Clinical Results of Automatic Slope Adaptation in a Dual Sensor VVIR Pacemaker

Manual slope programming in rate adaptive pacemakers can be time consuming. This may become worse with dual sensor devices. The remedy is to let the pacemaker automatically learn the slopes. Fast learning replaces initial manual slope programming. Daily learning is a continuous process to determine and optimize slopes during daily life. Both methods are known for a QT sensing pacemaker. Fast learning is known for other single sensor devices. The aim of this study was to follow daily learning in a QT and activity dual sensor pacemaker, starting with factory slope settings. Six patients were studied for about 8 weeks. The daily learning algorithm appeared to be elective, showing the desired regulation processes. It took 2–5 weeks to reach full rate response.     

Sensor-Driven Rate Smoothing in a DDDR Pacemaker

DDD pacemakers may have large cycle-to-cycle variations in rate at the upper rate limit because of 2:1 block or Wenckebach-type block. Rate smoothing was introduced as an option to eliminate these large variations. Now, DDDR pacemakers can produce similar electrocardiographic displays through a different mechanism that uses an activity sensor. This is termed "sensor-driven rate smoothing" because it occurs only when the activity sensor is driving the pacemaker. In the case described, as the atrial rate exceeded the maximum tracking rate and reverted to Wenckebach-type block, the RR interval varied only from 600 msec to 680 msec (13.3% rate-smoothing value) because of sensor-driven pacing. Maximal sensor-driven rate smoothing requires optimal programming of the rate response indicators. This sensor-driven rate-smoothing effect is an electrocardiographic manifestation that will undoubtedly be seen more frequently as DDDR devices come into widespread clinical use.     

Myopotential Interference in Unipolar Rate Responsive Pacemakers

The effects of myopotential interference on unipolar rate responsive pacemakers were assessed in 22 patients. Six types of pacemakers (from four manufacturers) were studied: five TX2 (QT sensing), seven Biorate (five RDP3 and two MB-1, respiratory rate sensing), seven Activitrax (activity sensing), two Medtronic 2503 (dP/dt sensing), and one Sensolog P703 (activity sensing). Provocative tests using arm exercises were performed in both VVI and rate responsive modes. At nominal sensitivity settings (1.8-2.5 mV), 55% of these patients were myopotential positive for at least 1 provocative test. Pressing the palms together was found to be the most sensitive provocative test. Rate response was achieved with treadmill exercise (all patients), hyperventilation (RDP3 and MB-1) and tapping (Activitrax) or wobbling the pacemaker in its pocket (Sensolog). During continued rate acceleration, myopotential interference was induced by arm exercises. The duration of inhibition was shorter when the provocative tests were performed during rate response compared to that occurred at rest. Short periods of myopotential interference resulted in temporary inhibition of pacing but rate response continued immediately on removal of the interference. In one patient with a RDP3 pacemaker, a prolonged episode of myopotential interference during treadmill exercise resulted in reversion of the pacemaker to the interference mode. Appropriate adjustment of the sensitivity setting effectively controlled the symptoms in most patients. However, one patient with a QT sensing pacemaker and symptomatic myopotential interference required programming to the VVT pacing mode. Two out of five patients with RDP3 required pacemaker replacement because of uncontrolled myopotential interference.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of a New Rate Adaptive Ventricular Pacemaker Controlled by Double Sensors

The LEGEND-PLUS, a new rate adaptive pacemaker that combines activity and minute ventilation sensing for automatic rate adaptation was implanted in the right ventricle (VVIR) in 11 patients (mean age 62 ± 9 years). Initial programming was performed using the Programmer Exercise Protocol (a 3-minute walk). This programming was evaluated by treadmill tests, up-stairs and down-stairs walking, and Holier recordings. Results: Following the final programming of LEGEND-PLUS, the mean upper activity rate was 102 ± 7 beats/rain (range 90–120 beats/min), while the mean upper minute ventilation rate was 125 ± 16 beats/min (range 100–150 beats/min). The mean rate responses during the exercise protocol and the final programming in minute ventilation and activity sensing modes were 5.4 ± 2.3 (range 1–9), versus 4 ± 2.4 (range 1–8; P < 0.01) and 7.6 ±1.1 (range 5–9), versus 7.5 ± 0.8 (range 6–9; P = 0.8), respectively. In the combined sensing mode, the acceleration rate was identical to the activity rate response and the deceleration rate mimicked the minute ventilation. Conclusion: Dual sensor VVIR pacemakers have the potential to improve rate adaptation to exercise. The rate response to exercise in patients fitted with activity and minute ventilation sensors, VVIR pacemakers closely mimics the physiological rate response.     

