A blended sensor restores chronotropic response more favorably than an accelerometer alone in pacemaker patients: the LIFE study results

Background: Adaptive rate sensors used in permanent pacemakers incorporate an accelerometer (XL) to increase heart rate with activity. Limited data exists regarding the relative benefit of a blended sensor (BS) (XL and minute ventilation) versus XL alone in restoring chronotropic response (CR) in chronotropically incompetent (CI) patients. Methods: One thousand five hundred thirty‐eight patients from the limiting chronotropic incompetence for pacemaker recipients (LIFE) study were implanted with a pacemaker and 1,256 patients had data collected at 1 month. Patients performed a treadmill test 1‐month postimplant while programed in nonrate responsive mode (DDD‐60) to determine CI. Only patients who completed at least three exercise stages and achieved a peak perceived exertion ≥16 were included in the analyses. The metabolic chronotropic relationship (MCR) slope was used to evaluate CR in 547 patients. Patients were randomized to XL or BS with a conservative fixed rate response factor (XL = 8, MV = 4). CI patients performed a follow‐up 6‐month treadmill test. Results: CI prevalence in this patient population (n = 547) was 34%. No differences in baseline characteristics existed between groups. Although both groups showed significant within‐group improvements in MCR slope from 1 to 6 months (both P < 0.001), the BS group had a significantly higher MCR slope at 6 months compared to the XL group (P = 0.011). Improvement in quality of life (QOL) did not differ between groups . Conclusions: In this general pacemaker population with CI, a BS programed empirically restores CR more favorably than an XL sensor programed nominally. Further studies are needed to determine if individual sensor optimization would lead to improvement in functional capacity, higher MCR slopes, and QOL.     

Correlation of Impedance Minute Ventilation with Measured Minute Ventilation in a Rate Responsive Pacemaker

Although rate responsive pacing based on impedance minute ventilation (IMV) is now standard, there is almost no data confirming the relationship between IMV from an implanted pacemaker and measured minute ventilation (VE) during exercise. Nineteen completely paced adults implanted with Medtronic Kappa 400 pacemakers underwent symptom-limited maximal metabolic treadmill testing using a modified Minnesota Pacemaker Response Protocol. Minute ventilation (VE, L/min) was simultaneously measured using the flowmeter of a respiratory metabolic gas analysis system and the transthoracic impedance minute ventilation circuitry of the pacemaker. Correlation coefficients (r) were used to find the best fit line to describe the relationship between the two measurements. Mean (+/- SD) r values for the first, second, and third order polynomial equations and for log and exponential equations were: 0.92 +/- 0.08, 0.94 +/- 0.04, 0.95 +/- 0.04, 0.91 +/- 0.06, and 0.91 +/- 0.07, respectively. None of the r values were statistically different from the first order equation. Transthoracic IMV as measured by the Medtronic Kappa 400 is closely correlated to measured minute ventilation and is represented well by a first order (linear) equation.     

New Algorithms to Increase the Initial Rate Response in a Minute Volume Rate Adaptive Pacemaker

Background : Minute volume is a truly physiological sensor for rate adaptive pacing that correlates with metabolic expenditure throughout the range of physical activity. Criticism has centered on the slow initial response compared to less physiological sensors. A new algorithm, consisting of rate augmentation factor and programmable speed of response, has been incorporated in the 1206 META III pacemaker generator and was designed to improve the rate response at lower levels of exertions. Rate augmentation factor increases the programmed rate response factor by 3, 6, or 10 when set to low, medium, or high, respectively; this augmentation lasting to 50% of the maximum programmed rate. Response time can be programmed to medium or fast. Methods : Nine patients were studied during the first 3 minutes of an exercise test (Bruce protocol) in a single blind manner. The pacemaker generator was randomly programmed with rate augmentation factor at off, low, or high and speed of response to medium or fast, giving six possible combinations. Heart rates were recorded continuously for the duration of the test and until resting heart rate was achieved during recovery. The test was repeated until all six combinations had been tested. Results : During exercise significant differences appeared in response time from 30 seconds onward. Fast response and rate augmentation factor contributed to an improved rate response with greatest speed of response seen with fast response time and high rate augmentation factor. During recovery decreases in recovery time were seen with fast response time but rate augmentation factor prolonged recovery. Conclusions : Rate augmentation factor improves initial rate response in the early stages of exercise. Fast response gives an improved time to initial rate increase and shortens the duration of inappropriate postexercise tachycardia. These features improve the pattern of response of the minute ventilation sensor.     

