The ventricular depolarization gradient: effects of exercise, pacing rate, epinephrine, and intrinsic heart rate control on the right ventricular evoked response

Comparison between ventricular gradient and a new descriptor of the wavefront direction of ventricular activation and recovery

BACKGROUND

Total R T cosine (TCRT) is a new descriptor of repolarization heterogeneity that quantifies the deviation between the directions of ventricular depolarization and repolarization. It revives the old concept of ventricular gradient (VG).

HYPOTHESIS

Our goal was to examine whether TCRT and VG contain nonredundant information by comparing their reaction to autonomic tests, namely, postural changes and Valsalva maneuver.

METHODS

Digital 12-lead electrocardiograms were recorded in 16 patients with cardiovascular syndrome X (SX, chest pain, exercise-induced ST-depression, normal coronary arteries, 3 men, age 60 +/- 9 years) and 40 healthy volunteers (31 men, age 33 +/- 7 years) during postural changes and Valsalva maneuver. The angle (VGA) [degrees] and magnitude (VGM) [ms.mV] of VG in reconstructed XYZ leads and TCRT (average cosine of the angles between the QRS and T vectors in mathematically reconstructed three-dimensional space) were calculated.

RESULTS

(mean +/- standard of the mean): In healthy subjects, VGM and TCRT decreased, whereas VGA increased in the sitting and standing compared with supine position (TCRT: 0.61 +/- 0.05,0.47 +/- 0.06,0.29 +/- 0.08, supine, sitting, and standing, p < 0.05) and during phase II Valsalva (TCRT: 0.47 +/- 0.06 vs. 0.61 +/- 0.05, p < 0.01 in supine, 0.24 +/- 0.08 vs. 0.37 +/- 0.07, p < 0.01 in standing). In patients with SX, VGM decreased in the standing position, VGA did not change significantly, while TCRT decreased only in patients without T-wave abnormalities (n = 9) (TCRT in standing and supine: 0.55 +/- 0.09 vs. 0.68 +/- 0.08, p < 0.05). VG(M) increased during Valsalva in patients with SX. Total R T cosine correlated strongly with VGA (r = -0.84, p < 0.00001) and, unlike VGM, did not correlate with heart rate.

CONCLUSIONS

Ventricular gradient and TCRT contain nonredundant information. In healthy subjects, they react sensitively to autonomic provocation. In patients with SX, their reaction is attenuated, which suggests disturbance of the autonomic control of repolarization.

AutoCapture with Dual-Coil Leads of Implantable Cardioverter Defibrillator

AutoCapture (AC) can confirm ventricular capture with true bipolar single coil leads of implantable cardioverter defibrillators (ICD). The compatibility of AC with a new, true bipolar, dual-coil ICD lead needed to be evaluated. This multicenter study enrolled 46 patients (69 +/- 10 years, 37 men) undergoing ICD implantation. All patients received a true bipolar, dual-coil lead. Evoked response (ER) sensitivity and AC threshold tests were performed using a pulse generator with the AC algorithm. Mean capture threshold was 0.85 +/- 0.67 V, pacing impedance 612 +/- 225 Omega, R wave amplitude 13.85 +/- 6.17 mV, and defibrillation threshold 14.4 +/- 5.1 J. AC was recommended in 45 patients (97.8%) with ER and polarization values of 14.86 +/- 7.32 mV and 0.87 +/- 0.69 mV, respectively. The AC algorithm was highly compatible with true bipolar, dual-coil ICD leads. An AC algorithm specifically designed for an ICD may improve the generator longevity. Further examination of AC compatibility with other leads is warranted.

