Acta Mech Sin (2007) 23:275–280 DOI 10.1007/s10409-007-0076-3
RESEARCH PAPER
In vivo mechanical study of helical cardiac pacing electrode interacting with canine myocardium Xiangming Zhang · Nianke Ma · Hualin Fan · Guodong Niu · Wei Yang
Received: 12 December 2006 / Revised: 30 March 2007 / Accepted: 2 April 2007 / Published online: 30 May 2007 © Springer-Verlag 2007
Abstract Cardiac pacing is a medical device to help human to overcome arrhythmia and to recover the regular beats of heart. A helical configuration of electrode tip is a new type of cardiac pacing lead distal tip. The helical electrode attaches itself to the desired site of heart by screwing its helical tip into the myocardium. In vivo experiments on anesthetized dogs were carried out to measure the acute interactions between helical electrode and myocardium during screw-in and pull-out processes. These data would be helpful for electrode tip design and electrode/myocardium adherence safety evaluation. They also provide reliability data for clinical site choice of human heart to implant and to fix the pacing lead. A special design of the helical tip using strain gauges is instrumented for the measurement of the screw-in and pull-out forces. We obtained the data of screw-in torques and pull-out forces for five different types of helical electrodes at nine designed sites on ten canine hearts. The results indicate that the screw-in torques increased steplike while the torque–time curves presente saw-tooth fashion. The maximum torque has a range of 0.3–1.9 N mm. Obvious differences are observed for different types of helical tips and for different test sites. Large pull-out forces are frequently obtained at epicardium of left ventricle and right ventricle lateral wall, and the forces obtained at right ventricle apex and outflow tract of right ventricle are normally small. The differences in pull-out forces X. Zhang · N. Ma · H. Fan · W. Yang Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China G. Niu Fuwai Angiocardiopathy Hospital, Beijing 100037, China W. Yang (B) Zhejiang University, Hangzhou 310027, China e-mail:
[email protected]
are dictated by the geometrical configuration of helix and regional structures of heart muscle. Keywords Cardiac pacing · Helical electrode · Acute · Interactions · Myocardium
1 Introduction Since the first permanent pacemaker was implanted in 1958 [1], device therapy has continued to grow. More than 2,000,000 [2] cardiac pacing leads have been implanted into patients worldwide and about 400,000 [2] leads are implanted annually. A helical electrode configuration is a new type of the heart pacing lead distal tip [3]. The helical electrode attaches itself to the desired site of heart by screwing its helical tip into the myocardium. This type of electrode tip has a more secured attachment than the normal passive fixation electrode. If the designs of the lead, its delivery system and the implant procedures are inappropriate, however, myocardium perforation or penetration to the heart may occur during the lead implant. So the research of the electrode/myocardium interactions is rather important and valuable to the cardiac pacing design and safety evaluation. It can also be used to help the clinical physicians to decide which site is most suitable for a certain type of electrode. The investigators traditionally performed in vitro testing of medical devices to predict in vivo mechanical performance. Other investigators developed 3-D finite elements analysis (FEA) modeling to study the heart/device interaction and analyzed the mechanical performances of electrode or heart muscle. Zhao [3] studied the mechanical forces applied on the pacing lead for it to go through the vein by using finite element method analysis software—ABAQUS.
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Mond et al. [4] studied the influences of the shape, size and material properties on the functions and safety of the screwin active fixation electrode. Baxter [5] developed an image analysis method based on finite element model to study the pacing lead mechanics. The present work aims to develop a method to measure the in vivo transvenous implantable pacing helical electrode/ myocardium interaction on anesthetic dogs. The method is based on a unique design of the lead tip without altering its mechanical properties that appropriately uses the tiny strain gauges. The acute interactions during two processes: screwin electrode and pull-out electrode were measured. These tests simulated the bounds for the in vivo load threshold. The results can be used to analyze the electrode/myocardium adherence safety. Five types of helical electrodes and ten canine hearts were applied in the tests. Nine different designed sites in heart were tested.
