Rate Responsive Pacemaker Research Articles

Electrocautery-induced Pacemaker Tachycardia: Why Does This Error Continue?

In Reply:—We thank Drs. Rozner and Nishman for taking the time to increase the amount of issues about our case report that we were unable to address because of word limitations.In response to issue 1, we believe that anesthesiologists in general do not have in-depth knowledge of pacemakers, especially newer pacemakers with features such as rate responsiveness. Drs. Rozner and Nishman cited a number of references reporting upper activity rate pacing in response to electromagnetic interference. However, the majority of these were case reports in nonanesthesia journals, such as Pacing and Clinical Electrophysiology . One needs only to refer to major anesthesia textbooks to see the lack of details regarding these devices. We practice in an academic center with a large cardiovascular program. Although most anesthesiologists are aware of potential problems with pacemakers, many are not familiar with the details for this type of pacemaker. The patient in our case report was referred to a cardiologist at a pacemaker clinic in another academic center who did not reprogram the pacemaker out of the rate-responsive mode preoperatively. Therefore, we believe that it is worthwhile to draw wider attention to electromagnetic interference in rate-responsive pacemakers with our case report.Drs. Rozner and Nishman implied that a gradual onset of pacemaker-driven tachycardia is not possible. A company representative from St. Jude Medical in California stated that a gradual onset of pacemaker driven tachycardia is the usual response to cautery interference in the Telectronics META II pacemaker (Englewood, CO). This pacemaker has two programmable response times: medium (36 s) or fast (18 s). The response time determines the time required to reach 50% of the metabolically indicated rate in response to an instantaneous change in the measured minute ventilation. Therefore, it would typically take four response times to reach greater than 90% of maximum programmed rate with real or perceived step changes in minute ventilation. This abnormal response to electromagnetic interference usually results from erroneous interpretation of the mixture of bioimpedance signals rather than a direct effect on the pulse generator itself; thus, a gradual rather than a sudden response is expected.In response to issue 2, the pacemaker spikes were clearly visible on the monitor screen and to a lesser extent on the printout tracing. Therefore, recognition of pacemaker tachycardia is not the central issue—prevention and management are.In response to issue 3, we intentionally kept our recommendations brief because of word limitations and at the request of the editor. Detailed recommendations could be obtained from the Web page *cited in the case report reference.In response to issue 4, we agree with the recommendations of Drs. Nishman and Rosner.We believe that practicing anesthesiologists need to maintain knowledge of and be familiar with the potential complications of rate-responsive pacemakers, and we hope the case report and its correspondence will heighten awareness of this subject.

Optimal atrioventricular delay setting determined by QT sensor of implanted DDDR pacemaker.

QT interval (QTI) may change when cardiac function is improved by optimizing the AV delay. QTI is used as the sensor for rate responsive pacemakers. Evoked (e)QTI is measured as the time duration from the ventricular pace-pulse to the T sense point, which is the steepest point of the intracardiac T wave. The relationship between AV delay and eQTI and cardiac function was studied in 13 patients (74.2 +/- 9.3 [SD] years old) with an implanted QT-driven DDDR pacemaker. A special pacemaker software module was downloaded into the pacemaker memory for eQTI data logging. AV delay was set at 100, 120, 150, 180, 210, and 240 ms. Cardiac output (CO) was measured by continuous Doppler echocardiography. eQTI was 343.3 +/- 22.4, 345.1 +/- 22.5, and 343.4 +/- 23.2 ms (P < 0.01, repeated ANOVA) and CO was 4.2 +/- 0.8, 4.6 +/- 0.8, and 4.2 +/- 0.8 L/min (P < 0.0001, repeated ANOVA) when AV delay was set at the AV delay shortened by one step (AV[-]) and prolonged by one step (AV[+]) from the AV delay at which QT interval was maximum (AV[max]) in seven patients, in whom the peak AV delay at which the eQTI was maximal could be identified. eQTI decreased from 341.1 +/- 20.9 to 339.4 +/- 21.1 ms (P < 0.0001) and CO decreased from 4.4 +/- 1.4 to 4.1 +/- 1.3 L/min (P < 0.005) when AV delay was prolonged from AV(max) to AV(+) in all patients. eQTI decreased from 345.1 +/- 22.5 to 343.3 +/- 22.4 ms (P < 0.0005) and CO decreased from 4.6 +/- 0.8 to 4.2 +/- 0.8 L/min (P < 0.05) when AV delay was shortened from AV(max) to AV(-) in seven patients. Thus, CO was maximal when AV delay was set at the AV delay at which eQTI was maximal. In conclusion, the optimal AV delay can be predicted from the eQTI sensed by an implanted pacemaker, and automatic setting of the optimal AV delay can be achieved by the QT sensor of an implanted pacemaker.

