Effects of Sensor Selection on Exercise Stroke Volume in Pacemaker Dependent Patients

The effects of sensor selection and sensor blending on the cardiovascular response to graded exercise was evaluated in 10 patients (age 74 ± 2 yrs; 7 men and 3 women) implanted with a dual sensor rate adaptive VVIR pacemaker (Vitatron Topaz(tm) model 515). Patients underwent three graded exercise tests (GXT) with sensor programming randomly assigned. For a given graded exercise text the pacemaker was programmed into activity sensing (ACT), QT sensing, or dual sensing (ACT = QT). Data were recorded at rest and during each stage of the graded exercise text. Oxygen uptake (VO₂) was measured continuously using a Q Plex I system. Heart rate (HR), stroke volume (SV), and cardiac output (Qc) were measured by impedance cardiography. Systolic time intervals were calculated from simultaneous recordings of the ECG. phonocardiogram, and the impedance cardiogram. In response to the GXT no differences in peak VO₂ were observed across the three sensor settings. Regardless of the sensor setting Qc increased linearly with each increment in VO₂. The HR response to ACT only pacing was significantly higher than in the other two pacing conditions. During ACT only pacing SV failed to rise in response to exercise. The increased exercise Qc during QT and ACT = QT pacing were mediated by significant increases in both HR and SV. The QT and dual pacing conditions were also associated with longer diastolic filling times. The data indicate that the mechanisms responsible for the increase Qc during exercise were different for ACT versus ACT = QT or QT sensor-driven pacing.     

Failure of Rate Responsive Ventricular Pacing to Improve Physiological Performance in the Univentricular Heart

The physiological efficacy of single chamber, rate responsive ventricular pacing (VVIR) is unknown for symptomatic patients following the Fontan procedure for univentricular hearts. A total of six postoperative children, ages 6–21 years (mean 13), with symptomatic bradycardia requiring pacing therapy, underwent comparative treadmill exercise testing in randomized fixed rate (VVI) and VVIR pacing modes. In all instances, implanted activity pulse generators (Medtronic Model 8403) were programmed to identical age-appropriate low paced rates during WI and VVIR modes with the upper rate response at 150 ppm. All studies were performed at least 2 weeks apart. Physiological values of heart rate, blood pressure, work rate (watts), oxygen comsumption (VO₂), carbon dioxide production (VCO₂), and respiratory exchange ratio (RER) were monitored continuously during each test using a 1 minute incremental treadmill protocol. Ventilatory anaerobic threshold (VAT) was calculated from VO₂, VCO₂, and minute ventilation. The results demonstrated that although there was a significant increase in paced heart rate per minute throughout exercise (P < 0.01) with VVIR pacing, maximum watts, VO₂, and VAT remained unchanged. These findings indicate that in spite of an improved chronotropic response to exercise, children with Univentricular hearts following the Fontan procedure continue to demonstrate altered hemodynamics which negate potential benefits of VVIR pacing.     

Measurement of Minute Ventilation with Different DDDR Pacemaker Electrode Configurations

A rate responsive minute ventilation (VE) pacemaker was implanted in 49 patients (70.8 ± 40.0 years). A Chorus RM 7034 pacemaker was implanted in 43 patients and an Opus RM 4534 in six patients. Four sensor configurations were compared: atrial configuration (bipolar atrial lead) in 34 patients; ventricular configuration (bipolar ventricular lead) in 6 patients; unipolar configuration (double unipolar leads) in 6 patients; and floating configuration (VDD single-pass lead) in 3 patients. The patients carried out 57 exercise tests in all with cardiopulmonary recording (CPX). Real VE and oxygen consumption (VO₂) were recorded by the CPX, the VE measured by the sensor (VEsensor) was recorded in the pacemaker memory. The mean correlation between VE and VEsensor was 0.90 ± 0.08 (P < 0.001) and between VO₂ and VEsensor was 0.86 ± 0.10 (P < 0.001). The mean correlation between VE and VEsensor by configuration type were as follows: atrial configuration = 0.89 ± 0.08; ventricular configuration = 0.95 ± 0.05; unipolar configuration = 0.87 ± 0.14; and floating configuration = 0.88 ± 0.05. In conclusion, VE may be reliably measured using different electrode configurations. A study conducted in a larger population should allow one to conclude that uniploar electrodes can be used in VDDR, AAIR, VVIR, or DDDR modes to measure VE.     

