Initial Clinical Experience with Rate Adaptive Cardiac Pacing Using Two Sensors Simultaneously

LANDMAN, M.A.J., ET AL.: Initial Clinical Experience with Rate Adaptive Cardiac Pacing Using Two Sensors Simultaneously. In the rate adaptive pacemakers, all presently available sensors show one or more drawbacks. Combining two sensors in a single pacemaker, we tried to optimize its rate responsive characteristics. In this study, we present the rate adaptive behavior of a two sensor pacemaker system, using both QT interval and activity sensing. In addition, we compared the rate response with that of each sensor alone. Nine patients with an implanted QT interval sensing pacemaker, and an externally attached activity sensing pacemaker performed three exercise stress tests on treadmill. The QT interval, measured by the implanted pacemaker, and the activity level, were transmitted to an external computer. This computer contained the two sensor rate adaptive algorithm, and reprogrammed the implanted pacemaker on beat-to-beat basis. Conclusion: In the two sensor mode the rate increases immediately at the onset of exercise, caused by the prompt response of the activity sensor. Further rate increase is driven by the QT interval sensor and therefore proportional to the level of exercise. Furthermore, the rate decay during the recovery phase is more physiological.     

Clinical Experience with the Rate Responsive Pacemaker Sensolog® 703

Pacemaker therapy with rate responsive systems becomes increasingly more important. Numerous clinical and experimental trials have been conducted thus far to investigate different sensors of metabolic demands of the body, functioning independently of an atrial electrogram. To transform mechanical energy into electrical energy, activity-triggered systems utilize a piezo-quartz as their sensor. Thirty jive patients suffering from pathological bradyarrhythmias and therefore from an inadequate increase of the heart rate during exercise were supplied with a Sensolog® 703. Clinical follow-up was first performed with the help of treadmill ergometry, later on by walking on flat ground and climbing steps. The settings due to the first test caused inadequately high stimulation rates. A 24-h-Hoher-ECG was necessary to control the programming and the rate profile in the patient's daily life. Using only the histogram and the rateread function, all rate adaptive parameters of the Sensolog® 703 could be set by walking on flat ground and climbing steps. We defined a frequency of 75–85 min⁻¹ to be appropriate during walking, respectively "low work." Individual programming ranged from 2–5/8–15/high-low/very fast-medium/fast-medium (slope, threshold, gain, reaction and recovery time). Reprogramming based on 24-h-Holter ECG was only necessary if the pacemaker was formerly adjusted with the help of treadmill ergometry. Besides, we observed an increase of the stimulation rate due to vibrations not related to exercise, e.g., by driving in a car.     

Rate Responsive Pacemakers: Assessment after Two Years

Rate responsive pacemakers (RRPM) for patients without sufficient response of the heart rate (HR) to exercise represent an alternative to improve cardiac output (CO) and capacity for exercise via an increase of HR. From 1983 until December 1985, we implanted 30 QT-related (TX), 25respiratory dependent (RDP), and 35 body activity directed (ACT) pacemakers. The follow-up examination consisted of Holter-ECG, treadmill and/or bicycle workload, and determination of CO (TX: thermodilution technique, n = 11; 6 months after implantation. RDP: equilibrium radionuclide ventriculography, n = 13; 1 month after implantation). The capacity for exercise of patients with ACT was studied using a climbing step. Adaptation of HR could be achieved with TX. RDP, and ACT. There was a significant increase in CO during exercise TX or RDP versus VVI-mode (TX: δ= 1.7l/min, RDP: δ= 2.1 l/min).Although all systems exhibited weak points, RRPM are reliable devices. We abide by using these pacemakers.     

