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J Thorac Cardiovasc Surg 1995;110:1725-1731
© 1995 Mosby, Inc.
SURGERY FOR ACQUIRED HEART DISEASE |
Bergen, Norway
Received for publication July 31, 1994. Accepted for publication Jan. 5, 1995. Address for reprints: Ole-Jörgen Ohm, MD, FACC, Professor of Cardiology, Medical Department A, University of Bergen, School of Medicine, Haukeland Sykehus, N-5021 Bergen, Norway.
Abstract
We have studied two new temporary pacing leads (Medtronic 6491 and 6492) intended for pacing after cardiac operations. The conductor has stainless steel strands coated with polyethylene connected to a 4 mm 2surface area, stainless steel, smooth, tapered electrode. A soft 4-0 coiled polypropylene fiber served as a fixation mechanism in the heart. The study included 15 children (aged 3 months to 7 years, body weight 4.4 to 20 kg) with a variety of congenital heart defects and 15 adults (aged 45 to 78 years) with coronary artery disease (n = 13) and aortic valve disease (n = 2). A pair of leads each was placed in a bipolar fashion in the right atrial wall and nonsystemic ventricle in the children (median implant duration 12 days) and in the right atrial wall only in the adults (median implant duration 9 days). The atrial current threshold values in children increased from 0.61±0.34 mA immediately after implant to 2.08±1.86 mA at explant (p <0.002). In the adults the threshold values increased from 0.95±1.44 mA immediately after implant to 2.76±2.76 mA at explant (p <0.002). In the ventricle the threshold values increased from 0.38±0.13 mA immediately after implant to 2.22±1.63 mA at explant (p <0.002). Tissue resistance immediately after implant measured 809±182
in the atrium and increased to 820±204at explant (children, p = not significant). Corresponding values in adults were 778±190and 599±91(p <0.004). In the ventricle resistances changed from 1019±143to 876±137(p <0.05). P wave amplitudes measured 1.8±1.5 mV immediately after implant and decreased to 1.6±1.2 mV at explant (p = not significant, children) and 2.0±1.3 mV to 1.8±1.1 mV (p = not significant, adults). R wave amplitudes were 13.1±3.0 mV immediately after implant and fell to 8.7±4.5 mV at explant (p <0.005). Thus threshold values, tissue resistances, and electrogram amplitudes assured a safe pacemaker function. The small diameter and pliable texture of these leads provided a smooth surgical handling. They were found particularly suitable in children. (J THORAC CARDIOVASC SURG 1995;110:1725-31)
Temporary pacing leads are routinely used after cardiac operations. The electrical properties of these leads are important for a safe pacemaker function. We have previously shown that the braided multifilament wire electrodes have a high failure rate both for sensing of spontaneous heart activity and safe heart stimulation. 
1,2
The introduction of a temporary lead with a localized fixed surface area was a great improvement that offered superior pacing and sensing performance and minimized chances of dislodgement. 3,4 Still, commercially available leads have not been found suitable for small children because of a large diameter and poor pliability.
The purpose of the present study was to evaluate the efficacy of two new temporary pacing leads designed specifically for implantation in the atrial and ventricular muscle of the smaller pediatric heart and in the adult to be affixed directly within the atrial myocardium without the need for sutures or special techniques such as use of a silicon disk or atrial plication technique. 4,5
PATIENTS AND METHODS
Patients
The patients included in the study were 15 children and 15 adults undergoing cardiac operations. To qualify for the pediatric portion of the study the patients had to weigh 25 kg or less. They had a variety of congenital heart diseases (Table I). The age range was 3 months to 7 years, and the body weight varied between 4.4 and 20 kg. The 15 adult patients consisted of 14 men and 1 woman aged 59.1 ± 10.4 years (range 45 to 78 years). Thirteen patients underwent coronary bypass grafting (7 patients had 3-vessel disease, 6 had 2-vessel disease) and two underwent aortic valve replacement (1 aortic stenosis, 1 combined aortic stenosis and insufficiency).
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Methods
Leads studied (Fig 1)
The leads used have a small 135-degree (3/8 circle) curved needle with a length of 18 mm and a diameter of 0.4 mm. A soft 4-0 coiled polypropylene fiber is connected to the distal end of the electrode to serve as a fixation mechanism in the heart. The conductor consists of multifilament stainless steel strands coated with color-coded polyethylene insulation (yellow for pediatric wire [6491] and purple for adult wire [6492]). The diameter measured over the insulation is 0.5 mm. The stainless steel electrode diameter is 0.5 mm and its surface area is 4 mm 2 . A stainless steel needle is connected to the proximal end portion of the conductor to bring the conductor wire outside the thorax. The chest needle has a diameter of 0.7 mm and a length of 60 mm (pediatric) or 90 mm (adult). The needle is grooved to enable breakaway of the sharp section and is slightly curved distal to the breakaway marking to ease smooth chest passage. The remaining part functions as a contact sleeve for an electrocardiographic recorder or temporary pulse generator.
