© 2005 The European Society of Cardiology. Published by Elsevier Ltd. All rights reserved.
Pacing with capture verification in candidates for resynchronisation therapy: A feasibility study
aInstitute of Cardiology, University of Bologna Via Massarenti 9, 40138 Bologna, Italy; bSt. Jude Medical Italy
Manuscript submitted 22 October 2004. Accepted after revision 23 January 2005.
*Corresponding author. Tel.: +39 051 6363531; fax: +39 051 344859. E-mail address: mbiffi{at}orsola-malpighi.med.unibo.it (M. Biffi).
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Abstract |
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BACKGROUND: Devices for cardiac resynchronisation therapy (CRT) deliver energy into 3 output channels. Such a burden can significantly reduce device longevity. AutocaptureTM has been shown to improve pacemaker longevity and safety of right ventricular pacing in clinical studies. The aim of this study was to investigate the application of AutocaptureTM during biventricular pacing (BIV) to decrease the energy cost of CRT.
METHODS: During implantation of BIV devices, an acute study was performed to test the hypothesis that the evoked response (ER) elicited by each delivered stimulus is correctly detected and measured either on the right ventricular (RV) channel during BIV pacing with the left ventricular (LV) channel pacing first, or in the LV channel with the RV channel pacing first. A reliable measurement of ER is the critical requirement for the correct performance of AutocaptureTM.
RESULTS: ER amplitude in the right ventricle during BIV pacing was not significantly decreased compared with RV pacing in the VVI mode (16.36 ± 5.27 mV vs 17.09 ± 6.12 mV). ER amplitude in the left ventricle during BIV pacing was not significantly decreased compared with LV pacing in the VVI mode (12.4 ± 8.95 mV vs 12.25 ± 8.97 mV). Three patients in atrial fibrillation had a DDDR pacemaker with the LV lead connected to the atrial port, and received BIV pacing with AutocaptureTM turned on in the RV channel. AutocaptureTM performance in the long term, as assessed by the trend of RV threshold over 20 ± 8 months, showed that LV depolarisation was never sensed as an ER on the RV channel.
CONCLUSIONS: Our observations support the feasibility and safety of capture verification during BIV pacing on the ventricular channel paced secondly, which could increase the longevity of CRT devices, and decrease the costs of this new therapy for heart failure patients.
Key Words: heart failure, resynchronisation therapy, autocapture, evoked response
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Introduction |
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Cardiac resynchronisation is a new therapeutic resource in the treatment of patients with advanced heart failure (NYHA class III and IV) and marked left ventricular conduction delay [1,
2]. Technological improvements in lead manufacture have made left ventricular (LV) pacing through the coronary sinus feasible and reliable in the majority of patients. There remain some unresolved issues in device technology, namely device longevity, and diaphragmatic stimulation in a small subset of patients. Device longevity is important in cardiac resynchronisation therapy (CRT) candidates, since the current drain of 3 output channels can significantly shorten the expected lifetime of the device, particularly where there is a high LV pacing threshold, thus increasing the cost of CRT itself.
In this study we sought to investigate a strategy to decrease the current drain of pacing therapy, by applying AutocaptureTM RV pacing to biventricular pacing (BIV).
AutocaptureTM is a pacing algorithm brought into clinical practice in 1994 by St. Jude Medical. Briefly, it provides beat-to-beat verification of right ventricular (RV) stimulation at outputs only 0.25 V above threshold, with a 4.5 V back-up pulse in case of exit block [3,4]. To achieve this feature, the device automatically monitors the evoked response (ER) elicited by each impulse delivered: measurement of the ER is hence the pivotal aspect of this algorithm's performance. ER needs to be reliably distinguished from the polarisation induced by the pacing pulse at the tissue-electrode surface. This goal was achieved using bipolar low-polarisation leads.
Beyond monitoring the evoked response, pacemakers with AutocaptureTM check daily stimulation threshold either at programmed intervals or in case of two consecutive failures of the low-energy stimulus, and adjust ventricular output to maintain effective low-energy pacing. Improvements in both pacemaker longevity and safety of RV pacing have been reported in clinical studies [3–8].