Adequacy of Pacing Rate During Exercise in Rate Responsive Ventricular Pacing

Our objective was to determint; the adequate pacing rate during exercise in ventricular pacing by measuring exercise capacity, cardiac output, and sinus node activity. Eighteen patients with complete AV block and an implanted pacemaker underwent cardiopulmonary exercise tests under three randomized pacing rates: fixed rate pacing (VVJ) at 60 beats/min and ventricular rate-responsive pacing (VVIR) programmed to attain a heart rate of about 110 beats/min ar 130 beats/min (VVIR 110 and VVIR 130, respectively) at the end of exercise. Compared with VVI and VVIR 130, VVIR 110 was associated with an increased peak oxygen uptake(VVIR 110:20.3 ± 4.5 vs VVI: 16.9 ± 3.1; P < 0.01; and VVIR 130: 19.0 ± 4.1 mL/min per kg, respectively; P < 0.05) and a higher oxygen uptake at anaerobic threshold (15.3 ± 2.7, 12.7 ± 1.9; P < 0.01, and 14.6 ± 2.6 mL/min per kg; P < 0.05). The atrial rate during exercise expressed as a percentage of the expected maximal heart rate was lower in VVIR 110 than in VVI or VVIR 130 (VVIR 110: 75.9%± 14.6% vs VVI: 90.6%± 12.8%; P < 0.01; VVIR 110 vs VVIR 130: 89.1%± 23.1%; P < 0.05). There was no significant difference in cardiac output at peak exercise between VVIR 110 and VVIR 130. We conclude that a pacing rate for submaximal exercise of 110 beats/min may be preferable to that of 130 beats/min in respect to exercise capacity and sympathetic nerve activity.     

Adaptive Rate Pacing Controlled by the Right Ventricular Preelection Interval: Clinical Experience With a Physiological Pacing System

In the Precept pacing system, the right ventricular intracardiac impedance waveform is used to evaluate either of two indicators of metabolic demand relative right ventricular stroke volume and preejection interval (PEI). PEI is known to reliably parallel contractility changes, which is reflective of physical and emotional stress. The stability and dynamic behavior of PEI were tested in ten patients with a Precept pacing system under various forms of exercise and during postural changes. Although significant patient-to-patient variability of the sensor values was observed, reflecting individual physiological differences, the chronic stability of PEI was excellent in the total device experience of 147 months. In all patients, PEI shortened significantly during bicycle ergometry from a mean value of 137.7 +/- 17.8 (range 96-162) to a mean value of 103.0 +/- 21.6 (range 92-109) (P less than 0.05). Low level bicycle exercise of short duration resulted in a prompt decrease in PEI and increase in pacing rate in all patients. There were no uniform postural responses overall, although some posture related rate changes were observed in two patients. We conclude that the first generation of a PEI based pacing system holds promise for adaptive rate pacing.     

The Range of Sensors and Algorithms Used in Rate Adaptive Cardiac Pacing

Implantable sensors play an important role in physiological cardiac pacing. Sensors can be classified according to the technical methods in which sensing is achieved: the sensing of the evoked ventricular response, intrathoracic impedance and body acceleration forces, and the incorporation of special sensors on pacing electrodes. These sensors differ in their relative merits in terms of speed, proportionality, sensitivity, and specificity of rate response. The efficacy of a sensor can be significantly modified by the algorithm used in relating sensor signal to a pacing rate change. The currently available types of sensors and algorithms are summarized and compared in this review article. The relative merits of these sensors and algorithms form the basis for designing a multisensor pacing system.     

Clinical Experience with Sensolog 703: A New Activity Sensing Rate Responsive Pacemaker