The influence of mental and physical stress on the autocapture function in children

The Autocapture function detects the evoked response signal (ERS) to verify beat-to-beat capture, and optimizes the output of ventricular pulse amplitude automatically. We had experience concerning the instability of the Autocapture recommendation in some patients. Evoked response is subject to variation as it is a biological event. However, the present knowledge about the Autocapture function lability is very limited. The purpose of this study was to evaluate whether mental stress, body positions or exercise influence the ERS and PS in children. Study was performed in 15 consecutive patients [13.4 +/- 4.1 (5-20) year] with VVIR (n = 10) and DDD/VDD pacemakers with the Autocapture function (n = 5), had received ventricular leads including Membrane-E-1450T (n = 6), Membrane-EX-1470T (n = 2), Tendrill-DX-1388T (n = 3), Tendrill-SDX-1488T (n = 1), AV-Plus-DX-1368 (n = 1), Accufix-II-DEC (n = 1) and Vitatron (n = 1), and followed more than six months. Autocapture functions were measured during arithmetic mental stress test (MST), in different body positions, and during symptom-limited treadmill exercise. MST was applied in all except two (5 and 8 year old) who didn't have ability to perform. Activating autocapture was not recommended in only one with Accufix-II-DEC due to high PS. ERS was 10.5 +/- 6.3 mV during supine and increased to 11.9 +/- 7.5 mV during sitting (p = 0.017) and standing 12.1 +/- 7.2 (p = 0.002). However, ERS remained stable before, during and after both exercise and MST, which were 12.6 +/- 7.2 mV, 12.8 +/- 7.8 mV, 13.6 +/- 9.4 mV (p > 0.05) and 10.5 +/- 5.5 mV, 10.9 +/- 6.7 mV, 10.4 +/- 5.5 mV (p > 0.05) respectively. In addition, PS and recommendation about the Autocapture remained unchanged during the study. In conclusion, MST, different body positions and exercise do not have any clinically important influence on the Autocapture function in children.

Changes in evoked QT intervals according to variations in atrioventricular delay and cardiac function in patients with implanted QT-driven DDDR pacemakers

In patients with implanted DDD pacemaker, cardiac output is maximal when atrioventricular (AV) delay is set to give the maximum QT interval (QTI). QTI is used as a sensor of a rate-responsive pacemaker and the evoked QTI (eQTI) is measured as the time duration from the ventricular pace-pulse and the T sense point, which is the steepest point of the intracardiac T wave. The relationship between the changes in eQTI according to AV delay variations and cardiac function was studied in 13 patients (74.2+/-9.3 [SD] years old) with an implanted QT-driven DDDR-pacemaker. A special software module was downloaded into the pacemaker memory and a personal computer equipped with the special software was connected to the programmer for eQTI date-logging. AV delay was set at 100, 120, 150, 180 and 210 ms. Delta eQTI was defined as maximal eQTI - minimal eQTI. The ejection fraction (EF) was measured by echocardiography. When the AV delay was prolonged, eQTI gradually increased and reached a peak, and then decreased. Delta eQTI in patients with reduced cardiac function (EF <40%) was significantly greater than that in normal cardiac function (EF >55%, 7.6+/-4.9 vs 2.7+/-9.8 ms, p<0.05). There was significant negative correlation between EF and delta eQTI (r=-0.63, p<0.05). The peak of changes in eQTI according to AV delay variations was steeper in patients with reduced cardiac function than in those with normal cardiac function. In conclusion, changes in eQTI according to AV delay variation are greater in patients with reduced cardiac function than in those with normal cardiac function, and the AV delay that gives the maximal eQTI can be easily determined in patients with reduced cardiac function.

Use of the AutoCapture Pacing System with implantable defibrillator leads

INTRODUCTION

Previous studies using various bipolar pacemaker leads have shown that the AutoCapture (AC) Pacing System is able to verify ventricular capture and regulate pacing output, increasing patient safety with respect to unexpected threshold changes and potentially prolonging device longevity. An increasing number of patients with implantable cardioverter defibrillators (ICDs) require ventricular pacing that contributes to a shortening of longevity of these systems. This prospective study tested the compatibility of the AC system with bipolar ICD leads.

METHODS

The AC algorithm was evaluated prior to ICD testing in 30 ICD recipients. A single coil, active fixation, true bipolar ventricular lead was implanted in 21 patients, and a dual coil, passive fixation, integrated bipolar ventricular lead was implanted in 9 patients. A ventricular evoked response sensitivity test and an AC threshold test were performed using a pacemaker with the ventricular AC algorithm.