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.     

Atrial Natriuretic Factor Release During Exercise in Patients Successively Paced in DDD and Rate Matched Ventricular Pacing

Dual chamber pacemakers were implanted in nine patients with permanent second or third degree AV block feight had complete retrograde block). Two identical exercise tests were performed after at least 1 month after implantation. During the first test (T₁) the pacemaker was programmed to the DDD mode and heart rates were recorded every 15 to 30 seconds during exercise and 30 minutes after exercise. Following 30 minutes of rest, the implanted pacemaker was programmed to the VVT mode and driven by an external pacemaker via a skin electrode. The second exercise test (T₂) was then performed and the rate of the external pacemaker was progressively changed to reproduce exactly the rate observed during T₁ at the same exercise stress. Atrial natriuretic factor (ANF) levels were determined at rest, at regular intervals during exercise, and 30 minutes after exercise. ANF levels and release were statistically higher during rate matched ventricular, than DDD pacing. It is concluded that preservation of AV synchrony reduces ANF release induced by heart rate acceleration during exercise.     

Impact of Adaptive Rate Pacing Controlled by a Right Ventricular Impedance Sensor on Cardiac Output in Response to Exercise

COOK, L., et al. : Impact of Adaptive Rate Pacing Controlled by a Right Ventricular Impedance Sensor on Cardiac Output in Response to Exercise. This study examined the effects of adaptive rate pacing controlled by closed-loop right ventricular impedance sensing on exercise hemodynamics. Twelve patients in whom Biotronik INOS²⁺ pacemakers had been implanted 4–6 weeks earlier participated in the study. All patients completed two graded, symptom-limited exercise tests. The pacemaker was programmed to DDDR with an upper rate limit of 75–85% of the age-predicted maximum heart rate and a lower rate limit of 45–60 ppm. Heart rate was recorded continuously. An average of 5 beats during the last 10 seconds of each exercise stage was used in the analysis. Oxygen uptake (VO₂) was measured using open circuit spirometry. The VO₂ values from the final 15 seconds of each exercise stage were used for analysis. Stroke volume and cardiac output were measured during the last minute of each stage using impedance cardiography. The test-retest reliability of heart rate and cardiac output responses to graded exercise was assessed using repeated measures analysis of variance, for which the reliability coefficients were r = 0.993 and r = 0.954, respectively (P < 0.01). There were significant correlations (P < 0.01) between VO₂ and heart rate and between VO₂ and cardiac output, with correlation coefficients of r = 0.907 and r = 0.824, respectively. This method of adaptive rate pacing produced reliable, positive hemodynamic responses to graded exercise on a test-retest basis. (PACE 2003; 26:[Pt. II]:244–247)     

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

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

Rate-Adaptive Cardiac Pacing in Children Using a Minute Ventilation Biosensor

YABEK, S.M., ET AL.: Rate-Adaptive Cardiac Pacing in Children Using a Minute Ventilation Biosensor. Chronotropic integrity is required for a normal cardiac output response to exercise. We evaluated a rate-adaptive ventricular demand pacemaker (Telectronics, META-MV) which uses minute ventilation as the sensed physiological variable for adjusting pacing rate, in seven young patients with a mean age of 11.4 years. All patients had clinically significant bradycardia related to complete heart block (n = 4) or sinus node dysfunction (n = 3). For the entire group, paced heart rates increased from 70 ± 10 beats/min to 151 ± 19 beats/min with exercise testing. The onset of rate adaptation took < 30 seconds. Changes in paced rate were linearly related to workload, VO₂ (5.9 to 20.7 mL/min/kg) and minute ventilation (8–65 L/min). The decline in pacing rate after exercise was related directly to the gradual decrease in minute ventilation and VO₂. Our data show that minute ventilation closely and accurately reflects the metabolic demands of varying workloads in children and can be used to achieve physiological, rate-adaptive pacing.     