Sensolog 703 is a new activity sensing rate responsive pacemaker which detects body vibration during physical exercise and uses the vibration as an indicator of the physiological need for a rate increase. This pacemaker was implanted in 11 patients with complete heart block and atrial arrhythmias. Their mean age was 58 (range 39-72) years. With appropriate rate response, exercise capacity, as assessed by the duration of graded treadmill exercise using the Bruce protocol, was significantly improved over the VVI pacing mode (mean +/- SEM, 462 +/- 52 s in the rate responsive mode and 368 +/- 34 s in the VVI mode, P less than 0.02). Cardiac output at peak exercise, as assessed by continuous wave Doppler sampling of aortic root blood flow, was also significantly increased compared to the resting value in both piecing modes. However, the increase was more marked when exercise was performed in the rate response mode (93 +/- 22% increase over resting cardiac output in the rate responsive mode and 57 +/- 13% increase in the VVI mode, P less than 0.05). The rate responses of this pacemaker were compared with those of a Medtronic Activitrax pacemaker. Although both pacemakers responded to an increase in walking speed, neither responded appropriately to walking up different gradients, In both cases, ascending and descending four flights of stairs resulted in similar pacing rates. There was no response to physiological activities with minimal body movements such as isometric exercise and the Valsalva maneuver. Technical problems were encountered in two implanted Sensolog pacemakers: one had spontaneous rate acceleration at rest immediately following implantation and one showed intermittent rate acceleration while the patient was at rest. Both units were programmed to the VVI mode. In conclusion, satisfactory rate response, improvement in exercise duration and increase in cardiac output were achieved with the Sensolog 703 pacemaker. However, as body vibration is not closely related to physiological needs, it has similar limitations in rate response as the Activitrax pacemaker.     

A New Mechanical Sensor for Detecting Body Activity and Posture, Suitable for Rate Responsive Pacing

In the past, thought about rate responsive pacing mainly focused on rate increase with exercise but did not consider that a rate increase with postural changes also is mandatory in order to prevent orthostatic reactions. A nightly decrease in pacemaker rate when the body is at rest and in a supine position is a further advantage for the patient's sleep and recovery. Therefore, we developed a sensor that could detect not only rest and body activity but also discriminate between a supine and an upright position. This sensor is a muiticontact tilt switch containing a small mercury ball, as shown in the left panel of the figure below. The principle of discrimination between rest and low and high body activity is realized by the movement of the mercury ball resulting from body motion, which causes openings and closures within the sensor as the ball touches the numerous sensor contacts. In the upright position, a distinct number of contacts at the bottom of the tilt switch are closed. In the supine position, there is no closure of the bottom contacts and a postural discrimination can he achieved. We studied 12 volunteers and 10 pacemaker patients with this new device both at rest and during physical exercise. The right panel of the figure illustrates that the contacts per second correlate to the increase of physical exercise, such as walking on the treadmill. Further studies with an external pacemaker containing a small sensor suitable to fit into the pacemaker are in preparation.     

Clinical Observations with a Dual Sensor Rate Adaptive Single Chamber Pacemaker

The Topaz model 515 (Vitatron B.V.) is a dual sensor rate responsive pacemaker for single chamber stimulation. It can be driven by activity counts (ACT) and QT interval measurements. Inappropriate rate modulation due to one sensor can be corrected by "sensor cross-checking." It was implanted in ten patients (20-86 years) of whom seven had complete heart block and atrial arrhythmias. After implantation T-wave amplitude ranged from 0.9mV-3.5 mV. T-wave sensing ranged from 88%–99% in 9/10 patients at the follow-up of 3 weeks. Eight patients remained in default setting of the activity threshold, after evaluation with a short walking test. An exercise test was performed on all patients. In one test, QT sensing was marginal because of lead implantation in the right ventricular outflow tract. Therefore, this pacing rate was only modulated by ACT sensing. All others were tested with equal contribution of information from both sensors (ACT = QT). In 7/9, rate response was satisfactory. When the treadmill was repeated with ACT in five of these seven patients, rate generally accelerated too fast. In one patient the setting was adjusted to "QT > ACT," because of inappropriate acceleration due to activity sensing, in another it was adjusted to "QT < ACT" because of delayed response to activity. The pacing rate and the ACT during treadmill tests in "QT = ACT" mode were more closely correlated in the first 3 minutes, compared with the last 3 minutes. We feel that rate modulation with this new pacemaker is adequate. Sensor blending and sensor cross-checking are of clinical importance.     

Is Local Myocardial Contractility Related to Endocardial Acceleration Signals Detected by a Transvenous Pacing Lead?