2 Methods 2.1 Animal preparation Ten healthy dogs were tested in our experiments. Each dog had a weight of 20–25 kg, and was about 1.5 years old. The tested canine was fasted overnight prior to the experiment and pre-sedated with acepromazine before testing. Anesthesia was induced through anterior tibial vein using ketamine at dose of 25 mg/kg to result in general anesthesia. The dog was intubated and placed on gas anesthesia: isofluorane (at 1.5–2.0%) and 100% O2 . A respirator was used. The dog was clipped at the neck, inguinal region, and left lateral thorax and be positioned into the left lateral recumbancy for the implant procedure of pacing lead with force sensors. An ECG monitor was connected to monitor the heart beating rate and the rhythm. The relevant local regions with suds and disinfect were cleaned with iodine if necessary. The surgical region was anaesthetized with procaine at dose of 1%. A right lateral cephalic vein was cut open and an intravenous catheter was placed into the vein and moved to the certain site in the heart chambers. A pacing lead with force sensors was delivered into the heart chambers through the catheter and reached a designed site of heart with the help of X-ray imagery. The experiments conducted the animal welfare regulations and guidelines in China. The animal testing is conducted in accordance with the ethical principles contained in World Medical Association Declaration of Helsinki. The dog number in the text below was labeled by the animal lab. 2.2 Specifications of the pacing lead tips Five types of cardiac pacing electrode manufactured by Medtronic Cardiac Rhythm Management were applied in
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X. Zhang et al. Table 1 Geometries of five different pacing lead tips Type of lead tip
5076
3830
2F
H1
H2
Out diameter/mm
1.17
1.02
0.76
1.17
1.02
Wire diameter/mm
0.25
0.25
0.25
0.25
0.25
Pitch length/mm
1.00
1.00
0.41
1.00
1.00
Total length/mm
4.32
3.00
0.59
7.11
6.45
the tests. They were labeled as 5076, 3830, 2F, H1 (elongated from 5076) and H2 (elongated from 3830). The basic geometries of the lead tips are listed in Table 1. 2.3 Test sites in canine heart Nine clinical valuable sites in heart were tested in our experiments: 1—right ventricle apex; 2—right ventricle lateral wall; 3—outflow tract of right ventricle; 4—interventricular septum; 5—high ventricular septum; 6—His bundle; 8— right atrium lateral wall; 10—right atrium septum; 11—epicardium of left ventricle. In the text below, we used the site number to represent the sites if not announced especially. 2.4 Design of force sensors When the pacing lead was delivered to a certain site of the heart and its lead tip screwed into the myocardium, the force sensors collected the data of lead/heart interactions. The force sensors were mainly assembled by miniature strain gauges which were connected with a dynamic strain indicator to identify and to amplify the electric signals. The details of force sensors are illustrated in Fig. 1.
3 Results and discussion 3.1 Screw-in torque When the helical electrode was screwed into the heart muscle at a designed site, the force sensor recorded the torque applied on the electrode. The value of the torque was dictated by the tissue structure of the heart and the geometrical shape of the helix. The following figures (Figs. 2, 3, 4, 5, 6, 7) show the typical curves of torque versus time. From these curves, one observes that the torques escalated in saw-tooth fashion when the helix was screwed into the heart muscle, with few exceptions where the torques only oscilated. Each peak represented one cycle of helix being screwed into the myocardium. The more cycles were screwed in, the larger attaching interface was between the electrode and the myocardium, and the larger friction was generated. Accordingly, larger torque was required for further screwing of the electrode into the
In vivo mechanical study of helical cardiac pacing electrode interacting with canine myocardium
Fig. 1 Details of the force sensors mounted on the electrode tip. The left photo is a lead tip without force sensors. The right two pictures delineate the structure of the specially designed lead tip with force sensors to measure the lead/heart interaction. We used a thin walled cylinder made of silicone adhesive to connect the helical electrode and the metal filament. Two strain gauge foils along the cylinder axis (labeled as sensors 1 and 2 of size 0.6 mm×0.8 mm) were attached to the outside surface of the cylinder and opposite to each other. They were used to measure the pull-out forces. Another two strain gauge foils along the 45◦ directions were used to measure the screw-in torques. The usage of silicon gel insulated the sensors from the blood in canine heart. All the sensors were calibrated before tests
