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Transthoracic defibrillation: does electrode adhesive pad position alter transthoracic impedance?
Resuscitation 37 (1998) 139 – 143
Transthoracic defibrillation: does electrode adhesive pad position alter transthoracic impedance? Lawrence A. Garcia, Richard E. Kerber * Cardio6ascular Di6ision, Uni6ersity of Iowa Hospital, 200 Hawkins Dri6e, Iowa City, IA 52242, USA Received 1 September 1997; received in revised form 20 March 1998; accepted 20 March 1998
Abstract Successful termination of ventricular fibrillation by transthoracic shocks is dependent on achieving adequate current flow, which in turn is governed by transthoracic impedance (TTI). The American Heart Association (AHA) Advanced Cardiac Life Support textbook recommends three electrode positions for defibrillation: (1) anterior-apex, (2) apex-posterior and (3) anterior-posterior. However, there are few data available comparing TTI of these positions. To study this, we applied large (78 cm2) self-adhesive monitor-defibrillator pads to 20 subjects (ten male, ten female, ages 21 – 79) and measured TTI using a validated test-pulse technique which does not require actual shocks. We performed two studies. In Study 1 (all 20 subjects) the electrode pads were applied in the three positions recommended by the AHA, with the posterior electrode placed in the right infrascapular location. All TTI measurements were made at end-expiration and body surface area (BSA) was recorded. The results (TTI, V, mean 9S.D.) for the respective positions were the following: anterior-apex, 82.0 9 24.7; apex-posterior, 71.2 9 23.5; anterior-posterior, 77.0 9 24.7 (P NS). In Study 2 (six subjects) we compared the effect of right vs. left infrascapular posterior electrode placement (TTI, V): apex-right infrascapular (RIS), 76.8 9 18.4; apex-left infrascapular (LIS), 72.1 9 18.7; anterior-RIS, 72.5 919.4; anterior-LIS, 71.6 918.6 (P NS). Correlation of TTI (anterior-apex placement) with BSA: TTI = 15.9 (BSA)+ 46.7, r =0.60, P B0.01; the correlations of TTI and BSA were similar in the other two electrode positions. Thus, the three AHA-recommended electrode positions for transthoracic defibrillation have equivalent and acceptable TTIs; current flow should be similar using any of these positions. Furthermore, the posterior electrode may be placed in either the right or the left infrascapular position without affecting TTI. TTI is related to BSA in any of the three recommended positions; patients with high BSA and TTI may require higher energy selection to achieve defibrillation. © 1998 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Transthoracic impedance; Defibrillation; Electrodes
1. Introduction Successful termination of ventricular fibrillation by transthoracic defibrillation is dependent upon delivery of sufficient current; current flow is determined by the shock energy and by the transthoracic impedance (TTI). TTI is dependent upon multiple factors including chest size, electrode size and electrode/chest contact pressure and interface [1,2].
* Corresponding author. Tel.: +1 319 3562739; fax: + 1 319 3564552.
The American Heart Association (AHA) recommends three electrode positions for defibrillation: anterior-apex, anterior-posterior and apex-posterior [3]. However, there are only limited data available comparing TTI associated with these various electrode positions [4]. If there are systematic differences in TTI between these positions then the position with the lowest TTI might be preferred, as more current would be generated by equal shock energies. Furthermore, there are no data available comparing TTI of these various positions when using self-adhesive monitor-defibrillator electrode pads, an increasingly popular electrode technology which will become even more
0300-9572/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0300-9572(98)00050-1
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L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
common as automatic external defibrillators become widely available [5,6]. This study sought to determine any differences among TTI of the various electrode positions using self adhesive monitor-defibrillator pads.
2. Methods This study was approved by the Human Research Committee of the University of Iowa Hospitals and Clinics. Informed written consent was obtained from each subject. In Study 1, we applied large (78 cm2) Hewlett-Packard (HP M1749A) self-adhesive electrode defibrillation pads at the three AHA-recommended positions to 20 volunteer subjects (ten male and ten female; ages 21–79; median 42.19 18.2). Body surface area (BSA) ranged from 1.4 to 3.0 (median 1.8, mean 2.290.9) m2. Ten men and women over age 42 (median age) were patients seen in our cardiology clinic. The ten men and women under age 42 were volunteers obtained from fellow physicians and medical students. In Study 2 we used the same electrode pads in six subjects under age 42 (three new, three from the Study 1 patients).