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Methods |
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In this study we investigated the possibility of achieving RV pacing by AutocaptureTM during biventricular pacing. To ensure efficacy and safety of this approach, we performed an acute trial during the implantation of biventricular ICDs or pacemakers in patients referred to our centre.
During the study, a bipolar lead for LV pacing became available (St. Jude 1055T): in those patients who received it, the feasibility of LV pacing with AutocaptureTM was also evaluated.
All patients gave written informed consent to the stimulation protocol. All patients had bipolar, low-polarisation RV leads to allow measurement of RV evoked response by AutocaptureTM (Table 1).
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As no external device for measurement of ER during implantation exists, a commercially available DDDR unit with AutocaptureTM (Entity DRTM) was used to measure ventricular threshold and ER.
Acute RV measurements
After RV and LV leads were placed, the LV lead was connected to the atrial port of this pacemaker, whereas the RV lead was connected to the ventricular port. Then, RV evoked response and pacing threshold were measured twice in randomised order by tests "commanded" via the pacemaker programmer. A major limitation of this setting is that ventricular offset is the pacemaker AV interval, thus simultaneous biventricular pacing is not possible (shortest programmable AV = 25 ms). Moreover, the AV interval is fixed (60 ms) during "commanded" testing of ER and threshold. Testing other AV intervals (different from that during "commanded" testing) requires waiting for an automatic threshold test, the minimum programmability of which is 1 h and thus was not feasible.
RV evoked response and pacing threshold were measured twice by tests "commanded" in randomised order to detect differences in AutocaptureTM performance in three different conditions: RV pacing alone in VVI mode; BIV+ (DDD mode programmed: consistent LV pacing with LV output twice LV threshold); BIV– (DDD mode programmed: LV non-capture, output set 0.25 V below LV threshold). This was done to understand if LV pacing coming first affects ER measurement in the RV channel, compared with RV pacing alone.
Acute LV measurements
In the subgroup of patients receiving the 1055T lead for LV pacing, we repeated the same measurements with RV lead connected to the atrial port pacing first, and LV lead connected to the ventricular channel, to understand if AutocaptureTM can function with left ventricular pacing. In these patients, left ventricular ER and threshold were measured in randomised order in VVI mode (LV pacing alone), BIV+ (DDD mode programmed: consistent RV pacing with RV output twice RV threshold), and BIV– (DDD mode programmed: RV non-capture, output set 0.25 V below RV threshold). In this way, we sought to understand if RV pacing first affects ER measurement on the LV channel.
During these acute measurements, pacing rate was at least 20 bpm above intrinsic rhythm to avoid fusion or pseudofusion beats in those patients with normal atrioventricular conduction. The test rate was 100 bpm in 22 patients and 110 bpm in three patients. Pulse duration was 0.5 ms.
"Commanded" threshold measurement automatically decreases ventricular amplitude in 0.25 V steps from 2 V to 1 V, then in 0.125 V steps from 1 V to 0.125 V: loss of capture is confirmed when two consecutive beats at the same amplitude fail to pace the ventricle (in these cases the back-up stimulus is elicited).
The interventricular interval (which is the atrioventricular interval of this DDDR pacemaker) during a "commanded" test is non-programmable and fixed at 60 ms.
In case of delivery of the back-up pulse after effective biventricular stimulation, loss of ER detection was noted.
The blanking period between the two channels was set at 24 ms, and sensitivity of the ventricular channel was programmed at one-third of the ventricular amplitude obtained while pacing the opposite ventricle in AAI mode. This setting was chosen to disclose any potential pitfall in the algorithm for measurement of the evoked response, on which depends the determination of ventricular pacing threshold.
RV measurements during long-term follow-up
Beyond this acute study, three patients in permanent atrial fibrillation received a DDDR pacemaker (unipolar LV lead connected to the atrial port, bipolar RV lead connected to the ventricular port), allowing us to evaluate AutocaptureTM RV pacing during chronic biventricular pacing.
