediated Tachycardias Unrelated to Retrograde Atrial Activation

Pacemaker mediated tachycardias (PMTs) are a well known complication of P synchronous pacing. Although the initiating mechanisms are severel, all of them are associated with retrograde atrial activation, which is sensed by the atrial sensing channel, resulting in ventricular pacing. In 19 patients suffering from symptomatic AV conduction disturbances and normal sinus node function, a VDD pacing system connected to a single pass ventricular lead with dual chamber electrodes was implanted. The bipolar atrial electrode, floating in the right atrium, was used to detect endocardial atrial electrograms that were differentially processed within the pacemaker for optimal discrimination and filtering of undesirable signals. The widely programmable atrial sensitivity (amplitude and filtering) allowed stable P synchronized ventricular pacing in all patients, but in five of them, sustained PMTs not related to retrograde atrial activation was documented during the follow-up. The common mechanism for the onset and maintenance of these PMTs was traced to the abnormal sensing of the terminal forces of ventricular activation and for of the T wave. The possibility of interferences between ventricular and atrial electrodes (crosstalk) was also considered, The reduction of atrial channel sensitivity represented in all cases the only effective procedure to prevent this type of PMT. In conclusion, the best signal to noise ratio is an important endpoint (o assure the proper function of a single lead VDD pacing system. Furthermore, using the differential amplifier built within the pacemaker, consideration should be given to the optimal mode of rejection of the terminal forces of the QRS and T wave.     

Single Lead Atrial Synchronous Ventricular Pacing: A Dream Come True

Single lead, atrial synchronous pacing systems were developed in the late 1970s. Clinical experience has demonstrated the need to position the "floating" atrial electrode in the mid-to-high right atrium and the need for a specially designed pulse generator (with very high atrial sensitivity) to provide a high quality and amplitude atrial electrogram for consistent sensing. A 12-year experience with different electrode configurations, from the first unipolar designed in 1980 to the most recent atrial bipolar electrodes, has confirmed the validity of the original concept and the long-term reliability of the single lead atrial synchronous pacing system, which can reliably produce long-term atrial sensing and ventricular stimulation in the presence of normal sinoatrial function.     

Combined Third-Generation Implantable Cardioverter Defibrillator with Permanent Unipolar Pacemakers: Preliminary Observations

As implantable Cardioverter defibrillators (ICDs) are strictly contraindicated in the presence of unipolar pacemakers, currently available options in patients having such chronic pacing systems include: abandoning the implanted pacemaker and selecting an ICD with ventricular demand (VVI) pacing; or replacing the chronic (dual chamber) unipolar pacing system with a dedicated bipolar version prior to ICD implantation. In three patients with previously implanted unipolar pacemakers, we challenged the premise that all ICD systems are incompatible by combining with a third-generation transvenous ICD system (Medtronic 7217B PCD® incorporating true bipolar sensing, a self-limiting auto-adjusting sensitivity, and a tolerant VF detection algorithm. The potential for pace-maker-ICD interaction was minimized by separating the tip of the ICDs transvenous right ventricular pace/sense-defihrillation coil lead from that of the chronic pacemaker lead by > 2–3 cm, and by performing “worst case” intraoperative testing. Although ICD double-counting of the dual chamber pacemaker's atrial and ventricular pacing spikes could be provoked at extreme high output settings, it did not occur at clinically appropriate settings. More importantly, continuous high output asynchronous pacing during ventricular fibrillation (VF) did not interfere with ICD detection. During a mean follow-up period of 18 months, one patient has had VF appropriately terminated bv the ICD. In the remaining two patients, proper VF detection and ICD function was reassessed at 3 months and/or at 1 year during noninvasive testing. Conclusion: These preliminary findings demonstrate that this transvenous ICD system's VF sensing and detection features combined with careful implant technique, rigorous “worst case” testing for possible pacemaker-ICD interaction with regular follow-up, may permit implantation of this ICD system in patients with chronic unipolar pacing systems. Further studies are needed to validate the long-term clinical safety of this promising revised approach to a currently contraindicated device combination.     

