Atrial Evoked Response Integral for Automatic Capture Verification in Atrial Pacing

BORIANI, G., et al.: Atrial Evoked Response Integral for Automatic Capture Verification in Atrial Pacing . Beat-by-beat Autocapture is currently limited to operation in the ventricle with bipolar leads. The authors investigated the integral of the negative-going portion of the atrial evoked response integral (AERI) as a potential resource for verification of atrial capture. Intracardiac electrogram signals were collected from 59 patients (ages 67.8 ± 15.1 years) with bipolar, low polarization atrial leads. The signals were collected over a mean period of 6.1 months (minimum 4 days) after lead implantation. St. Jude Medical Affinity pulse generators were used to perform automatic capture threshold tests while the electrogram signals were recorded by a Model 3510 programming device. These signals were transferred to a personal computer in digital form for later analysis. The AERI was calculated at each programmable pacing voltage until capture was lost. The difference between the polarization integral at loss of capture and evoked response integral with successful capture was sufficient to justify enabling the atrial Autocapture feature in 53 of 59 patients in whom bipolar pacing and unipolar sensing was performed. The authors developed a calibration routine to identify automatically those patients in whom atrial Autocapture could be programmed On, based on the polarization integral at loss of capture, the estimated maximum polarization integral, and the AERI. Preliminary analysis indicated that the AERI is a practical resource for beat-by-beat atrial capture detection when used with low polarization leads. (PACE 2003; 26[Pt. II]:248–252)     

A Method for Analysis of the Local Atrial Evoked Response for Determination of Atrial Capture in Permanent Pacing Systems

We developed a method for detection and analysis of the local atrial evoked response. Eleven patients undergoing permanent dual chamber pacemaker implantations for standard clinical indications were included in the study. Using a pacing system emulator, charge balancing using a variable triphasic stimulus waveform to reduce polarization artifact amplitude was performed first. This could not be completed in one patient because of a poor signal-to-noise ratio. Subsequent analysis of the local atrial evoked response in the remaining ten patients showed the typical signal to be a biphasic waveform with an initial negative deflection followed by a positive deflection nearly equal in amplitude. The mean amplitude of the atrial evoked response was 3.1 ± 1.4 mV, while the intrinsic P wave amplitude in these same patients averaged 5.6 ± 3.0 mV. The summed evoked response, a parameter that is directly proportional to the area of the signal, was used to determine atrial pacing threshold. The median atrial pacing threshold determined by the algorithm was 1.00 V, There was no instance of failure to detect loss of capture, nor was loss of capture inaccurately determined when there was still successful atrial pacing. Atrial capture in permanent pacing systems can thus be determined using an algorithm to record and analyze the local atrial evoked response. This method could potentially be useful in the automatic determination of atrial pacing threshold.     

Atrial Capture Detection with Endocardial Electrodes

A new method of evoked response detection, previously demonstrated in the ventricle, has been studied in the atrium at the time of routine pacemaker implant in 16 patients. The atrial evoked response was readily detectable in all patients due to excellent recovery from poststimulus polarization. In six patients, as experimental threshold- tracking pacemaker was used to automatically verify atrial capture and to generate strength-duration curves. It is concluded that this pacing technique is both simple and reliable, and that automatic atrial threshold tracking is feasible.     

Alternate Methods for the Determination of Atrial Capture Threshold Utilizing the Telemetered Intracardiac Electrogram

Periodic determination of pacemaker capture threshold is important to ensure appropriate pacemaker function. During dual chamber pacing, it is sometimes difficult to identify evidence of atrial depolarization on surface electrocardiography (ECG), and this can interfere with the ability to ascertain atrial capture. We describe new methods for determining atrial capture threshold using a standard telemetered endocardial atrial electrogram (AEGM). For the first method, the atrial output is decremented until loss of atrial capture is demonstrated by the appearance of native P wave activity on the AEGM. The atrial capture threshold can then be accurately determined as the point at which a stepwise increase in atrial output results in extinction of the native P wave activity. The second method uses the direct visualization of the AEGM recorded between the ring electrode and pacemaker generator during unipolar (lead tip electrode) pacing. This requires the presence of a bipolar lead. Using this method of recording, it is possible to identify a signal after the atrial stimulus artifact during atrial capture, which disappears with loss of capture. This signal is consistent with a paced "evoked atrial potential" and allows verification of atrial capture. After validating the methods in two sets of test patients with clearly identifiable atrial depolarization on surface ECG, one method was successfully applied to a patient in whom atrial depolarization could not be reliably ascertained on surface ECG. These methods promise to be useful in selected patients in whom confirmation of atrial capture would otherwise be difficult.     