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Statistical analysis |
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Descriptive parameters are reported as mean ± standard deviation. Measurements of RV evoked response and pacing threshold during the acute trial were analysed by ANOVA; when significant differences were detected, Bonferroni's t-test was applied.
Measurements of LV evoked response and pacing threshold during the acute trial were analysed by ANOVA; when significant differences were detected, Bonferroni's t-test was again applied.
Comparisons of acute and chronic measurements of RV evoked response and threshold in patients who received a DDDR pacemaker were analysed by ANOVA; when significant differences were detected, Bonferroni's t-test was once again applied.
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Results |
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Twenty-five patients (20 males, five females) with left bundle branch block underwent implantation of a biventricular device: 17 patients received an ICD, eight received a pacemaker. Mean age was 70 ± 9 years (range 41–84), mean LV ejection fraction (LVEF) was 26 ± 5% (range 14–38), QRS width was 179 ± 22 ms (range 150–230), 21 patients were in New York Heart Association (NYHA) class III and four patients were in NYHA class IV.
The aetiology of heart failure was coronary artery disease (13 patients), idiopathic dilated cardiomyopathy (eight patients), hypertension (three patients), Chagas disease (one patient).
The indication for ICD implantation was sustained monomorphic ventricular tachycardia (VT) (eight patients), in-hospital ventricular fibrillation (VF) (two patients), prevention of sudden death in MADIT II [9] patients (four patients, all of whom had non-sustained VT (NSVT)), syncope in patients with NSVT and inducible sustained monomorphic VT (three patients). All these 17 patients received biventricular ICDs capable of offsetting the RV and LV channels (Epic HF, St. Jude Medical). The ventricular offset was tailored to achieve optimal CRT in each individual patient under echocardiographic guidance: simultaneous ventricular pacing was optimal in 2/17 patients, LV first was optimal in 8/17 (range 20–50 ms), and RV first was optimal in 7/17 patients (range 20–40 ms).
In addition to advanced heart failure, 3/8 patients treated with a pacemaker had atrial fibrillation with slow ventricular rate (average heart rate < 50 bpm on 24 h Holter recording) and sporadic R–R intervals > 4 s, 4/8 had complete AV block in sinus rhythm, and one had marked chronotropic incompetence (maximal heart rate off drugs = 72 bpm). The three patients in AF received a DDDR pacemaker, whereas the five in sinus rhythm received a biventricular pacemaker without ventricular offset.
The site of LV pacing was posterolateral in 12/25 patients, lateral in 8/25, and anterolateral in 5/25. The site of RV pacing was RV apex in all the patients.
Acute RV measurements
Leads for either RV or LV pacing, together with impedance, thresholds and evoked responses in the three different pacing modes (RV VVI, BIV+, BIV–) measured during the acute study are reported in Table 1.
No significant differences in RV evoked response amplitude and RV pacing threshold were observed either during consistent BIV pacing or during LV non-capture compared with VVI mode (Table 1, P = NS for evoked response, P = NS for RV threshold). LV pacing occurred 60 ms before RV pacing during "commanded" threshold testing.
Acute LV measurements
In five patients receiving a bipolar LV lead a similar evaluation was performed with RV pacing first by 60 ms. The ER was measured by "commanded" test on the left ventricular channel during LV pacing in VVI mode, and was not affected by RV stimulation (Table 2). AutocaptureTM was able correctly to identify LV threshold in all the three settings (Table 2).
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Loss of ER detection, reflected by delivery of an unnecessary back-up pulse, was not observed in either of the two configurations (LV first or RV first).