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Operative procedure
One pair of unipolar leads each was implanted in the interatrial groove or posteriorly in the right atrium and on the nonsystemic ventricle with an approximate spacing of 1 cm to obtain a bipolar configuration. All leads of each kind were implanted by the same surgeon (L. S. for the children, H. E. for the adults).
Measurements
Measurements were taken at the end of operation or immediately after admission to the intensive care unit, 3 to 6 hours after operation, and thereafter daily through explant.
Statistical methods
Threshold values are given both as mean plus or minus the standard deviation and as median values because of skewed distribution of the data with a few high values. Electrogram amplitudes, initial tissue resistance, and force used during lead removal are given as mean plus or minus the standard deviation. Student's t test was used for comparing the development in myocardial threshold, tissue resistance, and electrogram amplitudes over time. Differences were considered statistically significant at p < 0.05.
RESULTS
Overall, the results were favorable both for cardiac stimulation and sensing of spontaneous heart activity. The leads were used exclusively on the atria of the adult population. Only one male adult patient had an obvious dislocation, which occurred the first postoperative day as evidenced by both sensing failure and exit block. Data for this patient are excluded from further analyses. In four children intermittent sensing failures were observed in the atrial position in the early postoperative phase, and the ventricular leads functioned properly in all cases.
Myocardial threshold at constant current pacing (Table II)
With use of the constant current pulse generator, there was a threefold increase in myocardial threshold in the atrial position in children during the observation period, from a mean of 0.61 mA at implantation to 2.08 mA at explantation (p < 0.002). Although the values at implant were lower in the ventricle (mean 0.38 mA), the threshold increased about six times to a mean of 2.22 mA at explantation, which was also highly statistically significant (p < 0.002) and slightly higher compared with values in the atrium. The highest initial threshold values were found in the atrial position in the adults (mean 0.95 mA), and also in this group there was a threefold increase during the observation period (mean 2.76 mA at explantation, p < 0.002).
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(Table III)
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. Because the leads were applied in the bipolar fashion, the wire resistance equals 32 and thus represents a minor portion of the total resistance. The atrial tissue resistance was almost equal in children and in adults at implant. During the observation period there was a more marked and statistically significant drop in tissue resistance in the adults (p < 0.004) to a level of about 150 lower than that in the children and the value showed little variation after this initial drop through the study period.
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higher in the ventricle than in the atrium and it demonstrated a marked and statistically significant drop during the observation period (p < 0.05). Throughout the study the tissue resistances were about 100 higher in the ventricle than in the atrium for the children.
Electrogram characteristics (Table IV and Fig 3)
The atrial electrograms were of similar amplitude in the children and adults and showed little change in mean values over time (p = not significant). The ventricular electrograms showed a continuous and statistically significant drop through the observation period (p < 0.005).
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Removal of leads
At explant a dynamometer was used to measure the kilograms of force applied during removal of the leads. Because each pair of leads was tied together, they were removed simultaneously. The extraction force in the atrium varied between 0.08 and 0.83 kg (0.35 ± 0.18 kg, children) and 0.08 and 0.60 kg (0.34 ± 0.13 kg, adults). In the ventricle the force varied from 0.08 to 0.51 kg (0.26 ± 0.12 kg).
DISCUSSION
The primary goal of this study was to investigate the handling and mechanical and electrical properties of these new leads. Both in the atrium and ventricle adequate sensing and secure pacing could be obtained. With the exception of a 30 mm longer chest needle for the adults and color coding of the leads, the characteristics of the pediatric and adult versions of the leads are similar.
Handling and mechanical properties
The handling properties of the leads were advantageous. The maximal diameter of 0.5 mm of the lead, introduced with an even thinner distal needle, assured minimal trauma when the electrode was placed within the myocardial wall. No bleeding or dislocation was observed immediately after placement of the leads. This has been of special concern in infants and with thin atrial walls in adults. In some earlier models a silicon rubber disk has been recommended for the fixation of similar leads to the epicardium. 4,5 With the present models no additional fixation mechanism had to be used except for the retaining coil fastened to the distal end of the electrode. Obviously this coil did not result in additional damage to the tissue. Furthermore, the low force that had to be used during removal indicates a good balance in the memory of the retaining coil. A similar but somewhat higher extraction force had to be used when removing the leads from the atrium, both in children and adults, compared with the force required to remove the leads from the ventricles of the children. There was no indication of bleeding after lead removal. Although it has not been tested in this study, it is expected that the lead will function favorably also in the ventricle in the adult inasmuch as the electrode geometry and fixation mechanism are identical for the pediatric and adult lead versions.
