Temporary leadless pacing in heart failure patients with ultrasound-mediated stimulation energy and effects on the acoustic window
Article Outline
Background
Left ventricular stimulation for cardiac resynchronization therapy is largely limited by access and anatomy of coronary veins.
Objective
This study sought to apply ultrasound-mediated leadless pacing technology in heart failure patients and to evaluate the effects of respiration and body posture on the acoustic window.
Methods
Patients with advanced heart failure and ejection fraction ≤35% were studied. An electrophysiology catheter incorporating a receiver electrode to deliver ultrasound-mediated pacing was positioned in the left ventricle. Ultrasound-mediated pacing thresholds were determined. The acoustic windows on the chest wall with the patient lying supine, titled 30° leftward, 30° rightward, and 30° upward were determined. The acoustic windows were also determined with computed tomography and transthoracic echocardiography. Simulated receiver movement with respiratory movement and body positioning was assessed with computed tomography.
Results
Ten patients were studied. Ultrasound-mediated pacing was successful in all patients. The acoustic window measured 39.6 ± 18.2 cm2. The window size decreased with rightward tilt, and increased with leftward and upward tilt. They correlated with measurements made by transthoracic echocardiography and computed tomography. Target receiver movement of 1.2 ± 1.4 cm horizontally and 1.3 ± 0.8 cm vertically were estimated by computed tomography.
Conclusion
The feasibility of leadless left ventricular stimulation was shown acutely in heart failure patients. The acoustic window validated by computed tomography was predicted by transthoracic echocardiography. Effects of respiration and body posture were evaluated for development of the future implantable device.
Keywords: Cardiac resynchronization therapy, Heart failure, Ultrasound energy, Pacing
Introduction
Cardiac resynchronization therapy (CRT) has proven to be a major advance in the treatment of medically refractory heart failure patients.1 Apart from reducing morbidity and mortality,2, 3 it improves left ventricular function and promotes reverse remodeling4, 5 in patients with dyssynchrony. However, at least 30% of patients do not respond to treatment.6, 7 Although the underlying reasons for the lack of benefit in some patients are undoubtedly multifactorial,8, 9, 10 several problems have been identified and may have the potential to be ameliorated. Firstly, the site for left ventricular stimulation is an important factor for hemodynamic improvement, but current pacing technology using access via the coronary sinus limits the ability to select the optimum location for left ventricular stimulation. Secondly, stimulation is epicardial, which leads to delayed electrical activation of the left ventricle in comparison to endocardial stimulation, and has been shown to provide less hemodynamic benefit.11 In addition to the nonresponder issue, there are other disadvantages associated with the use of a pacing lead in the coronary sinus in relation to its higher complication rate, such as phrenic nerve pacing.12
The feasibility of a novel technology using energy transfer from an ultrasound transmitter to a receiver electrode to achieve cardiac stimulation without the use of a pacing lead has been reported previously.13, 14 This technology could be used to provide left ventricular stimulation from the endocardium at selected sites. An implantable CRT pacing system incorporating leadless left ventricular stimulation is now in development. The components of the system will include a receiver electrode implanted endocardially using a delivery catheter, an implantable pulse generator incorporating the ultrasound transmitter, and an external programmer/pacing system analyzer. The receiver electrode is preloaded onto a delivery catheter, inserted percutaneously via the femoral vessels, advanced into the right or left heart, positioned at the selected stimulation site, deployed, and attached with active fixation, and the delivery catheter is removed. The pulse generator is implanted subcutaneously in a left precordial location determined from preprocedural and intraprocedural testing.
This study was performed to investigate the feasibility of acute leadless stimulation in patients with heart failure and to delineate the factors that may affect application of this technology in the future implantable system.