AAIR Versus DDDR Pacing in Patients with Impaired Sinus Node Chronotropy: An Echocardiographic and Cardiopulmonary Study

The aim of this study was to compare AAIR and DDDR pacing at rest and during exercise. We studied 15 patients (10 men, age 65 ± 6 years) who had been paced for at least 3 months with activity sensor rate modulated dual chamber pacemakers. All had sick sinus syndrome (SSS) with impaired sinus node chronotropy. The patients underwent a resting echocardiographic evaluation of systolic and diastolic LV function at 60 beats/min during AAIR and DDDR pacing with an AV delay, which ensured complete ventricular activation capture. Cardiac output (CO) was also measured during pacing at 100 beats/min in both pacing modes. Subsequently, the oxygen consumption (VO₂at) and VO₂at pulse at the anaerobic threshold were measured during exercise in AAIR mode and in DDDR mode with an AV delay of 120 ms. The indices of diastolic function showed no significant differences between the two pacing modes, except for patients with a stimulus-R interval > 220 ms, for whom the time velocity integral of LV filling and LV inflow time were significantly lower under AAI than under DDD pacing. At 60 beats/min, CO was higher under AAI than under DDD mode only when the stimulus-R interval was below 220 ms. For stimulus-R intervals longer than 220 ms, and also during pacing at 100 beats/min, the CO was higher in DDD mode. The stimulus-R interval decreased in all patients during exercise. The time to anaerobic threshold, VO₂at ond VO₂at pulse showed no significant differences between the two pacing modes. Our results indicate that, at rest, although AAIR pacing does not improve diastolic function in patients with SSS, it maintains a higher CO than does DDDR pacing in cases where the stimulus-R interval is not excessively prolonged. On exertion, the two pacing modes appear to be equally effective, at least in cases where the stimulus-R interval decreases in AAIR mode.     

The Effect of DDD Pacing on Ergospirometric Parameters and Neurohormonal Activity in Patients with Hypertrophic Obstructive Cardiomyopathy

This study examined the acute and long-term effects of DDD pacing on ergospirometric parameters and neurohormonal activity in patients with hypertrophic obstructive Cardiomyopathy (HOCM). We studied eight patients (five males), aged 56 ± 7 years, with HOCM refractory to drugs. In all patients a DDD pacemaker was implanted and programmed with an atrioventricular (AV) delay that insured full ventricular activation. The patients underwent echocardiographic examination and exercise stress testing before and 3 days, 3 months, and 12 months after pacemaker implantation. Oxygen consumption was measured at the anaerobic threshold (VO_2AT) and peak exercise (pVO₂). Atrial natriuretic peptide (ANP) and cyclic adenosine monophosphate (c-AMP) levels were measured concomitantly. Left ventricular outflow tract (LVOT) pressure gradient decreased significantly from 70 ± 18 to 25 ± 12 mmHg (P < 0.05) 3 days after pacing and remained unchanged at 3 and 12 months. pVO₂ and VO_2AT increased significantly, from 20.1 ± 3 to 23.4 ± 3 mL/kg/min and from 16 ± 3 to 17.8 ± 2 mL/kg/min, respectively (P < 0.05). This improvement continued up to 3 months, and then remained stable until the end of the 12-month follow-up period. ANP levels decreased at 3 days from 85.4 ± 5.7 to 75.4 ± 7.3 fmol/mL (P < 0.05), and remained unchanged over the 12 months. c-AMP levels did not change significantly after the onset of pacing. DDD pacing in patients with HOCM not only reduces the LVOT pressure gradient but also causes a significant early and long-term improvement in exercise capacity and neurohormonal profile.     