RESULTS

AC was recommended in 22/30 (73.3%) of implants, including 20/21 (95.2%) with the single coil and 2/9 (22.2%) with the dual coil lead. Mean polarization was lower (1.23 +/- 0.95 mV vs 3.70 +/- 2.33 mV, P = 0.013) while the mean evoked response was higher (18.04 +/- 8.29 mV vs 10.13 +/- 4.22 mV, P = 0.002) with the single coil leads.

CONCLUSION

Automatic threshold tracking using the AC is compatible with ICD leads. Leads with lower polarization and greater evoked response are more likely to result in recommendation of AC use. Use of this system offers the potential for increasing ICD generator longevity and improving patient safety in response to late unexpected threshold increases.

A cardiac evoked response algorithm providing threshold tracking: a North American multicenter study. Clinical Investigators of the Microny-Regency Clinical Evaluation Study

The purpose of this study was to evaluate a pacing system using the recognition of cardiac evoked response for the automatic adjustment of pacing output. Patients were prospectively followed after primary implantation of VVIR pacemakers using AutoCapture (St. Jude Medical CRMD). Sensing and pacing thresholds, polarization signal, evoked response, and AutoCapture performance were evaluated with serial visits and 24-hour Holter monitoring. Three hundred ninety-eight patients (mean age 71 +/- 15 years) were followed for an average duration of 1 year (3 days-1.75 years) with the algorithm functional in > 90% of patients. Backup pacing in the event of exit block was confirmed in all patients. Pacing thresholds remained stable at 0.89 +/- 0.34 V with a pulse width of 0.31 ms (with chronic output autoset at 0.3 V above the actual threshold). Evoked response exhibited a small but statistically significant increase with time (8.92 mV at implant, 9.60 mV at 12 months), however, this finding did not result in any change in AutoCapture function during our follow-up period. The polarization signal remained stable with minimal variation (1.12 mV at implant, 1.18 at 12 months). No clinical adverse events were observed using the AutoCapture algorithm. In this initial experience with the AutoCapture algorithm the evoked response and polarization measurements remained adequate, allowing the system to function in the majority of patients with safe, low output pacing. High energy backup pacing provided an added safety feature over fixed output devices in cases of unexpected threshold rises. Longer follow-up is required for continued long-term validation of the algorithm.

Developments in sensor-driven pacing

This article reviews the recent major developments in the field of rate adaptive pacing. Including, the improved instrumentation of existing sensors, the use of multiple sensors to enhance sensor specificity or sensitivity, and the automation of sensor calibration. The physiologic benefits and programming of rate adaptive pacing are reviewed.

Effects of body position and exercise on evoked response signal for automatic threshold activation

The Autocapture function controls and optimizes the output of the ventricular pulse amplitude automatically. For this reason an automatic test has to be performed during follow-up to measure the evoked response signal and lead polarization for the calculation of the appropriate evoked response sensitivity setting. The aim of the study was to assess whether body position and exercise influence the evoked response and polarization. Both parameters were determined in the supine and upright position and subsequently during supine and upright symptom-limited ergometry. The study included 14 patients with the VVIR pacemaker Regency SR+ who had received the ventricular pacing leads Membrane E 1450 T (n = 8), CapSure Z 5034 (n = 4), or SX 60 (n = 2). The evoked response signal was 7.4 +/- 3.3 mV during supine and increased to 9.7 +/- 5.6 mV (+35%) during upright position (P < 0.05). The exercise tests were terminated at 105 +/- 36 W (supine) and 110 +/- 34 W (upright). There was a gradual insignificant decrease of the evoked response during each exercise test with a mean decrease of -1.1 +/- 0.9 mV (-15%; supine) and -1.6 +/- 2.1 mV (-16%; upright). The evoked response increased within 5 minutes during recovery to the initial values. Polarization remained unchanged during both tests. The pacemaker did not recommend activating autocapture in four patients who all had received high-ohmic pacing leads. In conclusions, the measurement of the evoked response in supine position seems to represent the worst case. Physical activities did not effect autocapture function in patients with the recommended lead, but the pacemaker did not always recommend Autocapture activation in some patients with high-ohmic pacing leads.