2.1. Study 1 Three positions were compared for TTI in all 20 subjects. The anterior-apex position was defined as the anterior electrode positioned in the right parasternal position at the second intercostal space and the apex electrode positioned in the left mid-axillary line, 5th intercostal space. The anterior-posterior position was defined as the anterior electrode positioned in the left parasternal position, 2nd intercostal space and the posterior electrode placed in the right infra-scapular position at the 5th intercostal space. The apex-posterior position was defined as the apex electrode placed in the left mid-axillary line, at the 5th intercostal space, and the posterior electrode placed in the right infra-scapular position at the 5th intercostal space.
2.2. Study 2 Six subjects were studied to evaluate whether placing the posterior electrode in the right infrascapular (recommended by the AHA) [3] versus the left infrascapular position altered TTI. Four electrode pad positions were evaluated in Study 2: apex-right infrascapular, apex-left infrascapular, anterior-right infrascapular, anterior-left infrascapular. We verified that repeated applications of the same pads to the same position would not affect TTI by repeating the TTI measurements in the anterior-posterior position at the end of all data collection for each subject.
We measured TTI in each electrode position by using a previously described and validated test pulse method [7,8]. Briefly, this process predicts TTI by applying a 5-V, 3.25-kHz square wave current through the chest. The low level current flows between the self adhesive pads and is monitored by a microprocessor. TTI is predicted by comparison of the current achieved to known references. No actual shocks are delivered, and the low-level current is painless. In each position TTI was measured at end expiration for three measurements and the average was taken as a single data point and BSA was calculated. Standard univariate and multivariate analyses were used to compare TTI and the various positions. Data are presented as mean9 standard deviation. Statistics were derived through the SPSS (Chicago, IL) statistical program. PB 0.05 is considered significant.
3. Results The subjects ranged in age from 21 to 79 years (median 42.19 18.2).
3.1. Study 1 The TTI’s of the various positions are shown in Fig. 1. TTI was not statistically different among the various positions: anterior-apex, 82.0924.6 V; anterior-posterior (right infrascapular) 71.29 23.6 V; apex-posterior (right infrascapular) 77.0924.7 V; and for the repeated anterior-posterior measurement, 72.49 22.5 V (P= NS) (Fig. 1). There were no differences in TTI based on gender: male, 77.49 20.5 V; female, 74.2926.8 V (P=NS). Correlation of TTI with BSA revealed the following linear relationship, TTI= 15.9 (BSA)+ 46.7, r=0.60, P B0.01 for the anterior-apex electrode position. The other two electrode positions showed similar relationship of TTI and BSA (Table 1). The overall relationship of TTI with BSA was TTI =14.3 (BSA) + 43.8, r= 0.55, PB 0.001 (Table 1). Correlation of TTI with age revealed the following relationship, TTI=0.9 (age)+45.6, r= 0.61, PB0.005 for the anterior-apex electrode position. The other two positions showed similar relationship of TTI to age (Table 2). The overall relationship of TTI with age was TTI= 0.8 (age)+ 39.5, r=0.61, PB 0.001 (Table 2). When accounting for BSA there was a difference in TTI with age. Based on the median age of 42, TTI was 55.39 9.0 V for subjects under 42 years of age and 86.6913.7 V for subjects over 42 years of age, both with BSAB 1.8, PB 0.001. TTI was 64.29 19.6 V for subjects under 42 years of age and 93.19 21.9 V for subjects over 42 years of age, both with BSA\1.8, PB 0.001 without interaction between BSA and age (Fig. 2).
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
141
Fig. 1. TTI of the three AHA-recommended electrode positions. There were no significant differences between the three positions.
3.2. Study 2 The TTI of the four positions were not significantly different: apex-right infrascapular, 76.89 18.4 V; apexleft infrascapular, 72.1918.7 V; anterior right infrascapular 72.59 19.4, V; anterior-left infrascapular 71.6 918.6, V (P =NS).