Cardiopulmonary Responses to Exercise in Children with Activity Sensing Rate Responsive Ventricular Pacemakers

The physiological benefits of activity sensing rate responsive ventricular pacing)VVIR) over fixed rate pacing)VVI) were investigated in 14 children during incremenlal cycle exercise. Based on their heart rhythm response during exercise, children were divided into two groups. Group I patients)13 ± 4 years) remained in a paced-only rhythm when exercised. Group II patients)16 ± 7 years) were paced at rest but converted to sinus rhythm with exercise. In Group I patients, the significant physioJogicol benefits of VVIR over VVI pacing were evidenced hy a 51% increase in peak heart rate)HRmax) and a 16% increase in exercise duration and maximum oxygen uptake)VO₂max). Additionally, a 27% reduction in peak oxygen pulse)O₂Pmax) was found, reflecting a similar decrease in stroke volume. The cardiorespiraiory responses of Group I and 11 patients were compared in terms of percent of predicted normal values. Although Group I patients in the VVIR mode attained a better exercise performance than in the VVI mode and a normal O₂Pmax)108% pred). their HRmax)62% pred) and VO₂max)70% pred) fell far below normal values. In comparison. Group II patients, who went into sinus rhythm, achieved normal values for HRmax)84% pred), VO₂max)90% pred), and O₂Pmax)97% pred). The higher pacing rates attained by Group I patients in the VVIR mode may have allowed them to reach not only a higher cardiac output but also a more normal stroke volume at peak exercise than in the VVI mode. However, the overall exercise performance of children paced in the VVI and VVIR modes were significantly diminished compared to the performance of children who went into sinus rhythm with exercise.)     

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

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

The Effects of Rate-Adaptive Atrial Pacing Versus Ventricular Backup Pacing on Exercise Capacity in Patients with Left Ventricular Dysfunction

Background: Atrial rate-adaptive pacing may improve cardiopulmonary reserve in patients with left ventricular dysfunction.
Methods: A randomized, blinded, single-crossover design enrolled dual-chamber implantable defibrillator recipients without pacing indications and an ejection fraction ≤40% to undergo cardiopulmonary exercise treadmill stress testing in both atrial rate-adaptive pacing (AAIR) and ventricular demand pacing (VVI) pacing modes. The primary endpoint was change in peak oxygen consumption (VO₂). Secondary endpoints were changes in anaerobic threshold, perceived exertion, exercise duration, and peak blood pressure.
Results: Ten patients, nine males, eight with New York Heart Association class I, mean ejection fraction 24 ± 7%, were analyzed. Baseline VO₂ was 3.6 ± 0.5 mL/kg/min. Heart rate at peak exercise was significantly higher during AAIR versus VVI pacing (142 ± 18 vs 130 ± 23 bpm; P = 0.05). However, there was no difference in peak VO₂ (AAIR 23.7 ± 6.1 vs VVI 23.8 ± 6.3 mL/kg/min; P = 0.8), anaerobic threshold (AAIR 1.3 ± 0.3 vs VVI 1.2 ± 0.2 L/min; P = 0.11), rate of perceived exertion (AAIR 7.3 ± 1.5 vs VVI 7.8 ± 1.2; P = 0.46), exercise duration (AAIR 15 minutes, 46 seconds ± 2 minutes, 54 seconds vs VVI 16 minutes, 3 seconds ± 2 minutes, 48 seconds; P = 0.38), or peak systolic blood pressure (AAIR 155 ± 22 vs VVI 153 ± 21; P = 0.61) between the two pacing modes.
Conclusion: In this study, AAIR pacing did not improve peak VO_2, anaerobic threshold, rate of perceived exertion, or exercise duration compared to VVI backup pacing in patients with left ventricular dysfunction and no pacing indications.     