Reduction of Pacing Output Coupling Capacitance for Sensing the Evoked Response

SPERZEL, J., et al. : Reduction of Pacing Output Coupling Capacitance for Sensing the Evoked Response. Sensing of the intracardiac evoked response (ER) after a pacing stimulus has been used in implantable pacemakers for automatic verification of capture. Reliable detection of ER is hampered by large residual afterpotentials associated with pacing stimuli. This led to the development of various technological solutions, like the use of triphasic pacing pulses and low polarizing electrode systems. This study investigated the effect of reducing the coupling capacitance (CC) in the pacemaker output circuitry on the magnitude of afterpotential, and the ability to automate detection of ventricular evoked response. A CC of 2.2 μF and four different blanking and recharge time settings were clinically tested to evaluate its impact on sensing of the ventricular ER and pacing threshold. Using an automatic step‐down threshold algorithm, 54 consecutive patients, aged 70 ± 10 years with acutely (n = 27 ) or chronically (n = 27 ) implanted ventricular pacing leads were enrolled for measurement testing. Routine measurements, using a standard pacing system analyzer (PSA), were (mean ± SD ) impedance 569 ± 155 Ω, R wave amplitude baseline to peak 9.8 ± 3.7 mV and threshold 0.9 ± 0.7 V at 0.4‐ms pulse width. This new capture verification scheme, based on a CC of 2.2 μF and recharge/blanking timing setting of 10/12 ms, was successful in 52 patients which is equivalent to a success rate of 96%. In a subgroup of 26 patients implanted with bipolar ventricular leads (10 chronic, 16 acute), data were collected in unipolar (UP) and bipolar (BP) pace/sense configurations. Also, ER signals were recorded with two different band‐pass filters: a wider band (WB) of 6–250 Hz and a conventional narrow band (NB) of 20–100Hz. WB sensing from UP lead configuration yielded statistically significant larger signal to artifact ratios (SAR) than the other settings (P < 0.01). A dedicated unipolar ER sensing configuration using a small output capacitor and a wider band‐pass filter enables adequate automatic capture verification, without any restrictions on pacing lead models or pacing/sensing configurations.     

Automatic Reduction of Stimulus Polarization Artifact for Accurate Evaluation of Ventricular Evoked Responses

The ventricular evoked response, the cardmc depolarization generated in response to a pacing stimulus, is potentially useful as a sensorforrate 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 poslcharge 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 < 0.005). Initial precharge duration was 3200 μsec and the mean final precharge duration was 3551 ± 516 μsec. 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 < 0.025). Final precharge duration averaged 3440 ± 310 μsec. 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.     

New Method of Atrial and Ventricular Capture Detection

Polarization as a result of the pacing stimulus has always been the main factor for complicating evoked response detection. Tri-phasic pulse schemes have been developed and used for a long time to solve the polarization issue and are used in commercial products today to enable evoked T-wave detection. For measurement of the evoked R wave in the ventricle and the evoked P wave in the atrium a further optimization step had to be taken. The results presented in this paper show that by individual optimization of the lead system in all cases the evoked P- or R wave could be detected. One aspect of the tri-phasic pulse scheme is the dynamic behavior of the polarization to a change in amplitude or pulse width of the pacing pulse. The time to rebalance the electrode tissue interface and the non-linear behavior of this system can lead to incorrect detections of evoked potentials. The response time of this phenomenon was investigated. A possibility to solve this problem is described with the introduction of a double pulse scheme where a test pulse is placed in front of the regular output pulse. Changes in this double pulse balanced system never lead to a loss of capture situation and the system is given time to re-balance before it can be decided which of the two pulses was responsible for a depolarization of the myocardium.     