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Fig. 3 Torque–time curves of screwing 2F helical electrode into atrium septum at No.165 canine heart
Fig. 4 Torque–time curves of screwing H1 helical electrode into his bundle at No.166 canine heart
Fig. 2 Torque–time curves of screwing 2F helical electrode into right ventricle lateral wall at No.165 canine heart
myocardium. The maximum torques needed to completely screw different helical electrodes into myocardium at different sites were listed in Table 2. From the data in Table 2, several observations can be drawn: 1. Large differences are observed among the torques obtained at different sites using different helical electrodes. They have an approximate range of 0.3–1.9 N mm. 2. The torques of H1 and H2 are larger than those of 5076 and 3830 at the same test sites. The length of the helix
Fig. 5 Torque–time curves of screwing H1 helical electrode into epicardium of left ventricle at No.166 canine heart
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X. Zhang et al. Table 2 Maximum torques to completely screw different helical electrodes into myocardium at different sites
Fig. 6 Torque–time curves of screwing H2 helical electrode into right ventricle lateral wall at No.167 canine heart
Fig. 7 Torque–time curves of screwing H2 helical electrode into atrium septum No.167 canine heart
influences the screw-in torque. Holding the other geometrical parameters of the helix, a longer helix requires a larger torque to be completely screwed into myocardium, since a longer helix has larger interface and frictions. 3. The torques of 3830 and H2 are larger than those of 5076 and H1 at the same test sites. The outer diameter of helix influences the screw-in torque. Holding the other geometrical parameters of the helix, the helix with a smaller outer diameter needs larger torque to be completely screwed into myocardium. Helixes with less outer diameters provide smaller space to contain the intruding myocardium with higher applied pressure on the helix wire surface. Consequently, larger torque is needed to screw in the helix. 4. Generally, H1 and H2 helical electrodes need larger torque to be screwed into ventricular sites than atrial sites. Whilest 2F helical electrode needs larger torque in atrial
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Electrode type
Dog number
Test site
Maximum torque/N mm
Test site
Maximum torque/N mm
5076
158
1
0.500
6
0.462
5076
158
2
0.303
10
0.785
5076
158
3
0.430
11
0.291
5076
158
4
0.636
5076
165
1
0.349
4
0.314
5076
165
2
0.246
5
0.233
5076
165
3
0.343
11
0.287
3830
162
2
0.796
4
1.63
3830
162
3
1.08
2F
163
1
0.415
6
0.698
2F
163
5
0.242
10
0.757
2F
165
1
0.453
6
0.810
2F
165
2
0.791
8
0.831
2F
165
4
0.423
10
0.913
H1
166
1
0.702
6
0.752
H1
166
2
0.761
10
0.589
H1
166
3
0.948
11
0.334
H1
166
5
0.461
H2
167
1
1.88
6
0.709
H2
167
2
1.76
10
0.965
H2
167
3
0.856
11
0.654
H2
167
5
0.433
sites than in ventricular sites. The tissues in atrium are thinner and denser than those in ventricle, so they can better adhere to 2F electrode with tiny helix. 5. All types of helical electrode need smaller torque to be screwed into epicardium of left ventricle than other sites in heart. The muscle fibers in this site are relatively smooth and regular in fiber directions, making the screw-in of the electrodes easier. 3.2 Pull-out force After screwed into the heart muscle to its maximum depth, the helical tip was pulled out. The force sensors and dynamic strain indicator jointly furnished the force curve. The force drop characterizes the minimum force pulling out the helical tip from the heart muscle. These tests simulate the critical status to measure the load threshold. The test data can be used to estimate the electrode/myocardium adherence safety and to judge the possibility for the electrodes to break off and displace from the work sites. They are also helpful for clinical physicians to choose different types of helical electrodes applied at certain sites and for engineers to design the cardiac pacing electrode.
