A Prospective Randomized Evaluation of Implantable Cardioverter-Defibrillator Size on Unipolar Defibrillation System Efficacy
Background The active can unipolar implantable cardioverter-defibrillator (ICD) has been shown to defibrillate efficiently, but its current 80-cc size limits use in the pectoral position in many patients. Decreasing can size will facilitate pectoral insertion and will soon be feasible as an inevitable consequence of technological advancements. However, decreasing the can size has the potential to compromise unipolar defibrillation efficacy. It is the purpose of this study, therefore, to prospectively and randomly compare unipolar defibrillation efficacy with 80-cc, 60-cc, and 40-cc can sizes in patients immediately before ICD surgery in anticipation of advances in technology that will make smaller ICDs possible.
Methods and Results Twenty-four consecutive patients underwent prospective, randomized evaluation of the effect of ICD can size on defibrillation efficacy during standard ICD surgery. Each patient had the unipolar defibrillation threshold (DFT) measured with 80-cc, 60-cc, or 40-cc active can placed in the left subcutaneous infraclavicular region. The system included a 10.5F tripolar right ventricular electrode that served as the shock anode. The shock waveform used in each instance was a single capacitor biphasic 65% tilt pulse delivered from a 120-μF capacitor. Stored energy at the DFT for the 80-cc, 60-cc, and 40-cc cans were 8.1±4.7 J, 8.7±5.8 J, and 9.5±4.8 J, respectively. There was no statistical significant difference between the DFTs for the three unipolar can electrodes (P=.39). Leading edge voltage also did not differ significantly among the three unipolar cans (356±92 V, 365±110 V, and 387±94 V, respectively, P=.29). There was, however, a slight progressive increase in resistance with decreasing can size (57±7 Ω, 60±7 Ω, and 65±9 Ω, respectively, P<.001).
Conclusions Decreasing can volume from 80 cc to 60 cc to 40 cc does not compromise unipolar defibrillation efficacy despite a slight rise in shock resistance. These findings indicate that technological advances that allow for smaller-volume ICDs will not compromise defibrillation efficacy for unipolar systems.
The development of single-lead unipolar transvenous defibrillator systems has made the use of implantable cardioverter-defibrillators (ICDs) significantly easier and as effective or more effective than previous ICD systems.1 2 3 4 5 6 7 The prototype unipolar ICD has an 80-cc volume that is similar in size to early model pacemakers.5 Although this system is practical for use in the pectoral region in most patients, its size requires more surgical skill than is necessary for today’s pacemaker implants and may force subpectoral or abdominal implantation in approximately 20% of patients with small body habitus.8 With the miniaturization that will follow further technological advancements, unipolar defibrillation systems should approach the ease of implantation of a pacemaker. However, the smaller volume and surface area of the “active can” electrode may have an adverse effect on defibrillation efficacy, possibly countering any benefit that might derive from smaller-sized ICDs. Consequently, it was the purpose of this study to explore the effect of ICD size on unipolar defibrillation efficacy.
After informed verbal and written consent was obtained, 24 consecutive patients resuscitated from cardiac arrest with syncopal ventricular tachycardia and/or ventricular fibrillation (VF) underwent a prospective, randomized comparative study of unipolar, active can defibrillation efficacy with the use of can electrodes of 80-cc, 60-cc, and 40-cc size during ICD surgery.
A tripolar 10.5F right ventricular transvenous lead (Medtronic model 6936) was the endocardial component of the unipolar ICD system. This lead incorporated a 5-cm-long coil defibrillation electrode with standard bipolar pace/sense electrodes at the tip. The active can component of the unipolar ICD system was provided by 80-cc, 60-cc, and 40-cc titanium cans modeled after the Medtronic model 7219C unipolar ICD. The surface areas of the active can electrodes were 112 cm2, 88 cm2, and 62 cm2, respectively. The active can was positioned subcutaneously on the anterior fascia of the left pectoralis major muscle, 2 to 3 cm inferior to the left clavicle and 2 to 3 cm medial to the humoral head (Fig 1⇓). The long axis of the can was placed parallel to the long axis of the body, with the opening of the portals in the connector block directed medially. The skin edges of the incision site were approximated with multiple towel clips to ensure that the entire can electrode was encapsulated by tissue during testing. All air and serosanguinous fluid were expressed or aspirated before defibrillation testing. Finally, all defibrillation pulses were delivered at end expiration to avoid the possible variable effect of respiration on pulse impedance.