Performance Evaluation of a Right Atrial Automatic Capture Verification Algorithm using Two Different Sensing Configurations

Background: This acute data collection study evaluated the performance of a right atrial (RA) automatic capture verification (ACV) algorithm based on evoked response sensing from two electrode configurations during independent unipolar pacing. Methods: RA automatic threshold tests were conducted. Evoked response signals were simultaneously recorded between the RA_Ring electrode and an empty pacemaker housing electrode (RA_Ring→Can) and the electrically isolated Indifferent header electrode (RA_Ring→Ind). The atrial evoked response (AER) and the performance of the ACV algorithm were evaluated off‐line using each sensing configuration. An accurate threshold measurement was defined as within 0.2 V of the unipolar threshold measured manually. Threshold tests were designed to fail for small AER (< 0.35 mV) or insufficient signal‐to‐artifact ratio (SAR < 2). Manual threshold measurements were obtained during RA unipolar and bipolar pacing and compared across device indications. Results: Data were collected from 38 patients with RA bipolar leads from four manufacturers. AER signals were analyzed from 34 patients who were indicated for a pacemaker (five), implantable cardioverter‐defibrillator (11), or cardiac resynchronization therapy pacemaker (six) or defibrillator (12). The minimum AER amplitude was larger (P < 0.0001) when recorded from RA_Ring→Can (1.6±0.9 mV) than from RA_Ring→Ind (1.3±0.8 mV). The algorithm successfully measured the pacing threshold in 96.8% and 91.0% of tests for RA_Ring→Can and RA_Ring→Ind, respectively. No statistical difference between the unipolar and bipolar pacing threshold was observed. Conclusions: The RA_Ring→Can AER sensing configuration may provide a means of implementing an independent pacing/sensing method for ACV in the RA. RA bipolar pacing therapy based on measured RA unipolar pacing thresholds may be feasible.     

Determination of Pacing Capture in Implantable Defibrillators: Benefit of Evoked Response Detection Using RV Coil to Can Vector

SPLETT, V., et al. : Determination of Pacing Capture in Implantable Defibrillators: Benefit of Evoked Response Detection Using RV Coil to Can Vector. Automatic detection of capture in ICDs would be useful for ensuring normal pacing function and lead integrity and may increase device longevity. Evoked response detection can be difficult due to postpace polarization. Polarization on the RV coil to can vector, however, should be absent when pacing with a true bipolar lead (pace tip to ring). Polarization on the RV coil to can vector should be low in an integrated bipolar lead due to the large surface area of the coil. Ventricular-paced responses were prospectively recorded in 20 patients during ICD implantation or replacement. Capture and loss of capture responses were noted during threshold searches with electrograms recorded between the RV coil and can. A detector was designed to discriminate between capture and noncapture-paced responses using data from the first 11 patients and validated on the remaining 9. The detector had a sensitivity of 99.9% (detected capture on capture beats), and had a specificity of 100% (detected no capture on noncapture beats) for all lead configurations. There was no measurable polarization with true bipolar leads. In integrated bipolar leads, maximum polarization ranged from 0.0 to 16.7mV. In conclusion, paced evoked responses can be detected in ICDs using the RV coil to can vector using standard pacing waveforms. Special polarization reducing pacing waveforms are not required. These observations could be used to design ICDs with automatic pacing threshold detection.     

Adjustment of the evoked response sensitivity after hospital discharge in pacemaker patients with automatic ventricular threshold tracking activated