2型糖尿病患者的心率变时性功能不全及其意义

目的:检测2型糖尿病患者运动时心率变时性功能不全情况。方法:2型糖尿病患者进行运动平板试验,计算其心率变时性反应指数,并与非糖尿病患者进行比较。结果:69例2型糖尿病患者心率变时性反应指数为0.74±0.16,明显低于对照组0.90±0.17(P<0.0001),提示2型糖尿病患者存在心率变时性功能不全,并且与有无心血管并发症及有无缺血性ST改变无关。结论:2型糖尿病患者较普遍存在运动心率变时性功能不全情况,可能与心脏自主神经损害有关。     

Comparison of impedance minute ventilation and direct measured minute ventilation in a rate adaptive pacemaker

Respiration rate (RR) and minute ventilation (MV) provide important clinical information on the state of the patient. This study evaluated the accuracy of determining these using a pacemaker impedance sensor. In 20 patients who were previously implanted with a Guidant PULSAR MAX group of pacemakers, the telemetered impedance sensor waveform was recorded simultaneously with direct volume respiration waveforms as measured by a pneumatometer. Patients underwent 30 minutes of breathing tests while supine and standing, and a 10-minute ergonometer bicycle exercise test at a workload of 50 W. Breathing tests included regular and rapid-shallow breathing sequences. RR was determined by a computerized algorithm, from impedance and respiration signals. The mean RR by impedance was 21.3 +/- 7.7 breaths/min, by direct volume was 21.1 +/- 7.6 breaths/min, range 7-66, the mean difference of RR measured by the impedance sensor, as compared with the true measurement, being 0.2 +/- 2.1 breaths/min. During the entire exercise, the mean correlation coefficient between impedance (iMV) and direct measured MV was 0.96 +/- 0.03, slope 0.13 +/- 0.05 L/Omega and range 0.07-0.26 L/Omega. Bland-Altman limits of agreement were +/- 4.6 L/min for MV versus iMV with each patient calibrated separately. The correlation coefficient for iMV versus MV over the entire 10 minutes of exercise, including the initial 4 minutes of exercise, was 0.99. The transthoracic impedance sensor of an implanted pacemaker can accurately detect respiration parameters. There was a large variation between subjects in the iMV versus MV slope during a bicycle exercise test, whereas for each subject, the slope was stable during submaximal bicycle exercise.     

Rate responsive pacing using transthoracic impedance minute ventilation sensors: a multicenter study on calibration stability

Previous studies showed that transthoracic impedance minute ventilation (IMV), as measured by a pacemaker sensor, is closely correlated to actual minute ventilation (VE) determined by standard methods. The aim of this study was to analyze the changes in the calibration between IMV and VE at rest and during exercise over time. Fifteen patients (age 60 +/- 13 years) with Medtronic Kappa 400 pacemakers completed a baseline visit followed by two visits separated by 1 month and 1 week, respectively. In each patient, VE (L/min) was monitored at rest in the supine and sitting positions and during graded bicycle ergometer exercise using a standard cardiopulmonary metabolic gas analysis system with simultaneous recording of IMV (omega/min) using DR-180 extended telemetry monitors. Calibration at rest was defined as the ratio of IMV to VE, calculated from 1-minute average values in the supine and sitting positions. Calibration during bicycle exercise was defined as intercept (IMV value at VE = 10 L/min-typical VE value at beginning of exercise), and slope of the IMV/VE regression line. The calibration of IMV showed individual variability over time. The magnitude (absolute value) of observed fractional changes in calibration at 1 month was 0.23 +/- 0.20 (rest-supine), 0.20 +/- 0.15 (rest-sitting), 0.18 +/- 0.19 (exercise-intercept), 0.28 +/- 0.35 (exercise-slope), and 0.18 +/- 0.15, 0.15 +/- 0.09, 0.28 +/- 0.39, and 0.27 +/- 0.15, respectively, at 1 week. The magnitude of change at 1 month was not statistically different from the magnitude of change at 1 week. In conclusion, the calibration of IMV, as measured by a pacemaker sensor, versus actual VE may demonstrate variability. However, this study also suggests that the observed changes are not cumulative over time. These results have implications for patient monitoring applications using these sensors and for development of future pacemaker rate response algorithms.     