Rate Control with an External SO2 Closed Loop System

In 20 volunteers (mean age 35.5 y) and 12 pacemaker patients (mean age 68.7 y), central venous oxygen saturation (SO₂) was monitored continuously by means of an optical sensor integrated in an external transvenous pacing lead placed in the right ventricular cavity. From the SO₂ signal recorded at rest and during various modalities of exercise, an algorithm for controlling pacing rate of an external pacing system was developed. An open loop system was used in the volunteers, allowing the comparison of the computed pacing rate with the individual intrinsic heart rate. There was an excellent correlation between the two frequencies as far as the dynamic characteristics and the steady state relationship were concerned. In five pacemaker patients who were stimulated via the external lead, a closed loop control of pacing rate was used. In one patient with a DDD pacemaker implanted for third degree AV-block, the rate response of the SO₂ driven pacemaker was well in accordance with the rate attained with the implanted atrial triggered system. With both pacing modes, exercise capacity as determined on a symptom limited treodmill test was identical. In four patients (3 AV block III, 1 bradyarrhythmia) an improvement in exercise tolerance up to 65 percent could be demonstrated with the rate responsive pacing mode. In all patients, it could he shown that an autoregulating pacemaker system with SO₂ is an open possibility.     

Intrinsic Conduction Maximizes Cardiopulmonary Performance in Patients with Dual Chamber Pacemakers

Dual chamber pacemaker programmability allows the possibility of atriallytracked ventricular pacing in patients who would otherwise have intrinsic atrioventricular (AV) conduction. Thirteen patients with permanent AV sequential pacemakers (ages 50–79) were evaluated with paired exercise tests to determine the Cardiopulmonary effects of pacemaker induced right ventricular activation compared with normal AV and intraventricular conduction. Peak oxygen uptake (VO₂), oxygen pulse (O₂P), respiratory rate (RR), and respiratory exchange ratio (RER) were determined using breath by-breath analysis of expired gases. Patients exercised to fatigue and exercise tests were performed in random sequence. For patients with intrinsic AV conduction (group I, n = 8) the AV delay was programmed to preserve intrinsic conduction during one study; the alternate test used AV delay programming to produce ventricular pacing. Five patients with chronic AV block (group II) acted as a control for the effects of a rate adaptive AV delay compared to a fixed AV delay. Paired t-testing showed a significantly lower peak VO₂ (P < 0.015) and O₂P (P < 0.01) in patients with atrially-tracked ventricular pacing compared to intrinsic conduction. In contrast, group II showed a significant improvement in peak VO₂ with rate adaptive AV delay compared to fixed AV delay programming (P < 0.05). In conclusion, intrinsic conduction should be preserved in patients with dual chamber pacemakers whenever possible.     

Effects of Chronotropic Responsive Cardiac Pacing on Ventilatory Response to Exercise in Patients with Complete AV Block

To identify the effect of chronotropic responsive cardiac pacing on the ventilatory response to exercise, ten selected patients with complete atrioventricular block underwent paired cardiopulmonary exercise tests in fixed rate ventricular (WI) and dual chamber (DDD) or rate responsive ventricular (VVIR) pacing modes. Compared to VVI pacing, DDD or VVIR pacing increased peak oxygen uptake (P < 0.005) and augmented anaerobic threshold (P < 0.001), In eight patients, dyspnea was the major symptom limiting exercise with VAT pacing and this was markedly attenuated with DDD or VVIR pacing. In all patients, ventilation (VE) and the ratio of ventilation to CO₂ production (VE/VCO₂) were consistently higher with VVI pacing during exercise. To compare the response of the two pacing modes at the same workloads in an aerobic condition, we measured ventilatory variables 1 minute prior to the anaerobic threshold obtained with VVI pacing. When DDD or VVIR pacing was compared with VVI pacing, VE and VE/VCO₂ significantly decreased from 20.5 ± 5.3 L/min to 18.3 ± 5.0 L/min (P < 0.005) and from 35.9 ± 5.8 to 31.9 ± 5.0 (P < 0.003), respectively. Respiratory frequency rose significantly more with VVI pacing (P < 0.001) despite an unchanged tidal vohame. Although peak VE did not differ between the two pacing modes, VE/VCO₂ at the peak exercise increased significantly more with VVI pacing (P < 0.005). Respiratory frequency also rose more with VVI pacing (P < 0.005) and tidal volume did not change. This study suggests that chronotropic responsive cardiac pacing attenuates the exertional dyspnea by improving the ventilatory response to exercise as well as increasing the cardiac output in patients with complete atrioventricular block.     