Methods
Patients
The study was in compliance with the Declaration of Helsinki and was approved by the Institutional Review Board of the University of Hong Kong/Hospital Authority Hong Kong West Cluster. The study was conducted as a nonrandomized prospective feasibility study in patients with heart failure undergoing routine diagnostic coronary and left ventricular angiography for standard clinical indications. Patients who were 18 years or older, had an ejection fraction of ≤35%, and were in New York Heart Association (NYHA) functional class III or IV were eligible to participate. All patients gave written informed consent. Patients were excluded if they had any of the following conditions: pregnancy, active infection, myocardial infarction within 2 weeks, cardiac surgery within 2 weeks, percutaneous coronary intervention within 2 weeks, unstable angina, acute myocardial ischemia, intracardiac thrombus, severe valvular heart disease, or suboptimal clinical status making them unfit to undergo additional testing in the cardiac catheterization laboratory.
Study equipment
The investigational system consisted of the following: (1) an ultrasound generator with an externally applied ultrasound transmission transducer, (2) a catheter incorporating a receiver electrode into the distal end, and (3) a data collection and display system that included an electrophysiology recording system and a special instrument to monitor and record electrical data for ultrasound transmission and electrode output.
The ultrasound generator and external transmitter produced a timed ultrasound field, coupled to the external surface/chest of the patient using coupling gel (Intelect, Chattanooga Corp., Chattanooga, Tennessee). The transmitter was similar in appearance and function to an ultrasound imaging probe. The catheter containing the receiver electrode was a custom 6-F steerable electrophysiology catheter (EBR Systems, Inc., Sunnyvale, California). The receiving transducer and circuitry converted ultrasound energy to electrical energy and output the electrical energy to a pair of bipolar electrodes. The cathode electrode was a 4-mm-long hemispherical platinum/iridium electrode located at the tip and the anode was a 2-mm-long ring platinum/iridium electrode located 1 cm proximally.
The ultrasound generator had a display screen and control buttons to provide adjustable settings for ultrasound transmission frequency, amplitude, transmit burst duration (pulse width), and transmit interval (cycle length). The ultrasound transmission frequency to be used was optimally adjusted for each receiver electrode. The frequency selected was determined based on water tank calibration testing during catheter construction, and ranged from 322 to 376 kHz. Selection of the ultrasound transmission amplitude was limited to an output corresponding to a mechanical index of <1.9 in the heart. This limit was selected because it is the maximum amplitude typically used for ultrasound imaging systems.
The distal and proximal electrodes were connected to either an electrical pacing stimulator, an electrophysiology laboratory recording system (Maclab, GE Healthcare, Waukesha, Wisconsin), or an instrumentation for monitoring the electrical output voltages on the electrodes during ultrasound-medicated pacing (Tektronix TDS 3014B, Beaverton, Oregon). Electrical pacing was accomplished using a programmed system analyzer (Medtronic Model 5311, Shoreview, Minnesota).
Study protocol
Ultrasound-mediated pacingPatients underwent diagnostic testing with coronary angiography and left ventriculography in the cardiac catheterization laboratory using all standard preparation and procedures. The research study was performed after completion of the clinical catheterization procedure. Unfractionated heparin was given by intravenous bolus injection to maintain an activated clotting time between 200 and 300 seconds throughout the procedure. The receiver electrode catheter was positioned in the left ventricle at a conventional CRT location via the femoral arterial sheath using the retrograde transaortic approach (Figure 1).
Figure 1.
Fluoroscopic appearance (left anterior oblique view) of the receiver electrodes (arrows) on the delivery catheter positioned endocardially on the lateral wall of the left ventricle in 1 patient.