A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker

BAIG, M.W., ET AL.: A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker. We have compared the pacing rate responses during cardiopulmonary exercise testing in 11 patients (mean 59 years, six female) with implanted QT sensing rate adaptive pacemakers who were randomly programmed to 1-month periods in the linear and nonlinear algorithms using a double-blind, cross-over design. Exercise testing was performed at the end of each month block and symptoms were scored with the MacMaster questionnaire. With exercise, the time to a 10 beats/min increment in rate was significantly less with the nonlinear compared to the linear algorithm (126 sec vs 255 sec, P = 0.02) but there were no significant differences in exercise duration, the peak pacing rate, the peak VO₂, the VO₂ at the anaerobic threshold or the mean correlation coefficients of the pacing rate VO₂ relationship. Rate oscillation occurred in seven patients in the linear algorithm and in two patients in the nonlinear setting. Initial deceleration of the pacing rate at the onset of exercise occurred in seven patients in the linear algorithm and in four patients in the nonlinear setting. The nonlinear algorithm is associated with a faster response time during exercise and fewer instances of rate instability. However, it has not overcome the problem of a dip in the pacing rate at the beginning of exercise. The major difference in the function of the two algorithms is faster initial acceleration with the nonlinear algorithm. This is explained by the significantly higher values of the slope setting at the lower rate limit for the nonlinear versus the linear algorithm (6.3 ms/ms vs 5.1 ms/ms).     

Superior Cardiac Hemodynamics of Atrioventricular Synchrony Over Rate Responsive Pacing at Submaximal Exercise: Observations in Activity Sensing DDDR Pacemakers

LAU, C.-P., ET AL.: Superior Cardiac Hemodynamics of Atrioventricular Synchrony Over Rate Responsive Pacing at Submaximal Exercise: Observations in Activity Sensing DDDR Pacemakers. The relative hemodynamic profile between dual chamber pacing (DDD) and activity sensing rate responsive pacing (VVIR) was compared in ten patients with dual chamber rate responsive pacemakers (Synergist 11). With a double blind, randomized exercise protocol, DDDR pacemakers were programmed into VVI, VVIR, and DDD (AV interval 150 msec) modes and in seven patients the test in the DDD mode was repeated with the AV interval programmed at 75 msec. A treadmill exercise test of 6-minutes duration (2 stages, Stage 1 at 2 mph, 0% gradient and Stage II at 2 mph, 15% gradient) was performed at each of the programmed settings, with a rest period of 30 minutes in between tests. Cardiac output was assessed using continuous-wave Doppler sampling ascending aortic flow and expressed as a percentage of the value achieved during VVI pacing. During exercise, pacing rate between DDD and VVIR pacing was similar but was higher with DDD at the first minute of recovery (91 ± 4vs 81 ± 3 beat/min, respectively). Cardiac output was significantly higher at rest, during low level exercise, and recovery with DDD pacing compared with VVIR pacing (resting: 21 ± 14 vs -2 ± 7%; Stage I: 36 ± 6 vs 16 ± 7%; Stage II: 25 ± 15 vs 10 ± 8%; recovery: 26 ± 12 vs 4 ± 9%; p < 0.05 in all cases). Systolic blood pressure was significantly higher during low level of exercise in the DDD mode. Shortening of the AV interval to 75 msec did not significantly affect cardiac output during exercise, but cardiac output after exercise was reduced (2 ± 6 vs 23 ± 6% at an AV interval of 150 msec, p < 0.02). By enhancing the stroke volume, DDD pacing improves cardiac hemodynamics at rest, during low level exercise, and early postexercise recovery.     

Comparison of Sinus Node Response to Exercise with Responses From Two Different Activity-Based Rate Adaptive Pacemakers in Healthy Subjects of Different Age Groups

The pacing rates from accelerometer-based (Excel™) and piezoelectric-based (Legend™) activity sensing rate adaptive pacemakers, both strapped externally in the pectoral position, were compared with sinus rate response in normal volunteers of two different age groups (group 1, mean age 35 ± 16 years; group 2, mean age 72 ± 9 years) during various physical activities. Both pacemakers were programmed in manufacturers' nominal rate adaptive settings. Both types of activity sensing pacemakers programmed in this way showed chronotropic deficiencies to metabolic demand in healthy young subjects and in those matching the usual age of pacemaker implant, especially during "burst" activities. These data suggest that present recommended activity sensing rate response algorithms for accelerometer and piezoelectric pacemakers are inappropriate for many physical activities.     

