Stability of the DDD Pacing Mode in Patients 80 Years of Age and Older

We reviewed the records of 119 consecutive patients aged 80 years or older (mean age 84 ± 3.7 years) in whom a dual chamber pacemaker was implanted between 1984 and 1991. Follow-up data was available up to February 1993. Immediate postimplantation complications were rare. Nine patients were lost to follow-up, all within 6 months of implantation. An additional seven patients died within 6 months of implantation. Long-term follow up for at least 6 months from implantation was available for 103 of the 119 patients (87%). Of these 89 (66%) remained in functioning DDD mode for a mean of 22 ± 15 months from implantation. Nine patients were reprogrammed to VVI mode, six due to atrial fibrillation and three due to failure of atrial sensing or pacing. One patient was programmed DVI for failure of atrial sensing; 94 of 112 patients (84%) whose status was definitely known in February 1993 remained in functioning DDD mode until death or last follow-up. Cumulative survival in DDD mode was 78% at 30 months. We conclude that DDD pacing is stable in the great majority of patients in their nineth and tenth decades who present with rhythms amenable to dual chamber pacing and who have no history of sustained atrial fibrillation.     

Efficacy and Safety of Bipolar Sensing with High Atrial Sensitivity in Dual Chamber Pacemakers

In dual chamber pacemakers, atrial sensing performance is decisive for maintenance of AV synchrony. Particularly, the efficacy of mode switching algorithms during intermittent atrial tachyarrhythmias depends on the sensitive detection of low potential amplitudes. Therefore, a high atrial sensitivity of 0.18 mV, commonly used in single lead VDD pacemakers, was investigated for its efficacy and safety in DDD pacing. Thirty patients received dual chamber pacemakers and bipolar atrial screw‐in leads for sinus node syndrome or AV block; 15 patients suffered from intermittent atrial fibrillation. Pace makers were programmed to an atrial sensitivity of 0.18 mV. Two weeks, 3, 9, and 15 months after implantation, P wave sensing threshold and T wave oversensing thresholds for the native and paced T wave were determined. The myopotential oversensing thresholds were evaluated by isometric contraction of the pectoral muscles. Automatic mode switch to DDIR pacing was activated when the mean atrial rate exceeded 180 beats/min. The patients were followed by 24‐hour Holter monitoring. Two weeks after implantation, mean atrial sensing threshold was 1.81 ± 0.85 mV (range 0.25–2.8 mV) without significant differences during further follow‐up. Native T wave sensing threshold was < 0.18 mV in all patients. In 13% of patients, paced T waves were perceived in the atrial channel at the highest sensitivity. This T wave sensing could easily be avoided by programming a postventricular atrial refractory period exceeding 300 ms. Myopotential oversensing could not be provoked and Holter records showed no signs of sensing dysfunction. During a 15‐month follow‐up, 1,191 mode switch events were counted by autodiagnostic pacemaker function. Forty‐two of these events occurred during Holter monitoring. Unjustified mode switch was not observed. In DDD pacemakers, bipolar atrial sensing with a very high sensitivity is efficient and safe. Using these sensitivity settings, activation of the mode switch algorithm almost completely avoids fast transmission of atrial rate to the ventricle during atrial fibrillation.     

Oversensing from Electrode ''Chatter'' in a Bipolar Pacing Lead: A Case Report

An 81-year-old man was treated for high degree AV block and syncope with an AV universal (DDD) pacemaker. Bipolar active fixation atrial and ventricular leads were used. Intermittent oversensing from the ventricular lead was detected on Holter monitoring following implantation. This resulted in inappropriate inhibition of ventricular pacing. Ventricular electrogram showed spurious signals between 5 mV and 15 mV in magnitude coincident with ventricular lead inhibition. Treatment consisted of reprogramming the device to the VVT mode. Six weeks after implantation normal pacemaker function in the DDD mode was demonstrated on Holter monitoring and inappropriate pacer inhibition could not be demonstrated. We postulate that electrode 'chatter' between the cathodal ring electrode and the helix of the ventricular lead resulted in artifactual potentials sufficient to inhibit pacing. We hypothesize that fibrosis and fixation of the ventricular lead tip over a period of weeks eliminated these electromechanical artifacts and resulted in the oversensing problem being self-limited. Physicians should be aware of electrode 'chatter' as a cause of pacemaker oversensing.     