Myocardial threshold
In none of the patients was there indication of wire breakage or insulation defects after a maximal observation period of 23 days. Compared with those in earlier studies these new leads showed similar values of myocardial thresholds for temporary heart wires with a controlled surface area. 4-7 Also, the myocardial threshold values showed little difference in the atrial and ventricular positions. Although there was a continuous increase in myocardial threshold values, the values were low with a high safety margin for pacing (20 mA for the constant current pulse generator, 10 V for the constant voltage pulse generator). There was somewhat higher atrial threshold values in the adult than in the pediatric hearts, which can possibly be explained by structural changes in the myocardium caused by long-standing heart disease or less direct contact with excitable myocardial tissue as a result of more fatty tissue in the interatrial groove.
Tissue resistance
The tissue resistance was of similar magnitude to that found for permanent pacemaker leads with a comparable surface area 8 and on average 200 to 300 higher than that found for earlier temporary electrode models with a localized stainless steel electrode with a surface area of 7.5 mm 2 . 3,4 This is of great importance to obtain an adequate current waveform during heart stimulation and thus secure optimal pacemaker function. This will furthermore decrease the current drain on the pulse generator, although this latter factor is of lesser importance in a temporary system. Also, the stable tissue resistance values throughout the study period indicated the integrity of the leads. There is no obvious explanation for the more marked drop and lower tissue resistance in the adults, because the electrodes are identical. Of note also is the higher tissue resistance found in the ventricular myocardium compared with that in the atrial myocardium.
Electrogram amplitudes
Atrial electrogram amplitudes were of similar size to those found for earlier models of temporary pacing leads in which different application techniques had been used. 3-5 In four children the atrial electrograms intermittently showed borderline amplitudes for adequate sensing (H0.5 mV) with the pulse generator used. In two or three of these cases this could be ascribed to temporary instability in the childrens' clinical condition. The ventricular electrogram findings were comparable to values found for permanent and temporary pacemaker leads. 3,9 In the ventricle there was a high sensitivity margin although there was a continuous drop in amplitude during the observation period.
Applicability of the leads
Heart wires can effectively be used for diagnosis and treatment of cardiac arrhythmias in the postoperative phase of cardiac operations. Not only can pacing be used in case of bradyarrhythmia to improve the hemodynamic situation, but also by use of these wires episodes of supraventricular and ventricular tachycardias can be treated immediately without additional invasive procedures. Furthermore, antiarrhythmic drugs with negative inotropic and potential proarrhythmic effects and direct cardioversion necessitating the use of general anesthesia can be avoided. These wires can also be used for electrophysiologic testing before hospital discharge: for example, in patients who have had surgery for ventricular tachycardia.
Recognition of atrioventricular synchrony
With the now available external DDD pacemaker it is important to have a reliable atrial lead to take advantage of improved hemodynamics of atrioventricular synchrony, especially for the patient with poor left ventricular function, reduced compliance, or complex congenital heart corrections (Fig. 3). Because the leads are applied in the bipolar fashion a high pulse generator sensitivity can be used with little risk for oversensing or cross-talk between the atrial and ventricular channels. Furthermore, interference from external sources seems unlikely.
Conclusion
The new temporary pacing leads have shown good performance in patients undergoing cardiac operations. Because of the slender construction and pliable texture of the leads trauma to the heart is minimal. The special memory coil secures safe positioning without additional fixation mechanisms. The leads seem particularly favorable for application in small children and for atrial application in the adult.
Acknowledgments
The support of Mr. Wim van Driel, Manager Heart Valve Department, Bakken Research Center, Maastricht, The Netherlands, is highly appreciated.
Footnotes
From Medical Department A a and the Surgical Institute,b University of Bergen, School of Medicine, Haukeland Sykehus, Bergen, Norway. 
References
This article has been cited by other articles:
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  S. Hasan and C. T. Lewis A New Method of Temporary Epicardial Atrioventricular Pacing Utilizing Bipolar Pacing Leads Ann. Thorac. Surg., April 1, 2005; 79(4): 1384 - 1387. [Abstract] [Full Text] [PDF]
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