Temporal Relationship Between Exercise and QT Shortening in Patients with QT Pacemakers

The present study was undertaken to examine the temporal relationship between exercise and QT interval shortening as one of the principal determinants for the functioning of QT pacemakers. Ten patients (mean age of 72.6 years) with implanted QT pacemakers were subjected to supine bicycle exercise with two different slopes, 90% and 80%. The QT interval as seen by the pacemaker was monitored by telemetry and stored on magnetic tape. After the beginning of exercise QT prolongation of a few msec occurred up to 40 sec in most patients. The earliest QT shortening of 4 msec was noted after 63.4 sec with 90% slope and 75.7 sec with 80% slope. The difference was not significant. The further time course was dependent on slope and pacemaker algorithm. Maximal QT shortening was 65.9 msec with 90% and 69.8 msec with 80% slope. It was seen 29.2 sec after termination of exercise with 90% slope and 69.5 sec with 80% slope (P < 0.05). There was no correlation of the measured delays with age. Earliest rate response in QT driven pacemakers is determined by earliest QT shortening on one hand and by the slope setting of the pacemaker on the other, where the limiting parameter appears to be QT shortening, which occurs after the first minute of exercise.     

Automatic Sensor Adjustment in a Rate Modulated Pacemaker

A new feature (AutoSlope) has been introduced that can automatically adjust the sensor slope based on the chronic activity level of the patient. The algorithm adjusts the slope once per week so that 99% of the sensor response is maintained between the base rate and 23% of the difference between the programmed Base Rate and the Max Sensor Rate. Offsets are available for fine titration of sensor response in individual patients. The AutoSlope feature was evaluated in 93 patients with DDDR pacemakers (Trilogy DR+, Pacesetter). Patients were seen at 1, 3, and 6 months for a total of 178 evaluations. At each evaluation, the AutoSlope value was recorded. Patients then performed a brisk walk at sensor values equivalent to the AutoSlope value. Desired sensor rate was compared to the rate achieved by AutoSlope for the exercise period. Long-term sensor performance was evaluated by analyzing the sensor histogram. AutoSlope provided the desired sensor rate in most patients. Use of AutoSlope offsets allows fine titration of rate modulation in individual patients. Ongoing changes in sensor performance provided by AutoSlope allow patients to achieve a desired sensor rate from one evaluation to another without changes in permanent programmed settings. Programming a low maximum sensor rate may limit sensor response in some patients.     

Impaired Activity Rate Responsiveness of an Atrial Activity-Triggered Pacemaker: The Role of Differential Atrial Sensing in its Prevention

The physiological benefit of rate responsive, single-chamber cardiac pacing is well documented. We studied the activity response of nine atrially placed Activitrax II pacemakers. Seven patients were noted to have an inadequate activity-rate response with maximal pacing rates of 85 to 101 beats/min. Marker Channel analysis revealed that the upper rate timeout was reset by far-field R wave sensing, even when sensing occurred in the atrial refractory period. These 9 pacemakers were tested by atrial sensitivity adjustment for ability to exclude far-field R wave sensing, while preserving P wave sensing. Unipolar implantation data were then examined for predictors of this differential far-R and P-wave sensing. Differential atrial sensing occurred in 4/9 pacemakers (2/2 bipolar in the right atrial appendage; 0/1 bipolar in the coronary sinus; and 4/9 unipolar). An empirically developed index utilizing unipolar implant parameters discriminated outcomes for 8/9 unipolar pacemakers. We conclude that: (1) the rate responsiveness of the atrial Activitrax II pacemaker is limited by far-field R wave sensing even when this occurs during atrial channel refractoriness; (2) reprogramming atrial sensitivity to differentially sensed P and far-field R waves may restore appropriate rate responsiveness; and (3) although a unipolar implant discriminant index may correctly identify adequacy of future rate responsiveness, the atrial application of the Activitrax II pacemaker is cautioned until further validation is forthcoming, particularly when used in unipolar and coronary sinus applications.     

