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.     

Activity-Sensing, Rate-Responsive Pacing: Improvement in Myocardial Performance with Exercise

A sensor that detects body activity by low frequency sonic impulses has been incorporated in a pacemaker so that body activity may be translated to an increased pacing rate in response to exercise. The pacemaker is designed for patients who may benefit from an increased cardiac output mediated by an increased heart rate during exercise. Following permanent pacemaker implantation, six patients (mean age 69 years) entered a single blind, randomized, crossover trial for comparison of activity-sensing, rate-responsive pacing (A) to fixed rate demand pacing (D). Ventricular function was assessed by gated radionuclide ventriculography at rest and at exercise, while exercise capacity was assessed by treadmill performance, along with measurements of oxygen consumption and carbon dioxide production. Total treadmill duration and maximum oxygen consumption were similar in the two pacing modes (A = 284 +/- 244 s, 13.4 +/- 3.4 ml O2/min/kg; D = 256 +/- 250 s, 11.7 +/- 3.7 ml O2/min/kg). Anaerobic threshold, however, was significantly improved (A = 266 +/- 199 s, (p less than .05), 13.0 +/- 2.2 ml O2/min/kg (p less than .01); D = 231 +/- 208 s, 10.8 +/- 2.3 ml O2/min/kg).(ABSTRACT TRUNCATED AT 250 WORDS)     

Central venous oxygen saturation for the control of automatic rate-responsive pacing

A rate-adjusting pacemaker system is described which uses central venous oxygen saturation (SO2) for the regulation of the pacing rate. This system was tested externally in ten patients with chronically implanted VVI units. There was an average increase in cardiac output of 18% with the SO2-regulated pacemaker as compared to the situation during fixed rate stimulation. Central venous oxygen saturation appears to be an ideal biological parameter for autoregulating the pacing rate. It represents the only sensor suitable for the realization of a closed feedback loop concept.     

Cardiac Pacemaker Regulated by Respiratory Rate and Blood Temperature

A new method using respiratory rate and temperature as the guides for optimal pacing is proposed. A pacemaker was fabricated which senses these two parameters simultaneously. The pacemaker functions by calculating the cardiac rate, which would be derived from the respiratory rate and the blood temperature. The higher of the two rates is adopted as the cardiac pacing rate, i.e., at which stimuli will be delivered. The operation was tested in a mongrel dog with complete atrioventricular block. After the induction of anesthesia, a thermistor temperature probe was inserted into right atrium and a respiratory rate sensor was attached around the chest. After administration of a pyrogenic drug, both respiratory rate and blood temperature increased. The pacing rate was increased from 178 beats/minute(bpm) at 36.4 degrees C, blood temperature, and 26.5 acts/minute(apm), respiratory rate, to 233 bpm at 40.1 degrees C and 40.0 apm. Cardiac output was increased from 2.15 liters/minute(l/pm) at the beginning to 2.50 l/pm at maximum. The transition of the guide from respiratory rate to temperature was observed at about 38 degrees C.     

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.     

Improved Rate Responsive Algorithm in QT Driven Pacemakers—Evaluation of Initial Response to Exercise

The QT pacemaker is a rate modulated pacemaker that uses the evoked QT interval as an indicator to determine its optimal pacing rate. Despite the generally favorable clinical results with this form of pacing, some flaws in the system have been reported, such as the frequently observed rather slow initial response of the pacing rate to physical exercise, and the phenomenon of oscillation of the heart rate. These problems can be attributed to the rate adaptive algorithm used in the current QT pacemaker. Recently, in a reexamination of the relationship between evoked QT interval and pacing rate, a curvilinear relationship between these parameters has been demonstrated. As a result, a new algorithm has been developed for the next generation of the QT pacemaker. Before this new algorithm was implemented in new implantable devices, it was evaluated in a multicenter clinical investigation, with emphasis on the initial response of the pacing rate to exercise. This study was carried out by means of special software in the programmer of the QT pacemaker. By employing real-time bidirectional telemetry, it was possible to submit the study population, consisting of 37 patients with implanted QT pacemakers of the current generation, to identical exercise tests. Comparing these exercise tests, it appears that a considerable gain in speed of response to exercise can be achieved by using the same sensor with a faster reacting, nonlinear rate adaptive algorithm.     