Diagnosis of Atrial Undersensing in Dual Chamber Pacemakers: Impact of Autodiagnostic Features

Atrial undersensing occurs in a considerable number of patients, both with single lead VDD pacemakers and with DDD devices. The aim of this study was to investigate the diagnostic efficacy of electrocardiographic methods and autodiagnostic pacemaker features to detect atrial sensing dysfunction. Two hundred and thirty-one patients with AV block received single lead VDD pacemakers or DDD devices. Atrial sensitivity was programmed to 0.1 or 0.18 in VDD devices and to 0.5 mV in DDD devices; the rate limits were set to 40 and 160 beats/min. Twelve-lead ECG recording for 1 minute during deep respiration and change of body position, 24-hour Holter ECG recording, and treadmill exercise were performed 2 weeks and 15 months after pacemaker implantation. AV synchrony and, if available, P wave amplitude histogram were sampled by autodiagnostic pacemaker features and compared to electrocardiographic findings. Atrial undersensing was assumed, if AV synchrony was below 100% or if minimal P wave amplitude (PWA) was equal to the programmed atrial sensitivity. Intermittent atrial undersensing occurred in 20.7% of patients. The diagnostic sensitivities of the various methods used to detect atrial sensing failures were: 24-hour Holter monitoring 97.5%, P wave amplitude histogram 90.0%, stored AV synchrony 68.0% without significant difference between the various devices, treadmill exercise testing 58.8%, and 12-lead ECG recording 21.3%. In one patient, atrial undersensing was exclusively detected by exercise testing. In conclusion, autodiagnostic pacemaker features facilitate the evaluation of atrial sensing performance. However, to exclude intermittent atrial malsensing, routine Holter monitoring and treadmill exercise are still needed .     

A Simplified Approach to Temporary DDD Pacing Using a Single Lead, Balloon-Tipped Catheter with Overlapping Biphasic Impulse Stimulation

Temporary DDD pacing offers significant hemodynamic benefits in emergency management of bradyarrhythmias but is underused because of its complexity (two leads) and unreliability. Single lead VDD pacing with atrial sensing via a floating dipole is feasible, but atrial pacing is limited by high thresholds and phrenic nerve stimulation (PNS). Overlapping biphasic impulse (OLBI) stimulation may avoid these problems. The authors designed a single lead balloon-tipped catheter for temporary transvenous DDD pacing, incorporating noncontact atrial dipoles for OLBI stimulation. This catheter was deployed using fluoroscopic guidance in 74 patients (43 men, 31 women) with mean age 56.9 +/- 17.0 years. Pacing parameters were assessed at implantation and follow-up. The median procedural time was 6.6 (range 1.2-25.0) minutes and fluoroscopy time 1.9 (range 0.2-7.8) minutes. Stable VDD pacing was achieved in all cases. Atrial capture was achieved in 73 of 74 cases with both modes at maximum output but was restricted by PNS at outputs below atrial capture threshold in 3 of 74 cases with OLBI and 10 of 74 cases with standard bipolar mode (P = 0.04). At outputs > or = 1.0 V above atrial threshold, reliable DDD pacing without PNS was achieved and maintained in 67 (91%) of 74 patients in OLBI compared to 53 (72%) of 74 patients in bipolar mode (P = 0.003). Pacing parameters were stable during follow-up (median 53 hours, range 6-168 hours). In conclusion, the single lead catheter with OLBI stimulation allows temporary VDD and DDD pacing without PNS to be achieved in > 90% of patients. This rapid and convenient approach should facilitate DDD pacing in emergency settings.     