DAS DROMOTROPE SCHRITTMACHERKONZEPT: PARAMETRISCHE SYSTEMIDENTIFIKATION

The focus of this paper is the design of a rate-responsive pacemaker which uses the atrio-ventricular conduction time (AVCT) as an exertion related sensor signal. AVCT can be easily obtained from an intracardial electrogram. During atrial pacing AVCT shortens with exercise but increases with the stimulation rate. Hence, an AVCT based pacemaker always establishes a closed-loop system. General design rules for an AVCT pacemaker have been developed from our experimental results and a system-theoretical treatment. Topics addressed were the controller gain, uncertainties concerning the dynamics of the AVCT, the attenuation of disturbances, the closed-loop bandwidth and stability.

Assessment of Pacemaker Chronotropic Response: Implementation of the Wilkoff Mathematical Model

There are few practical guidelines for proper adjustment of rate responsive pacemaker sensor parameters. This study describes the application of the chronotropic assessment exercise protocol (CAEP) and the Wilkoff model of chronotropic response to assess the adjustment of pacemaker sensor parameters. In 31 patients implanted 1 month previously with a dual sensor pacemaker, pacemaker sensor parameters were adjusted to yield a peak sensor rate of 100 beats/min on a simple 6-minute walk (low intensity treadmill exercise [LITE] protocol); the maximum sensor rate was set to the age predicted maximum heart rate (220-age). The rate response behavior of the pacemaker was then assessed using the slope of metabolic-chronotropic relation (MCR) during CAEP exercise. After adjustments based on the LITE protocol, CAEP exercise yielded MCR slopes of 0.92 +/- 0.25 for the entire study group, which compares well with the predicted normal slope of 1. However, 7 of the 31 patients had sensor MCR slopes during CAEP exercise that were 2 SD or more below expected. To test the sensitivity of this approach to suboptimal pacemaker programming or suboptimal exercise, simulations were performed with the maximum sensor rate programmed below age-predicted maximum heart rate or with exercise truncated before maximum exertion; with these conditions, MCR slopes were sharply lower for the entire group. The authors conclude that a simple treadmill walk (LITE) allowed for optimum programming of sensor parameters in most patients, but in a minority the chronotropic behavior was underresponsive. Failure to appropriately adjust pacemaker maximum sensor rate or failure to achieve peak exercise sharply limits the accuracy of this methodology.