Initial Clinical Experience with a New Dual Sensor SSIR Pacemaker Controlled by Body Activity and Minute Ventilation

Fourteen patients were implanted with a single chamber dual sensor pacemaker (Legend Plus®) that measures minute ventilation (VE) via variations in impedance between a bipolar lead and the pacemaker case, and activity via a piezoelectric crystal bonded to the pacemaker case. Chronotropic incompetent patients were exercised an a treadmill and a bicycle in dual sensor mode. Activity only indicated pacing rate was measured using a strap-on pacemaker. Both implanted and strap on pacemakers were adjusted to yield a steady-state pacing rate of 100 beats/min during hall walk. Pacing rate, VE, and oxygen uptake (VO₂) were measured continuously. Linear curve fit analysis slopes for plots of VE versus pacing rate during exercise (1.33-1.49) compared favorably to values reported in normals. Peak pacing rates achieved for treadmill and bicycle testing for dual sensor mode were higher than activity mode alone. Slopes of heart rate to VE or VO₂ were not significantly different (P < 0.05) for dual sensor mode in contrast to activity alone. In conclusion, the Legend Plus dual sensor rate adaptive pacing therapy delivered pacing rates more proportional to VE and VO₂ under different types of exercise than rates indicated by a strap-on pacemaker in activity mode.     

Trends in Pacemakers Which Physiologically Increase Rate: DDD and Rate Responsive

Exercise (rate) responsive pacemakers benefit patients by providing increased cardiac output when needed and permitting lower rate during rest. This paper briefly reviews trends in reported studies on rate responsive pacemakers. For patients with reliable atrial rhythms, atrial-triggered pacemakers (DDD) provide physiological ventricular rates unless complications arise. At low rates, A-V synchrony benefits patients with refractory cardiac decompensation; however, in patients with healthy myocardiums, achieving higher pacing rates is more significant than maintaining synchrony. If atrial rhythms are unreliable, an alternative sensor for determining pacing rate is indicated. Pacemakers that respond to changes in right ventricular blood temperature, respiratory rate, QT interval, body vibration (motion), and pH have been implanted in humans. Clinical evaluations have shown that increased pacing rate leads to increased exercise tolerance and cardiac output when needed, independent of the sensor type (DDD, QT, respiratory rate, etc.). The effectiveness of any sensor type to increase pacing rate appropriately requires reliable sensors that respond specifically to the need for increased pacing rate. Sensors for stroke volume, venous oxygen saturation, right atrial or ventricular pressure and catecholamines are also under preclinical investigation. The availability of a reliable, long-term sensor is a key limitation to several techniques including pH. stroke volume, oxygen saturation, pressure, and catecholamines. Sensor technology and clinical effectiveness are the keys to rate responsive pacing.     

Dual Sensor VVIR Mode Pacing: Is It Worth It?

Dual sensor ventricular demand rate responsive (VVIR mode) pacing was compared with single sensor rate responsive pacing to assess whether this new development should be more widely incorporated in modern pacemaker devices. A within patient randomized, double-blind crossover study involving ten patients, mean age 67.4 years (70% male), had Medtronic Legend Plus dual sensor VVIR pacemakers implanted for high grade A V block and chronic or persistent paroxysmal atrial fibrillation. Performance values were compared to 20 healthy control subjects of a similar age and gender. Patients were both subjectively and objectively assessed after 2 weeks of out-of-hospital activity in VVIR mode (minute ventilation sensing), VVIR mode (activity sensing), VVIR mode (dual sensor), and VVI mode (no rate response). All patients were assessed for subjective preference for, and objective improvement in, any pacing modality as assessed by standardized daily activity protocols and graded exercise treadmill testing. Subjective perception of exercise capacity and functional status was significantly lower in VVI mode (P < 0.05) compared to any of the VVIR modes, which did not differ. After completion of the study 70% of patients chose VVIR as their preferred mode, with 30% expressing no preference. Forty percent preferred activity sensor WIR mode pacing, 30% preferred dual sensor VVIR mode pacing, and 70% found either dual sensor WIR mode, minute ventilation sensor WIR mode, or both modalities least acceptable. No patient found activity sensing WIR mode least acceptable. Graded treadmill testing revealed significantly lower exercise tolerance during WI mode pacing (P < 0.01) compared to the VVIR modalities, which did not differ. Overall, chronotropic response was best with dual sensor pacing during standardized daily activity protocols and during the standard car journey. The data from this study suggest that there is no marked clinical advantage obtained from the use of dual sensor devices over current activity sensing ventricular demand rate responsive pacemakers, but with the probable added disadvantages of increased size, complexity, cost, and decreased longevity.     