The position of the receiver electrode catheter was recorded on cine images. Sensing and pacing from the catheter electrodes was tested to confirm contact with viable myocardium. The electrical pacing threshold was obtained at a pacing cycle length approximately 20% below the inherent cycle length at 0.5 msec pulse width. If acceptable (≤2.5 V), 12 seconds of consistent capture was recorded for documentation. The catheter electrodes were then connected to the oscilloscopes to monitor the voltage output during ultrasound-mediated pacing. The ultrasound transmitter was placed on the chest wall with acoustic get used for coupling. Acoustic energy was delivered transcutaneously at a constant intensity in ultrasound bursts transmitted at the identical pacing cycle length and pulse width used for electrical pacing. The transmitter was moved across the chest wall to map out the area within which consistent pacing was observed. This area was referred to as the acoustic window, similar to the term used to describe the chest wall area used for transthoracic echocardiographic imaging. Areas overlying the bone were included if ultrasound-mediated pacing was successful. The corners of the acoustic window were documented using anatomic references such as centimeters lateral to the mid-sternal line and the level of the intercostal space (ICS) or rib. The acoustic window was determined with the patient lying supine, tilted 30° to the right, 30° to the left, and 30° upward. Pacing capture was attempted from 3 locations on the chest wall: a standard location at the fifth ICS just left to the left parasternal line, a subcostal location, and a best location defined as the spot with the highest observed receiver output voltages during acoustic window mapping. The best location was documented using anatomic references as above. At each of the 3 transmission locations, the transmission angles in the left–right plane and the superior–inferior plane were determined using a protractor placed on the chest tangential to the chest wall surface. At each of the 3 acoustic transmission locations, 12 seconds of consistent acoustic-mediated pacing capture were recorded on surface electrocardiogram (ECG) and receiver output waveforms were recorded on oscilloscope tracings. Patients were enquired for any tactile or audible sensation during the transmission of ultrasound-mediated pacing.
Safety proceduresPreprocedural and postprocedural safety evaluations included a complete cardiovascular examination, a 12-lead ECG, serum creatine phosphokinase (CK) with CK-MB isoenzyme levels if elevated, and a transthoracic echocardiogram to assess for intracardiac thrombus, valvular disease, pericardial effusion, or other abnormalities.
Thoracic computed axial tomogram (CT)Either before or after the procedure, the patients obtained noncontrast CT scans of the thorax under 3 conditions: supine position during end inspiration, and right lateral position during both end inspiration and end expiration. The right lateral position was selected for testing based on the assumption that this position would maximize cardiac movement within the chest.
Transthoracic echocardiographyAfter the procedure, transthoracic echocardiography was performed to determine the echocardiographic acoustic window. The measurement was acquired with the patient lying in the same 4 positions as those tested during ultrasound-mediated pacing, supine, 30° left lateral, 30° right lateral, and 30° upward. Using a commercially available ultrasound imaging system (iE33, Philips, Bothell, Washington) with a vascular transducer operating at frequency range of 3–11 MHz (L11-3, Philips\), the echocardiographic acoustic window that allowed clear visualization of the near-field endocardium was determined along the lower ICS and measured on the chest wall directly with a measuring tape. The acoustic window determined during ultrasound-mediated pacing was also marked on the chest wall and a direct tape measurement taken along the same ICS.
Data analysis
The primary study efficacy end point was ECG-documented stable ultrasound-mediated pacing at left ventricular locations that consistent direct electrical pacing was achieved. The triggered oscilloscope recording of 12 seconds of ultrasound-mediated pacing was analyzed for the minimum and the maximum output voltages at the terminal portion of the pacing pulse. The minimum receiver electrode output voltage at the terminal portion of the pacing pulse was defined as the ultrasound-mediated pacing threshold. The distance from the transmitter to the receiver was calculated by multiplying the speed of sound through tissue (1.54 mm/μs) with the time delay on oscilloscope recording from the onset of the ultrasound transmission burst to the onset of the receiver electrode output.13
DICOM files from CT scans were imported for 3-dimensional (3D) reconstructions using Mimics version 10.0 software (Materilise, Ann Arbor, Michigan). The location and size of the acoustic window on the chest wall was determined from the supine CT scan using anatomic references in a manner analogous to the clinical measurements. The area on the chest wall mapping the transmission path to the heart without any intervening lung tissue was defined as the CT-determined acoustic window. The receiver electrode location and the standard transmitter location were simulated on the 3D reconstruction, and the horizontal and vertical movements of the receiver relative to the transmitter were determined in all 3 CT scanning conditions.
