A cardiac evoked response algorithm providing threshold tracking: a North American multicenter study. Clinical Investigators of the Microny-Regency Clinical Evaluation Study

The purpose of this study was to evaluate a pacing system using the recognition of cardiac evoked response for the automatic adjustment of pacing output. Patients were prospectively followed after primary implantation of VVIR pacemakers using AutoCapture (St. Jude Medical CRMD). Sensing and pacing thresholds, polarization signal, evoked response, and AutoCapture performance were evaluated with serial visits and 24-hour Holter monitoring. Three hundred ninety-eight patients (mean age 71 +/- 15 years) were followed for an average duration of 1 year (3 days-1.75 years) with the algorithm functional in > 90% of patients. Backup pacing in the event of exit block was confirmed in all patients. Pacing thresholds remained stable at 0.89 +/- 0.34 V with a pulse width of 0.31 ms (with chronic output autoset at 0.3 V above the actual threshold). Evoked response exhibited a small but statistically significant increase with time (8.92 mV at implant, 9.60 mV at 12 months), however, this finding did not result in any change in AutoCapture function during our follow-up period. The polarization signal remained stable with minimal variation (1.12 mV at implant, 1.18 at 12 months). No clinical adverse events were observed using the AutoCapture algorithm. In this initial experience with the AutoCapture algorithm the evoked response and polarization measurements remained adequate, allowing the system to function in the majority of patients with safe, low output pacing. High energy backup pacing provided an added safety feature over fixed output devices in cases of unexpected threshold rises. Longer follow-up is required for continued long-term validation of the algorithm.     

Use of automatic threshold tracking function with non-low polarization leads: risk for algorithm malfunction

The AutoCapture (AC) function of new pacemakers (PM) from St Jude Medical (SJM) was originally recommended for use with low polarization (LP) ventricular leads only.However, recent reports have encouraged the use of the AC function with various leads, including those lacking a special LP design. The objective of this study was to analyze the reliability and safety of the AC algorithm application with different types of pacing leads.The study group comprised 30 consecutive patients with AC PMs connected to three different types of non-LP leads. Ten patients with SJM LP leads served as the control group. The study protocol included a complete AC function test using four different pulse widths (PW). The pacing threshold was independently assessed by a manual/semiautomatic check. Erratic behavior of polarization measurements with increasing PWs was demonstrated in 43% (n = 13) of the study group. Invalid polarization measurements resulted in erroneous algorithm recommendation to apply AC function in 17% (n = 5) of the study patients. Subsequent AC function activation lead to incorrect threshold determination due to missed noncapture in three patients. AC function should be applied with caution with non-LP leads. "Off label" use of these leads may cause erroneous polarization signal measurements which, in some cases, may result in incorrect pacing threshold determination, rendering a potential risk to dependent patients.     

Adjustment of the evoked response sensitivity after hospital discharge in pacemaker patients with automatic ventricular threshold tracking activated