Dual Chamber Pacing with a Single-Lead DDD Pacing System

The successful application of single-lead VDD pacing during the last few years has generated the idea of single-lead DDD pacing. Preliminary data from several single-lead VDD studies attempting to pace the atrium by a floating atrial dipole are unsatisfactory, causing an unacceptably high current drain of the device. We studied the feasibility as well as the short- and long-term stability of atrioventricular sequential pacing, using a new single-pass, tined DDD lead. In eight consecutive patients (age 73+/-16 years) with symptomatic higher degree AV block and intact sinus node function, this new single-pass DDD lead was implanted in combination with a DDDR pacemaker. Correct VDD and DDD function was studied at implantation; at discharge; and at 1, 3, and 6 months of follow-up. At implant, the atrial stimulation threshold was 0.6+/-0.1 V/0.5 ms. During follow-up, the atrial pacing thresholds in different every day positions averaged 2.1+/-0.5 V at discharge, 2.9+/-0.5 V at 1 month, 3.8+/-0.4 V at 3 months, and 3.4+/-0.4 V at 6 months (pulse width always 0.5 ms). The measured P wave amplitude at implantation was 4.5+/-2.2 mV; during follow-up the telemetered atrial sensitivity thresholds averaged 2.1+/-0.3 mV. Phrenic nerve stimulation at high output pacing (5.0 V/0.5 ms) was observed in three (38%) patients at discharge and in one (13%) patient during follow-up; an intermittent unmeasurable atrial lead impedance at 3 and 6 months follow-up was documented in one (13%) patient. This study confirms the possibility of short- and long-term DDD pacing using a single-pass DDD lead. Since atrial stimulation thresholds are still relatively high compared to conventional dual-lead DDD pacing, further improvements of the atrial electrodes are desirable, enabling lower pacing thresholds and optimizing energy requirements as well as minimizing the potential disadvantage of phrenic nerve stimulation.     

Atrial Lead Dislodgement with a DDD Pacemaker

A 48-year-old man with previous aortic valve surgery and aortic root repair had a DDD pacemaker inserted (using transvenous leads) for the treatment of complete heart block. An atrial J active fixation electrode was used. Four weeks following implantation the patient returned with an unusual electrocardiographic rhythm demonstrating two separate QRS morphologies. Both PA and lateral chest x-ray failed to demonstrate a change in lead position. The MARKER CHANNEL of the DDD pulse generator confirmed that the alternating QRS morphologies were due to atrial lead dislodgement. Although uncommon, displacement of atrial active fixation leads may occur and lead to unusual electrocardiographic rhythms. Use of a MARKER CHANNEL may aid in the diagnosis.     

VDD Pacing with a Previously Implanted Single Lead System

Three patients who had undergone implantation of a rate modulated, afrial sensitive RS4 pacemaker, with a single orthogonal lead underwent replacement of a depleted unit with a DDD pulse generator, reusing the original lead with an adapter that allowed conversion of the bipolar atrial electrode into unipolar configuration. The mean atrial electrogram amplitude was 1,8 mV and no significant atrial sensing defects were found during Holler monitoring. As the RS4 pulse generator is no longer available, continued VDD pacing is possible by replacing it with a DDD pulse generator using the previously implanted single lead system.     

Efficacy of single lead VDD pacing in patients with impaired and normal left ventricular function

Atrial synchronous ventricular pacing seems to be the best pacing mode for patients with advanced AV block and impaired LV function. The long-term follow-up of single lead VDD pacing was studied in 33 patients with impaired LV function and compared to 42 patients with normal LV function. All patients received the same VDD lead and VDDR pacemaker. The lead model with 13-cm AV spacing between the atrial and ventricular electrode was implanted in 89% of the patients. Follow-ups were 1, 3, 6, and 12 months after implantation. The percentage of atrial sensing and the P wave amplitude were determined at each follow-up. Minimal P wave amplitude at implantation was 2.0 +/- 1.4 mV in patients with impaired and 1.7 +/- 0.9 mV with normal LV function (not significant). At the 12-month follow-up, 33 patients with normal and 23 patients with depressed LV function remained paced in the VDD mode. The remaining patients died in five (impaired LV function) and seven cases (normal LV function) or their pacemakers were programmed to the VVI/VVIR pacing mode in four (impaired LV function) and three cases (normal LV function). P wave amplitude did not differ in the two groups (e.g., at month 12: impaired: 1.17 +/- 0.42 mV; normal: 1.09 +/- 0.49 mV). The atrial sensitivity was programmed in most patients to sensitive settings with no differences between the two groups (e.g., at month 12: impaired: 0.13 +/- 0.06 mV; normal: 0.13 +/- 0.05 mV). The diagnostic counters indicated nearly permanent atrial sensing (e.g., at month 12: impaired: 99.3 +/- 2.2%; normal: 99.0 +/- 1.0 mV). In conclusions, single lead VDD pacing restored AV synchronous ventricular pacing in patients with normal and with impaired LV function indicating that it could be an alternative to DDD pacemakers, but not to dual-chamber pacing.     