Statistical analysis
Data are presented as mean ± standard deviation. Student 2-tailed paired t test was applied to compare pacing threshold voltages. Student paired t test with the Bonferroni correction was used to compare distances and angles from the 3 transmitter locations. A probability value <0.05 was considered statistically significant.
Results
Twelve patients signed informed consent, and 10 patients underwent testing of ultrasound-mediated pacing. Two patients were not studied, 1 because of ventricular irritability and catheter-induced ventricular fibrillation during catheter manipulation, and 1 because of a markedly calcified and tortuous aorta. The clinical characteristics of the 10 study patients are shown in Table 1. The receiver electrode site was positioned at the midlateral wall of the left ventricle in 4 patients, at the midposterolateral wall in 4 patients, and at the posterior wall in 2 patients. The endocardial pacing site was chosen once a stable catheter position was achieved. Consistent electrical and ultrasound-mediated pacing with the transmitter positioned at the standard location was successful in all patients during the first attempt. The mean direct electrical pacing threshold was 1.34 V at pulse width of 0.5 msec, which was not significantly different from the thresholds obtained during ultrasound-mediated pacing from the 3 different transmitter locations (Table 2). The transmitted energy, received energy, and overall energy efficiency of ultrasound-mediated pacing for each transmission location are also shown in Table 2. Transmitter locations on the body surface evaluated for receiver electrode targeting are compared in Table 3. The standard location, defined as the fifth ICS at the left parasternal line, ranged 2 to 6 cm from the midsternal line because of the varying sternum widths. The best transmitter locations showed substantial variability among patients. In 2 patients, the best transmitter location was the standard location. As would be anticipated, the best transmitter location had the shortest transmitter to receiver distance (9.9 ± 2.1 cm). Also, the least transmitter angulation was required for targeting from the best transmitter location.
Table 1. Patient demographic data
| Patient | Gender | Age | Cardiac diagnosis | NYHA class | LVEF (%) | LVEDD (cm) | LVESD (cm) | QRS duration (ms) |
|---|---|---|---|---|---|---|---|---|
| 1 | M | 57 | ICM | III | 25 | 6.8 | 5.6 | 113 |
| 2 | M | 48 | NIDCM | III | 25 | 7.6 | 6.8 | 163 |
| 3 | M | 48 | NIDCM | III | 25 | 6.1 | 5.1 | 83 |
| 4 | F | 41 | NIDCM | III | 30 | 6.0 | 5.0 | 130 |
| 5 | F | 73 | NIDCM | III | 35 | 5.8 | 4.3 | 85 |
| 6 | F | 73 | NIDCM | III | 29 | 6.3 | 5.4 | 100 |
| 7 | M | 49 | NIDCM | III | 35 | 5.8 | 4.8 | 92 |
| 8 | M | 25 | NIDCM | III | 35 | 6.0 | 5.6 | 86 |
| 9 | F | 31 | NIDCM | III | 30 | 5.7 | 4.3 | 91 |