The availability of sensors monitoring cardiac function parameters may offer many interesting new applications in cardiac pacing. A microaccelerometer sensor (BEST, Biomechanical Endocardial Sorin Transducer) located at the tip of a pacing lead (PL) has been developed by Sorin Biomedica. The signal detected by the accelerometer, peak Endocardial acceleration (PEA), was shown to reflect cardiac contractility and to be related to the dP/dt signal. Whether the PEA detected by the BEST sensor in different cardiac locations is the expression of local acceleration forces or reflects the whole heart contractility has not yet been demonstrated in humans. Endocardial acceleration and PEA were evaluated in five patients (4 males, 1 female, mean age 68 years) who underwent cardiac catheterization. Sinus rhythm was present in four patients and chronic atrial fibrillation was present in one. The BEST PL was introduced through the left subclavian vein and PEA signals were recorded: (1) at the apex of the right ventricle (RV), (2) within the coronary sinus (CS), (3) at the right atrial appendage (RAA), and (4) floating in the right atrium. The PEA signals were recorded simultaneously with surface ECG, intracardiac electrograins, and RV pressure. At each recording site, PEA signals with significant amplitude were always recorded during the preelection period, during the isovolumic contraction phase, independently of the recording site and cardiac rhythm. The PEA amplitude was higher in the RV (mean value 1.32 g) and it decreased in the RAA and CS (0.75 and 0.45 g, respectively). The same behavior of PEA was observed during sinus rhythm or atrial fibrillation. The amplitude and the timing of the PEA signals detected by the BEST accelerometer were independent of the recording site and atrial rhythm; they appeared to be strictly related to the global ventricular contractility. These results suggest that the BEST could be used either as an effective sensor in closed loop pacing systems, or primarily as a diagnostic hemodynamic sensor.     

Comparison of Ventricular Function in Atrial Rate Adaptive Versus Dual Chamber Rate Adaptive Pacing During Exercise

The hemodynamic effects of two different pacing modes—rate adaptive atrial (AAIR) versus dual chamber (DDDR) pacing—were assessed in 12 patients with DDDR pacemakers during upright bicycle exercise first-pass radionuclide angiography using a multiwire gamma camera with tantalum-178 as a tracer. All patients had sinus node disease with intact AV conduction. Patients exercised to the same heart rate in random order in these two different pacing modes, AAIR and DDDR with AV delay (of 100 msec) selected to maintain 100% ventricular capture. Cardiac output in creased significantly above baseline values during exercise in both pacing modes: 154 ± 41% (mean ± SEM, P = 0.002) with AAIR, versus 95 ± 24% (P = 0.004) with DDDR (P = NS between the two modes). The peak filling rate, likewise, increased in both pacing modes (2.3 ± 0.21 end-diastolic volumes/sec to 3.8 ± 0.31 end-diastolic volumes/sec in AAIR [P = 0.0004] and 2.2 ± 0.18 end-diastolic volumes/sec to 3.4 ± 0.27 end-diastolic volumes/sec in DDDR [P = 0.0008]). LV ejection fraction was normal at rest (60 ± 4%, SEM) and did not significantly change with submaximal exercise in either pacing mode (both 56%, P = NS). No significant changes in end-diastolic volume or stroke volume indexes occurred with exercise in either pacing mode. Our study demonstrates that in patients with normal resting LV function, AAIR and DDDR pacing are equally effective in attaining appropriate increases in cardiac output and LV filling during exercise.     

Upper Limit Ventricular Stimulation in Respiratory Rate Responsive Pacing due to Electrocautery

Transient programmed upper limit stimulation (150 bpm) was observed during repetitively utilized electrocautery in the beginning of an open-heart surgical procedure in a patient with a minute ventilation rate responsive ventricular pacemaker. This tachycardia caused severe hemodynamic deterioration, and was also initiated by internal heart massage and manual ventilation. Considering the recommendations of the manufacturer, this series of serious events could have been prevented, when reprogramming to the inhibited mode had been executed in anticipation of the operation.     