Defibrillation Threshold Testing
Testing of the single-lead unipolar defibrillation system with each of the three active can electrodes was done in a prospective, randomized fashion with the use of a 120-μF capacitor, asymmetrical biphasic pulse delivered at 65% tilt for both phases. Order of can size testing was based on a numerical randomization scheme derived from the patient’s hospital number. After determining the order of testing, each can was inserted in the subcutanous pocket with closure of the skin edges as described above. The right ventricular (RV) electrode served as the anode for the initial phase of the biphasic waveform in each instance. After VF was induced with alternating current, defibrillation thresholds were obtained. The first defibrillation pulse was given starting with a 400-V leading edge pulse delivered 10 seconds after induction of VF. If the transvenous pulse was unsuccessful, a 100- to 200-J transthoracic rescue pulse was immediately delivered via a precharged external defibrillator (Physio-control Lifepack 6s) between anterior-posterior cutaneous pads (Darox Corporation).
Before reinduction of VF, a minimum rest period of 3 minutes was required. In addition, arterial blood pressure, heart rate, ECG ST segment morphology, and O2 saturation were monitored and were required to return to baseline values before reinduction of VF. If the initial 400-V shock was unsuccessful, pulse voltages were increased in 100-V steps up to 900 V. If the initial pulse shock was successful, shock strength was decreased in 50-V steps.
The defibrillation threshold was defined as the minimum energy that successfully terminated VF 10 seconds after its induction. After the defibrillation threshold was determined for one system, the remaining methods were tested in a likewise and consecutive fashion. The defibrillation threshold was measured only once for each method, given the concerns over repetitive fibrillation and defibrillation in human subjects.
Repeated-measures ANOVA was used to compare defibrillation thresholds among the three active can electrodes as well as to evaluate for linear correlation. Statistical significance was defined as a value of P<.05. The population studied was designed to provide a power of 80% to detect a difference of 2.5 J for defibrillation threshold among the three can size groups.
There were 20 men and 4 women. Seventeen had coronary artery disease, 5 had idiopathic dilated cardiomyopathy, 1 had hypertrophic cardiomyopathy, and 1 had right ventricular dysplasia. The mean ejection fraction as assessed by radionuclide ventriculography was 0.41±0.16, with a range of 0.15 to 0.65. Indications for ICD implantation included 12 patients resuscitated from VF, 8 with syncopal ventricular tachycardia (VT), and 4 with both VT and VF. No patient was on antiarrhythmic drug therapy at the time of defibrillation testing.
Mean defibrillation threshold data are shown in the Table⇓. The mean defibrillation threshold stored energy for the 80-cc can was 8.1±4.7 J, for the 60-cc can was 8.7±5.8 J, and for the 40-cc can was 9.5±4.8 J. There was no statistically significant difference among the three active can sizes in terms of energy requirements for defibrillation (P=.39, Fig 2⇓). In addition, no significant linear correlation was seen between can size and stored energy (P=.16). Similarly, there was no statistically significant difference in leading edge voltage (356±92 V, 365±110 V, and 387±94 V, respectively, P=.29) or leading edge current (6.3±1.5 A, 6.2±1.8 A, and 6.0±8.8 A, respectively, P=.68) at the defibrillation threshold among the three can sizes. There also was no linear correlation between can size and leading edge voltage or current (P=.10 and P=.35, respectively). There was, however, a slight but significant difference in the pulsing resistance at the defibrillation threshold between the different sized cans (56.6±6.9 Ω, 60.2±7.4 Ω, and 64.5±8.8 Ω, respectively, P<.001; Fig 3⇓). No significant difference in defibrillation threshold was noted based on the order of testing: The defibrillation threshold for the cans tested first was 7.9±3.8 J, 9.0±6.0 J for the cans tested second, and 9.4±5.3 J for those tested last (P=.63). The number of shocks delivered to measure the defibrillation threshold for each can size was 3.2±0.6, 3.3±0.7, and 3.3±0.8, respectively (P=.90).