Restoration of cardio-circulatory regulation by rate-adaptive pacemaker systems: the bioengineering view of a clinical problem

In the past, the development of rate-adaptive (sensor-controlled) pacemaker systems seems to have been determined primarily by the availability, compatibility and other properties of the technical sensor. This paper, however, focuses on the system-physiological aspect in an attempt to answer the question to what extent physiological cardiovascular control is restored by the pacemaker system. This is a question which should be asked before attempting to design a sensor-controlled system and especially before designing multisensor systems with infinite combinations. Four categories are defined: direct bridging ("shunting"), open loop systems, closed systems using cardiorespiratory or metabolic coupling and those using cardiac signals. Further subdivisions are shown. From the bioengineering as well as from the physiological viewpoint a system should preferably not combine sensors from one and the same of these categories. At present direct bridging is available only for the atrioventricular (AV)-block, so that for sick-sinus-syndrome (SSS) patients feedback control via cardiac signals ("inotropic" pacemaker) comes nearest the goal without, however, ideally bridging the gap. Open-loop systems should no longer be developed as single-sensor systems. A well developed activity sensor, however, which quickly pinpoints the most prominent stressor of cardiovascular control is best suited to complement another sensory system achieving closed-loop control. New and promising concepts orientated toward direct bridging are the analysis of monophasic action potentials and the "dromotropic" concept, both of which seek direct correlation with the "chronotropic" information not available in SSS patients.

Pacemakers

Rapid advances in pacing technology will continue to affect the quality of life of many patients with cardiovascular disease. A truly "smart" device that seemed fanciful 30 years ago now seems to be a virtual certainty by early in the next century. The surgical contributions and expertise of individuals trained in cardiothoracic surgery in these bradypacing developments is highly desirable to minimize morbidity to the greatest possible degree, to optimize the outcome of the procedure for the individual patient, and to conserve health care costs as much as possible. To maintain this cardiothoracic presence in cardiac pacing, acquisition of knowledge and expertise in the basic electrophysiology and technology of cardiac pacing, to go along with surgical expertise, is necessary on the part of individuals with the interest and opportunity to do so.

New and combined sensors for adaptive-rate pacing

A new potential indication for cardiac pacing is chronotropic incompetence, that is, an inadequate cardiac rate response to exercise and other metabolic demands. Many patients who have been paced for indications such as complete heart block or sick sinus syndrome also have chronotropic incompetence. Such patients are not adequately treated when fitted with a constant rate pacemaker. Adaptive-rate pacemakers increase the pacing rate in proportion to signals derived from a biosensor which is sensitive to exertion and possibly to other metabolic requirements. These pacemakers have proven valuable for patients with overt chronotropic incompetence. However, no single sensor/algorithm is ideal and improvement has been sought by introducing new sensors, adjusting the algorithms by which biosensor signals are converted to the most appropriate pacing rate, or by combining sensors in such a way that a composite biosensor signal is derived which bears a close linear relationship with the appropriate heart rate. An example of a new sensor is the accelerometer, which is sensitive to a fuller range of movements than the piezo crystal. A successful new algorithm is the rate augmentation algorithm for use with minute ventilation, which provides a better initial pacing rate response. A combination of minute ventilation sensed by impedance changes and movement sensed with piezo crystals maintains the rapid response from the piezo crystal and overcomes its lack of proportionality. Another successful new combination of sensors is QT sensing from the evoked ventricular potential and motion sensing with a piezo crystal. As yet, these innovations have not been exhaustively tested and shown to confer clinical benefit but the improvements are such that an advantage can be expected.

The measurement of the paced depolarization integral using the braided endocardial lead

Previous studies have shown that the paced depolarization integral (PDI) data recorded in unipolar configuration could potentially improve the specificity of tachyarrhythmia classification in an implantable cardioverter defibrillator (ICD). However, the defibrillation protection would be compromised if the ICD case were used as an indifferent electrode. Since transvenous defibrillation leads are being investigated to be used with ICDs, this study determined if reliable PDI data could be obtained using the braided endocardial defibrillation lead (BEDL). The results demonstrated that comparable PDI values and PDI changes with epinephrine induced sinus tachycardia were obtained with all three tested sensing configurations: conventional unipolar, tip electrode to right ventricular defibrillation electrode, and tip electrode to superior vena cava defibrillation electrode. Therefore, the BEDL can be used to measure PDI data, which possibly may improve tachyarrhythmia classification in an ICD, without compromising its defibrillation protection.