| 10 | M | 56 | NIDCM | III | 20 | 6.1 | 5.7 | 102 |
| Mean±SD | 50 ±16 | 29±5 | 6.2±0.7 | 5.3±0.7 | 105±25 |
Table 2. Pacing thresholds
| Electrical threshold | Ultrasound-mediated pacing threshold | |||
|---|---|---|---|---|
| Standard location | Best location | Subcostal location | ||
| Number of patients | 10 | 10 | 10 | 8 |
| Voltage threshold (V) | 1.34±0.31 | 1.08±0.36 | 1.18±0.51 | 0.93±0.48 |
| Energy received (μJ) | 2.49±1.28 | 2.66±1.24 | 2.05±1.34 | |
| Energy transmitted (mJ) | 10.63±8.22 | 5.22±2.31 | 12.46±7.57 | |
| Energy efficiency (%) | 0.03±0.03 | 0.07±0.06 | 0.02±0.01 | |
Table 3. Transmitter targeting distances and angles
| Patient number | ICS | Standard location | ICS | Best location | Subcostal location | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Distance (cm) | Angulation (°) | Distance (cm) | Angulation (°) | Distance (cm) | Angulation (°) | ||||||||
| MSL | T-R | L-R | S-I | MSL | T-R | L-R | S-I | T-R | L-R | S-I | |||
| 1 | 5 | 4 | 12.6 | −10 | −10 | 5 | 7 | 12.0 | 0 | −3 | 13.6 | −10 | −45 |
| 2 | 5 | 4 | 12.1 | 0 | −7 | 5 | 4 | 12.1 | 0 | −7 | 13.9 | −25 | −28 |
| 3 | 5 | 2 | 9.4 | −5 | −6 | rib 5 | 5 | 7.45 | 0 | −5 | NA | NA | NA |
| 4 | 5 | 3 | 16.3 | −20 | −10 | 4 | 4 | 13.1 | 5 | −5 | 14.5 | −10 | −25 |
| 5 | 5 | 4 | 11.3 | −10 | −10 | 4 | 10 | 8.38 | −5 | −10 | 13.4 | −20 | −39 |
| 6 | 5 | 4 | 11.2 | −5 | −3 | 6 | 12.5 | 8.44 | 0 | 5 | 10.8 | −22 | −32 |
| 7 | 5 | 3.5 | 10.0 | 10 | −5 | 5 | 3.5 | 10.0 | 10 | −5 | 13.0 | −30 | −40 |
| 8 | 5 | 2.5 | 9.0 | 0 | −5 | 5 | 5 | 9.2 | 10 | −10 | 10.8 | −22 | −45 |
| 9 | 5 | 3 | 7.8 | −10 | −5 | rib 6 | 0 | 7.0 | 0 | −5 | 7.6 | −15 | −40 |
| 10 | 5 | 6 | 11.0 | 5 | 12 | 6 | 5.5 | 11.0 | 2 | 4 | NA | NA | NA |
| Mean±SD | 11.1±2.4 | −4.5±8.6 | −4.9⁎±6.4 | 9.9†±2.1 | 2.2‡±4.8 | 4.1#±5.1 | 12.2±2.3 | 19.0±7.0 | −36.8±7.6 | ||||
⁎P = 0.012 compared to subcostal location. |
†P = 0.04 compared to standard location, P = 0.005 compared to subcostal location. |
‡P = 0.05 compared to standard location, P = 0.001 compared to subcostal location. |
#P = 0.000016 compared to subcostal location. |
Table 4 shows the acoustic window location, area, and length along the ICS in the study patients. Transmission of acoustic energy enabled consistent pacing in all 10 patients at the fifth ICS, 7 patients at the fourth ICS, and 6 patients at the sixth ICS. All patients had acoustic windows demonstrated in more than one ICS. The acoustic window lengths along the fourth, fifth, and sixth ICS were 3.0 ± 2.6, 5.2 ± 1.9, and 4.2 ± 4.5 cm. As anticipated, the acoustic window tended to decrease with rightward tilt because of loss along the left lateral margin, and the window tended to increase with leftward tilt because of gain along the left lateral margin. With upward tilt, the window tended to increase because of gain in the sixth ICS.