Cardiac Rehabilitation in Patients with Rate Responsive Pacemakers

This study investigated the suitability of our oxygen pulse reserve (OPR) method for tailoring parameters of rate responsive pacemakers and planning aerobic rehabilitation programs. We selected 11 patients, NYHA Classes I-III (7 males and 4 females; mean age 60 years, range 18–83), with rate responsive pacemakers implanted for high degree AV block and chronotropic incompetence. Five pacemakers had activity sensors, 4 had temperature sensors, and 2 had dual sensors. All patients underwent a cardiopulmonary stress test (CPX). We determined anaerobic threshold time (ATT), VO₂/AT, total exercise time (TET), VO₂ Max, and VO₂AT/VO₂ Max. OPR (mL-O₂/beat) was calculated for each patient using the formula OPR = (VO₂Max - VO₂ resting)/(HR Max - HR resting). During CPX, this slope was used as the rate response tailoring guideline by comparing the mL-O₂/beat equation and the pacing rate. Rate response settings were modified until the optimal relationship between theoretical and paced rate was obtained. The work protocol yielded rate response settings capable of providing pacing rates within ± 10 beats of the theoretical values. Data relating to theoretical and measured rates have been analyzed statistically. Patients underwent an aerobic rehabilitation program that followed the American College of Sports Medicine guidelines. Pacing rate at the anaerobic threshold was considered the border for an aerobic workload. After 2–7 months, all patients showed an evident improvement in ATT (92%), VO₂/AT(48%), TET(49%), VO-,Max (23%), and VO₂ AT/VO₂ (21%). Particularly, two patients improved from Weber Class C to A, and three improved from C to B. At each CPX step, the mean values of pacing rate and theoretical rate almost overlap, and the relevant coefficients suggest an excellent correlation (P < 0.001). Our OPR method for tailoring rate response provides the basis for an effective rehabilitation program with functional advantages for patients.     

An Integrated Dual Sensor System Automatically Optimized by Target Rate Histogram

The use of combined sensors and advanced algorithms using different principles can improve rate performance over a single sensor system. Combinations of sensors and more sophisticated algorithms, however, invariably increase the complexity of pacemaker programming. An automatically optimized combined minute ventilation and activity DDDR pacemaker was developed to minimize repeated sensor adjustment. The device used subthreshold (below cardiac stimulation threshold) lead impedance to detect lead configuration at implantation automatically, followed by "implant management," including setting of lead polarity and initiation of DDDR pacing. Automatic sensor adaptation was achieved by programming a "target rate histogram" based on the patient's activity level and frequency of exertion, and the rate profile optimization process matched the recorded integrated sensor response to the target rate histogram profile. In nine patients implanted with the DX2 pacemakers, the implant management gave 100% accuracy in the detection of lead polarity. Rate profile optinuzation automatically increased the pacing rate during exercise between discharge and 3-month follow-up (hall walk: 78 ± 3 vs 98 ± 3 beats/min, and maximal treadmill exercise: 89 ± 6 vs 115 ± 5 beats/min, P < 0.001) with a significant increase in exercise duration during maximal exercise (7.18 ± 1 min vs 9.56 ± 2 min, P = 0.05). The accuracy of rate profile optimization versus manual programming was assessed at 1 month, and there was no significant difference between pacing rate kinetics and maximal pacing rate between the two methods of programming. In conclusion, pacemaker automaticity can be initiated at implantation and the self-optimized rate adaptive response appeared to be comparable to that derived from a manual programming procedure, which may reduce the need to perform time consuming sensor programming.     

Clinical Experience with a New Activity Sensing Rate Modulated Pacemaker using Autoprogrammability

Sensolog 703 is a new single chamber activity sensing rate modulated pacemaker that offers an automatic adjustment of settings called Autoset. Units were implanted in 11 patients (mean age: 67 years) for atrioventricular block (two patients), sinoatrial block (three patients), sick sinus syndrome (four patients), chronotropic incompetence (one patient), and atrial fibrillation with slow ventricular response (one patient). The devices were programmed in VVIR mode using Autoset. The accuracy of the settings was verified by the built-in histogram function. In 6/11 patients, these settings were not satisfactory. Autoset was repeated at 6 months (nine patients) and 10 months (five patients) after implantation. External telemetric recordings during daily life activities, Holter monitoring, bicycle or treadmill stress tests helped in the evaluation of the rate response obtained with the automatic programming. The following problems were encountered: maximum pacing rate for a low level of exercise (four patients), insufficient rate increase (four patients), higher pacing rate during low than during heavy exercises (four patients). A time-consuming (15 to 48 minutes) manual programming was necessary in eight out of nine patients (6 months) and five out of five patients (10 months). In our study, Sensolog 703 algorithm tended to behave as an on/off system; automatic programming was time consuming and only indicative.     