Automatic threshold tracking in cardiac pacemakers allows ventricular capture verification and self-adaptive pacing output regulation. The Autocapture algorithm detects the evoked response (ER) signal immediately after the pacing pulse to verify the efficacy of ventricular pacing. Before hospital delivery, the ER sensitivity must be programmed individually so that the pacemaker detects the ER signal adequately without sensing lead polarization. The aims of the study were to assess the frequency of patients in whom Autocapture could be activated and whether the ER sensitivity had to be adjusted after hospital discharge. The study included 44 patients who received the VVIR pacemaker Regency SR+ (St. Jude Medical) connected to the model 1450 T pacing lead. ER signal, lead polarization, and ER sensitivity were evaluated before hospital discharge and 1, 3, and 6 months after implantation. The system recommended activating Autocapture in 42 of 44 patients. The mean ER signal was 8.4+/-1.2 mV at discharge, 9.0+/-3.9 mV at month 1, 8.9+/-4.9 mV at month 3, and 9.3+/-4.5 mV at month 6. Polarization was 1.0+/-0.1 mV at discharge, 1.1+/-0.5 mV at month 1, 1.1+/-0.2 mV at month 3, and 1.1+/-0.5 mV at month 6. Mean ER sensitivity was 3.7+/-1.8 mV at discharge, 4.0+/-1.8 mV after 1, 4.1+/-2.2 mV after 3, and 4.1+/-1.8 mV after 6 months. ER sensitivity could remain unadjusted in 14 patients. Programming to a less sensitive ER setting from 2.9+/-1.2 mV to 4.3+/-1.5 mV was possible in 21 patients. Programming to a more sensitive ER setting from 4.1+/-1.1 mV to 2.5+/-0.9 mV was required in nine patients because of the decrease of the ER signal. The automatic threshold tracking algorithm Autocapture could be activated in 95% of patients. Programming to more sensitive ER settings was recommended in 21% of the patients after hospital discharge. Therefore, ER signal and polarization must be checked at each follow-up, as a decrease in ER signal amplitude can make reprogramming of the ER sensitivity necessary. There is no risk for the patient if the ER is not sensed, as high voltage backup stimulation is present.     

Clinical performance of a ventricular automatic capture verification algorithm

BACKGROUND: This study was conducted to evaluate the clinical performance of the ventricular automatic capture feature as implemented in the Insignia I Ultra pacemaker system (Guidant) utilizing a variety of ventricular leads. Currently, the optimal programming of the pacemaker output considers both pacemaker efficiency (prolonging battery longevity) and patient safety (adequate safety margin). The ability of a pacemaker to automatically adjust the ventricular output above the pacing threshold while maintaining the appropriate safety margin has been explored since the early 1970s and is only available today in conjunction with a specific low polarization lead system. METHODS: One hundred and five patients were enrolled from 17 European centers utilizing 31 different types of ventricular leads were followed through their 3-month follow-up visit. There were no restrictions on the type of ventricular leads used. RESULTS: The average mean difference between the commanded autothreshold test (0.652 + 0.335 V) and the manual threshold test (0.651 + 0.335 V) was 0.001 + 0.49 (P < 0.0001). The average mean difference between the ambulatory autothreshold test (0.696 + 0.322 V) and the commanded autothreshold test (0.682 + 0.315 V) was 0.002 + 0.74 (P < 0.0001). Holter recordings confirmed that there were no loss of capture incidences without a backup pulse being delivered. In addition, the mean number of backup pulses delivered in a 24-hour period was less than 0.1% of the total number of paced beats. CONCLUSIONS: This study provided that the automatic capture feature while using a variety of leads accurately determines the ventricular stimulation threshold and safely delivers a backup pulse when required.     

Underestimation of pacing threshold as determined by an automatic ventricular threshold testing algorithm

In this case report, we describe markedly different pacing thresholds determined by a manual threshold test and the automatic Ventricular Capture Management algorithm. The discrepancy in pacing threshold values reported was due to the difference in the AV intervals used with the different testing methods. We propose that the differences in right ventricular dimensions with altered diastolic filling periods affected the threshold in this patient with a new passive fixation lead in the right ventricular apex.     

Effects of body position and exercise on evoked response signal for automatic threshold activation

The Autocapture function controls and optimizes the output of the ventricular pulse amplitude automatically. For this reason an automatic test has to be performed during follow-up to measure the evoked response signal and lead polarization for the calculation of the appropriate evoked response sensitivity setting. The aim of the study was to assess whether body position and exercise influence the evoked response and polarization. Both parameters were determined in the supine and upright position and subsequently during supine and upright symptom-limited ergometry. The study included 14 patients with the VVIR pacemaker Regency SR+ who had received the ventricular pacing leads Membrane E 1450 T (n = 8), CapSure Z 5034 (n = 4), or SX 60 (n = 2). The evoked response signal was 7.4 +/- 3.3 mV during supine and increased to 9.7 +/- 5.6 mV (+35%) during upright position (P < 0.05). The exercise tests were terminated at 105 +/- 36 W (supine) and 110 +/- 34 W (upright). There was a gradual insignificant decrease of the evoked response during each exercise test with a mean decrease of -1.1 +/- 0.9 mV (-15%; supine) and -1.6 +/- 2.1 mV (-16%; upright). The evoked response increased within 5 minutes during recovery to the initial values. Polarization remained unchanged during both tests. The pacemaker did not recommend activating autocapture in four patients who all had received high-ohmic pacing leads. In conclusions, the measurement of the evoked response in supine position seems to represent the worst case. Physical activities did not effect autocapture function in patients with the recommended lead, but the pacemaker did not always recommend Autocapture activation in some patients with high-ohmic pacing leads.     