Automatic threshold tracking in cardiac pacemakers allows ventricular capture verification and self-adaptive pacing output regulation. The Autocapture algorithm detects the evoked response (ER) signal immediately after the pacing pulse to verify the efficacy of ventricular pacing. Before hospital delivery, the ER sensitivity must be programmed individually so that the pacemaker detects the ER signal adequately without sensing lead polarization. The aims of the study were to assess the frequency of patients in whom Autocapture could be activated and whether the ER sensitivity had to be adjusted after hospital discharge. The study included 44 patients who received the VVIR pacemaker Regency SR+ (St. Jude Medical) connected to the model 1450 T pacing lead. ER signal, lead polarization, and ER sensitivity were evaluated before hospital discharge and 1, 3, and 6 months after implantation. The system recommended activating Autocapture in 42 of 44 patients. The mean ER signal was 8.4+/-1.2 mV at discharge, 9.0+/-3.9 mV at month 1, 8.9+/-4.9 mV at month 3, and 9.3+/-4.5 mV at month 6. Polarization was 1.0+/-0.1 mV at discharge, 1.1+/-0.5 mV at month 1, 1.1+/-0.2 mV at month 3, and 1.1+/-0.5 mV at month 6. Mean ER sensitivity was 3.7+/-1.8 mV at discharge, 4.0+/-1.8 mV after 1, 4.1+/-2.2 mV after 3, and 4.1+/-1.8 mV after 6 months. ER sensitivity could remain unadjusted in 14 patients. Programming to a less sensitive ER setting from 2.9+/-1.2 mV to 4.3+/-1.5 mV was possible in 21 patients. Programming to a more sensitive ER setting from 4.1+/-1.1 mV to 2.5+/-0.9 mV was required in nine patients because of the decrease of the ER signal. The automatic threshold tracking algorithm Autocapture could be activated in 95% of patients. Programming to more sensitive ER settings was recommended in 21% of the patients after hospital discharge. Therefore, ER signal and polarization must be checked at each follow-up, as a decrease in ER signal amplitude can make reprogramming of the ER sensitivity necessary. There is no risk for the patient if the ER is not sensed, as high voltage backup stimulation is present.     

A New Atrial Lead with Improved Stability and P-Wave Detection

A new segmented polyether polyurethane atrial pacemaker lead has been developed and tested acutely and chronically in dogs. This lead was constructed so that its tip could q uick-ly and accurately be positioned in close proximity to the S-A node and provide long-term stability without the use of active fixation devices such as tines or screws. The acute intrinsic P-wave potentials seen at the S-A node area (9.7 ± 4.7 mV) were superior to those defected in the appendage (4.7 ± 2.8 mV) and coronary sinus (6.8 ± 4.1 mV). There were no significant differences in pacing thresholds between sites. In long-term studies, transvenous (juguiar) leads maintained their position for over three months without dislodgment in active unrestrained dogs with only a 25% decrease in intrinsic P-wave potentials. The chronic pacing thresholds for this lead were similar, or superior, to those reported for other pacemaker lead placement sites. The polyurethane coating material produces no adverse tissue reaction or thrombosis.     

Electrical Performance and Automatic Capture Characteristics of a 3.5-mm2 Passive Fixation Lead during 1-Year Follow-Up

Background: Bipolar low polarization electrodes are recommended for a regular AutoCapture™ (St. Jude Medical, Inc., Sylmar, CA, USA) function in order to effectively detect the evoked response (ER) signal. The objective of this national multicenter registry was to evaluate the electrical performance and the AutoCapture™ characteristics of the bipolar ventricular pacing lead IsoFlex S, model 1636T or 1646T (St. Jude Medical), in combination with single- and dual-chamber pacemakers.
Methods: Ventricular pacing and sensing thresholds, lead impedance, ER amplitude, and polarization signals were measured at discharge and routine follow-up visits after 1, 3, 6, 9, and 12 months. AutoCapture™ activation was recommended based on the results of the ER sensitivity test.
Results: Of the 252 patients initially included, 109 (43%) have completed the follow-up. The mean ventricular pacing threshold was 0.43 ± 0.19 V at discharge and 0.68 ± 0.32 V at 12 months postimplant. The values for the ventricular sensing threshold were between 9.51 ± 4.12 and 9.99 ± 4.09 mV at discharge and at the 12-month follow-up. The unipolar lead impedance decreased from 533 ± 94 to 476 ± 73 ohms during the follow-up. The mean ER amplitude was 16.47 ± 6.70 mV at discharge and 17.42 ± 7.43 mV after 12 months, and the corresponding mean polarization signals were 0.59 ± 1.00 and 0.74 ± 1.24 mV, respectively. AutoCapture™ activation was recommended in at least 95% of the patients investigated over the 12-month follow-up.
Conclusion: The bipolar ventricular pacing lead IsoFlex S 1636/1646T shows a good electrical performance and is mostly compatible with the AutoCapture™ algorithm.     