RV measurements during long-term follow-up
The three patients in permanent AF had pacemakers programmed in DDDR mode with AutocaptureTM turned on in the RV channel and with interventricular delay set at 25 ms, based on echocardiographic guidance. The performance of the AutocaptureTM algorithm at an average follow-up of 20 ± 8 months (range 13–29) was observed by the trend of RV pacing threshold (Fig. 1) and Holter recording. In case any device had misdiagnosed LV depolarisation as an ER on the RV channel, loss of capture could not be identified during RV threshold search. This would turn off AutocaptureTM automatically, and the threshold trend would not be available. This event was not observed in any patient; fusion beats were recorded by Holter in <5% of paced beats in one patient. Sensing of LV depolarisation as an R wave on the RV channel (which would result in AR pacing) was never observed (retrieved from pacemaker diagnostics).
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In these three patients, measurements of RV evoked response and pacing threshold at follow-up were repeated by "commanded" testing (interventricular delay 60 ms) in the three pacing modes (VVI, BIV+, BIV–) as in the acute study (Table 3). No difference was found in RV evoked response compared with implantation (P = NS), or in RV (P = NS) and LV (P = NS) thresholds (Table 3). These three patients have not had worsening heart failure symptoms since pacemaker implantation. A significant improvement in exercise tolerance was observed in all three patients at the last follow-up, from 270 ± 42 m to 410 ± 21 m on the 6-min walk test. One of them died 33 months after implantation because of gastric cancer.
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Discussion |
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This is the first trial to evaluate the feasibility and safety of ventricular pacing by automatic capture verification of ventricular threshold in patients receiving cardiac resynchronisation therapy.
Feasibility and safety of biventricular pacing with capture verification
We observed that capture verification is feasible during biventricular pacing in either RV or LV channels. By testing the same protocol in each ventricle, we demonstrated that ventricular pacing by AutocaptureTM is effective and safe, in the acute setting, in the right ventricle when it is paced after the left ventricle (or in the left ventricle when paced after the right ventricle). Safety is ensured by the fact that failure of stimulation in the ventricle paced first does not affect the detection of the ER in the ventricle paced second, and that back-up pulses are correctly delivered in case of failure of the low-energy stimulus (Figs. 2 and 3). Moreover, the back-up pulse in the ventricle paced second falls within the QRS in case of low-energy pulse failure during effective stimulation of the ventricle paced first, without any risk of stimulating in the ventricular vulnerable period (Figs. 2 and 3).
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It appears from our study that the applicability of our findings is restricted to CRT devices capable of ventricular offset. In fact, AutocaptureTM performance was not evaluated during simultaneous biventricular pacing due to technical limitations. This is a major limitation of our investigation, since it cannot be excluded that the interference from either ventricle could occur in this situation. On a theoretical basis, ER misdetection during sequential biventricular pacing may be more likely to occur if AutocaptureTM is applied to the second pulse rather than the first, so the results observed in our experience can be considered very promising. Nonetheless, testing AutocaptureTM in the ventricle paced first and during simultaneous pacing is mandatory before AutocaptureTM can be implemented in CRT devices. This aspect has important clinical implications, since CRT devices employed in large-scale trials [1,
2] provided only simultaneous biventricular pacing.
Recent studies have shown that individual tailoring of CRT using the ventricular offset improves left ventricular EF and cardiac output, and reduces the extent of delayed LV contraction compared with simultaneous biventricular pacing [10–12].
Devices with ventricular offset have been made available in Europe since the second half of 2002; depending on the method of evaluation (tissue Doppler imaging, echo Doppler, acute haemodynamics) it has been reported that more than 50%, and up to 100% of patients are best treated by optimisation of ventricular offset [10–12]. It appears that the majority of CRT candidates could be paced using ventricular offset, in the near future, and could possibly benefit from a capture verification algorithm to increase battery longevity. Although the feasibility of AutocaptureTM during simultaneous biventricular pacing or in the channel paced first remains an unresolved issue in our investigation, a capture verification algorithm in the ventricle paced second could be an interesting technological improvement for a large number of patients.