Automatic threshold tracking activation without the intraoperative evaluation of the evoked response amplitude. AUTOCAP Investigators

Automatic threshold tracking (Autocapture) controls the amplitude of the pacing pulse and adjusts it to the actual pacing threshold. The algorithm is based on the proper detection of the evoked response (ER) amplitude after the pacing pulse. For this reason an intraoperative evaluation of ER and polarization is recommended. The aims of the study were to evaluate the ER signal and polarization and the performance of automatic threshold tracking without any intraoperative testing of the ER signal. In addition, the ER amplitude was correlated with the pacing threshold, pacing impedance, spontaneous R wave amplitude, and with the clinical data. The study included 60 patients who received the VVIR pacemaker Regency connected to the Membrane E 1450/1452 pacing lead (St. Jude-Pacesetter). At implantation, a pacing threshold < 0.7 V at 0.5 ms was achieved in all patients. ER and polarization were assessed for the first time at hospital predischarge testing. Follow-up measurements were conducted at month 1, 3, and 6. The ER amplitude at hospital discharge was 8.4 +/- 4.2 mV and increased to 9.4 +/- 4.8 mV at the 6-month follow-up. The pacemaker recommended not to program automatic threshold tracking on in one patient permanently and in three patients intermittently. The ER amplitudes were not differently distributed in men compared with women or in right-sided compared to left-sided implants. The correlation between age and the evoked response was r = 0.15. The correlation between ER amplitude and pacing threshold was r = -0.08, with pacing impedance r = 0.02, and with R wave amplitude r = 0.44. In conclusion, despite no operative evaluation of the ER amplitude being performed, the mean ER amplitude was about 9 mV at 6-month follow-up. Automatic threshold tracking could be programmed on in 93% of the patients throughout the time. Neither the clinical data nor the conventional electrical parameters help to predict patients who will have low ER amplitude or to optimize the ER signal at implantation.     

Clinical study of the laser sheath for lead extraction: the total experience in the United States

The laser sheath uses optical fibers, delivering pulsed ultraviolet excimer laser light, to vaporize fibrotic tissue binding intravenous cardiac leads to the vein or heart wall during lead extraction from the implant vein. The total investigational experience with laser sheaths is reported. During the period from October 1995 to December 1999, 2,561 pacing and defibrillator leads were treated in 1,684 patients at 89 sites in the United States with three sizes of laser sheath. Endpoints were complete removal of the lead, partial removal (leaving the tip behind), or failure (abandoning the lead, onset of complications, change to transfemoral or transatrial approach). Minimal follow-up at 30 days was recorded. Of the leads, 90% were completely removed, 3% were partially removed, and the balance were failures. Major perioperative complications (tamponade, hemothorax, pulmonary embolism, lead migration, and death) were observed in 1.9% of patients with in hospital death in 13 (0.8%). Minor complications were seen in an additional 1.4% of patients. Multivariate analysis showed that implant duration was the only preoperative independent predictor of failure; female sex was the only multivariate predictor of complications. Success and complications were not dependent on laser sheath size. At follow-up, various extraction related complications were observed in 2% of patients. The learning curve showed a trend toward fewer complications with experience. Lead extraction with the laser sheath can be safely practiced with high success rates. Success is independent of laser sheath size. Major complications can be expected in < 2% of patients, and occur more often during an investigator's early experience.     