In vivo mechanical study of helical cardiac pacing electrode interacting with canine myocardium
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Table 3 Site groups according to the pulling-out forces categories Electrode type
Fig. 8 Pull-out force–time curves obtained at different sites in canine heart
Dog number
Site groups according to the pulling-out force categories I(> 2 N)
II(1–2 N)
III (0–1 N) 1, 4, 6
5076
158
2, 11
3, 5
5076
165
2, 3, 11
1, 5
4
3830
162
2, 11
4, 5
3
3830
164
2, 11
1, 3, 4, 10
5, 6
2F
163
6, 10, 11
2F
165
2
2F
167
3, 11
H1
156
H1
157
H1
166
H2
161
H2
162
H2
167
1, 5 1, 4, 6, 10
8
6
11
1, 4
2
4, 6, 11
1
8, 10
1, 2, 3, 5, 6, 11 10
3, 5 1, 4, 6
3, 10
2, 6, 8, 10
1, 5
tests. While the right ventricle apex, outflow tract of right ventricle and atrial sites offer smaller pull-out forces, making the electrode there easier to break off in acute tests. From the data in Table 3, the suitable sites for each helical electrode can be ranked as follows:
Fig. 9 Pull-out force–time curves obtained at different sites in canine heart
Figures 8 and 9 show typical curves of pull-out force versus time at different sites in canine hearts. Most of the curves exhibit sharp drops that indicate the brittle nature for the helical electrodes to break off from the myocardium. The forces at different sites have distinct difference. Two issues, the electrode/myocardium adherence and the mechanical properties of regional myocardium, determine the forces. The latter is typically the main factor. Figure 5 also depicts the difference of the force curves obtained from two dogs. Although the force data differ among various dogs to reflect bio-diversity, common trends are observed for the responses in each dog. We classify the data into three categories according to the quantity of the force (Table 3). Obviously, a higher value for the pull-out force refers to a safer electrode/myocardium adherence. Accordingly, the sites with larger forces should be selected for clinical applications with priority. The sites in ventricle, especially epicardium of left ventricle and right ventricle lateral wall, exhibit larger pull-out forces and are consequently more suitable for electrode fixations in acute
1. 2. 3. 4. 5.
The 5076 helical electrode: 2, 11 > 1, 3, 5 > 4, 6; The 3830 helical electrode: 2, 11 > 1, 4, 10 > 3, 5, 6; The 2F helical electrode: 2, 3, 11 > 4, 6, 10 > 1, 5, 8; The H1 helical electrode: 8, 10 > 2, 3, 5, 6, 11 > 1, 4; The H2 helical electrode: 3 > 6, 8, 10 > 1, 4, 5, 6.
4 Conclusions The following conclusions can be drawn from the in vivo tests of electrode/myocardium interactions under acute state: 1. Large differences exist among the torques obtained at different sites using different helical electrodes. They have an approximate range of 0.3–1.9 N mm. 2. The length and the outer diameter of the helix influence the screw-in torque. The longer helix needs larger torque to be completely screwed into myocardium. On the other hand, the helix with smaller outer diameter needs larger torque to be completely screwed into myocardium. 3. H1 and H2 helical electrodes need larger torque to be screwed into ventricular sites than atrial sites. While 2F helical electrode requires larger torque in atrial sites than in ventricular sites. All types of helical electrode need smaller torque to be screwed into epicardium of left ventricle than other sites in heart.
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4. The break-offs of helical electrode from the myocardium are normally brittle. 5. The pull-out forces at different sites differ substantially. The sites in ventricle, especially epicardium of left ventricle and right ventricle lateral wall, exhibit larger pullout forces while the right ventricle apex, outflow tract of right ventricle and atrial sites offer smaller pull-out forces. Each helical electrode has its suitable sites of implants. Acknowledgment The authors thank Dr. Zhang Shu and the animal lab in Beijing Fuwai Angiocardiopathy Hospital for their clinical supports and animal preparing. The authors greatly acknowledge the financial and facility supports from the Medtronic Cardiac Rhythm Management, USA, as well as the suggestion of Y. Zhao on conducting this research.
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