AutocaptureTM performance during long-term biventricular pacing
At present, no biventricular device is capable of performing AutocaptureTM in any channel, to prove effectiveness during chronic therapy. However, we had the possibility of observing that AutocaptureTM is feasible in AF patients during chronic biventricular pacing when LV stimulation occurs first, at 25 ms interventricular delay (Fig. 1, Table 3). AutocaptureTM performance in the long term confirmed the findings observed in the acute study (Table 3).
Possible clinical implications of our observations
The practical implications of these findings may be relevant to candidates for CRT in terms of device longevity, which is compromised by the current drain of 3 output channels, particularly when pacing thresholds higher than optimal are found. Avoiding the need for a "safety margin" by AutocaptureTM at least in one channel decreases current drain while providing safety in ventricular pacing [3–8].
In this way, expensive devices such as biventricular ICDs are most likely to benefit in terms of longevity. Until now, AutocaptureTM has not been implemented in ICDs for technical reasons (unipolar pacing was mandatory), and for clinical reasons (RV pacing was minimised due to its unfavourable clinical outcome) [13–15]. On the contrary, recent technological improvement has enhanced AutocaptureTM so that unipolar pacing is no longer mandatory [16,17], and the algorithm can run in a bipolar pacing configuration. Furthermore, a novel Autocapture conception has been developed, which allows ER detection with leads of any type in any configuration [18–20], and also makes a distinction between captured and fusion beats in ICD patients [20]. This is possible thanks to a reduced coupling capacitor, which prevents sensing the polarisation induced by the pacing stimulus [18–20]. In ICD leads, the use of RV coil-can detection configuration is also helpful to detect the evoked response during RV pacing regardless of lead type (dedicated bipolar or integrated bipolar) [21,22]. In this configuration, the large electrode area allows minimisation of the effect of lead polarisation [21,22]. Implementation of AutocaptureTM algorithms has proven feasible in ICDs [20–22], and should be considered in the very near future to decrease pacing-related current drain in biventricular devices, which are required to provide 100% pacing for effective CRT. Following the recent observation of 43% reduction in overall mortality by biventricular ICD [2] in advanced heart failure patients, the economic impact of CRT is likely to be relevant for its acceptance. Thus, the issue of reducing CRT costs by increasing device longevity is likely to be relevant. In fact, the observed geographic differences in ICD use are largely dependent on socio-economic reasons [23–25].
Automatic output adjustment by capture verification has proved beneficial, from the economic point of view, in conventional RV pacing since the introduction of AutocaptureTM featured pacemakers, which has allowed either the downsizing of devices or increase in their longevity [6–8].
In the subgroup of patients with a bipolar LV lead, we have observed that pacing using capture verification is also feasible and safe in the left ventricle (Table 2).
A further application of AutocaptureTM algorithms should be LV pacing, particularly when suboptimal pacing thresholds have to be accepted for effective CRT: avoiding the need for the safety margin on LV output could substantially increase device longevity. The development of bipolar LV pacing leads and the feasibility of AutocaptureTM either with unipolar leads or with leads of any polarity [16–20,22] have set the basis for starting this type of investigation, as recently reported [26].
Study limitations
This is an acute study which simply discloses potential applications of technologies currently used in cardiac pacing to CRT. The addition of AutocaptureTM to complex devices such as biventricular ICDs needs accurate evaluation to assess its feasibility. A major limitation of the study is the inability to assess AutocaptureTM performance during simultaneous biventricular pacing, which was the only pacing configuration available for CRT until the second half of 2002. Another important issue is that our observations apply only to sequential biventricular pacing, where AutocaptureTM is being used in the ventricle paced second. An accurate evaluation in devices with ventricular offset is needed to assess its performance in the ventricle paced first, and possibly in both ventricles. This is the major technical limitation of our investigation, since no device to test AutocaptureTM in either channel is actually available.
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Conclusion |
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Our data support the concept that pacing with capture verification is feasible and safe during biventricular pacing in the ventricle paced second. Further research is needed to develop an algorithm suitable for clinical application. Despite the above limitations, we believe that our observations offer a real basis for technical development, towards a safer and less expensive device to provide CRT for heart failure patients.
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