Intraoperative study of polarization and evoked response signals in different endocardial electrode designs

Some new generation pacemakers use an algorithm based on evoked response (ER) detection to verify beat-to-beat capture and to enable automatic adjustment of output. This is a prospective acute study of polarization signal (PS) and ER in nine currently available electrodes. Intraoperative testing of ventricular bipolar electrodes used the Autocapture (AC) algorithm. The intrinsic R wave, PS, ER, acceptance of AC function, and stimulation thresholds (STs) were obtained. Ventricular electrodes were categorized as follows: titanium nitride (TiN)-coated passive and active fixation, high impedance (HI), passive fixation (VP), iridium oxide-coated titanium (IROX) (VI), and platinum helix (PH) active fixation. Acute testing was performed in 217 patients with an average age of 74.26 years, 59.6% were men with primary pacing indication-SSS (46.3%). There were no significant differences found with respect to R wave and threshold between the various electrodes. PH active-fixation electrodes had significantly higher ER and PS than other groups including the TiN-coated active-fixation electrodes. TiN-coated electrodes (active and passive fixation) had significantly lower PS than other electrodes. As a result, TiN electrodes had a significantly higher functional rate of AC (91.7%), whereas PH had the lowest rate (0%). In conclusion, (1) polarization characteristics are significantly different for commercially available ventricular electrodes, (2) certain physical features at the tissue to electrode interface like TiN coating appears to be more important in determining PS than electrode tip size and fixation method, and (3) the current algorithm for AC requires electrodes that provide low polarization for satisfactory performance.     

Clinical evaluation of two different evoked response sensing methods for automatic capture detection in the left ventricle

BACKGROUND: This acute feasibility study compared two different automatic capture detection methodologies, the reduced coupling capacitor (RCC) and the independent pace/sense (IPS) methods, for the left ventricle (LV). METHODS: LV threshold tests were performed in DDD mode, with LV-only and bi-ventricular (BiV) pacing using an external cardiac resynchronization therapy (CRT) defibrillator. Evoked response (ER) signals from LV leads were recorded using the LV(Tip) (LV(Tip)-->Can) and LV(Ring) (LV(Ring)-->Can) to empty pulse generator (Can) housing sensing vectors to evaluate the two methodologies. Pacing vector, pulse duration, atrioventricular delay, and interventricular delay were varied to assess their effects on ER. The minimum ER amplitude (ER(min)), signal-to-artifact ratio (SAR), and ER amplitude voltage dependence were evaluated. ER(min)>2 mV and SAR(min)>2 define potential automatic LV capture detection for the two methodologies. RESULTS: Data collected from 43 patients (63.7 +/- 11.0 years) were analyzed, including unipolar and bipolar (14/29) LV leads. Neither ER sensing method was affected by changing the pacing vector. The LV(Tip)-->Can ER(min) was significantly decreased at the 1.0-ms pulse duration when compared to 0.4-ms (p < 0.05). During BiV pacing, LV(Tip)-->Can ER(min) increased at negative interventricular delays and decreased at positive interventricular delays relative to simultaneous pacing. LV(Tip)-->Can resulted in fewer patients with sufficient ER characteristics for capture detection, albeit only significantly at the extended pulse duration (79% vs 97%, p < 0.05) and at simultaneous and positive interventricular delays (81% vs 97%, p < 0.05). CONCLUSIONS: Though LV capture detection was feasible using both investigated methods, the RCC method (LV(Tip)-->Can) sensitivity to the evaluated pacing parameters suggests the IPS method (LV(Ring)-->Can) provides a more robust performance.     

Unexpected loss of bipolar pacing with implanted dual chamber pacemakers

Bipolar leads are most commonly used in the current practice of pacemaker therapy. In our study of 124 patients implanted with Guidant/Cardiac Pacemakers (CPI) Vigor dual chamber pacemakers, 5 patients had unexpectedly abrupt increases in bipolar lead impedance and pacing threshold 2 weeks to 18 months postimplantation without changes in sensing function. With the lead configuration reprogrammed to unipolar, the lead impedance and pacing threshold were restored to appropriate ranges. The changes in bipolar lead parameters can be caused by the CPI's "Quick Connect" (QC1) header lead system incorporated in these pacemakers.     