With the use of a defibrillation threshold ≤24 J as an implant criterion, only one patient would not have met implantation criterion and only in the case of the 60-cc size can (Fig 2⇑). This finding is probably consistent with statistical variation in defibrillation efficacy.
A unipolar single electrode pectoral transvenous defibrillation system has been shown to be an effective means to treat VF and can be inserted into the left infraclavicular subcutaneous region.5 6 7 8 However, in small individuals or those with limited subcutaneous space, present ICD can size may force an abdominal insertion or a subpectoralis muscle implant and thereby increase the complexity of the procedure. Thus, a smaller can size could have significant surgical advantage if it does not compromise defibrillation efficacy for this otherwise simple-to-use, single-lead ICD.
Previous defibrillation studies with epicardial lead systems, using either monophasic or biphasic pulses, have shown that the surface area of electrodes is a significant factor for defibrillation efficacy.9 10 11 12 Given that the current unipolar system uses the active can as an electrode, the surface area of the can could play a significant role in defibrillation efficacy. In this study, however, there was no significant difference in defibrillation efficacy among active cans varying in volume between 80 cc, 60 cc, and 40 cc with surface areas of 112 cm2, 88 cm2, and 62 cm2, respectively. The mean defibrillation threshold in each of the systems tested was below 10 J. In addition, there was no significant difference among the varying can sizes in leading edge voltage or current required for defibrillation. The one factor that did differ among the different sized cans was pathway resistance. Although the larger electrode afforded a lower pathway resistance, this did not result in improved defibrillation efficacy.
One explanation as to why there was no statistically significant change in the defibrillation threshold despite a 50% reduction in surface area may be related to the observation that the electrode tissue interface impedance is a small component of overall shock resistance, within practical limits.13 14 It should be noted, however, that electrode-tissue interface impedance is nonlinear.15 Consequently, as electrode surface area is reduced beyond that evaluated in this study, electrode-tissue interface impedance may become more important and lead to more significant differences in defibrillation efficacy than seen in this study. Hence, there probably will be limits as to how small an active can could be. The trend seen in our data suggests that eventually defibrillation energy could increase unacceptably as the can size falls below 40 cc.
The results of this study on ICD size also have implications for ICD energy output. Because defibrillator size is significantly affected by capacitor technology, it would not be possible to provide a 34-J, 40-cc can ICD at this time with the 120-μF K-film capacitor used in this study. Although smaller 60-μF and 90-μF K-film capacitors have a modest effect on improving defibrillation efficacy in unipolar ICD systems, it is not sufficient to decrease capacitor size and therefore ICD size to 40 cc while maintaining a 34-J output.16 17 The direct relationship between capacitor size and energy cancels the slight advantage provided by use of a smaller capacitance. Thus, without a change in the basic capacitor technology, a 40-cc unipolar active can system will not be practical in the near future unless the maximal output of the device is reduced to 25 J or less. This study therefore anticipates either a reduced output ICD or improved capacitor technology before ICDs can be smaller.
This work was supported in part by grants from the National Institutes of Health (R01-HL-48814-03), from the Tachycardia Research Foundation, Seattle, Wash, and from Medtronic Corporation.
↵1 Dr Gust H. Bardy serves as a consultant to Medtronic, Inc, manufacturer of the unipolar defibrillation system.
- Received January 30, 1995.
- Revision received April 24, 1995.
- Accepted June 23, 1995.
- Copyright © 1995 by American Heart Association
Bardy GH, Hofer B, Johnson G, Kudenchuk PJ, Poole JE, Dolack GL, Gleva M, Mitchell R, Kelso D. Implantable transvenous cardioverter-defibrillators. Circulation. 1993;87:1152-1168.