Optimal Cardiac Pacing in Patients with Coronary Artery Disease

Pacemaker patients with coronary artery disease and angina pectoris fare better with devices providing AV synchrony and rate increase on exercise provided the programmed upper rate is not excessive. Optimal programming requires knowledge of the factors influencing pacemaker rate response, MVO₂ and cardiac sympathetic activity. Inappropriately high rates during rate adaptive pacing can be controlled by new multisensor systems with sensor cross-checking to avoid false positive responses with inappropriate increases in the pacing rate. Permanent pacing in patients with intractable angina who are unsuitable for interventional procedures permits more aggressive pharmacological therapy.     

Multicenter Clinical Evaluation of a New SSIR Pacemaker

A multicenter clinical evaluation of Sorin Swing 100, a new SSIR pacemaker with a gravimetric sensor, was performed by seven different centers enrolling a total of 89 patients, 56 men and 33 women, mean age 73.1 years, for pacemaker implantion (73 patients) or pacemaker replacement (16 patients). Pacing mode was VVIR in 73 patients and AAIR in 16. The behavior of pacing rate was evaluated 3 months after the implant by performing a 24-hour Holter monitor, an exercise stress test, and tests for the assessment of mechanical external interference (MEI). A physiological behavior of the paced rate was always observed during Holter monitoring. In 52 completely paced patients mean diurnal, nocturnal, and maximal heart rate were, respectively, 74.9 ± 5.7 ppm, 58.1 ± 5.8 ppm, and 113.4 ± 12.7 ppm; a paced rate exceeding 100 ppm was reached on the average 5.6 times/Holter monitor. In all but two patients the sleep rate (55 ppm) was reached during the night or long resting time. During exercise stress test a direct correlation between the increase in pacing rate and the increase in workload was observed; the mean maximal heart rate reached in 49 completely paced patients was, respectively, 102.8 ± 9 ppm in 17 patients who accomplished stage 1, 116.2 ± 13.6 ppm in 28 patients who accomplished stage 2, and 133 ± 6.7 ppm in 10 patients who accomplished stage 3 of the Bruce protocol. MEI testing never increased the pacing rate over the noise rate (10 ppm over the basic rate). In only seven patients the results obtained suggested to change the nominal set up of the pacemaker. Our experience clearly indicates that Swing 100 is an effective, reliable, and easy to use SSIR pacemaker. The availability of the sleep rate allows a more physiological pattern of pacing rate and can lead to significant energy saving.     

A Rate-responsive Pacemaker Controlled by Right Ventricular Blood Temperature

To demonstrate the capabilities of a rate-responsive pacemaker controlled by right ventricular blood temperature, a prototype pacemaker was developed and tested in dogs with complete atrioventricular (AV) block. Using a previously obtained data base of right ventricular blood temperature recorded both during rest and during treadmill exercise from dogs with either induced AV block or normal AV conduction, a control algorithm which identified periods of exercise was developed. This algorithm was implemented in a microprocessor-based, rate-responsive pacemaker which generates two pacing rates: a basal rate (86 bpm) during rest and a higher rate (113 bpm) during moderate exercise. With repeated trials of submaximal treadmill exercise (2.25, 3.0, 5.0 mph, 16% grade) the pacemaker correctly detected the onset and cessation of exercise with latencies ranging from 35 to 124 seconds. Pacemaker rate increase latency decreased as the exercise level was increased. Pacemaker response latency was not affected by the choice of resting or exercise pacing rate. These preliminary data indicate that right ventricular blood temperature can provide a reliable index to control pacemaker rate. An implantable temperature-controlled pacemaker using this algorithm is presently under long-term evaluation in dogs performing untethered exercise.     