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.     

Is Accurate Rate Response Programming Necessary?

Exercise capacity and general well-being are improved by appropriately programmed rate responsive pacemakers when compared to fixed rate units. Ten patients had activity sensing DDDR units implanted for combined AV block and sinus node incompetence. Ten patients had Sensolog activity sensing VVIR units implanted for complete heart block. The effects of over and under programming of rate response in both dual and single chamber activity sensor rate adaptive pacemakers has been assessed subjectively by visual analog scales and specific activity questionnaires and objectively by graded treadmill testing and the performance of standardized daily activities. Patients were randomly programmed to absent rate response (VVI in the Sensolog group), hyporesponsive (DDD in the dual chamber group), appropriate response (VVIR, DDDR according to Manufacturer's instructions) and over responsive (VVIR+, DDDR+) in a double-blind crossover design. Thirty percent of patients demanded early crossover from VVI, 30% from DDDR+ and 50% from VVIR+. Perception of Exercise Capability was similar to objective exercise treadmill times which were shorter in VVI than in VVIR or VVIR+ (P less than 0.05) or control subjects (P less than 0.001). There was no difference between any dual chamber mode or control subjects. General well-being was poorest in DDDR+ and VVIR+ modes despite objective improvement in exercise capacity. Symptoms were least in VVIR and DDDR and all but one patient chose appropriate programming as their overall preferred mode. Thus, even inaccurate rate response programming results in similar and improved exercise capacity compared to absent rate response but overprogramming is unacceptable to most patients, confirming that appropriate programming and sensor specificity is critical in rate responsive pacing.     

Reliability of Minute Ventilation as a Parameter for Rate Responsive Pacing

A minute ventilation sensing rate responsive pacemaker was implanted in 11 patients with bradycardias. Their mean age was 59 +/- 4 years (mean +/- SEM). The pacemaker measures minute ventilation by sensing intravascular impedance using a standard bipolar electrode. The rate responsive programming was simple: apart from ascribing an upper and lower rate, the only programmable parameter was the slope of rate response. This could be derived approximately by assessing the suggested slope value during an exercise test in the 'adaptive VVI' mode. Compared with exercise in the VVI mode, symptom limited treadmill tests in the rate responsive mode showed a 33% improvement of exercise capacity and a 44% improvement of cardiac output as determined noninvasively by continuous wave Doppler measurements of the ascending aortic blood flow. The pacing rate was appropriately increased during a variety of daily activities such as walking at different speeds and gradients, and ascending and descending stairs. Voluntary interference of the respiratory pattern such as during coughing and hyperventilation increased the pacing rate from a resting rate of 70 bpm to 111 +/- 10 and 86 +/- 4 bpm respectively. Continuous talking during exercise attenuated the expected rate response. The pacemaker can sense activity induced by arm swinging. In conclusion, the Meta pacemaker improved cardiac output and exercise capacity in patients with bradycardias. Its rate response was related to workload. Although voluntary interference affected the pacing rate, excessive rate acceleration was not encountered.     

Programming of the Pacing Impulse in Pacemakers Connected to Steroid Lead Systems

The purpose of the study was to compare the feasibility of low amplitude output programming (2.5 V/0.5 msec) 3 or more months after pacemaker implantation in patients receiving steroid and nonsteroid lead systems. Chronic pacing voltage, current, and energy thresholds were determined from 0.05- to 1.0-msec pulse duration in 44 patients with steroid lead systems, and in 36 patients with nonsteroid lead systems; all patients received pacemakers from the same manufacturer, which utilized the same programming and telemetry features. Chronaxie, pulse duration at the lowest pacing current, and energy were assessed from individual threshold curves. Steroid-eluting leads had significantly lower pacing voltage, current, and energy thresholds than nonsteroid leads. A 100% safety threshold margin could be achieved in 43 (98%) patients with steroid lead systems and in 27 (75%;P < 0.05) patients with nonsteroid lead systems with output programming of 2.5 V/0.5 msec. Chronaxie (0.22 ± 0.17 msec vs 0.44 ± 0.31 msec; P < 0.05), pulse duration at lowest pacing current (0.28 ± 0.12 msec vs 0.49 ± 0.22 msec; P < 0.05), and pulse duration at lowest pacing energy (0.31 ± 0.17 msec vs 0.53 ± 0.22 msec; P < 0.05) were significantly shorter for steroid than for nonsteroid lead systems. In 42 patients of the former group, a 100% safety margin could be maintained either with a 2.5 V/0.3 msec or with a 1.6 V/0.5 msec output. Conclusions Low amplitude output programming can be obtained in almost all pacemakers connected to steroid-eluting lead systems, and in a significantly higher number of patients than when connected with nonsteroid leads systems. Moreover, in most cases, a 100% safety margin can be achieved at settings as low as 1.6 V/0.5 msec or 2.5 V/0.3 msec; these different combinations offer potential different opportunies to select definitive programming and reduce pacing current requirements in patients undergoing pacemaker implantation.     