Double Ventricular Pacing in a Patient with a "DDD" Universal Pacemaker

A patient with a DDD universal pacemaker presented with dyspnea. The electrocardiogram showed pacing artifacts both at the start of the QRS complex and 110 ms later. Also, 400 ms following sensed ventricular extrasystoles, there were normal single paced QHS complexes of slightly different morphology. An x-ray showed that the atrial electrode had displaced into the ventricle, and at exploration this was found to be due to late retraction of the electrode's active fixation helix. Following implantation of a new atrial lead, normal DDD pacing function was restored and the patient became asymptomatic.     

Long-term complication rates in ventricular, single lead VDD, and dual chamber pacing

A higher incidence of pacemaker related complications has been reported in DDD systems as compared to VVI devices. The implantation of single lead VDD pacemakers might reduce the complication rate of physiological pacing in patients with AV block. In a retrospective study, the data records of 1,214 consecutive patients with pacemaker implantation for AV block between 1990 and 2001 (VVI 36.5%, DDD 32.9%, VDD 30.6%) were analyzed. Complications requiring surgical interventions were compared during a follow-up period of 64 +/- 31 months. Operation and fluoroscopic times were longer in DDD pacemaker implantation compared to VDD and VVI devices:58 +/- 23 versus 39 +/- 10 and 37 +/- 13 minutes (P<0.001), 9.2 +/- 5.2 versus 4.1 +/- 2.4 and 3.5 +/- 2.3 minutes, respectively. Differences remained significant after correction for covariates. In a multivariate Cox regression model, the corrected complication hazard of a DDD pacemaker implantation was increased by 3.9 (1.4-11.3) compared to VVI and increased by 2.3 (1.1-4.5) compared to VDD pacing. Higher complication rates in DDD pacing were mainly due to a higher incidence of early reoperation for atrial lead dysfunction, whereas the long-term complication rate was not different from VDD or VVI pacing. Early and long-term complication rates did not differ between VDD and VVI pacemaker systems. In conclusion, operation time and complication rates of physiological pacing are reduced by VDD pacemaker implantation achieving values comparable to VVI pacing. Thus, single lead VDD pacing can be recommended for patients with AV block.     

Atrial Fibrillatory Electrogram Measurement Allows Atrial Lead Placement in Patients Who Develop Atrial Fibrillation During Permanent Dual Chamber Pacemaker Implantation

The benefit of DDD(R) pacing is proven even in patients with intermittent atrial fibrillation. Atrial fibrillation developing during dual chamber pacemaker implantation creates a difficult problem. Maneuvers to reestablish a stable atrial rhythm often are required if atrial fibrillation sets in. This study was performed to determine if atrial lead placement can be performed with acceptable long-term results in the presence of atrial fibrillation. Twenty-one patients in whom atrial fibrillation developed during permanent pacemaker implantation were included in this study. In 12 patients, episodes of intermittent atrial fibrillation had been documented before the procedure. Screw-in leads were used in 15 patients and J-shaped passive fixation leads in 6 patients. AH leads were bipolar. The intraoperative atrial fibrillation electrogram amplitudes ranged from 0.9 to 3.2 mV (mean 1.8 ± 0.6 mV). One patient required lead revision due to a high atrial pacing threshold after conversion to SR. One patient remained in atrial fibrillation at 3-month follow-up. The other 20 patients converted to SR, 11 of whom had intermittent atrial fibrillation with successful mode switch activation. P wave amplitudes were 2.8 ± 6 mV (range 1.4 to 4.0 mV) after conversion to SR. The mean atrial pacing threshold was 1.1 ± 0.5 V (range 0.5 to 3.5 V). Placement of atrial leads in patients who develop atrial fibrillation during pacemaker implantation is feasible; fibrillatory electrogram amplitudes showed a good correlation with the atrial signal after conversion to an organized atrial rhythm (r = 0.698). Acceptable atrial pacing thresholds can be expected as well.     