Time Course of Transvenous Pacemaker Stimulation Impedance, Capture Threshold, and Electrogram Amplitude

To examine the time course of atrial and ventricular stimulation impedance, capture threshold, and electrogram amplitude, we obtained noninvasive telemetric data in 63 patients who underwent implantation of unipolar, endocardial pacing leads and a second-generation dual chamber pacemaker with expanded bidirectional telemetry, including stimulation impedance, endocardial electrograms, and automatic capture threshold determination. On follow-up of 9-20 months (mean, 15 months), all but six patients continued to pace in the DDD mode. To validate measurements made with telemetry, invasive measurements made directly with a pacing system analyzer at time of implant were compared with immediate postimplant telemetric measurements. Significant correlation of acute stimulation impedance was noted in both atrial (r = .7, p less than .001) and ventricular (r = .8, p less than .001) lead systems. The atrial stimulation impedance decreased from 538 ohms at implant to 471 ohms at 13 months (p less than .01); the ventricular stimulation impedance similarly declined from 545 ohms to 485 ohms at 13 months (p less than .01). Capture thresholds peaked at one month, then declined: atrial, 1.2 V at implant vs 2.2 V at 1 month (p less than .008) and 1.4 V at 13 months; ventricular, 1.1 V at implant vs 1.9 V at 1 month (p less than .001) and 1.3 V at 13 months. There were no significant changes noted in atrial or ventricular electrogram amplitude following implantation. We conclude that there is close correlation of invasive recordings with those made telemetrically with this pacemaker at time of implant.(ABSTRACT TRUNCATED AT 250 WORDS)     

Single Pass Lead VDD Pacing in Children and Adolescents

Implantation of permanent pacemakers in children and adolescents is possible but usually is limited to single chamber generators. The natural growth of these patients may require physiological pacing, but until recently two leads were required for this type of pacing. The single pass lead VDD pacing mode makes posible physiological pacing by using only one lead, for both atrial sensing and ventricular sensing and pacing. The feasibility of VDD pacing using endocardial lead was evaluated in 16 children and adolescents with congenital or postsurgical atrioventricular block. Their mean age was 7.9 ± 4.9 years (range 1-16 years) and the smallest patient's weight was 8.2 kg. In all the patients, a single pass pacing lead with atrial sensing rings and bipolar ventricular pacing and sensing capability was implanted through the left or right subclavian vein. The pacemaker generator was implanted in a rectopectoral position. The mean atrial electrogram during the implantation was 4.2 ± 2.1 mV and 2.6 ± 1.9 mV after a mean of 1 week. The ventricular pacing threshold was 0.5 ± 0.2 V; the ventricular pacing impedance was 560 ± 95 Ω and the ventricular electrogram amplitude was 9.9 ± 2.1 mV. This is a first report to demonstrate the feasibility of atrial synchronous ventricular endocardial pacing using a single pass lead in a relatively large group of children and adolescents.     

Atrioventricular cross-talk in biventricular pacing: a potential cause of ventricular standstill

The aim of the study was to evaluate, in recipients of biventricular pacing systems, the risk of asystole due to ventricular pacing inhibition by sensing the left atrial signals by the LV lead at conventional sensitivity. Long-term ventricular sensitivity was programmed at > or = 4 m V in 17 consecutive recipients of ventricular resynchronization systems implanted for chronic management of congestive heart failure. Ventricular pacing inhibition due to AV cross-talk on spontaneous left atrial electrogram (AVCSA) was tested at a 2 mV ventricular sensitivity immediately after implantation of the stimulation system and 1 month later. Pacemaker dependence was also tested during temporary VVI pacing at a rate of 30 beats/min. AVCSA was observed in three patients. It was present on the day of implantation in one patient, and developed within the first month in two others. Asystole was observed in two of the three cases of AVCSA. Three pacemaker nondependent patients at the time of system implantation had become pacemaker dependent at 1 month. AVCSA was observed only with LV leads positioned in the great cardiac vein. In conclusion, asystole due to AVCSA was observed in 11% of recipients of ventricular resynchronization stimulation systems. Care should be taken in these patients to minimize the risk of atrial sensing by the LV lead, preferably avoiding its placement in the great cardiac vein. This phenomenon could be eliminated by the programmability of a right ventricular only sensing configuration.     

Amplitude and Direction of Atrial Depolarization Using a Multipolar Floating Catheter: Principles for a Single Lead VDD Pacing

VDD stimulation using a single catheter for atrial sensing and ventricular sensing and pacing has become a reality. In order to compare the quality of the cavitary atrial electrogram (AEG) and to determine the intraatrial P wave direction and conduction time (CT), we compared, in an acute study, three different types of atrial electrode systems using four different leads, in 53 patients in sinus rhythm. The three electrode systems were: (1) one experimental system with quadripolar orthogonal electrodes using the Goldreyer concept; (2) one experimental system with quadripolar whole ring electrodes; (3) two systems with diagonally oriented half-ring electrodes, one experimental quadripolar and one bipolar CCS commercial (Polysafe A-Track lead). For the experimental systems, the four electrodes forming two independent bipolar pairs were situated on the intraatrial floating portion of a single lead and one supplemental electrode was distally positioned in the right ventricular apex. Bipolar AEGs were recorded at the high and at the low levels of the right atrium. For the CCS lead, the single bipolar AEG was recorded at the high level of the right atrium only. The highest AEG amplitude and the highest values for ventricular far-field rejection were provided by both diagonally oriented half-ring electrodes at the high atrial level and by the whole ring electrodes at the low atrial level. For both atrial levels, the orthogonal electrode system provided the smallest AEG amplitudes, the highest ventricular electrogram amplitudes, and therefore, the smallest values for ventricular far-field rejection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Complications after Single versus Dual Chamber Pacemaker Implantation