Yee R, Klein GH, Leithch JW, Guiraudon GM, Guiraudon CM, Jones DL, Norris C. A permanent transvenous lead system for an implantable pacemaker cardioverter-defibrillator: nonthoracotomy approach to implantation. Circulation. 1992;85:196-204.
Bardy GH, Allen MD, Mehra R, Johnson G, Feldman S, Greene HL, Ivey TD. Transvenous defibrillation in humans via coronary sinus. Circulation. 1990;81:1252-1259.
Fromer M, Brachmann J, Block M. Efficacy of automatic multimodal device therapy for ventricular tachyarrhythmias as delivered by a new implantable pacing cardioverter-defibrillator: results of a European multicenter study of 102 implants. Circulation. 1992;86:363-374.
Bardy GH, Johnson G, Poole J, Dolack GL, Kudenchuk PJ, Kelso D, Mitchell R, Mehra R, Hofer B. A simplified, single-lead unipolar transvenous cardioversion-defibrillation system. Circulation. 1993;88:543-547.
Bardy GH, Dolack GL, Kudenchuk PJ, Poole JE, Johnson G, Kelso D, Mitchell R, Mehra R, Hofer B. Prospective, randomized comparison in humans of a unipolar defibrillation system with that using an additional superior vena cava electrode. Circulation. 1994;89:1090-1093.
Kudenchuk PJ, Bardy GH, Dolack GL, Poole JE, Mehra R, Johnson G. Efficacy of a single-lead unipolar defibrillator compared with a system employing an additional coronary sinus electrode. Circulation. 1994;89:2641-2644.
Bardy GH, Raitt MH, Jones GK. Unipolar defibrillation systems. In: Singer I, ed. Implantable Cardioverter-Defibrillator. Armonk, NY: Futura Publishing Co; 1994:365-376.
Dixon EG, Tang AS, Wolf PD, Meador JT, Fine MJ, Calfee RV, Ideker RE. Improved defibrillation thresholds with large contoured epicardial electrodes and biphasic waveforms. Circulation. 1987;76:1176-1184.
Kallok MJ, Bourland JD, Tacker WA, Jones DL, Klein GJ, Wessale JL. Optimization of epicardial electrode size and implant site for reduced sequential pulse defibrillation thresholds. Med Instrm. 1986;20:36.
Mehra R, DeGroot P, Norenberg S. Three-dimensional finite element model of the heart for analysis of epicardial defibrillation: effect of surface area. PACE Pacing Clin Electrophysiol. 1989;12:652.
Jorgenson DB, Haynor GH, Bardy GH, Kim Y. Computational studies of transthoracic and transvenous defibrillation in detailed 3-D human thorax model. IEEE Trans Biomed Eng. 1994;42:172-184.
Schimpf PH, Johnson G, Jorgenson DB, Haynor DR, Bardy GH, Kim Y. Effects of electrode interface impedance on finite element modeling of transvenous defibrillation. Med Biol Eng Comput. In press.
Mehra R, Cybulski Z. Tachyarrhythmia termination: lead system and design. In: Singer I, ed. Implantable Cardioverter-Defibrillator. Armonk, NY: Futura Publishing Co; 1994:109-133.
Bardy GH, Poole JE, Kudenchuk PJH, Dolack GL, Mehra R, Raitt MH, Jones GK, Johnson G. A prospective randomized comparison in man of biphasic waveform 60-μF and 120-μF capacitance pulses using a unipolar defibrillation system. Circulation. 1995;90:90-95.
Bardy GH, Poole JE, Kudenchuk PJ, Dolack GL, Raitt MH, Jones GK, Mehra T, Troutman C, Anderson J, Johnson G. A prospective randomized comparison in humans of 90μF and 120μF 65% tilt biphasic pulse defibrillation using a unipolar pectoral transvenous defibrillation system. J Am Coll Cardiol. 1994;1A-484A:13A. Abstract.