DDD Pacing: An Effective Treatment Modality for Recurrent Atrial Arrhythmias

We performed atrial EP studies (atrial substrate evaluation) on 10 patients. These patients had evidence of paroxysmal, sustained, recurrent atrial arrhythmias (7 men and 3 women with a mean age of 64 ± 15 years). All patients combined a brady-tachy syndrome; 7 patients had a sick sinus syndrome (SSS) and 3 patients a typical vagally induced atrial arrhythmia. No anti-arrhythmic drug was allowed in 3 patients with SSS, 1 drug failed in 4 patients and the combination of 2 drugs failed in 3 patients during the first to eighth years prior to pacemaker implantation. Atrial substrate evaluation was feasible in all these patients off anti-arrhythmic therapy and showed important abnormalities of atrial loco-regional conduction parameters and long refractory periods (RP). The remarkable point was, in 7 patients, a paradoxical improvement in intra-atrial conduction delay at rapid pacing rate. The DDD pacing mode was chosen in all patients. No technical problem occurred during implantation. Atrial pacing rate was programmed to be slightly higher than the mean diurnal heart rate calculated on Holter monitoring. After implantation, the mean follow-up period was 18 ± 25 months with an average of one Holter every 4 months during the first 2 years. The 7 patients who improved intra-atrial conduction at rapid pacing rate were controlled without drugs, 2 patients were controlled with 1 drug, and 1 patient with 2 drugs. Atrial pacing in the DDD mode in a selected group of patients prevents paroxysmal and drug-resistant atrial arrhythmias. Atrial substrate evaluation is a sensitive tool for assuring the long-term benefit of atrial pacing. In this subset of patients, maintenance of AV synchrony by DDD pacing is preferable to catheter ablation of the His bundle.     

不同起搏模式对病窦综合征患者血浆脑钠肽的影响

[目的]探讨心脏起搏器不同起搏模式对病态窦房结综合征（病窦综合征）患者血浆脑钠肽（BNP）水平的影响.[方法]常规植入起搏器的病窦综合征患者30例,根据不同起搏模式分为心房起搏组[AAI（R）组] 12例和双腔起搏组[DDD（R）组] 18例;分别于术前、术后3个月、术后6个月测定血浆BNP水平.[结果]术后3个月时两组血浆BNP水平均升高,但差异无显著性[（113.8±31.3） pg/mLvs （95.7±53.2） pg/mL,（92.8±87.2） pg/mL vs （70.1 74.6±69.3） pg/mL;均P〉0.05];6个月时AAI（R）组血浆BNP水平与3个月时[（84.8±73.7）pg/mL vs （113.8±31.3 ）pg/mL]比较无升高,而DDD（R）组血浆BNP水平较3个月时进一步升高[（98.8±61.7）pg/mL vs （92.8±87.2 ）pg/mL],但差异仍无统计学意义（P〉0.05）.[结论]病窦综合征患者采用心房起搏AAI（R）和双腔起搏DDD（R）两种工作模式血浆BNP水平虽无明显改变,但长期双腔起搏模式下的右心室心尖部起搏可能有导致血浆BNP水平增高的趋势.     

Single lead DDD pacing using electrodes with longitudinal and diagonal atrial floating dipoles