To compare the complication rate in patients having a dual chamber versus a single chamber pacing system, 337 consecutive procedures performed during a 3-year period were analyzed prospectively. Two hundred fifty-eight patients (77%) received a VVI pacemaker and 75 (23%) a DDD unit. Thirteen VVI (5%) and 4 DDD (5.3%) needed reintervention. Lead displacement with reoperation was required for three ventricular leads (1%) and one atrial lead (1.3%). Infection occurred in two VVI units (0.77%) and one DDD (1.33%) unit. Muscular stimulation was noticed among three DDD (4%) and nine VVI systems (3.5%). Urgent reprogramming was needed for 23 VVI (9%) and six DDD units (8%). There was no increase in complications with dual chamber pacing compared to single chamber systems.     

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.     

Analysis of mode switching algorithms in dual chamber pacemakers

Dual chamber systems are currently implanted in an increasing number of patients with a pacing indication and paroxysmal atrial tachyarrhythmias (ATAs). To avoid tracking of high atrial rates during ATA while providing AV synchronous pacing during sinus rhythm and AV block, automatic mode switching (MS) to a nontracking mode has been developed. Several concepts of MS algorithms have been introduced differing in sensitivity and specificity of ATA detection, delay between ATA onset and MS, pacing mode during MS, rapidity of MS termination upon detection of sinus rhythm, and special algorithms pertinent to MS. This review classifies and analyzes algorithms that are integrated into MS concepts and outlines considerations on optimal MS performance. Based on simulations and clinical studies, fast MS concepts avoiding high paced ventricular rates seem preferable but require special functions to avoid loss of AV synchrony by inappropriate MS. Similarly, fast return to tracking mode after ATA cessation seems preferable but requires algorithms to prevent mode oscillations. Sudden ventricular rate changes have to be avoided by rate smoothing functions, and an appropriate ventricular rate during nontracking mode has to be provided by rate responsive pacing. Programming of the device includes individual optimization of atrial sensitivity, atrial blanking times, and tachycardia detection rates. In addition, a high signal-to-noise ratio by careful atrial lead implantation, narrowly spaced bipolar atrial leads and a good sensing amplifier, together with special algorithms for atrial sensing adjustment improve the performance of MS algorithms.     

A Gastroesophageal Electrode for Atrial and Ventricular Pacing

Temporary transvenous cardiac pacing requires technical expertise and access to fluoroscopy. We have developed a gastroesophageal electrode capable of atrial and ventricular pacing. The flexible polythene gastroesophageal electrode is passed into the stomach under light sedation. Five ring electrodes, now positioned in the lower esophagus, are used for atrial pacing. A point source (cathode) on the distal tip of the electrode, now positioned in the gastric fundus. is used for ventricular pacing. Two configurations of atrial and ventricular pacing were compared: unipolar and bipolar. During unipolar ventricular pacing the indifferent electrode (anode) was a high impedance chest pad. For bipolar ventricular pacing the indifferent electrode was a ring electrode placed 2 cm proximal to the tip. Unipolar atrial pacing was performed with 1 of 5 proximal ring electrodes acting as cathode ("cathodic") or as anode ("anodic") in conjunction with a chest pad. Bipolar atrial pacing was performed using combinations of 2 of 5 ring electrodes. Atrial capture was obtained in all 55 subjects attempted. When all electrode combinations were compared, atrial capture was significantly more frequent using the bipolar approach (153/210 bipolar, 65/210 unipolar; t = 7.37, P < 0.001). For unipolar atrial pacing, cathodic stimulation (from esophagus) was more successful than anodic stimulation (cathodic 62/105, anodic 20/105; t = 5.81, P < 0.001). In 43 subjects attempted unipolar ventricular pacing resulted in a higher frequency of capture than the bipolar approach (unipolar 41/43 (95.3%), bipolar 19/43 (44.2%); P < 0.001). In conclusion, atrial pacing was optimal using pairs of ring electrodes ("bipolar") while ventricular pacing was optimal using the distal electrode tip (cathode) in conjunction with a chest pad electrode ("unipolar"). This gastroesophageal electrode may be useful in the emergency management of acute bradyarrhythmias and for elective electrophysiological studies.     