Table 4. Acoustic window measurement from clinical testing and CT reconstruction in all 10 patients
| Method | Window length (cm) | Window area (cm2) | |||||
|---|---|---|---|---|---|---|---|
| 4 ICS | 5 ICS | 6 ICS | Supine | Right tilt | Left tilt | Upward tilt | |
| Clinical testing (normal respiration) | 3.0±2.6 | 5.2±1.9 | 4.2±4.5 | 39.6±18.2 | 29.8±17.7 | 55.1±17.8 | 49.7±22.4 |
| CT (end inspiration) | 2.6±2.7 | 4.4±2.7 | 4.9±3.2 | 46.9±19.5 | 38.5±18.8 | ||
Acoustic window sizes and locations were also determined from 3D reconstructions of thoracic CT scans obtained in patients lying supine and on their right sides. The acoustic window lengths measured from the supine CT reconstructions were 2.6 ± 2.7 cm, 4.4 ± 2.7 cm, and 4.9 ± 3.2 cm along the fourth, fifth, and sixth ICS, not statistically different from the clinical measurements. However, in individual patients, there were some differences in the location and size of the window attributed primarily to lung volume changes associated with respiratory phases. Figure 2 shows an example of clinically determined acoustic windows superimposed upon the 3D reconstruction and CT-determined acoustic window in 1 patient. Note that the acoustic window moves rightward with right tilt, leftward with left tilt, and downward with upward tilt. However, an area exists in which all windows overlap, which would be a satisfactory location for implantation of a transmitter. Figure 3 shows an example of 3D CT reconstructions obtained from 1 patient in supine and right-sided positions. In panel A, the patient is supine at end inspiration (acoustic window in green). In panel B, with the patient turned on the right side at end inspiration, the left lung enlarges and the acoustic window is reduced in size (shown in red). In panel C, the upward movement of the lower border of the acoustic window with expiration is shown (black). The effects of both body positioning and respiratory phase on movement of the receiver site within the left ventricle were further evaluated with CT scans obtained in the supine position during end inspiration, and in the right lateral position during both end inspiration and end expiration. Comparison of CT scans obtained in these 3 conditions showed that the mean maximum horizontal movement of the receiver site was 1.2 ± 1.4 cm (range, 1.4 to 3 cm) and the mean maximum vertical movement was 1.3 ± 0.8 cm (range, 0.3 to 2.3 cm).
Figure 2.
An example of clinically determined acoustic windows in 4 body positions (in red with the patient lying supine; in green with 30° right tilt; in yellow with 30° left tilt; in purple with 30° upright tilt) superimposed on the CT-determined acoustic window (in light blue with the patient lying supine and during end inspiration) on 3D reconstruction CT of the thorax. 3D = three-dimensional; CT = computed tomography.
Figure 3.
An example of changes in acoustic windows obtained by computed tomography of the thorax with various body positioning and respiratory phrases. A: Acoustic window in green when the patient was lying supine and during end inspiration. B: Acoustic window in red during end inspiration when the patient was lying on the right side. C: Acoustic window in black during end expiration when the patient was lying on the right side.
Nine patients underwent echocardiographic assessment of their acoustic window. The acoustic window was identified on the chest wall along each ICS. The measurements are shown in Table 5. The values are statistically not significantly different from those obtained during ultrasound-mediated pacing.
Table 5. Acoustic window dimensions from clinical testing and transthoracic echocardiography in 9 patients
| 4 ICS (cm) | 5 ICS (cm) | 6 ICS (cm) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Clinical | Echo | P | Clinical | Echo | P | Clinical | Echo | P | |
| Supine | 3.2±2.5 | 3.7±2.1 | ns | 4.4±1.9 | 5.2±1.2 | ns | 4.0±4.7 | 2.2±3.6 | ns |
| 30° Right tilt | 2.0±1.7 | 1.4±1.5 | ns | 3.9±2.1 | 2.8±2.2 | ns | 3.3±4.7 | 1.3±2.2 | ns |
| 30° Left tilt | 5.3±5.3 | 5.3±1.7 | ns | 8.2±3.4 | 6.3±1.3 | ns | 2.7±5.2 | 4.5±3.5 | ns |
| 30° Upward tilt | 5.6±4.0 | 2.6±2.0 | ns | 5.6±2.8 | 5.1±1.5 | ns | 3.5±5.1 | 4.2±3.3 | ns |
The results of echocardiographic, ECG, and CK blood level safety testing performed before and after the research protocol did not reveal any abnormality attributable to the investigational system. One patient experienced an adverse event of transient right-hand numbness during the procedure with no clinical sequela; only equivocal but no definite cerebral ischemia was detected on magnetic resonance imaging of the brain. No patient perceived any tactile or audible sensation during testing.