Age and Sex Related Changes in Heart Rate to Ventilation Coupling: Implications for Rate Adaptive Pacemaker Algorithms

Minute ventilation (VE) controlled rate adaptive pacemakers determine the paced rate increase during exercise by measuring changes in transthoracic impedance that have been shown to correlate well with VE. To determine the normal coupling of heart rate (HR) to VE, this relationship was evaluated in 30 younger and 25 older, healthy subjects using peak cardiopulmonary exercise testing. After determining the anaerobic threshold (AT), the linear HR to VE slope was determined both below and above the AT. In addition, the entire curve of the HR to VE relationship was assessed by a "best fit" regression analysis method. The relationship of HR to VE was more often logarithmic in younger as compared to older subjects. The HR to VE slope below the AT was always steeper than above the AT in younger subjects. Females of both age subgroups demonstrated a significantly greater slope below and above the AT. For the appropriate programming of VE controlled, rate responsive pacemakers, one should take into consideration age- and sex-specific differences in the HR to VE relationship throughout exercise. Therefore, age- and sex-specific programmable features for rate responsive parameters should be incorporated into pacemakers using VE controlled rate adaptive algorithms.     

Pacemaker Mediated Tachycardias In Single Chamber Rate Responsive Pacing

LAU, C.-P., ET AL.: Pacemaker Mediated Tachycardias In Single Chamber Rate Responsive Pacing. Although pacemaker mediated tachycardias are classically associated with dual chamber pacemakers, single chamber rate responsive pacemakers are also susceptible to such tachycardias under special circumstances. A unipolar activity sensing rate responsive pacemaker (Activitrax 8403) was implanted in an 83-year-old man with complete atrioventricular block. The pacemaker was programmed at an output of 5 V, activity threshold medium, rate response 5, and lower and upper rates of 70 and 125 beats/min, respectively. He presented with palpitations at rest and muscle twitching of the pacemaker pocket 4 months after implantation. Examination confirmed that the pacemaker had flipped over, resulting in pocket pacing which in turn activated the activity sensor, resulting in a rate response. The increase in pacing rate lead to a higher frequency of pocket pacing, thus leading to positive feedback increase in rate. With the patient at rest, pacemaker mediated rates were 106, 91, and 74 beats/min at low, medium and high thresholds, respectively. Decreasing the output to 2.5 V eliminated pocket pacing and the tachycardia. As a result of the reversal of the pacemaker, a similar rate response during exercise could only be achieved at a more sensitive rate responsive setting. Thus, pacemaker mediated tachycardia can complicate pacemaker "flipping" in single chamber activity sensing rate responsive pacemakers. Methods for the avoidance and treatment of pacemaker flipping are discussed. A review of other sensor mediated tachycardias is also presented.     

Comparison of Externally Strapped Versus Implanted Accelerometer- or Vibration-Based Rate Adaptive Pacemakers During Various Physical Activities

The ability of externally strapped accelerometer(Excel [Cardiac Pacemakers, Inc.]) and vibration-based (Activitrax [Medtronic, Inc.]) rate adaptive pacemakers to reproduce the rate response of the same implanted devices with identical programming was evaluated in ten patients by ambulatory Holter monitoring. The resting and postexercise external pacemaker rates closely resembled those of the respective implanted devices. During short bursts and more prolonged exercise, both types of strapped-on devices underestimated maximal implanted pacemaker rate response by4%-10% when programmed to nominal rate adaptive settings. Studies evaluating chronotropic responses from either type of externally strapped activity sensor appear valid, provided the modest attenuation in maximal rate increase by this method is appreciated.