4. Discussion The major finding of this study was that there were no significant differences in transthoracic impedance among the three electrode defibrillation positions recommended by the AHA anterior-apex, anterior-posterior and apex-posterior. Furthermore, placing the posterior electrode in either the left or right infrascapular position did not significantly affect transthoracic impedance. Achieving cardiac defibrillation is dependent upon adequate current flow, which in turn is dependent on the energy selected and the TTI. The TTI is determined by many factors including: chest size, electrode chest Table 1 Relationship of transthoracic impedance (V) to body surface area Electrode position
Regression equation
Correlation
Anterior-apex
TTI=15.9 (BSA)+46.7 TTI =15.0 (BSA)+37.8 TTI = 13.0 (BSA)+48.0 TTI = 14.3 (BSA)+43.8
r= 0.60
PB0.01
r= 0.60
PB0.01
r= 0.50
PB0.01
r= 0.55
PB0.001
Anterior-posterior Apex-posterior All positions
wall interface (i.e. contact pressure), electrode area and edge length, number of time between shocks and others [1,2,9–12]. Previous studies on TTI and electrode position used hand-held electrode paddles [4]. Using this older system in patients undergoing elective cardioversion of atrial arrhythmias, Kerber et al. [4] found no significant difference between TTI of anterior-apex versis anteriorposterior electrode placement. They noted a decline in TTI with larger electrode paddles. Our present study differs from this previous study in the use of the new self-adhesive pads rather than electrode paddles. The use of the self-adhesive pads eliminates the variable of hand-held electrode pressure as a TTI determinant [1]. Further, our electrode size was constant, unlike the previous study [8] which evaluated electrode paddles of varying size. Despite these differences we found no difference in TTI among the three commonly used defibrillation positions. This is consistent with the previous investigation’s conclusions. Most previous studies have placed posterior electrodes in the left infrascapular position [4,13,14]. The American Heart Association recommendations have been inconsistent; the 1992 AHA Guidelines for CPR Table 2 Relationship of transthoracic impedance to age
P value Electrode position
Regression equation
Correlation
P value
Anterior-apex
TTI=0.9 45.6 TTI =0.7 38.0 TTI =0.9 35.2 TTI=0.8 39.5
(age)+
r =0.61
PB0.005
(age)+
r =0.59
PB0.01
(age)+
r= 0.70
PB0.001
(age)+
r= 0.61
PB0.001
Anterior-posteior Apex-posterior
TTI, transthoracic impedance (V); BSA, body surface area (m2).
All positions
142
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
Fig. 2. TTI versus age subdivided by BSA. ‘A’ and ‘C’ refer to the first and third columns.
and Emergency Cardiac Care [3] recommend a right infrascapular position, while the 1994 AHA Textbook of Advanced Cardiac Life Support [15] describes a left infrascapular position. Does it matter? We compared left versus right infrascapular positions with the other electrode placed in anterior or apex locations, and found no significant difference. The posterior electrode may be placed in either the left or the right infrascapular location. Our study supports previous work that TTI is dependent on BSA [1,7]. Kerber et al. [1] showed TTI was related to BSA (using hand-held paddles), but the relationship was stronger with chest width; increased inter-electrode distance increased TTI. This study also shows an effect of age on TTI. This was not simply due to an increase in BSA with age: accounting for BSA, TTI was higher in older individuals. This finding may be due to age-related changes in body habitus (kyphoscoliosis) with an increase in anterior-posterior chest diameter, thus increasing TTI with age. Two significant limitations of this study are: (1) although equivalent TTI will result in equivalent transthoracic current flow among different electrode positions, transcardiac current flow may differ. The proportion of the transthoracic current which traverses the heart is not necessarily constant. Lerman [16] has demonstrated transcardiac current to be as low as 4% of the transthoracic current using hand-held electrode paddles placed in the anterior-apex position. (2) No actual shocks were given in this study; the effectiveness in terminating ventricular fibrillation of electrodes placed in different positions was not assessed. In summary, the various recommended and commonly used electrode positions for defibrillation yield equivalent TTI when self-adhesive monitor-defibrillator
electrode pads are used. Automatic external defibrillators use such electrode pads; any of the AHA-recommended electrode placement positions are appropriate for these devices.
Acknowledgements Supported in part by NHLBI grants HL07121 (LAG) and HL53284 (REK).