In this study, an assessment was made of the possibilities of single lead DDD pacing in two groups: a group of 15 patients in whom a lead with a longitudinal atrial "floating" dipole was implanted, and another group of 10 patients with a lead with a diagonal atrial "floating" dipole. In both groups, the electrodes were connected to a SLD generator. At discharge, atrial capture was achieved with the unipolar mode in 17 of 25 patients, whereas in the group carrying the longitudinal atrial dipole, atrial capture was achieved with the overlapping biphasic impulses (OLBI) system in 12 of 15 patients and in all 10 patients in the group with the diagonal atrial dipole. At 3 months, atrial capture was achieved with the unipolar mode in 13 of 22 patients (5.75 +/- 1.77 V/0.5 ms), whereas with the OLBI system atrial capture was achieved in 8 of 13 patients carrying the longitudinal atrial dipole (3.11 +/- 1.13 V/0.5 ms), and in 8 of the 9 patients carrying the diagonal atrial dipole (2.80 +/- 0.69 V/0.5 ms). In this study, the use of the OLBI system led to a significant reduction of atrial threshold (P < 0.0001). Phrenic stimulation is the main untoward effect reported during single lead DDD pacing, a lower incidence being detected in the group carrying the diagonal atrial dipole (10 vs 35.7%, P = NS). Other limitations of this form of pacing could result from a crossed stimulation phenomenon detected in a patient during single lead DDD pacing.     

具有自动搜索心房滞后功能的起搏器对房性心律失常近期影响研究

目的 探讨具有自动搜索心房滞后功能的起搏器对房性心律失常近期影响,评估其临床疗效和安全性.方法 选择植入Identity Adx DDD 5286型双腔起搏器病态窦房结综合征的患者43例.起搏器植入后不打开带自动搜索的心房滞后,保持起搏器出厂设置;术后3个月随访时程控为在DDD模式下打开带自动搜索的心房滞后,共随访6个月.比较起搏器植入前后及打开心房滞后模式后心房起搏百分比、动态心电图（DCG）房性心律失常发生情况.结果 与术前比较术后3个月DCG24 h房性早搏（简称房早）次数和房性心动过速（简称房速）、心房颤动（简称房颤）阵数增加（P＜0.05）;房早、房速和房颤的发作例数增加（P＜0.05）.打开具有自动搜索功能的心房滞后模式与未打开该模式比较：心房的起搏百分比明显降低[0.54（0.41,0.71）vs.0.82（0.65,0.93）,P＜0.05];DCG24h房早次数和房速、房颤阵数明显减少（P＜0.05）.房早、房速和房颤的发作例数减少（P＜0.05）.结论 具有自动搜索功能的心房滞后模式可以明显减少心房起搏的比例,减少房性心律失常的发生;打开具有自动搜索功能的心房滞后模式安全可靠.     

Determination of the optimal atrioventricular interval in sick sinus syndrome during DDD pacing

BACKGROUND: Although the AAI pacing mode has been shown to be electromechanically superior to the DDD pacing mode in sick sinus syndrome (SSS), there is evidence suggesting that during AAI pacing the presence of natural ventricular activation pattern is not enough for hemodynamic benefit to occur. Myocardial performance index (MPI) is a simply measurable Doppler-derived index of combined systolic and diastolic myocardial performance. The aim of this study was to investigate whether AAI pacing mode is electromechanically superior to the DDD mode in patients with SSS by using Doppler-derived MPI. METHODS: Thirty-nine SSS patients with dual-chamber pacing devices were evaluated by using Doppler echocardiography in AAI mode and DDD mode. The optimal atrioventricular (AV) interval in DDD mode was determined and atrial stimulus-R interval was measured in AAI mode. The ratio of the atrial stimulus-R interval to the optimal AV interval was defined as relative AV interval (rAVI) and the ratio of MPI in AAI mode to that in DDD mode was defined as relative MPI (rMPI). RESULTS: The rMPI was significantly correlated with atrial stimulus-R interval and rAVI (r = 0.57, P = 0.0002, and r = 0.67, P < 0.0001, respectively). A cutoff point of 1.73 for rAVI provided optimum sensitivity and specificity for rMPI >1 based on the receiver operator curves. CONCLUSIONS: Even though the intrinsic AV conduction is moderately prolonged, some SSS patients with dual-chamber pacing devices benefit from the ventricular pacing with optimal AV interval. MPI is useful to determine the optimal pacing mode in acute experiment.     