Chronotropic incompetence in patients with an implantable cardioverter defibrillator: prevalence and predicting factors

Chronotropic incompetence (CI), which has not been systematically examined in the ICD patient population, may have implications for device programming. A total of 123 ICD patients were classified into three groups: single-chamber ICD with sinus rhythm, dual-chamber ICD with sinus rhythm, and single-chamber ICD with permanent atrial fibrillation. Heart rate response, maximum oxygen uptake, and oxygen uptake at the anaerobic threshold were measured during treadmill exercise testing. In addition, clinical variables such as antiarrhythmic drug therapy, underlying heart disease, and left-ventricular (LV) ejection fraction were recorded. Of the patients studied, 38% were chronotropically incompetent (47/123). Significant predictors of CI were as follows: presence of a coronary disease (P = 0.036), prior cardiac surgery (P = 0.037), chronic drug therapy with beta-blockers (P = 0.032), administration of amiodarone (P = 0.025), and a combination of these two forms of treatment (P = 0.01). Spiroergometry revealed reduced exercise capacity (P = 0.041) and lessened VO2max (P = 0.034) among chronotropically incompetent patients. A large percentage of ICD patients demonstrates CI with subsequently reduced physical stress tolerance. In light of the DAVID study, we believe that a closer examination of rate-adaptive modes for ICD patients is warranted under enhanced conditions: (1) optimized AV interval programming; (2) utilization of new algorithms to reduce ventricular pacing in combination with rate-adaptive atrial pacing, with the goal of addressing CI while minimizing ventricular pacing; and (3) an optimized upper heart-rate limit.     

Follow-Up of a Minute Ventilation Rate Adaptive Pacemaker

Although rate adaptive pacemakers are now frequently prescribed, there is limited information regarding long-term follow-up of patients with a pacemaker capable of rate adaptation. We have examined our patients in whom a pacemaker capable of rate adaptation via a sensor that determines minute ventilation has been implanted. After following a group of 42 patients for a mean of 13.2 months we have found this to be a reliable rate adaptive pacing system. The sensor was reliable long term, the system is easy to program, and sensor settings were changed infrequently.     

Comparison of Normal Sinus Rhythm and Pacing Rate in Children with Minute Ventilation Single Chamber Rate Adaptive Permanent Pacemakers

Rate adaptive pacemakers are used to achieve a better cardiac performance during exercise by increasing the heart rate and cardiac output. The ideal rate adaptive sensor should be able to mimic sinus node modulation under various degrees of exercise and other metabolic needs. Minute ventilation sensing has proven to be one of the most accurate sensor systems. In this study, alterations in sinus rhythm and pacing rates during daily life conditions in 11 children (median age 11 years, range 6–14 years) with minute ventilation single chamber pacemakers were investigated. Correlation of sinus rhythm with pacing rates was assessed. ECG records were obtained from 24–hour Holter monitoring. Average rates of five consecutive P waves and pace waves were determined every half hour. The average of the two values was then used to determine hourly rates. Correlation coefficients between the sinus rhythm and pacing rates were calculated. In nine patients, pacing rates correlated well to sinus rhythm (range 0.6793–0.9558. P < 0.001 and P < 0.05), whereas in two cases correlation was not sufficient (P > 0.05). Most of the patients, in whom rate response factor (RRF) measurements during peak exercise by treadmill with cnronotropic assessment exercise protocol were performed and pacemakers were programmed to these parameters, had more appropriate ventricular rates compared to spontaneous sinus rates. In these patients mean RRF value was 15.3 ± 2.7 (range 12–20, median 15). This study shows that during daily activities minute ventilation rate adaptive pacemakers can achieve pacing rates well correlated to sinus rhythm that reflects the physiological heart rate in children.     