Rate Modulated Pacing Based on Right Ventricular dP/dt: Quantitative Analysis of Chronotropic Response

Right ventricular contractility increases in response to catecholamine stimulation and greater ventricular preload, factors that increase with exercise workload. Thus, the maximum systolic dP/dt may be a potentially useful sensor to control the pacing rate of a permanent pacing system. The present study was designed to test the long-term performance of a permanent pacemaker that modulates pacing rate based on right ventricular dP/dt and to quantitatively analyze the chronotropic response characteristics of this sensor in a group of patients with widely varying structural heart diseases and degrees of hemodynamic impairment. A permanent pacing system incorporating a high fidelity pressure sensor in the lead for measurement of right ventricular dP/dt was implanted in 13 patients with atrial arrhythmias and AV block, including individuals with coronary artery disease, hypertension, severe obstructive pulmonary disease with prior pneumonectomy, atrial septal defect, dilated cardiomyopathy, restrictive cardiomyopathy, and mitral stenosis. Patients underwent paired treadmill exercise testing in the VVI and VVIR pacing modes with measurement of expired gas exchange and quantitative analysis of chronotropic response using the concept of metabolic reserve. The peak right ventricular dP/dt ranged from 238–891 mmHg/sec with a pulse pressure that ranged from 19–41 mmHg. There was a positive correlation between the right ventricular dP/dt and pulse pressure (r = 0.70, P = 0.012). The maximum pacing rate and VO_2max were 72 ± 6 beats/min and 12.61 ± 4.0 cc O₂/kg per minute during VVI pacing and increased to 124 ± 18 beats/min and 15.89 ± 5.9 cc 0₂/kg per minute in the VVIR pacing mode (P < 0.0003 and P < 0.002, respectively). The integrated area under the normalized rate response curve was 96.7 ± 45.7% of expected during exercise and 100.1 ± 43.4% of expected during recovery. One patient demonstrated an anomalous increase in pacing rate in response to a change in posture to the left lateral decubitus position. Thus, the peak positive right ventricular dP/dt is an effective rate control parameter for permanent pacing systems. The chronotropic response was proportional to metabolic workload during treadmill exercise in this study population with widely varying forms of structural heart disease.     

Temperature May Be an Appropriate Sensor for Chronotropically Incompetent Patients with Postural Syncope

Chronotropically incompetent patients benefit most from sensor driven rate response during exercise. Postural syncope may occur despite the chronotropic response because of the failure of currently available sensors to respond physiologically to postural changes. Seven chronotropically incompetent patients with postural syncope who had a dual chamber rate adaptive pacemaker (Circadia^R) that modulates heart rate in response to temperature change were studied with respect to: (1) response to exercise: and (2) head-up tilt (HUT). During exercise, continuous-wave Doppler of aortic velocities and two-dimensional echocardiographic derived measurements of left ventricular systolic function were used to assess cardiac function. Patients exercised longer (by an average of 168 sec) in the DDDF/compared to the DDl mode (P = 0.013). Increase in exercise duration was due mostly to the sensor driven increase during DDDH pacing. During DDDR pacing, heart rate increased from 71 ± 6 to 121 ± 17 ppm compared to 70 ± 1 to 103 ± 21 ppm for the DDl pacing (P = 0.038). Stroke volume as assessed by Doppler derived stroke distance (SD) contributed more significantly to the cardiac output increase during exercise in the DDl mode (SD increased from 13.4 ± 4 to 18 ± 7 cm in DDl compared to 13 ± 4 to 14 ± 2 cm in DDDR mode), although these mechanisms were insufficient to fully compensate for failure of appropriate chronotropic response. In response to the HUT, right ventricular temperature increased from 36.78°C ± 0.29°C to 36.89±± 0.28°C (P = 0.0002), and heart rate increased from 54 ± 3 to 71 ± 8 ppm (P = 0.0003) in the DDDR mode. No significant change in heart rate occurred in the DDl mode in response to the HUT. Strong positive correlation of temperature and heart rate was noted in all patients in response to HUT (P = 0.001, R²= 0.755–0.976). We conclude that temperature sensor responds physiologically to exercise and HUT. Therefore, temperature sensing rate adaptive dual chamber pacing may be appropriate for chronotropically incompetent patients with posture related syncope.     