Discussion
The present study focused on the feasibility of incorporating this new technology into a CRT system to address current limitations of left ventricular stimulation. Ultrasound-mediated pacing without leads was successful acutely in an appropriate patient cohort with New York Heart Association class III heart failure and low ejection fraction. Potential transmission locations and important targeting parameters were evaluated. The acoustic window size was adequate for ultrasound transmission from the chest surface to a receiver electrode in the left ventricle, regardless of patient position or respiratory phase. Thus, the ability to perform leadless left ventricular pacing in heart failure patients using the novel technology under development seems to be feasible.
One patient not studied had ventricular fibrillation during catheter manipulation, and 1 patient had a probable transient ischemic attack. These were likely catheter related. In the implantable system under development, the delivery catheter is specially designed and different from the electrophysiology catheter used in this study. Catheter stiffness and maneuverability will be improved. The level of anticoagulation should be carefully monitored in future procedures.
Comparison to prior study
We previously evaluated this technology in 24 patients without severe heart disease.14 In the previous study, the acoustic window size was not delineated and multiple locations were not evaluated. However, qualitatively, the acoustic window size was substantially larger in this study because of displacement of the left lung by the enlarged heart. The presence of cardiac enlargement in this study was not associated with increased distance to the receiver electrode compared with the previous study.
Energy efficiency and targeting
For the design of an implantable system, optimization of energy is imperative, and the ability to focus the transmission beam should substantially improve energy efficiency.
This study provides the initial data on targeting issues. CT scans provided further information regarding potential 3D receiver movement relative to a potential transmitter reference point in the chest wall. This indicated that the movement of the receiver would be relatively circumscribed within a 3-cm distance in any direction. Thus, considerable ultrasound beam focusing could help to alleviate the inherent energy conversion inefficiency of this technology. The CT data confirmed the accuracy of the results obtained from clinical ultrasound transmission testing. Future design of an implantable product will utilize CT scans with 3-dimensional reconstructions from a database of heart failure patients.
The results of this study have identified several important parameters that must be taken into consideration for the development of a future implantable system. The size and location of the acoustic window vary considerably among individual patients. The size and location of acoustic window are also affected by changes in body position and respiratory phase. Rather than obtaining CT scans, which are expensive and associated with radiation hazard, it seems possible to develop a preimplantation protocol with a commercially available transthoracic echocardiographic imaging system.
Study limitations
Most patients in this study did not have QRS duration prolongation, and all but 1 patient had nonischemic dilated cardiomyopathy, so they did not represent a standard population eligible for CRT. However, the thoracic anatomy was appropriate for evaluation of this technology.
More extensive safety testing will be required to address long-term safety. An implantable system will incorporate sensing capabilities that were not addressed in this study. The bipolar pacing electrodes used had a surface area that was not optimized for sensing.
Conclusion
The feasibility of leadless left ventricular stimulation was shown acutely in heart failure patients with large acoustic windows and moderate left ventricular site movement. The size and location of the acoustic window was determined. Transmitting distances and angles were obtained. Factors modifying the acoustic window and targeting of the stimulation site were assessed. If this technology is incorporated into an implantable system, it could enable endocardial stimulation with left ventricular site selection in target zones for cardiac resynchronization.
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This study was supported by a research grant from EBR Systems, Inc.
Dr. Echt is employed by EBR Systems, Inc.
PII: S1547-5271(09)00207-0
doi:10.1016/j.hrthm.2009.02.025
© 2009 Heart Rhythm Society. Published by Elsevier Inc. All rights reserved.