Assessment of the Chronotropic Response at the Anaerobic Threshold: An Objective Measure of Chronotropic Function

MEINE, M., et al. : Assessment of the Chronotropic Response at the Anaerobic Threshold: An Objective Measure of Chronotropic Function. The evaluation of the heart rate response to exercise is important for the diagnosis of chronotropic incompetence and the assessment of a rate responsive algorithm of sensorcontrolled pacemakers. The aim of the present study was to examine a classification of the chronotropic response at an individually moderate exercise level. Sixteen pacemaker patients (patient group, age 62.9 ± 7.6 years ) with sick sinus syndrome and 15 age‐matched healthy subjects (control group, age 57.6 ± 9.4 years ) underwent a maximum cardiopulmonary exercise test on a treadmill after a protocol with individually selected incremental steps. To analyze the patients' intrinsic heart rate response, the rate responsive mode of the pacemaker was switched off. Chronotropic incompetence was diagnosed in eight patients whose maximal heart rate was < 80% of the age‐predicted heart rate. The heart rate at the anaerobic threshold was significantly lower in the chronotropically incompetent subgroup than in the chronotropically competent patients and the healthy subjects (85.9 ± 6.6 beats/min vs 100.3 ± 9.9 beats/min and 112.9 ± 11.7 beats/min , respectively). The chronotropic slope of the heart rate reserve as a function of the metabolic reserve was significantly higher in the control group than in the patient groups with either mild or severe chronotropic incompetence (0.94 ± 0.17 vs 0.64 ± 0.08 and 0.43 ± 0.14 , respectively). Furthermore, the chronotropically incompetent response could be divided into a linear type with and without a threshold, an exponential, and a logarithmic type. The anaerobic threshold was an objectively detectable breakpoint at an individually moderate exercise level that could be used for characterization of chronotropic function. At the anaerobic threshold, a physiological heart rate response was about 220 ‐ age – 50 beats/min. A deviation of more than 10 beats/min below this physiological value characterized chronotropic incompetence.     

A Rate Responsive Pacemaker that Physiologically Reduces Pacing Rates at Rest

Current rate responsive pacemakers incorporate sensors such as minute ventilation (MV) for adapting to changing patient conditions during exercise and periods of exertion. However, for sleep and/or rest periods, the only pacemakers currently on the market that slow the pacing rate utilize an internal timer to determine a decrease in pacing rate. It would be advantageous if the pacing rate could be automatically lowered during periods of sleep or rest. This study utilized a rate responsive sensor, MV, to track the patient's sleeping and resting periods and to decrease the pacing rate at such times. A total of eight patients implanted with Sentri 1210 single chamber MV sensor pacemakers were studied. A sleep rate (SH) of 45 beats/min was selected. A sleep rate response function, which indicated the relationship between changes in MV and corresponding heart rate, was initially set at a value of 16 and continually and automatically updated in a 3-month study. Adaptation was based on the premise that 3 hours per day should be spent at the SR. The average decrease in pacing rates from onset to 3 month for the eight patients was 12.4%± 5.3%. Correspondingly, the histograms of the lowest datalog histogram (40–59 beats/min) increased from 0% to 15.4%± 0.9% of paced beats. Correlation between the patients' 24-hour diary and Holter recordings showed that the pacing rates during sleep were consistently lower than when the patients were awake and active. This was also the case with a patient whose nocturnal and daily routine was intentionally altered. In conclusion, the MV sensor has shown to be an accurate sensor for the automatic determination of periods of sleep or rest. Data demonstrates that the decreased average pacing rate will increase pacer longevity as well as potentially provide more physiological pacing rates at night.     