Effect of diltiazem on the recurrence rate of paroxysmal atrial fibrillation

P experimental and clinical studies have demonstrated that verapamil may attenuate the atrial electrical remodeling that is induced by atrial fibrillation (AF),1,2 thereby reducing the predisposition toward early recurrence of AF after cardioversion. This may explain why some clinical studies have found that treatment with calcium antagonist agents before cardioversion may reduce the incidence of immediate or early recurrences of AF.3–6 However, whether calcium antagonists help to delay recurrences of AF in patients with paroxysmal AF is unclear. Therefore, this prospective study evaluated the effect of oral diltiazem on the frequency of AF in patients with paroxysmal AF who underwent atrioventricular junction ablation and implantation of a dual-chamber pacemaker. • • • The study population consisted of 8 men and 12 women, mean age of 71 6 4 years, who underwent atrioventricular junction ablation and implantation of a dual-chamber pacemaker because of drug refractory paroxysmal AF with an uncontrolled ventricular rate. Their clinical characteristics are listed in Table 1. Patients who had been treated with amiodarone within 3 months before the ablation procedure were excluded from the study. The onset of paroxysmal AF occurred a mean of 60 6 48 months before ablation. Thirteen patients had no cardiovascular disease and 7 patients had systemic hypertension. The mean left ventricular ejection fraction was 0.58 6 0.05, and the mean left atrial diameter by echocardiography was 4.1 6 0.4 cm. Atrioventricular junction ablation was successful in all patients. A dual-chamber, rate-responsive pacemaker capable of mode-switching (model 7940, 7950, or 7960, Medtronic Inc., Minneapolis, Minnesota) was implanted in each patient. The pacemaker was programmed to have a lower rate limit of 70 ppm, an atrioventricular interval of 150 ms, and an upper rate limit of 120 to 130 ppm. The mode-switch detection rate was programmed at 180 beats/min. The study protocol was approved by the Human Research Committee, and informed consent was obtained from each patient. Two days after pacemaker implantation, the patients were randomly assigned to treatment with 240 mg/day of diltiazem, or to no treatment with diltiazem. All patients were in sinus rhythm during randomization. No patient was treated with any other antiarrhythmic agent, including b blockers. The mode-switch event counter was activated and used to quantitate the number of arrhythmia recurrences. The event counter had a saturation level of 254 episodes. The patients were examined at follow-up after 3 months, at which time the pacemaker was interrogated and the event counters were cleared. The patient was then crossed over to the alternate therapy. After another 3 months of follow-up, the patients were again examined during a clinic visit, at which time pacemaker interrogation was again performed. Continuous variables are expressed as mean 6 1 SD. Statistical comparisons were performed with a paired t test. A p value of ,0.05 were considered statistically significant. One of 10 patients randomly assigned to receive diltiazem during the first 3-month study period developed persistent AF, and this was also the case in 1 of 10 patients randomized to receive no treatment. After crossover, the only 2 patients who developed persisFrom the Cardiology Division, Department of Medicine, University of Hong Kong, Queen Mary Hospital, Hong Kong, China; and Division of Cardiology, Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor, Michigan. This study was supported in part by the Don Nouse Arrhythmia Research Fund, Ann Arbor, Michigan. Dr. Morady’s address is: Division of Cardiology, B1F245, University of Michigan Medical Center, 1500 East Medical Center Drive, Ann Arbor, Michigan 48109-0022. E-mail: fmorady@umich. edu. Manuscript received February 21, 2001; revised manuscript received and accepted April 9, 2001. TABLE 1 Patient Characteristics and Mode-Switch Events

Chronotropic incompetence: a case report.

Chronotropic incompetence is generally defined as an inadequate heart rate response to exercise, but manifestations can vary. The incidence depends on underlying cardiac pathology and, to a lesser degree, on the cut-off value of the predicted heart rate during exercise. Different pathologies induce chronotropic incompetence. Its presence indicates an adverse outcome and is strongly correlated with coronary artery disease. Treatment consists of rate-responsive pacemakers; dual-sensor, adaptive pacemakers are superior to single-sensor, rate-augmenting pacemakers. This case report illustrates the negative effect of chronotropic incompetence on daily activities and its amelioration by implantation of a rate-responsive pacemaker. (c)2001 by CHF, Inc.

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.

Pediatric Cardiac Electrophysiology

The rapid development of invasive cardiac electrophysi ologic procedures, including the insertion of rate-responsive pacemakers, radiofrequency catheter ablation (RFA), and the implantation of cardiac defibrillators has given pediatric cardiac electrophysiologists better techniques to treat in fants and children with arrhythmias. Expertise on the basic pathophysiologic mechanisms of arrhythmias is a requisite in performing invasive intracardiac electrophysiologic pro cedures. Techniques for invasive cardiac electrophysiologic procedures (EPS) in pediatric patients are similar to those for diagnostic cardiac catheterization. The choice of anes thesia is determined by institutional preference, patient's age, anticipated sites of intravascular access, and type of procedure. The goal is to avoid agents that depress myocar dial contractility or affect cardiac conduction or refractori ness. EPS equipment includes multielectrode catheters, a multichannel recording system, an electrical stimulator, and a fluoroscopy unit. EPS require the ability to induce, termi nate, and modify cardiac arrhythmias. The objectives of the EPS include definition of the mechanisms of observed and induced cardiac arrhythmias, determination of risk for sudden cardiac arrhythmic death, investigation of antiar rhythmic drug efficacy, and ablation of tachyarrhythmia sub strates. This is accomplished by evaluating electrophysi ologic properties, such as automaticity, conduction, and refractoriness; by initiating and terminating tachycardias; by mapping sequence of activation; and by evaluating patients for various forms of therapy and judging the response to therapy. Though generally quite low, the associated morbid ity and mortality in performing EP study and RFA in pediatric patients varies considerably, depending on the patient's age and size, diagnosis, associated cardiac defects, and prepro cedure condition.