A New Feature for Control of Inappropriate High Rate Tracking in DDDR Pacemakers

LEE, M.T., ET AL. A New Feature for Control of Inappropriate High Rate Tracking in DDDR Pacemakers. A limitation of current DDD and DDDR pacemakers is the inability to distinguish between inappropriate high rate atrial sensed events that are physiologically appropriate to track (e.g., elevated sinus rates resulting from exercise, emotional responses, etc.) and those that are physiologically inappropriate to track (e.g., paroxysmal atrial dysrhythmias, myopotentials, retrograde conduction, etc.). The sophistication of sensing circuitry is not yet sufficiently advanced to permit a pacemaker to distinguish appropriate atrial events by morphology. The addition of an independent sensor to a DDD pacemaker (i.e., DDDR) gives more information about the patient's condition, especially with respect to exercise. This information can be used to judge the appropriateness of a high sensed atrial rate, and to modulate the pacemaker's response. If the sensor input is below a specified level, indicating lack of exercise, the DDDR can track sensed atrial events only to a tolerably low limit-the conditional ventricular tracking limit (CVTL). Wenckebach-type behavior ensues at the CVTL until the sensor input increases, indicating that exercise is occurring, or until the sensed atrial rate decreases. If the sensor input indicates exercise, the DDDR pacemaker can track up to the programmed maximum rate. Two DDDR systems have been developed that incorporate this feature; one based on temperature, the other on activity. Currently the CVTL is set at a value about 30 ppm above the pacing rate, as a compromise to support emotional needs not seen by the sensor. Improved sensors could cause the decision to raise the tracking limit (i-e., recognition of physiological need for higher rates) to be more accurate, perhaps making the CVTL proportional to the sensor signal.     

A Central Venous Temperature Sensing Lead

Pacemaker rate responsiveness derived from changing central venous blood temperature requires the development of sensor leads that are stable and reliable. The relevant characteristics of one such design are described. Temperature response time, data acquisition time, temperature sensitivity, and long-term sensor shunt impedance have been studied both in vitro and in vivo. These parameters are analyzed with respect to the intrinsic temperature signal and to pacemaker implementation problems.     

The Central Venous Blood Temperature as a Guide for Rate Control in Pacemaker Therapy

True physiologic pacing is only feasible in patients with heart block and normal sinus rhythm using atrial synchronized pacing. In sick sinus syndrome (SSS) or atrial fibrillation no adequate sensing possibility is present. For these conditions something other than electrical signals must be found to guide rate control. In clinical experiments a thermistor integrated in a bipolar pacing lead was implanted in one of the authors as well as in a pacemaker patient. Treadmill tests at 3, 6, and 10 km per hour have shown no substantial increase of the central venous temperature (CVT) in the lower effort range; an increase of 0.6-0.75 degrees Celsius (degrees C) in the medium range; and an increase of 1.35 degrees C in the higher range, respectively. External pacing in the author, or rate variations by programming the VVI, M unit in the patient at rest do not influence CVT. CVT correlates well in the medium-range group, which mostly reflects the capabilities of the average pacemaker patient. In the higher effort range a delay between rate increase and increase of CVT is noticed. There is no increase in CVT beyond the physiologic variations in the lower effort range.     

Clinical Study of a New Activity Sensor for Rate Adaptive Pacing Controlled by Electrical Signals Generated by the Kinetic Energy of a Moving Magnetic Ball