Differentiation between monomorphic ventricular tachycardia and sinus tachycardia based on the right ventricular evoked potential

The differentiation between ventricular tachycardia (VT) and sinus tachycardia (ST) is problematic in some patients with implantable defibrillators and/or antitachycardia pacemakers. The integral of the ventricular endocardial evoked response, or paced depolarization integral (PDI), has been demonstrated to undergo characteristic changes with a variety of stimuli including catecholamines, pacing rate, and exercise. We hypothesized that the PDI recorded from a unipolar transvenous right ventricular endocardial catheter would differentiate VT from ST. The PDI was calculated from a unipolar pacing stimulus, delivered via a cathode in the right ventricular apex, and the reference electrode, a quadripolar catheter positioned in the superior vena cava. PDIs were measured in 22 patients during VT and sinus rhythm. The PDI measured during sinus rhythm was 579 +/- 240 microV-sec and the PDI during VT was 894 +/- 411 microV-sec (P < 0.001). In a subset of seven patients, PDIs were measured during VT, sinus rhythm, and ST induced by catecholamine infusion or exercise. In this subset, the PDI during sinus rhythm was 645 +/- 295 microV-sec, during ST 588 +/- 308 microV-sec (9% decrease from sinus, P = 0.05), and during VT 863 +/- 342 microV-sec (33.9% increase, P = 0.01). These data indicate that the measurement of the PDI is potentially useful in differentiating VT from ST.

The range of sensors and algorithms used in rate adaptive cardiac pacing

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

A New Pacemaker Algorithm for Continuous Capture Verification and Automatic Threshold Determination: Elimination of Pacemaker Afterpotential Utilizing a Triphasic Charge Balancing System

A new pacemaker algorithm designed to automatically verify pacemaker capture and determine pacing threshold by detection of a stimulus evoked potential was studied in 20 patients undergoing permanent pacemaker implantation. To eliminate pacing stimulus afterpotential and detect an evoked response, a hardware feedback circuit and a software template matching algorithm were used to produce a triphasic charge-balanced pacing pulse. After charge balancing the pacing lead, a residual artifact is measured. A capture window is defined as the area integral of the first 24 msec of the evoked depolarization, and a capture threshold as one third the amplitude of the capture window. The maximum allowable residual artifact is one eighth the amplitude of the capture window. Once the stimulus afterpotential is eliminated and the evoked response detected, capture threshold is automatically and continuously determined and the algorithm adds a 0.8-V safety margin to the pacemaker output. This algorithm was run automatically and after simulated loss of capture, produced by manually decreasing pacer output below threshold, in the bipolar (13 patients) and unipolar (20 patients) pacing modes. In each patient loss of capture was immediately detected. The data were consistent (P = NS) between algorithm runs. During unipolar pacing the area integral of the first 24 msec of the evoked response was 412 +/- 137 versus 413 +/- 144 and the residual artifact 5.8 +/- 4.8 versus 8.1 +/- 7.5. The resulting ratio (signal/noise) of the two parameters was 150 +/- 141 versus 145 +/- 181. Automatically determined threshold was 0.69 +/- 0.43 V versus 0.69 +/- 0.42.(ABSTRACT TRUNCATED AT 250 WORDS)

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 +/- 10.1 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 less than 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.