Proportionality of rate response to metabolic workload provided by a rate adaptive pacemaker with automatic rate profile optimization

The rate response of a pacemaker (PM) was compared with the sinus rate in patients during repeated exercise tests, at different settings of the rate response parameters. In patients with paroxysmal sick sinus syndrome (n=3) or atrioventricular block (n=8), a rate responsive PM was implanted. The activity-dependent pacing rate is represented by the sensor indicated rate (SIR). Each patient performed a treadmill test at 1 month, 1 year, and 2 years after implantation. Prior to the 1 and 2 year tests PM parameters were changed to produce a larger rate increase, especially at moderate levels of daily life activity. During the tests the O(2) consumption and CO(2) production were measured, breath-by-breath, to determine the workload and the anaerobic threshold. On average the workload (oxygen consumption), the patient's sinus rate, and the SIR, showed a linear increase with the workload imposed by the treadmill. In the 1 month and 1 year test the SIR was much lower than the spontaneous rhythm, especially at low or moderate workloads. On the more dynamic setting of several rate adaptive parameters at 2 years, the SIR changed significantly and was close to the spontaneous HR. The examined PM provides a paced heart rate that is proportional to the workload. For the first time the effect of reprogramming rate response parameters to produce an SIR that is similar to the sinus rate is shown in this study.

Rate-responsive pacing improves exercise tolerance in heart transplant recipients: a pilot study.

Chronotropic incompetence is one cause of diminished exercise capacity in heart transplant recipients. If reinnervation occurs, it often is late after transplantation and is not always accompanied by functional improvements in peak heart rate and appropriate tachycardia during exercise. To determine the efficacy of rate-responsive pacing on peak heart rate and exercise capacity, the authors studied eight male heart transplant recipients (age 57 +/- 12 years; 23 +/- 9 months after transplantation) that had either atrial or dual-chambered pacemakers. All subjects completed two maximal graded exercise tests (GXT) using the Naughton treadmill protocol. During the first GXT, pacemakers were programmed for bradycardia support only and without rate responsiveness (unpaced). After a 14-day regimen of beta blockade with metoprolol to nullify the influence of circulating catecholamines on heart rate, subjects performed the second GXT with pacemakers programmed to respond optimally in the rate-responsive mode (paced). Peak heart rate (149 versus 129 bpm), peak oxygen uptake (18.9 versus 15.4 mL/kg/min), treadmill time to exhaustion (14.6 versus 12.4 min), and minute ventilation (76.7 versus 66.2 L/min) were significantly increased (P < or = 0.05) during the paced versus unpaced GXT. The results of this study demonstrate that chronotropic support of the transplanted heart using a rate-responsive pacemaker, with activity-based sensors programmed for maximal sensitivity, improves both peak heart rate and exercise capacity in heart transplant recipients significantly more than circulating catecholamines alone.

Clinical evaluation of a pacemaker algorithm that adjusts the pacing rate during sleep using activity variance.

Even though rate responsive pacemakers are able to regulate pacing rates based on sensor activity, they are set with a minimum rate that is not adjusted to provide rate decreases during sleep. The aim of this study was to evaluate the performance of the "Sleep Rate" feature, as compared to patient diaries and a validated method that identifies sleep from wrist actigraphy. In 19 patients (15 men; age 69 +/- 8 years) with Pacesetter Trilogy DR+ pacemakers, the base rate and the sleep rate were set to 80 and 50 ppm, respectively. When the patients returned 2 days later, data recorded by the pacemaker and wrist actigraph were analyzed to find the agreement in corresponding sleep/wake periods. In 17 (89%) patients, the pacemaker went into the sleep mode. The total sleep time derived from actigraphy significantly exceeded the time during which the pacemaker was in sleep mode (1156.8 +/- 83.4 vs 307.3 +/- 77.2 minutes). Frequent reversions out of the sleep mode limited the total sleep time derived from the pacemaker. Cumulative analysis of the pacemaker data showed that the maximum time in the sleep mode was 78 minutes, and exceeded 1 hour in six instances, 30 minutes in 32 instances, and 15 minutes in 83 instances. Epoch by epoch comparisons revealed a good agreement (93.6 +/- 1.8%) during wakefulness between the corresponding actigraph and pacemaker epochs. However, only 24.6 +/- 3.7% of the corresponding epochs during sleep were identical, and the overall agreement was 54.4 +/- 3.7%. Except for one patient who reported palpitations, patients did not suffer from a pacemaker rate change. The Sleep Rate feature provides rate reduction during sleep, while assuring rapid frequency response during physical activity. However, the current algorithm does not allow long periods of slow pacing rate during continuous sleep, possibly due to its conservative design and the presence of movement arousals, which has to be improved in future generation algorithms.