The Effect of Rate Responsive Pacing in Patients with Angina Pectoris on the Extent of Ischemia on 201-Thallium Exercise Scintigraphy

VAN CAMPEN, L.C.M.C., et al. : The Effect of Rate Responsive Pacing in Patients with Angina Pectoris on the Extent of Ischemia on 201-Thallium Exercise Scintigraphy. In patients with coronary artery disease (CAD), rate responsive pacing is considered to be contraindicated because an increase in heart rate may increase oxygen demand. Although previous studies have shown no subjective increase in ischemia during rate responsive pacing, data from objective assessment have not been documented. The goal of this study was to determine if there was an increase in ischemia on 201-Thallium (²⁰¹TI) exercise scintigraphy in this mode of pacing in patients with CAD and angina. Eighteen consecutive patients with chronic atrial fibrillation and symptomatic bradyarrhythmias with a pacemaker for more than 6 months participated in the study. In VVI and VVIR modes a symptom-limited exercise ²⁰¹TI scintigram was performed in a single blind randomized crossover fashion. Exercise duration, anginal attacks, use of nitroglycerine (NTG) tablets, blood pressure, and analysis of the scintigrams were assessed during each pacing mode. Fifteen men and three women were included (  age 65.9 ± 4.9 years, LVEF 0.44 ± 0.07  ). Four were in Class III angina pectoris, and 14 in class II. The mean exercise duration increased 28% in the VVIR group without an increase in anginal attacks per week or the use of NTG tablets. On scintigrams, no differences were seen between the two groups. One patient was withdrawn from the study because of an increase in angina pectoris (AP) attacks during VVIR pacing. Rate responsive pacing is safe and effective in patients with CAD without an increase in subjective and objective signs of ischemia.     

Activity-Sensing Rate Responsive Versus Conventional Fixed-Rate Pacing: A Comparision of Rate Behavior and Patient Well-Being During Routine Daily Exercise

Rate responsive single chamber pacing (WIR) may be the pacemaker of choice in pafients who are not suitable candidates for a dual chamber system. Several studies, most of them performed in an exercise laboratory, have shown a significantly higher exercise capacity demonstrating an improvement in cardiac output and anaerobic threshold compared to conventional fixed rate pacing (VVT). Expressing our idea that stress testing in an “artificial environment” on a bicycle or motor driven treadmill has its limitations and may be difficult to extend into patient's daily life, we designed an outdoor study imitating patient's daily activity. Twenty-one patients with an activity-sensing rate responsive pacemaker performed in a double blind fashion in VVI and VVIR mode the following test circuit: walking 170 meters on flat ground, 210 meters incline, climbing a flight of stairs, and the same circuit in reverse order, and therefore “downhill”. Heart rate behavior was recorded by Holter monitoring and patients subjective feelings of well-being, i.e. fatigue and dyspnea were also evaluated, VVIR pacing responded promptly to exercise, i.e., walking on a flat ground, but no further significant increase in pacing rate was observed in relationship to the strength of physical activity while walking inclined or climbing stairs. While patients became exhausted, a nonphysiological decrease in heart rate sometimes occurred. Despite these limitations 6 of 12 patients who had a paced-only rhythm while exercising in both VVI and VVIR mode reported feeling significantly belter in the VVIR mode, expressing less dyspnea and fatigue. In contrast, two of nine patients having only intermittently paced rhythm preferred the VVIR mode. Patients with lower ejection fraction (EF) were more likely to show subjectively a benefit while exercising in VVIR mode, compared to those with less reduced or normal EF. Despite the technical limitations of using a piezo crystal for rate adaptation, WIR pacing is an important option in paced-only patients, but it seems less beneficial in patients with only intermittent paced rhythm.     

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.     

Rate Responsive Cardiac Pacing Using a Minute Ventilation Sensor

A minute ventilation sensing rate responsive pacemaker was implanted in 15 patients (8 males and 7 females)with bradycardia. The mean age was 72.8 ± 8.7 years. The single chamber system measures transthoracic impedance between the tip electrode of a standard bipolar lead and the pulse generator case. In the adaptive mode the pulse generator calculates a rate responsive factor or slope during maximal exercise but /unctions as in the VVI mode. The patients exercised maximally on an upright cycle ergometer with the pacemaker programmed to VVI mode, adaptive mode, and rate responsive mode. Exercise and gas exchange data were collected continuously and analyzed using an automated breath-by-breath system. The slope, heort rate, and ventilation were measured every 20 seconds. Heart rate in pacemaker dependent patients correlated well to minute ventilation (correlation coefficient ranging from 0.72–0.95, P < 0.0001). This study demonstrates that minute ventilation is a good metabolic sensor in rate responsive pacing.     