Electrical Performance and Automatic Capture Characteristics of a 3.5-mm2 Passive Fixation Lead during 1-Year Follow-Up

Background: Bipolar low polarization electrodes are recommended for a regular AutoCapture™ (St. Jude Medical, Inc., Sylmar, CA, USA) function in order to effectively detect the evoked response (ER) signal. The objective of this national multicenter registry was to evaluate the electrical performance and the AutoCapture™ characteristics of the bipolar ventricular pacing lead IsoFlex S, model 1636T or 1646T (St. Jude Medical), in combination with single- and dual-chamber pacemakers.
Methods: Ventricular pacing and sensing thresholds, lead impedance, ER amplitude, and polarization signals were measured at discharge and routine follow-up visits after 1, 3, 6, 9, and 12 months. AutoCapture™ activation was recommended based on the results of the ER sensitivity test.
Results: Of the 252 patients initially included, 109 (43%) have completed the follow-up. The mean ventricular pacing threshold was 0.43 ± 0.19 V at discharge and 0.68 ± 0.32 V at 12 months postimplant. The values for the ventricular sensing threshold were between 9.51 ± 4.12 and 9.99 ± 4.09 mV at discharge and at the 12-month follow-up. The unipolar lead impedance decreased from 533 ± 94 to 476 ± 73 ohms during the follow-up. The mean ER amplitude was 16.47 ± 6.70 mV at discharge and 17.42 ± 7.43 mV after 12 months, and the corresponding mean polarization signals were 0.59 ± 1.00 and 0.74 ± 1.24 mV, respectively. AutoCapture™ activation was recommended in at least 95% of the patients investigated over the 12-month follow-up.
Conclusion: The bipolar ventricular pacing lead IsoFlex S 1636/1646T shows a good electrical performance and is mostly compatible with the AutoCapture™ algorithm.     

Impact of fusion avoidance on performance of the automatic threshold tracking feature in dual chamber pacemakers: a multicenter prospective randomized study

The Autocapture algorithm enables automatic capture verification on a beat-by-beat basis by recognizing the evoked response signal following each pacemaker stimulus. The algorithm intends to increase patient safety while decreasing energy consumption. However, the occurrence of fusion beats, particularly during dual chamber pacing, may limit the energy saving effect of Autocapture. The aim of this multicenter, prospective, randomized study was to evaluate the impact of the Fusion Avoidance (FA) algorithm on the incidence of fusion beats. Thirty-eight patients (mean age 69 +/- 13 years) with intrinsic AV conduction who were implanted with an Affinity DR were studied. After programming a PV/AV delay of 120/190 ms, patients were randomized to FA On or Off. Each group was further randomized with respect to activation of the AutoIntrinsic Conduction Search (AICS) algorithm. The total number of beats, ventricular paced beats, fusion beats, backup pulses, and threshold searches were analyzed from 24-hour Holter recordings. The number of total beats was comparable in both FA groups. The number of total ventricular paced beats, fusion beats, backup pulses, and threshold searches were significantly reduced in the FA On group (% reduction: 68% P < 0.001, 75% P < 0.01, 95% P < 0.01, and 94% P < 0.05, respectively). The number of ventricular paced beats with full capture was significantly reduced when AICS was activated (P < 0.05). In conclusion, the FA algorithm substantially reduces the amount of ventricular paced beats, fusion beats, unnecessary backup pulses and threshold searches, and therefore, provides added benefits in energy saving obtained by Autocapture.     