Automatic reduction of stimulus polarization artifact for accurate evaluation of ventricular evoked responses

The ventricular evoked response, the cardiac depolarization generated in response to a pacing stimulus, is potentially useful as a sensor for rate responsive pacing and automatic threshold tracking. It is necessary to minimize the polarization artifact that results from pacing in order to sense cardiac depolarizations from the same electrodes that pace the heart. To accomplish this, a triphasic stimulus waveform consisting of precharge, stimulus, and postcharge was used. An algorithm was developed that introduced pacing stimuli during the refractory period of sensed beats, when cardiac depolarization could not occur by definition and polarization artifact could be evaluated. Precharge duration was varied until the amplitude of the polarization artifact was small compared to the evoked response. In 18 patients with temporary electrode catheters, polarization artifact was reduced from 6.8 +/- 3.4 mV to 1.9 +/- 1.1 mV after balancing (P less than 0.005). Initial precharge duration was 3200 mu sec and the mean final precharge duration was 3551 +/- 516 mu sec. In 14 patients with permanent bipolar pacing leads, polarization artifact was reduced from 3.2 +/- 3.5 mV to 0.7 +/- 0.6 mV (P less than 0.025). Final precharge duration averaged 3440 +/- 310 mu sec. Under a wide variety of pacing conditions, this algorithm simply and quickly reduces polarization artifact to a minimum to allow accurate analysis of evoked responses.

Effects of stress and beta 1 blockade on the ventricular depolarization gradient of the rate modulating pacemaker

Prism-CLR is a closed loop, rate modulating pacemaker that uses ventricular depolarization gradient (Gd) to continuously adjust heart rate. Heart rate response to a formal mental stress protocol, esmolol (500 mcg/kg bolus, 75-125 mcg/kg/min infusion), and mental stress during esmolol infusion were studied in six patients to investigate if Gd and paced heart rate response are under direct beta-adrenergic control. Paced heart rates increased in response to mental stress in a physiological manner (P less than 0.001). Response to esmolol infusion was paradoxical, with increased paced heart rates during esmolol bolus and infusion (P less than 0.05). There was no significant alteration in either systolic or diastolic blood pressure during mental stress or esmolol infusion (P greater than 0.05). Paradoxical increase in paced heart rates during esmolol administration suggests a primary or secondary effect of esmolol to decrease the ventricular depolarization gradient. This hypothesis was supported in four dog studies in which direct Gd measurements were made during esmolol infusion. Mental stress during esmolol infusion resulted in significantly increased paced heart rates (esmolol effect) with blunted changes in heart rate in response to the mental stress. The results of this study suggest that the physiological rate response during mental stress is attributable to sympathetic autonomic response.

Detection of ventricular tachycardia using scanning correlation analysis

Cross correlation is an accurate method for distinguishing normal sinus rhythm (NSR) from ventricular arrhythmias. The computational demands of the method, however, have prohibited development of an implantable device using correlation. In this study, temporal data compression prior to correlation analysis was used to reduce the total number of computations. Unipolar and bipolar intracardiac electrograms of NSR and 23 episodes of ventricular tachycardia (VT) from 23 patients were obtained from a right ventricular apex electrode catheter during routine electrophysiology studies. The data were filtered (1-11 Hz), digitized (250 samples/sec) and temporally compressed to 50 samples/sec. Data compression removed four out of every five samples by only saving the sample with the maximum excursion from the last saved sample. The average squared correlation coefficient (r2) was computed for the NSR and VT episodes using each patient's NSR waveform as a template. In all 23 patients, the r2 values showed large separation between NSR versus VT in both unipolar (0.93 +/- 0.05 vs 0.20 +/- 0.16, P less than 0.005) and bipolar (0.91 +/- 0.07 vs 0.17 +/- 0.11, P less than 0.005) electrode configurations using template lengths of 80% the intrinsic interval (avg +/- SD). Narrow templates (40% intrinsic interval or less) often resulted in multiple r2 peaks during each heart cycle and degraded the r2 separation (n = 10, P less than 0.005). High pass filtering at 3 Hz also degraded the r2 separation (n = 10, P less than 0.05). Standard noncompressed correlations indicated that data compression had negligible effects on the results. Thus, a computationally efficient cross correlation method was found to be a reliable detector of VT.(ABSTRACT TRUNCATED AT 250 WORDS)