Ablate and pace for drug refractory paroxysmal atrial fibrillation: Is ablation necessary?

Background: Atrio-ventricular junctional ablation with pacemaker insertion has been shown to improve quality of life in patients with drug refractory paroxysmal atrial fibrillation. It is unknown whether this improvement is secondary to the ablation procedure or to the pacemaker mode utilised. To investigate this we reviewed our experience of implanting a dual chamber rate responsive pacemaker with mode switching (DDDR/MS) alone on quality of life in this patient group. Methods and results: Over a 1-year period, 19 patients (mean age 62±9 years, 13 female) with drug refractory paroxysmal atrial fibrillation (mean duration of symptoms 8.7±7 years, failed 3.1±0.9 anti-arrhythmic drugs, amiodarone in 15) were recruited. Quality of life was assessed at baseline and after 1 month using a cardiac specific questionnaire, the modified Karolinska questionnaire. The mean score for all patients significantly improved by 39% at follow up (baseline 59±24, 1 month 36±24, P=0.001). Individually 15 patients (79%) had an improvement in their score, whilst for 13 patients (68%) their symptoms were sufficiently improved after pacing that ablation was not required. The benefit was maintained to a mean follow up of 12±5 months (score 31±20, P<0.001). Six patients remained symptomatic after pacing and requested further treatment. Benefit was unrelated to symptoms at baseline or the number and total duration of paroxysmal atrial fibrillation episodes recorded on pacemaker Holter. Conclusions: Patients with drug refractory paroxysmal atrial fibrillation, DDDR/MS pacing alone can improve quality of life without concurrent atrio-ventricular junctional ablation in a significant proportion of patients.

Automatic Pace-Sense Polarity Switch as an Indicator of Early Lead Corrosion: The Usefulness of Impedance Trend Graphing

A rate responsive dual chamber pacemaker system (Medtronic Inc.) was implanted without complications. At 6-week postimplantation a routine pacemaker check showed a spontaneous switch from programmed bipolar pace-sense to unipolar pace-sense on the atrial and ventricular leads. Pacing and sensing thresholds were not significantly changed from implantation. The atrial and ventricular lead impedances increased from 680 and 720 ohms at implantation to 1,290 and 2,400 ohms, respectively. The device was reprogrammed to bipolar pace-sense and the continuous lead telemetry trend option was programmed On. Evaluation of the system 1 month later revealed a decrease in atrial and ventricular lead impedances, 680 and 2,100 ohms, without a change in pace-sense polarity. One month later, the lead polarity had again switched from programmed bipolar to unipolar pace-sense. The lead trend data revealed stable atrial impedances with sporadic increases in the ventricular lead impedance to values > 3,000 ohms. The pacemaker lead system was invasively investigated and visible gross corrosion of the ventricular lead distal connector pin was discovered.

Contractility rate responsive pacemaker: An important step toward an artificial sinus node for severe chronotropic incompetence

Contractility rate responsive pacemaker: An important step toward an artificial sinus node for severe chronotropic incompetence