Accelerometer-Based Adaptive-Rate Pacing: A Multicenter Study

A new adaptive rate pacemaker, using an acceleromeler to detect body motion as an indicator of metabolic demand, was evaluated in 13 centers located in five countries. During the study, 55 patients with a broad range of indications were implanted with the EXCEl™ VR single chamber pacemaker. One month evaluations were completed on a subgroup 0f 37 patients. The pacemaker response was optimized before discharge using a simple walking test. An "exercise test" recording feature in the programmer eliminated the need for ECG monitoring. At nominal settings, the mean (± SD) pacing rate increased 29 ± 9 ppm while walking slowly and 44 ± 11 ppm (n = 33) during a brisk walk. This information was used to optimize the parameter settings. Without further changes to the programmed settings the patients were subsequently tested 2–4 weeks postimplant with a symptom-limited treadmill test using the chronotropic assessment exercise protocol (CAEP). Mean pacing rate increased from 74.5 ±2.5 ppm (n = 24) at rest to 118.6 ± 12.2 ppm (n = 21) at 3 mph/6% grade to 143.5 ± 3.5 ppm [n = 2) at 3.8 mph/8% grade. Conclusion: The individual responses were proportional to the physical exertion imposed on the patients. Pacing rates were considered to be appropriate using the chronotropic response zone as a criterion for appropriate rate modulation. A simple walking test utilizing the exercise test facility, results in appropriate optimization of the pulse generator to the individual patient.     

Reference values for bicycle exercise tests in patients with various degrees of ischaemic heart disease

Summary. This investigation was carried out in an effort to analyze the influence of various degrees of ischaemic heart disease (IHD) on cardiovascular and physical performance. Assessment of the severity of IHD was based on observations obtained routinely during exercise tests (ST segment response and systolic blood pressure response). The study group included 926 subjects with known or suspected IHD, who were referred for an exercise testing; 268 females, mean age 54 years (range 19–89 years), and 658 males, mean age 52 years (range 16–88 years). We found that increasing IHD severity was associated with significant reductions of cardiovascular performance. The mean maximum work-load (MWL) was lower in females than in males, and MWL as well as mean maximum heart rate (MHR) and mean maximum change in systolic blood pressure (M Δ SBP) decreased with increasing IHD and age. The present results may be used to assess the cardiovascular response to exercise in patients with IHD so that altered responses due to causes other than IHD may be identified. Furthermore the result may prove useful in the adjustment of rate responsive pacemakes (RRP) in patients with IHD.     

The Effect of Maximum Heart Rate On Oxygen Kinetics and Exercise Performance at Low and High Workloads

The normal heart rate is lineurly related to oxygen consumption during exercise. The maximum heart rate of the normal sinus node is approximated by the formula: HR_max= (220-age) with a variance of approximately 15%. However, the nominal upper rate of most permanent pacemakers is 120 beats/min, a value that remains unchanged for many patients. As this nominal setting falls well below the maximum predicted heart rate for most patients, it is possible that the chronotropic response of rate adaptive pacemakers during moderate und maximal exercise workloads may be less than optimal. The purpose of this study was to determine the effect of the upper programmed rate on oxygen kinetics during submaximal exercise workloads and maximum exercise performance during symptom-limited treadmill exercise. Exercise performance with an upper rate programmed to 220-age was compared with an upper rate of 120 beats/min. Eleven patients (5 men and 6 women, mean age 54 ± 10 years) with complete heart block following catheter ablation of the atrioventricular junction for refractory atrial fibrillation who were implanted with permanent, rate-modulating VVIR pacemakers comprised the study population. The rate adaptive sensors were based on activity in 8 patients, minute ventilation in 2 patients, and mixed venous oxygen saturation in 1 patient. After performing a symptom-limited treadmill exercise test to determine maximum exercise capacity and to optimize programming of the rate adaptive sensor, each subject performed two treadmill exercise tests in random sequence with a rest period of at least 1 hour between tests. During one of the tests the upper rate was programmed to a value calculated by the formula: HR_max= (220-age). During the other exercise test the upper rate was programmed to 120 beats/min. Patients were blinded as to their programmed values and to the hypothesis of the study. A novel treadmill exercise protocol was used that consisted of a 6 minute, constant-workload phase at approximately 50% of maximum workload followed immedictely by incremental, symptom-limited exercise using a modified Chronotropic Assessment Exercise Protocol (CAEP) with 1 minute stages until peak exertion. Breath-by-breath analysis of expired gases was performed with subjective scoring of exertional difficulty at the end of the constant workload phase and during each stage of incremental exercise using the Borg Perceived Exertion Scale. Exercise duration was significantly longer (6.37 ± 47 vs 611 ±48 seconds. P < 0.005) with the higher programmed upper rate. Oxygen kinetics were also significantly improved with an age predicted upper rate with a lower O₂ deficit (258 ± 88 vs 395 ± 155 ml, P = 0.002) and higher VO₂ rate constant (3.6 ± 1.0 vs 2.4 ± 0.7. P < 0.001.). The V0_2maxduring peak exertion was higher with an age predicted upper rate than with an upper rate of 120 beats/min (1807 ± 751 vs 1716 ± 702 mL/min, P = 0.01). The mean Borg score was lower during the last common treadmill stage during maximum exercise with an age predicted upper rate than with an upper rate of 120 beats/min (15.7 ± 2.0 vs 16.5 ± 1.9. P = 0.04). The mean Borg score during submaximal. constant workload exercise was also lower with a higher upper rate (9.0 ±2.5 vs 9.6 ± 2.2, P = 0.10). Programming the upper rate of rate adaptive pacemakers based on the age of the patient improves exercise performance and exertional symptoms during both low and high exercise workloads as compared with a standard nominal value of 120 beats/min.     