Electrocardiographic Characteristics of Pacing From the Right Atrial Appendage During Atrioventricular Sequential Pacing

In the interpretation of electrocardiograms recorded during atrioventricular sequential pacing, uncertainty frequently arises in the assessment for evidence of atrial capture. In the present study, electrocardiographic characterization of pacing from the right atrial appendage as a component of atrioventricular pacing was performed on tracings obtained from 16 patients with bipolar dual chamber pacing units, and from 18 patients with unipolar dual chamber pacing units in which large overshoot potentials occurred following the atrial pacing spike. Atrial complexes resulting from bipolar pacing of the right atrial appendage were found to be uniformly prolonged and of diminished amplitude compared to those in sinus rhythm; they were also noted to contain sequential inferoposterior and leftward-posterior component vectors. The exponential overshoot-decay complex associated with unipolar atrial pacing appeared as a vector directed along the axis from the pulse generator to the pacing lead; the degree to which this deflection interfered with identification of atrial capture in various leads was thus largely dependent on pulse generator location. It was concluded that careful systematic inspection of multiple electrocardiographic leads will generally permit the characteristic features of pacing the right atrial appendage to be recognized, thus facilitating correct interpretation of atrial capture during atrioventricular sequential pacing.     

Differential Bipolar Sensing of a Dual Chamber Pacemaker

Differential bipolar sensing was evaluated in 10 consecutive patients with symptomatic heart block managed with dual chamber pacing. During pacemaker implantation atrial and ventricular electrograms were recorded using unipolar (UP) and differential bipolar (DBP) sensing amplifiers. The mean peak-to-peak amplitudes of the UP and DBP atrial electrograms were 3.3 +/- 1.2 mV and 4.2 +/- 1.2 mV, respectively. The difference was statistically significant (p less than 0.05). The mean peak-to-peak amplitudes of the ventricular electrograms were, respectively, 6.8 +/- 1.5 mV and 7.5 +/- 1.4 mV (p less than 0.01). Within 6 weeks after pacemaker implantation, patients visited the outpatient clinic. Isometric exercise tests were performed during UP and DBP sensing of the pacing system. Myopotential sensing in the ventricle occurred in nine patients during UP sensing and in none of the patients during DBP sensing (p less than 0.01) at a sensitivity setting of 0.5 mV. In addition, chest wall stimulation was performed to assess the effects of far-field signals on the ventricular sensing circuit of the pulse generator. Chest wall stimuli inhibited ventricular output during UP sensing in all 10 patients, whereas during DBP sensing inhibition of the ventricular channel occurred in three patients and then only at high output (greater than 8 V) settings. The susceptibility of the pacing system to crosstalk was also determined. However, neither during UP sensing nor during DBP sensing could cross-stimulation or cross-inhibition be demonstrated. In conclusion, DBP sensing is superior to UP sensing in terms of myopotential and far-field sensing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intermittent Pacemaker Syndrome: Revision of VVI Pacemaker to a New Cardiac Pacing Mode for Tachy-Brady Syndrome

A patient with tachy-brady syndrome manifested by paroxysmal atrial fibrillation and symptomatic sinus bradycardia and treated by VVI pacing developed pacemaker syndrome during episodes of ventricular pacing. His cardiac pacemaker was revised to a dual chamber system utilizing the new AV sequential DDI pacing mode which eliminated pacemaker-related tachycardias and totally abolished the pacemaker syndrome symptoms. There have been no further episodes of atrial fibrillation, possibly due to elimination of temporal dispersion of refractory periods during bradycardia. The propensity for atrial fibrillation has also been minimized by excluding competitive atrial stimulation during DVI pacing. The DDI mode provides the clinician increased utility and flexibility in the use of AV sequential pacing therapy.     

Cross-Stimulation: The Unexpected Stimulation of the Unpaced Chamber

The ability to stimulate one chamber through a lead or output circuit to the opposite cardiac chamber is termed cross-stimulation. Three examples of this phenomenon are presented. The first involves the close proximity of the atrial lead to the ventricular myocardium with ventricular capture occurring at sufficiently high outputs; the second is due to the basic design of dual unipolar pacing systems which have output circuits that share a common anode; the third is a self-limited eccentricity of one device that occurs only during the first phase of magnet-induced asynchronous pacing. The mechanism and clinical significance of these observations are discussed.