Multicenter experience with transvenous lead extraction of active fixation coronary sinus leads

Background/Objective: Active fixation coronary sinus (CS) leads limit dislodgement and represent an attractive option to the implanter. Although extraction of passive fixation CS leads is a common and frequently uncomplicated procedure, data regarding extraction of chronically implanted active fixation CS leads are limited. Methods: We performed a retrospective cohort study of patients undergoing active fixation CS lead extraction at six centers. Patient and procedural characteristics, indications for extraction, use of extraction sheath (ES) assistance, and outcomes are reported. Results: Between January 2009 and February 2011, 12 patients underwent transvenous lead extraction (TLE) of Medtronic StarFix® lead (Medtronic Inc., Minneapolis, MN, USA). The cohort was 83% male with mean age 71 ± 14 years. Average implant duration was 14.2 ± 5.7 months (2.3–23.6). All leads but one were removed for infectious indications (67% systemic infection). At the time of explant, the fixation lobes were completely retracted in only one of the 12 cases and ES assistance was required for lead removal in all cases (58% laser, 25% cutting, 25% mechanical, and 25% femoral). The majority of cases required advancement of the sheath into the CS (75.0%) and often into a branch vessel (41.7%). One lead could not be removed transvenously and required surgical lead extraction. There were no major complications. Examination of the leads after extraction frequently revealed significant tissue growth into the fixation lobes. Conclusions: Although TLE of active fixation CS leads can be a safe procedure in select patients and experienced hands, powered sheaths and aggressive techniques are frequently required for successful removal despite relatively short implant durations. This raises significant concern regarding future TLE of active fixation CS leads with longer implant durations. (PACE 2012; 35:641–647)     

Automatic Capture Verification by Charge-Neutral Sensing

Automatic capture verification can prolong pulse generator longevity and increase patient safety. However, the detection of evoked response following pacing is complicated due to afterpotentials caused by polarization of electrodes. This study describes a new capture verification scheme, which neutralizes the charges between the pacing electrodes. The hypothesis of the charge-neutral sensing is that the afterpotentials in the ring and the tip are opposite in polarity when pacing in a bipolar mode between ring and tip. Summing the unipolar signals sensed at the tip and the ring should effectively cancel the afterpotentials. This scheme was implemented in an external computer based system and tested during pacemaker implant/replacement on 23 patients during VVI pacing (17 acutely implanted leads and 6 chronic leads). Surface ECG was recorded to provide a marker for capture and noncapture. The pacing voltage was gradually decreased until a noncapture beat was noted. To avoid fusion beats, the pacing rate was programmed ˜50% higher than the intrinsic rate. The evoked response was high pass filtered and the integral average was calculated for both capture and noncapture beats. The system signal to noise ratio (SNR) was expressed as ratio of the minimum integral average of all capture beats to the maximum integral average of all noncapture beats. The system SNR was 8.6 ± 1.3 (mean ± S.E.M; range 1.5–22.8), indicating that the charge-neutral sensing method has, on average, a ninefold safety margin in providing capture verification. Further, evaluation is needed to fully assess this feature in patients with chronic leads.     

Clinical Evaluation of VDD Pacing with a Unipolar Single-Pass Lead

Twenty patients with advanced AV block and normal sinus node function underwent pacemaker implantation, randomly receiving a CPI 910 ULTRA II model VDD pacemaker. The first 13 patients received the implantation of a single lead with a screw-in positive ventricular fixation tip and a unipolar ring floating atrial electrode spaced 13 cm from the tip. A subsequent group of seven patients received a conventional porous tinned-tip lead with a pair of unipolar ring floafing electrodes. The second solution was adopted because the best atrial signal was not always in the high or mid-high atrium portion, but sometimes in the middle or mid-low position. With the modified double-electrode lead, the floating atrial electrode that detects the best signal can be selected, cutting out the pin of the one not used. The comparisons between minimal atrial slew rate and maximal ventricular slew rate, as well as those between minimal P wave amplitude and maximal R wave amplitude, show a highly significant range difference, as large as P < 0.01. Surface electrocardiograms, stress tests, and 24-hour Holter monitoring showed the correct functioning of the system with an average sensing failure from 0.05 to 1%. In conclusion, VDD stimulation is feasible with a single unipolar lead and a floating atrial electrode in conjunction with a pacemaker generator (CPI 910 ULTRA II) originally designed for permanent twin-lead implantation.     