Rate-modulated pacing

The primary role of cardiac rate in adapting cardiac output to changing physiological needs has been more clearly recognized in recent years. Previously, the rate of cardiac stimulation had been determined either at pacemaker manufacture, by programming a single rate, or by sensing the atrium. More recently, sensing another physiological or nonphysiological function that changes in response to body need has become possible. Exercise changes blood oxygen saturation, central venous pH, central venous temperature, minute ventilation and respiratory rate, stroke volume, circulating catecholamines, QT interval, evoked endocardial response to a stimulus, and the mechanics of myocardial contraction. Some sensors respond to muscle work but not to intellectual effort or emotion. Pacemaker-based sensors of physiological function or activity allow a change in cardiac stimulation rate in response to need. Whichever sensor is used, increases in ventricular rate during exercise regularly produce a cardiac output response. Single-chamber, rate-modulated pacemakers in atrium or ventricle and dual-chamber devices are now implanted on a widespread basis. These drive the atrium, the ventricle, or both, sensing or pacing the atrium at a rate determined by the sensor.

Characteristic variation in evoked potential amplitude with changes in pacing stimulus strength

The evoked potential, the intracardiac signal generated by a pacing stimulus, shows promise as a sensor for rate-responsive pacing and automatic threshold determinations. Thus, it is important to understand factors that may alter the morphology of evoked potentials and affect accurate signal analysis. Using a computer-based pacing system emulator, stimuli at 2.5, 5.0 and 6.9 V were delivered to 12 patients through permanent bipolar pacing leads. At 2.5 V, the evoked potential amplitude measured -12.63 +/- 7.79 mV. When the pacing amplitude was increased to 5.0 and 6.9 V, the signal diminished in size or reversed in polarity, or both, averaging -0.83 +/- 7.82 mV and 0.64 +/- 7.0 mV, respectively (p less than 0.01 vs 2.5 V). Pacing at 2.5 V was performed in an additional 8 patients with temporary quadripolar electrode catheters. With the distal pole of the catheter as the cathode and the proximal 3 poles as a common anode, the evoked potential averaged -9.01 +/- 5.44 mV. With the proximal 2 poles of the catheter disconnected to make the anode equal in size and current density to the cathode, the evoked potential diminished to -0.94 +/- 11.27 mV (p less than 0.05). There is thus a decrease in the evoked potential at high stimulus amplitudes compared to that obtained at the cathodic threshold. This finding can be reproduced by manipulation of the size and current density of the anode, suggesting that anodal stimulation at the ring of permanent pacing leads may be responsible.

Concepts of rate responsive pacing

Various concepts for measuring (by means of biosensors incorporated into pacemakers) biologic parameters to determine the appropriate pacing rate are reviewed. They are pH, stimulus-to-T-wave interval, blood temperature, intercardiac blood pressure change, venous oxygen saturation, intercardiac impedance (stroke volume, ejection rate, preejection interval), thoracic impedance (respiratory rate, minute volume), R-wave area, and body vibration. Those which have been incorporated in an implantable pacemaker and studied in a significant number of patients include intracardiac blood temperature, respiratory rate, respiratory minute volume, stimulus-to-T-wave interval, and body vibration. Studies of intracardiac impedance, QRS complex area, venous oxygen saturation, and right ventricular pressure are in early stages. Because no single parameter has yet proved to be an ideal indicator of metabolic need, dual-chamber pacemakers, which use atrial rate and body vibration to control pacing rate, and multisensor pacemakers are under development.

Automatic functions in cardiac pacing: optimization of device and patient therapy

An overview of advances is presented. The focus is on automatic functions that have been developed to monitor the patient continuously and readjust the postventricular atrial refractory period, the atrioventricular (AV) delay, the ventricular refractory period, and the pacemaker stimulus intensity. The advent of automatic algorithms for the postventricular atrial refractory period may have broadened considerably the clinical application of dual chamber pacemakers. Automatic adjustment of the ventricular refractory period has been demonstrated in an implantable pacemaker and may eliminate the rhythm disturbance associated with inappropriate T-wave sensing. Automatic adjustment of the AV delay has been implemented, and there is evidence suggesting that improved hemodynamics may result. Automatic output regulation has emerged in an implantable pacemaker and promises to increase device longevity while providing important patient safety benefits.