Permanent, Direct His-Bundle Pacing

Direct His-bundle pacing (DHBP) produces synchronous ventricular depolarization and improved cardiac function relative to apical pacing. Although it has been performed transiently in the electrophysiology laboratory and persistently in open-chested canines, permanent DHBP in humans has not been achieved. A total of 18 patients aged 69+/-10 years who had a history of chronic atrial fibrillation, dilated cardiomyopathy, and normal activation (ie, QRS< or =120 ms) were screened for permanent DHBP using an electrophysiology catheter. In 14 patients, the His bundle could be reliably stimulated. Of these 14, permanent DHBP using a fixed screw-in lead was successful in 12 patients. Radiofrequency atrioventricular node ablation was performed in patients exhibiting a fast ventricular response. All patients received single-chamber rate-responsive pacemakers. Acute pacing thresholds were 2.4+/-1.0 V at a pulse duration of 0.5 ms. Lead complications included exit block requiring reoperative adjustment and gross lead dislodgment. Echocardiographic improvement in heart function was shown by reductions in the left ventricular end-diastolic dimension from 59+/-8 to 52+/-6 mm (P</=0.01) and in the end-systolic dimension from 51+/-10 to 43+/-8 mm (P<0.01), with an accompanying increase in fractional shortening from 14+/-7% to 20+/-10% (P=0.05). The left ventricular ejection fraction improved from 20+/-9% to 31+/-11% (P<0. 01), and the cardiothoracic ratio decreased from 0.61+/-0.06 to 0. 57+/-0.07 (P<0.01). Despite DHBP, 2 patients died at 8 and 36 months. Conclusions-Permanent DHBP is feasible in select patients who have chronic atrial fibrillation and dilated cardiomyopathy. Long-term, DHBP results in a reduction of left ventricular dimensions and improved cardiac function.

PROGRAMMING OF SENSOR DRIVEN PACEMAKERS

The chronotropic response is the most important means by which cardiac output is increased and oxygen delivery is maintained in response to increased oxygen consumption during exercise or stress. When the chronotropic response is suboptimal or absent, exercise intolerance results. This condition, called chronotropic incompetence can be treated effectively with a sensor-driven rate-responsive pacemaker. The effectiveness of this therapy assumes that the pacemaker is programmed appropriately. This article focuses on the programming of sensor-driven pacemakers and provides additional suggestions for follow-up testing to ensure maximal benefit from these devices.

Influence of the pacing rate on the atrioventricular conduction time during aerobic and anaerobic exercise: basic concepts for a dromotropically controlled rate responsive pacemaker.

The dromotropic pacemaker concept needs a rate responsive algorithm in which the pacing rate is controlled by the atrioventricular conduction time (AVCT). To develop basic concepts for such a rate responsive algorithm, the influence of the pacing rate on the AVCT was investigated. Seven patients (62 +/- 7.8 years) with sick sinus syndrome and intact atrioventricular conduction underwent two cardiopulmonary exercise tests (CPX) on a treadmill. According to the determination of the anaerobic threshold (AT) and the patients maximum capacity in the first incremental CPX the work rate for two exercise levels below and above the AT were chosen for the second constant workload CPX. The calculation of the optimal pacing rate (HRopt) was based on the oxygen uptake (VO2) during exercise after reaching steady-state conditions. According to the increase of the VO2 from 14.8 +/- 2.3 mL/min per kilogram during aerobic work (38.3 +/- 16.0 W) to 19.4 +/- 4.7 mL/min per kilogram during anaerobic work (80.6 +/- 32.3 W), the HRopt was calculated to be 98.6 +/- 6.9 beats/min and 116.4 +/- 4.7 beats/min. Starting from HRopt, the pacing rate was increased (overpacing) and decreased (underpacing) by about 5 beats/min every minute. At optimal pacing rate the AVCT decreased significantly from 233.0 +/- 30.5 ms during aerobic work and to 226.4 +/- 27.3 ms during anaerobic work (P < 0.05). Whereas overpacing induced a significant prolongation of the AVCT during aerobic work (4.17 +/- 1.78 ms per 10 beats/min) and anaerobic work (3.84 +/- 1.60 ms per 10 beats/min), underpacing yielded a significant shortening of the AVCT by about 4.49 +/- 2.64 ms per 10 beats/min during aerobic work and 4.75 +/- 1.87 ms per 10 beats/min during anaerobic work (P < 0.01). The slopes of the regression lines of the relationship between AVCT and pacing rate were not significantly. different. Based on the reciprocal relationship of heart rate (HR) and AVCT, basic concepts may be established for a dromotropic rate responsive algorithm.

Incidence and significance of chronotropic incompetence in patients with indications for primary pacemaker implantation or pacemaker replacement.