Aerobic Capacity in Rate Modulated Pacing

Whether heart rate or AV synchrony is the most important factor for an increase in aerobic capacity was evaluated in a comparative study between sinus bradycardia, VVIR, DDD, and DDDR stimulation. Sixteen patients (mean age 67 years) with chronotropic incompetence and impJanted DDDR pacemaker (Telectronics META 1250) were randomly studied by cardiopulmonary exercise testing. All patients were exercised to their anaerobic threshold (ATJ with the following heart rates: DDD 84 ± 3, WIR 110 ± 5, and DDDR 116 ± 6 beats/min. Mean oxygen uptake (VO₂, mL/kg per min) at AT was 7.4 ± 0.3 in DDD and WIR modes. A 12% increase was measured in DDDR mode (8.3 ± 0.4). Compared to VVIR work capacity in the DDDR mode was improved by 17% (41 vs 48 W/min). In patients with isolated sinus node disease (n = 9) the increase of VO₂ and work capacity at AT during DDDR mode was more pronounced (16% and 20%, respectively, compared to VVIR). In patients with intermittent second or third degree AV block (n = 7) the differences between the pacing modes were not significant. This might partly be due to a lesser degree of chronotropic incompetence in this subgroup. In conclusion only the conjunction of heart rate increase and preservation of AV synchrony provides a significant improvement in aerobic capacity during exercise.     

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.     

Rate-Responsive Pacing by Means of Activity Sensing Versus Single Rate Ventricular Pacing: A Double-Blind Cross-Over Study

The clinical appiicabiJity of rate-responsive pacing (RRP) by means of activity sensing has been tested in 15 patients. The patients (ages 24–85) had sinus node dysfunction (2), atrial fibrillation (7), or sinus rhythm (6) combined with complete atrioventricular block. Exercise capacity was investigated on a bicycle ergometer and on a treadmill in a double-blind cross-over study design following one week each of fixed rate ventricu/ar pacing (70 bpm) and rate-responsive pacing (60/125–150 bpm). The patients answered a questionnaire concerning subjective symptoms. A Holter ECG was recorded during 24 hours of all day activity on rate-responsive pacing. During exercise in the rate-responsive mode, heart rate increased more on the treadmill than on the bicycle. A majority of the patients (13 of 15) preferred rate-responsive pacing mainly due to less dyspnea and tiredness. Exercise capacity improved significantly both on bicycle (+7%; p < 0.01) and on treadmill (+19%; p < 0.01) during rate-responsive pacing. There were no complications during the follow-up period. In conclusion, the activitysensing pacemaker is a valuable supplement to existing types o/ pacemakers. It should be used in patients in whom an atrial electrogram cannot be used for rate triggering.     

New Concept in Activity-Controlled Pacemakers: Clinical Results With an Accelerometer-Based Rate Adaptive Pacing System

An accelerometer-based rate adaptive generator (EXCEL VR) has been introduced. A preclinical group of 22 subjects with strap-on devices was observed and reported. A clinical protocol including observation of rate adaptive response to typical daily activities and incremental exercise on a treadmill was administered in seven implanted patients. Indications for implantation in these patients was either second- or third-degree atrioventricular block (five patients, VVIR pacing mode) and sick sinus syndrome (two patients, AAIR pacing mode). Mean pacing rates were 50 ppm (supine), 56 ppm (standing), 77 ppm (descending the stairs), 81 ppm (slow walk), 83 ppm (slow stair climb), 91 ppm (fast walk), and 92 ppm (fast stair climb). When the arm proximal to the pulse generator was exercised, the rate rose to 92 ppm. When the distal arm was strained, the rate was 63 ppm. During treadmill testing, rates between 82 ppm (2 km/hour) and 104 ppm (5 km/hour) were observed. This accelerometer-based rate adaptive pulse generator provided a proportional response to graded activities of treadmill exercise and daily living in these groups of preclinical and clinical subjects.