Pacemaker auto-interference by thoracic impedance measurement for the rate response function

This report describes two cases of pacemaker auto-interference caused by thoracic impedance measurements for the rate response function of a dual chamber pacemaker. Mechanisms of this phenomenon and reprogramming strategies are discussed.     

12周运动康复对冠心病患者心脏变时性功能和心率恢复值的影响

目的:探讨12周运动康复对冠心病患者心脏变时性功能和心率恢复值(HRR1)的影响。方法:对30例男性冠心病患者进行递增负荷运动试验,根据实验结果将其分为两组,变时性功能正常组(G1,n=19,HRR%≥0.8);变时性功能不全组(G2,n=11,HRR%0.8),并测定受试者运动前、运动中和运动后心率(HR)、摄氧量(VO2)、ST段下降数值和血压。12周运动康复后再次进行递增负荷运动实验,对患者的运动能力和上述指标进行测定分析。结果:与康复程序前相比,12周康复程序后,G1和G2组的VO2peak、HRR1均有显著增加(G1,8%,P0.01;G2,4%,P0.05)、(G1,18.6±3.3—23.8±4.5,P0.01;G2,12.8±4.2—13.7±3.5,P0.05),而G1比G2提高的更加明显。ST段降低显著改善(G1,-0.7±0.5—-0.2±0.5,P0.05;G2,-0.9±0.5—-0.3±0.6,P0.05)。HRR%有显著增加(G1,84.2±2.5—88.9±6.1,P0.05;G2,60.2±8.3—75.8±3.8,P0.05)。结论:12周运动康复可以改善冠心病患者(包括变时性功能正常者和变时性功能不全者)的运动能力,提高冠心病患者运动时自主神经的调节功能,上述积极作用对变时性功能正常者的体现则更为明显。     

Lack of Physiological Adaptation of the Atrioventricular Interval to Heart Rate in Patients Chronically Paced in the AAIR Mode