Management of Pacemaker Circus Movement Tachycardias

DDD pacemakers were implanted in 11 patients of whom 5 had the capacity to conduct retrogradely to the atrium. Methods to prevent or terminate pacemaker circus movement tachycardia (PCMT) were evaluated in these patients. V-A conduction was assessed before implantation by incremental right ventricular pacing while recording right atrial electrograms. Following implantation and at quarterly outpatient clinic visits, V-A conduction and ability to initiate and sustain PCMT were systematically assessed by non-invasive techniques. PCMT could be induced non-invasively in all 5 patients. The methods used to reduce and terminate the incidence of PCMT were: 1) decreasing the atrial sensitivity; 2) stressing the V-A conduction system by programming a high upper rate with an appropriately short A-V interval; 3) programming a low lower rate; 4) avoiding the Wenckebacb response (by programming a high upper rate); 5) medication; and 6) occasionally by using a magnet. PCMT was controlled in all patients, in 2 patients by programming measures only and in 2 with the addition of medication. One patient who refused medication had to be programmed into another pacing mode. We conclude that : 1) the presence of V-A conduction is not an absolute contraindication to the use of a DDD pacing system; 2) pacing the ventricle early enough to cause V-A block was the most useful method to terminate PCMT; 3) future generation DDD pacemakers should prevent initiation of PCMTs while maintaining the possibility to synchronize to exercise-induced high atrial rates.     

Comparison of Unipolar and Bipolar Active Fixation Atrial Pacing Leads

The purpose of this investigation was to compare the acute pacing and sensing characteristics of a new bipolar active fixation atrial pacing lead with those of a unipolar atrial lead of similar design. Pacing threshold voltage and current, lead impedance, and atrial electrogram amplitude and slew rate were measured at the time of surgery in 28 consecutive patients undergoing DDD pacing system implantation. Eleven patients received a Medtronic 6957J-58 unipolar active fixation atrial lead and 17 patients were given a Medtronic 4016-58 bipolar lead. Both leads are polyurethane-insulated with the distal electrode being a platinum alloy screw-helix with a surface area of 8.0 mm2. There were no significant differences in the threshold voltage (1.01 V unipolar versus 1.05 V bipolar) or current (1.93 mA unipolar versus 1.78 mA bipolar) of the two leads. The mean impedance of the bipolar active fixation lead was 618 ohms compared to 479 ohms for the unipolar lead (p = 0.02). The mean amplitude of the atrial electrogram was 4.64 mV for the bipolar lead and 3.11 mV for the unipolar lead (p = 0.02). The atrial electrogram exceeded 5 mV in 10 of 17 patients with the bipolar lead but zero of 11 with the unipolar lead. There was no significant difference in the mean slew rate of the leads (1.09 V/s bipolar versus 0.73 V/s unipolar; p = 0.18). Over a follow-up period of up to 10 months, all patients remained in either the DDD or DDI modes with no episodes of atrial sensing or pacing failure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Limitations to Activation of the Antitachycardia Burst Pacing Function of the Symbios 7008 Pacemaker in the Universal (DDD) Mode

The Symbios 7008 antitachycardia pacemaker was implanted in five patients for control of supraventricular tachycardia. Shortly after implantation in the first two patients, it was noted that the burst pacing sequence was not automatically activated by tachycardia when the pacemaker was in the DDD mode. Data from these two and the subsequent three patients were evaluated to explain this observation. The problem was primarily related to the operation of the device during the postventricular atrial refractory period. In all patients, the atrial electrogram encroached upon the programmed postventricular atrial refractory period because VA conduction during SVT was less than the lowest programmable interval (155 ms). Atrial events occurring during this interval will not trigger the tachycardia termination sequence. In all five patients, the size of the atrial electrogram decreased substantially (48 +/- 10%; mean +/- SD) during supraventricular tachycardia compared to sinus rhythm. In at least two of the five patients, decreased atrial size during supraventricular tachycardia may also have resulted in intermittent failure of atrial sensing during tachycardia, even at the most sensitive setting (0.6 mV). The latter may remain a problem even if the technical fault in SVT detection in the DDD mode were corrected. Two related problems were noted in the DDD mode: ventricular events during rapid SVT do not reset the low rate interval, resulting in random low rate pacing; and, automatic prolongation of atrial refractory period by two successive ventricular events without an intervening atrial sensed event compounds problems of atrial sensing. All of these problems were easily circumvented in all patients by noninvasive reprogramming to the DVI mode in which supraventricular tachycardia detection is based on ventricular sensing. These findings have implications for the future design of such devices.