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.     

Automatic threshold tracking activation without the intraoperative evaluation of the evoked response amplitude. AUTOCAP Investigators

Automatic threshold tracking (Autocapture) controls the amplitude of the pacing pulse and adjusts it to the actual pacing threshold. The algorithm is based on the proper detection of the evoked response (ER) amplitude after the pacing pulse. For this reason an intraoperative evaluation of ER and polarization is recommended. The aims of the study were to evaluate the ER signal and polarization and the performance of automatic threshold tracking without any intraoperative testing of the ER signal. In addition, the ER amplitude was correlated with the pacing threshold, pacing impedance, spontaneous R wave amplitude, and with the clinical data. The study included 60 patients who received the VVIR pacemaker Regency connected to the Membrane E 1450/1452 pacing lead (St. Jude-Pacesetter). At implantation, a pacing threshold < 0.7 V at 0.5 ms was achieved in all patients. ER and polarization were assessed for the first time at hospital predischarge testing. Follow-up measurements were conducted at month 1, 3, and 6. The ER amplitude at hospital discharge was 8.4 +/- 4.2 mV and increased to 9.4 +/- 4.8 mV at the 6-month follow-up. The pacemaker recommended not to program automatic threshold tracking on in one patient permanently and in three patients intermittently. The ER amplitudes were not differently distributed in men compared with women or in right-sided compared to left-sided implants. The correlation between age and the evoked response was r = 0.15. The correlation between ER amplitude and pacing threshold was r = -0.08, with pacing impedance r = 0.02, and with R wave amplitude r = 0.44. In conclusion, despite no operative evaluation of the ER amplitude being performed, the mean ER amplitude was about 9 mV at 6-month follow-up. Automatic threshold tracking could be programmed on in 93% of the patients throughout the time. Neither the clinical data nor the conventional electrical parameters help to predict patients who will have low ER amplitude or to optimize the ER signal at implantation.     

Threshold and Polarization Properties of Modern Active Fixation Atrial Leads

The purpose of the study was to compare the stimulation characteristics of two modern active fixation leads (E1a 583F, vitreous carbon tip [ELA] and Intermedics 82–0008–1601, iridium oxide tip [IROX]) with a standard lead (Osypka KY 67 VC, carbon-covered elgiloy tip [OSY]). In three groups of ten patients each, minimum charge threshold δQ_min and polarization properties were determined via charge telemetry of the pacemaker (Intermedics Cosmos II and Relay) 0, 2, 5, 10, 28, 90, and 180 days after implant (dai). The polarization parameters global capacitance C_g, global resistance R_g, polarization voltage U_p, and a time constant t* (t*= C_gċ R_g) were obtained by nonlinear regression. U_p was always significantly (sig) lower in ELA and IROX (0.04–0.10 V) compared to OSY (0.54–0.76 V). R_g was sig lower in ELA (330–437 Ω) compared to OSY and IROX (414–588 Ω) from 0 to 28 dai. From 2 to 10 dai, C_g was sig higher in ELA and IROX (3.8–4.2 μF) compared to OSY (3.3–3.4 μF). In the three groups, BQ_min reached a comparable maximum (1–1.2 μC) at 5 dai. Therefore, vitreous carbon and iridium oxide atrial fixation leads exhibit low chronic polarization effects compared to a standard elgiloy lead, but do not show a sig reduction in charge threshold.     