Seventeen consecutive patients, aged 56 +/- 12, were chronically paced in the AAIR mode for a symptomatic sinus node disease with atrial chronotropic incompetence defined by a peak exercise heart rate (HR) less than 75% of the maximal predicted heart rate (MPHR) mean = 65 +/- 10%). Sensors used were activity sensing (n = 7), minute ventilation (n = 6), or respiratory rate (n = 4). Basic pacing rate was programmed at 71 +/- 5 beats/min and the maximal sensor rate at approximately 85% MPHR (143 +/- 10); other sensor parameters were programmed individually. Six months after implant, two standardized and symptom limited exercise tests were performed in random order, AAI and AAIR modes, respectively. AAIR pacing significantly improved peak exercise HR (139 +/- 14 vs 112 +/- 30 beats/min; P less than 0.01), maximal sustained workload (132 +/- 42 vs 110 +/- 38 watts; P less than 0.02), and total exercise duration (724 +/- 299 vs 594 +/- 245 sec; p less than 0.02) compared to the AAI mode. In all 17 patients, HR was continuously sensor driven in the AAIR mode, making it possible to precisely study the adaptation of the stimulus-R interval and of the stimulus-R:RR ratio during exercise. Six patients normally adapted with a progressive shortening. Six others did not adapt at all without any variation of interval. Five patients paradoxically increased their stimulus-R interval (286 +/- 10 msec at peak E vs 220 +/- 19 msec at rest) and their stimulus-R:RR ratio (67 +/- 20% vs 29 +/- 4%), producing P waves occurring immediately after, or even within the R wave of the preceding cycle; two patients complained of severe exercise related symptoms corresponding to the so-called "AAIR pacemaker syndrome." The principal factors involved in the nonadaptation of AV interval to HR were related to the patient (organic heart disease, with the particular problem of the denervated heart; the bradytachy syndrome; and the use of drugs, especially beta blockers and Class I antiarrhythmic drugs) or to the pacemaker ("overstimulation" phenomenon). These observations constitute an additional argument for wider indications of implanting DDDR units in these patients.     

Clinical Comparison of Acute Single to Dual Chamber Pacing in Chronotropically Incompetent Patients with Left Ventricular Dysfunction

Dual chamber, rate responsive (DDDR) pacing is felt to be superior to ventricular, rate responsive (VVIR) pacing since it more closely mimics the normal electrical and hemodynamic activity of the heart. This reasoning has been used to justify the higher initial costs and increased complexity of dual chamber systems. This study was designed to determine if objective criteria could be identified during acute testing to justify implanting a dual chamber instead of a single chamber system in patients with left ventricular dysfunction. Eight patients with DDDR pacemakers (implanted for chronotropic incompetence) and left ventricular dysfunction underwent exercise radionuclide angiography and graded exercise treadmill testing. Each patient performed the tests in the single (VVIR) and dual (DDDR) chamber modes in a randomized, blinded fashion. We found that objective parameters such as ejection fraction (31%± 13% vs 31%± 10%), exercise tolerance (6.1 ± 2.7 min vs 6.3 ± 2.9 min), oxygen consumption (VO₂) (941 ± 286 mL/min vs 994 ± 314 mL/min), carbon dioxide production (VCO₂) (995 ± 332 mL/min vs 1054 ± 356 mL/min), and maximum attainable workload (43 ± 24 W vs 46 ± 22 W) did not differ between the single and dual chamber pacing modes. These findings suggest that in the acute setting, the additional cost and complexity of dual chamber, rate responsive pacing cannot be justified by objective improvements in exercise tolerance in patients with underlying left ventricular dysfunction.     

Atrial Synchronized Ventricular Pacing: Contribution of the Chronotropic Response to Improved Exercise Performance

In contrast to asynchronous ventricular pacing (VOO, VVI), alrial synchronized ventricular pacing (VAT, VDD, DDD) maintains the normal sequence of cardiac chamber activation and permits a chronotropic response to exercise. thereby improving exercise performance. To assess the separate contributions of these two factors to improved work capacity. 14 patients with implanted programmable VAT pacemakers were exercised according to the Bruce protocol, in three different pacing modes, selected in a random orderand on a double blind basis: (a) VAT: (b) chest wall stimulation triggered ventricular (V-CWS-T) pacing, during which the pacemaker was programmed to VAT mode but driven externally using chest wall stimulation at rates fractionally above the patients'atrial rate, thereby providing a chronotropic response to exercise without atrioventricular synchronization; and (c) VOO mode at 70 beats per minute. There was a significant improvement in exercise performance in all patients during both VAT and V-CWS-T pacing as compared to VOO mode; the average increase in work capacity being similar: VAT: 44 ± 31, (range, 12 to 140) percent and V-CWS-T; 40 ± 24 (range, 5 to 85) percent. It is concluded that in patients with adaptive pacing systems, the chronotropic response is the major determinant of any improvement in exercise performance.