This prospective study was undertaken to evaluate the incidence and significance of chronotropic incompetence in 211 patients [age 71.1 6 10.6 years (mean 6 SD)] by means of maximum exercise test in order to determine the indication for rate-responsive pacing before primary pacemaker implantation (147 patients) or pacemaker replacement (64 patients). There were 112 (53%) patients with second- or third-degree AV block, 63 (30%) with sick sinus syndrome, and 36 (17%) with chronic atrial fibrillation. Chronotropic incompetence was defined as maximum heart rate lower than age-adjusted norm calculated by the formula: 0.7x(220 - age) and its significance as the difference between the two rates. The overall incidence of chronotropic incompetence was 42%. The incidence was significantly higher in patients with atrial fibrillation (67%, P<0.0005) and sick sinus syndrome (49%, P<0.012) than in those with AV block (30%). The mean difference between maximum heart rate and the age-adjusted norm was 18% (range 2%-63%). The mean difference was significantly higher in patients with atrial fibrillation (27%, range 8-63%) than in those with sick sinus syndrome (19%, range 2%-45%, P<0.01), or with AV block (12%, range 6%-26%, P<0.000001). The rate-responsive pacemakers were implanted in 44% of 211 patients studied and in 43% of 196 patients excluded from the study due to the apparent (contra)indication of rate-responsive pacing (NS). Thus, chronotropic incompetence seems to be common in the pacemaker patient population. The highest incidence and significance was found in patients with chronic atrial fibrillation. Systematic evaluation of chronotropic competence can double the rate of implantation of rate-responsive pacemakers; however, further studies are needed to clarify relation between the significance of chronotropic incompetence and functional benefit of rate-responsive pacing.

The utility of pacemaker evoked T wave amplitude for the noninvasive diagnosis of cardiac allograft rejection.

Previous work suggested that pacemaker evoked T wave amplitude (ETWA) may be a sensitive noninvasive marker of cardiac allograft rejection. A Topaz QT sensing rate responsive pacemaker (Vitatron Medical) was implanted at transplantation using epicardial ventricular leads in 45 recipients (35 males; median age 51 years, range 20-63). The median duration of follow-up was 129 days (range 4-327). The ETWA at a paced rate of 100 beats/min was measured daily during hospitalization and at each outpatient attendance (900 readings). Endomyocardial biopsies were at routine intervals or when otherwise clinically indicated (257 biopsies with concurrent ETWA data). There were 58 episodes of rejection > or = grade 3a in 28 patients. The biopsies were classed as either no rejection (grade < 3a) or rejection requiring treatment (grade > or = 3a). The median normalized ETWA was 100.8% (range 24.6-239.7) without rejection and 89.9% (17.0-189.7) with rejection (Mann-Whitney U Test: P = 0.028). The performance of ETWA monitoring as a diagnostic test for the individual recipient was evaluated with exponentially weighted moving average quality control charts. For the diagnosis of all rejection episodes, ETWA monitoring had a sensitivity of 55%, a specificity of 62%, a positive predictive value of 30%, and negative predictive value of 83%. It is concluded that although analysis of pooled data showed a significant reduction in normalized ETWA with biopsy proven rejection, ETWA monitoring requires further refinement to improve sensitivity before it can be considered a clinically useful technique for the non-invasive diagnosis of cardiac allograft rejection in individual recipients.

RATE RESPONSIVE CARDIAC PACING INCREASES EXERCISE CAPACITY IN HEART TRANSPLANT RECIPIENTS

1715 Chronotropic incompetence, secondary to surgical cardiac denervation, is one cause of diminished exercise capacity in heart transplant recipients (HTR). If reinnervation occurs, it is very late after transplantation (Tx) and reinnervation is not always accompanied by functional improvements in peak heart rate and the ability to mount a tachycardic response during exercise. To determine the effect of a rate-responsive pacemaker on heart rate and exercise capacity we studied 8 male cardiac denervated HTR (Age 57±12 yr; 23±9 mos post-Tx). All subjects completed 2 maximal GXT's using the Naughton treadmill protocol. During the first GXT pacemakers were programmed to support resting heart rate only and to be non-rate responsive during exercise (UNPACED). After a 14 day regimen of cardiac-specific beta blockade with Metoprolol to nullify the influence of circulating catecholamines on heart rate, subjects performed the second GXT with pacemakers programmed to respond optimally in the rate-responsive mode (PACED). Tabular data are peak exercise values (mean±SD). *p≤0.05 PACED vs UNPACED. (Table)TableTotal treadmill time to exhaustion, VO2max, and peak heart rate were significantly increased during PACED. The results of this preliminary study demonstrate that a rate responsive cardiac pacemaker improves both chronotropic incompetence and exercise capacity in denervated HTR.