The Effects of Rate Responsive Pacing on Exercise Performance in the Postoperative Univentricular Heart

Following the Fontan operation for definitive palliation of the univentricular heart, sinus node dysfunction, and/or atrioventricular block requiring pacemaker therapy is common. In previous studies ventricular rate responsive pacing (VVI, R) resulted in improved exercise performance over VVI pacing in anatomically normal hearts with either sinus node disease or atrioventricular block. In this study, the usefulness of both VVI, R and DDD, R pacing are evaluated in the postoperative univentricular heart following the Fontan operation. Eight postoperative Fontan patients with sinus node disease or atrioventricular block underwent exercise testing using a treadmill protocol. Six patients had single chamber ventricular pacemakers and two patients had dual chambered rate responsive pacemakers. Median age at exercise testing was 14 years. Patients were tested in the VVI, VVI, R, and DDD, R modes acting as their own controls. Heart rate, work rate, oxygen consumption, and respiratory exchange ratio were monitored continuously. Heart rate was significantly increased in the rate responsive modes compared to the VVI mode. In spite of the significant increase in heart rate, there was no change in maximal work rate or oxygen consumption. There was also no significant change in oxygen consumption at ventilatory anaerobic threshold. From these data we would conclude that VVI, R pacing in postoperative univentricular hearts does not result in improved exercise performance and that further study with DDD, R pacing is needed to determine its usefulness in this group of patients.     

Upper Rate Pacing After Radiofrequency Catheter Ablation in a Minute Ventilation Rate Adaptive DDD Pacemaker

A 58-year-old man with an implanted minute ventilation rate adaptive DDD pacemaker underwent RF ablation of the AV junction because of symptomatic supraventricular tachyarrhythmias. Immediately after ablation, while the pacemaker was programmed in the DDDR mode, AV sequential pacing at upper rate was observed. After programming the pacing system to the DDD mode and repeated ablation, no abnormalities were observed. It was concluded that AV sequential upper rate pacing was caused by false interpretation of the RF current by the sensor measuring transthoracic impedance as an indicator for minute ventilation.     

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.     

Efficacy and Safety of Ventricular Rate Responsive Pacing in Children with Complete Atrioventricular Block

Single chamber rate responsive pacing offers many potential advantages over the more complex dual chamber atrial tracking pacing mode in children, and the preservation of atrioventricular synchrony could be unnecessary in selected groups of pediatric patients. Twenty-two pediatric patients (age range 9 months to 12 years; mean 6.5 years) had implantation of ventricular rate responsive (VVIR) pacemakers over a 2-year period. All patients had chronic third-degree atrioventricular block, and a normal ventricular function at rest. During the follow-up each patient underwent a 24-hour Holter monitoring, and ten performed a graded treadmill test in both ventricular fixed rate (VVI) and rate responsive (VVIR) pacing mode. Paced ventricular rates were found to be normal for age in all 22 patients; maximum rate did not reach the higher programmed rate during daily activities in any patient. Comparing the mean paced ventricular rate to the mean rates of blocked P waves, six patients showed a difference of more than 20 beats/min, which induced the pacemaker parameters to be reprogrammed. In all patients a significant correlation was found between variations of paced ventricular rate and variations of spontaneous blocked atrial rhythm (P < 0.05); this correlation persisted in the subsequent Holter controls in the ten patients with longer follow-up. Exercise tolerance resulted normal in the ten patients who performed a treadmill test either in VVIR or VVI mode, with increased maximal heart rates and maximal systolic blood pressure in VVIR mode (P < 0.0013). Rate responsive ventricular pacemakers seem to adequately respond to the physiological needs of daily life of this selected group of children requiring permanent pacing.