Percutaneous lead implantation connected to an external device in stimulation-dependent patients with systemic infection--a prospective and controlled study

BACKGROUND: Permanent pacemaker implantation usually is contraindicated in patients with systemic infection. The aim of the present study was to compare two different techniques of transvenous temporary pacing to bridge the infectious situation until permanent pacemaker implantation under infection-free conditions is possible. METHODS AND RESULTS: Forty-nine patients with systemic infection and hemodynamic-relevant bradyarrhythmia/asystole were temporarily paced using either a conventional pacing wire/catheter (n = 26, reference group) or a permanent bipolar active pacing lead, which was placed transcutaneously in the right ventricle and connected to an external pacing generator (n = 23, external lead group). In both groups, there were no significant differences in patient characteristics. Whereas the sensing values were almost identical, the median pacing threshold was significantly higher in the reference group (1.0 V vs 0.6 V, P < 0.05). Within comparable duration of pacing (median: 8.2 vs 7.7 days), there were 24 pacing-related adverse events (including dislocation, resuscitation due to severe bradycardia, or local infection) in the reference group as compared to one event in the external lead group (P < 0.01). None of these complications resulted in cardiac death. CONCLUSION: Thus, transvenous pacing with active fixation is safe and associated with a significantly lower rate of pacing-related adverse events as compared to the standard technique of transvenous pacing using a passive external pacing catheter.     

Clinical Experience with a New Lead Comhining Active Fixation with Steroid Elution

The Medtronic lead engineering model number 10335A represents a new concept in lead design combining active fixation with steroid elution. It aims for immediate stability and low chronic thresholds. Twenty-one leads, 9 atrial and 12 ventricular, were implanted in 13 patients (10 males, mean age 68; range 22–91 years). The atrial leads showed no rise in pulse width threshold at a voltage of 1.6 volts (mean thresholds at implant, 1, and 26 weeks; 0.1 ± 0.09 msec, 0.15 ± 0.04 msec, and 0.1 ± 0.03 msec, respectively). The ventricular leads had a small but significant rise between implant and 1 week at an output of 1.6 volts (0.07 ± 0.03 msec increasing to 0.11 ± 0.04 msec; P < 0.02) but no significant later rise (0.1 ± 0.04 msec at 2 weeks and 0.1 ± 0.05 msec at 6 months). These low chronic thresholds would allow early reprogramming of the unit to low voltages resulting in a battery saving with prolongation of the unit's life. There were no significant changes in the P and R wave amplitudes, but there was a fall in lead impedance in the ventricular leads between implantation and 1 week (P < 0.02) but none subsequently, and there was no significant change in atrial impedance. There were no sensing failures and no lead displacements. Despite impressive pacing characteristics, the study was suspended because of a high level of mechanical complications. Of the 96 patients implanted worldwide with 136 leads there were eight helix deformations, which will require redesign. However, the concept of steroid elution combined with active fixation appears to be an attractive and valid concept and is supported by this data; further studies are planned.     

Dual Chamber Pacing with a Single Lead System: Initial Clinical Results

A new mode of biphasic pacing was used in 26 patients to assess the feasibility of atrial pacing by means of the floating atrial ring electrodes of a single lead VDD permanent pacing system. During implantation, atrial pacing was possible in 25 patients with a 1-ms total pulse duration, a mean atrial threshold of 1.70 ± 0.60 V (range, 0.6–3.0), and a mean diaphragmatic threshold of 6.7 ± 2.5 V (range, 2.5–10.0). At 3 months, the atrial threshold had increased beyond 4.8 V in three patients. In the 22 other patients, the mean atrial threshold was 2.2 ± 0.5 V (range, 1.50–3.50) in the supine position and 2.5 ± 0.8 V (range, 1.5–4.8) in the sitting position. Stable atrial capture without diaphragmatic stimulation was achieved in 76% of patients.