A new rate adaptive pacemaker (Sensorithm) controlled by an activity sensor providing electrical signals induced by a magnetic ball moving freely in an elliptical cavity surrounded by two copper coils, was implanted in ten patients; mean age of 75 years (range 64–89). Six patients had atrioventricular block and four had sinus node disease. In auto-set testing procedure during a 1-minute walk in the corridor, a slope resulting in a maximum rate of 95 beats/min was selected in every patient, and a medium reaction time was programmed. During graded treadmill exercise tests the heart rate increased 63 ± 7 beats/min to 135 ± 6 beats/min in rate adaptive pacing mode (VVIR), and 15 ± 6 beats/min (P < 0.0001) in ventricular pacing mode (VVI). The symptom-limited exercise time was 9.1 ± 1.1 minutes and 8.2 ±1.2 minutes (P = NS), and the exercise distance was 501 ± 95 meters and 428 ± 92 meters (P < 0.05) in VVIR and VVI pacing mode, respectively. The maximum oxygen uptake was 20.6 ± 2.6 mL/kg per minute in VVIR pacing and 18.1 ± 2.1 mL/kg per minute (P < 0.05) in VVI pacing. The delay time until the pacing rate increased 10% of the total rate increase at onset of treadmill exercise was 4.4 ± 0.7 seconds. Assuming a linear relation between metabolic workload and heart rate response from rest to the age predicted maximum heart rate, a deviation of heart rate ranging from 13.5 ± 11.2% to –1.6 ± 5.2% from the expected heart rate at mid-point and endpoint of each quartile of workload was observed during treadmill testing. Conclusions : By using a 1 -minute walk test for selecting an appropriate slope setting, Sensorithm provided a significant and proportional heart rate increase during exercise resulting in an improvement of exercise capacity during VVIR pacing compared to VVI pacing.     

Influence of Propranolol on the Ventricular Depolarization Gradient

Sensing of the ventricular depolarization gradient (VDG) has recently been used as The basis of a closed-loop rate responsive pacemaker. Factors influencing this aspect of the evoked response have not been fully evaluated although previous reports have suggested that sympathetic stimulation and circulating catecholamines are primarily responsible for the observed changes during stress and exercise. In five patients (Table I), four males and one female (mean age 60.4 ± 20.1 years) implanted with the Prism pacemaker, the pacing response to exercise and tilting was assessed before and after the infusion of propranolol. There was an increase in the pacing rate in all patients during the infusion of the drug (mean 27 ± 12.9 beats/min) suggestive of a direct drug effect on the VDG. The rate control parameter (RCP) of the pacemaker, the numerical equivalent of the VDG, was significantly different after the administration of propranolol (P < 0.01). However, exercise performance and pacing rate behavior were not different after beta blockade. The pacing rate increase observed when tilting patients to the supine position was not altered by propranolol. Out date suggest that factors other than adrenergic stimulation may be of importance in affecting the ventricular evoked response and accordingly the rate adaptation of the Prism pacemaker.     

Rate Response of a Closed-Loop Stimulation Pacing System to Changing Preload and Afterload Conditions

Closed-loop stimulation (CLS) is a new sensor concept for rate adaptive pacing measuring changes in the unipolar right ventricular impedance, which correlates to changes of the right ventricular contractility and reflects the autonomic nervous innervation of the heart. Some patients do not tolerate the CLS mode because of inappropriate tachycardia, mainly related to postural changes. This study tested if the rate response of the CLS sensor is influenced not only by myocardial contractility but also by rapid changes in right ventricular filling. In 12 patients (10 men, median age 77 years) with a Biotronik Inos²-CLS DDDR pacemaker and 14 controls (13 men, median age 59 years) head-up tilt and handgrip testing was performed to provoke rapid changes in pre- and afterload. Tilting the pacemaker patients resulted in a nonphysiological steep increase of the sensor rate (increase >20 beats/min, peak after 1 minute, return to baseline within 2–3 minutes), which was significantly different from the control group, showing only a slight rise in intrinsic heart rate immediately after tilting. Simultaneously to the rapid increase in sensor rate, the pacemaker patients showed a marked orthostatic decline of systolic blood pressure. During handgripping, heart rate and blood pressure curves were similar in both groups. In patients with this CLS pacemaker, rapid preload reduction during head-up tilting caused an overshooting sensor rate increase, reproducing the authors' clinical observation of postural pacemaker tachycardia in some patients. Consequently, they concluded that the rate response of the CLS pacing system can be inappropriately influenced by rapid shifts of blood volume, affecting right ventricular filling. (PACE 2003; 26[Pt. I]:1504–1510)