Comparative Evaluation of Rate Modulation in New Generation Evoked QT and Activity Sensing Pacemakers

Two new generation rate adaptive pacemakers, the Rhythmyx (Vitatron) using the evoked QT interval and the Legend (Medtronic) using vibration as indicators of metabolic demand, were compared for rate adaptive characteristics during different forms of exercise. While both showed improvements over previous generation pacemakers, they still show deficiencies in some aspects of rate modulation. Rhythmyx was slow to respond to changes in metabolic need and showed an "over-shoot" with increasing pacing rate on cessation of exercise. Legend was quick to adapt rate at beginning and end of exercise but showed a plateaus of rate modulation during the period of slowly increasing workload. Legend also showed only modest rate adaptation to changes in treadmill gradient and to bicycle ergometer exercise. Further developments in pacemaker technology are required if physiological rate adaptation to exercise is required.     

Initial Clinical Experience with a Rate Responsive Pacemaker

Prism-CLR is a new single chamber unipolar pace, bipolar or unipolar sense, rate responsive pacemaker that utilizes a closed-loop system based on the analysis of the evoked potential for rate adjustment. It also has an automatic output regulation feature for capture verification and threshold search. Five patients in whom this pacemaker was implanted exhibited an appropriate rate increase with exercise and psychological stress. Automatic output regulation functioned appropriately in three of five patients. Preliminary data suggest that Prism-ClR is an effective pacemaker for patients who may benefit from rate responsive pacing. The automatic output regulation recognition algorithm may need modification in some patients.     

Inconsistent Response to Rapid Atrial Rhythms in a DDD Pacemaker with the Fallback Feature

Dual chamber pacing may be problematic in patients with paroxysmal atrial arrhrfhmias. The fallback feature has been incorporated into some devices to avoid tracking of rapid atrial rhythms. A case is reported in which the initiation of fallback was inconsistent during paroxysms of atrial flutter. Evaluation of the ventricular output of an isolated device in response to various applied atrial rates revealed that fallback occurred consistently only when 2×TARP (ms) ≤ URL (ms).     

A New Pacemaker for Paroxysmal Atrial Fibrillation Treated with Radiofrequency Ablation of the AV Junction

Atrial fibrillation is a relative contraindication to atrial synchronous pacing because of the risk of the tracking of rapid atrial rhythms by the pacemaker. In this study, we describe the clinical results of an AV synchronous rate responsive pacemaker with an original algorithm, which is able to sense pathological increments in atrial rate and automatically to switch into a non-AV synchronous mode of pacing. This pacemaker was implanted in 12 patients who had undergone radiofrequency ablation of the A V junction in order to cure severely symptomatic, drug refractory, paroxysmal atrial fibrillation. In an acute, intrapatient comparison between the standard AV synchronous mode and the automatic switching mode, ventricular tracking of atrial fibrillation occurred in 35% and 4% of total beats at rest and in 24% and 2% of total beats during exercise, respectively (P < 0.001). During 5 ± 4 months of follow-up, no further tachyarrhythmia related symptoms occurred. In conclusion, the standard DDDR mode is unable to eliminate ventricular tracking of atrial fibrillation, thus undermining the efficacy of AV junction ablation therapy. The automatic switching mode eliminates this adverse effect of dual chamber pacing.     

Diagnosis of Early Cardiac Transplant Rejection by Fall in Evoked T Wave Amplitude Measured Using an Externalized QT Driven Rate Responsive Pacemaker

Reliable diagnosis of cardiac ailograft rejection is at present only possible using endomyocardial biopsy. We have serially measured epicardial evoked T wave amplitude during ventricular pacing with an externalized QT driven rate responsive pacemaker telemetered to a TP2 analyzer in 13 patients (12 males) followed for 19 (14–26) days after transplantation. A total of 228 records were analyzed. Rejection was defined on endom-yocardial biopsy. On 17 of the 31 occasions on which biopsy was performed during the study, specimens showed significant (moderate) rejection. In 11 patients the initial biopsy proven rejection episode was associated with a significant fall in the evoked T wave amplitude from 1.3 (0.7–2.3) mV to 0.6 (0.5–1.8) mV (P < 0.005), which began 2 (1–4) days earlier. One patient with uncontrolled diabetes mellitus had no change in evoked T wave amplitude during rejection. The evoked T wave amplitude did not fall in the absence of histologic rejection. These results suggest a noninvasive method for detecting cardiac rejection, which appears both sensitive (92%) and specific (100%) in the first rejection episodes.