References [1] Kerber RE, Grayzel J, Hoyt R, Marcus M, Kennedy J. Transthoracic resistance in human defibrillation: influence of body weight, chest size, serial shocks, paddle size and paddle contact pressure. Circulation 1981;63:676 – 82. [2] Connell PN, Ewy GA, Dahl F, Ewy D. Transthoracic impedance to defibrillator discharge. Effect of electrode size and electrode-chest wall interface. J Electrocardiol 1973;6:313– 7. [3] American Heart Association guidelines for CPR and emergency cardiac care. J Am Med Assoc 1992;268:2171– 302. [4] Kerber RE, Jensen SR, Grayzel J, Kennedy J, Hoyt R. Elective cardioversion: Influence of paddle-electrode location and size on success rates and energy requirements. New Engl J Med 1981;305:658 – 62. [5] Kerber RE, Martins JB, Kelly K, Ferguson DW, Jensen S, Newman B, Parke JD, Kieso R, Melton J. Self-adhesive pre-applied electrode pads for defibrillation and cardioversion. J Am Coll Cardiol 1984;3:815 – 20. [6] Weisfeldt M, Kerber RE, McGoldrick RP, Moss AJ, Nichol G, Ornato JP, Palmer DG, Reigel B, Smith SC. American Heart Association report on the public access defibrillation conference. Circulation 1995;92:2740 – 7. [7] Geddes LA, Tacker WA, Schoenlein W, Mitton M, Grubbs S, Wilcox P. The prediction of the impedance of the thorax to defibrillating current. Med Instrum 1976;10:159 – 62. [8] Kerber RE, Kouba C, Martins JB, Kelly K, Low R, Hoyt R, Ferguson D, Bailey L, Bennett P, Charbonnier F. Advance
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143 prediction of transthoracic impedance in human defibrillation and cardioversion: importance of impedance in determining the success of low-energy shocks. Circulation 1984;70:303 – 8. [9] Tarren DS, Ewy GA. Relative contribution of paddle electrode area and edge length to transthoracic impedance from a DC defibrillator discharge. Med Instrum 1979;13:183–4. [10] Dahl CF, Ewy GA, Warner ED, Thomas ED. Myocardial necrosis from direct current countershock: Effect of paddle electrode size and time interval between discharges. Circulation 1974;50:956 – 61. [11] Geddes LA, Tacker W, Cabler D, Chapman R, Rivera R, Kedder H. The decrease in transthoracic impedance during successive
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ventricular defibrillation trials. Med Instrum 1975;9:179–80. [12] Sirna SJ, Ferguson DW, Charbonnier F, Kerber RE. Electrical cardioversion in humans: factors affecting transthoracic impedance. Am J Cardiol 1988;62:1048 – 52. [13] Lown B. Electrical reversion of cardiac arrhythmias. Br Heart J 1967;29:469 – 89. [14] Resnekov L, McDonald L. Appraisal of electroversion and treatment of cardiac dysrhythmias. Br Heart J 1968;30:786–811. [15] American Heart Assocation Textbook of Advanced Cardiac Life Support, 3rd ed. Dallas, TX: AHA, 1994. [16] Lerman BB, Deale OS. Relation between transcardiac and transthoracic current during defibrillation in humans. Circ Res 1990;67:1420 – 6.
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Transthoracic defibrillation: does electrode adhesive pad position alter transthoracic impedance? Lawrence A. Garcia, Richard E. Kerber * Cardio6ascular Di6ision, Uni6ersity of Iowa Hospital, 200 Hawkins Dri6e, Iowa City, IA 52242, USA Received 1 September 1997; received in revised form 20 March 1998; accepted 20 March 1998
Abstract Successful termination of ventricular fibrillation by transthoracic shocks is dependent on achieving adequate current flow, which in turn is governed by transthoracic impedance (TTI). The American Heart Association (AHA) Advanced Cardiac Life Support textbook recommends three electrode positions for defibrillation: (1) anterior-apex, (2) apex-posterior and (3) anterior-posterior. However, there are few data available comparing TTI of these positions. To study this, we applied large (78 cm2) self-adhesive monitor-defibrillator pads to 20 subjects (ten male, ten female, ages 21 – 79) and measured TTI using a validated test-pulse technique which does not require actual shocks. We performed two studies. In Study 1 (all 20 subjects) the electrode pads were applied in the three positions recommended by the AHA, with the posterior electrode placed in the right infrascapular location. All TTI measurements were made at end-expiration and body surface area (BSA) was recorded. The results (TTI, V, mean 9S.D.) for the respective positions were the following: anterior-apex, 82.0 9 24.7; apex-posterior, 71.2 9 23.5; anterior-posterior, 77.0 9 24.7 (P NS). In Study 2 (six subjects) we compared the effect of right vs. left infrascapular posterior electrode placement (TTI, V): apex-right infrascapular (RIS), 76.8 9 18.4; apex-left infrascapular (LIS), 72.1 9 18.7; anterior-RIS, 72.5 919.4; anterior-LIS, 71.6 918.6 (P NS). Correlation of TTI (anterior-apex placement) with BSA: TTI = 15.9 (BSA)+ 46.7, r =0.60, P B0.01; the correlations of TTI and BSA were similar in the other two electrode positions. Thus, the three AHA-recommended electrode positions for transthoracic defibrillation have equivalent and acceptable TTIs; current flow should be similar using any of these positions. Furthermore, the posterior electrode may be placed in either the right or the left infrascapular position without affecting TTI. TTI is related to BSA in any of the three recommended positions; patients with high BSA and TTI may require higher energy selection to achieve defibrillation. © 1998 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Transthoracic impedance; Defibrillation; Electrodes
1. Introduction Successful termination of ventricular fibrillation by transthoracic defibrillation is dependent upon delivery of sufficient current; current flow is determined by the shock energy and by the transthoracic impedance (TTI). TTI is dependent upon multiple factors including chest size, electrode size and electrode/chest contact pressure and interface [1,2].
* Corresponding author. Tel.: +1 319 3562739; fax: + 1 319 3564552.
The American Heart Association (AHA) recommends three electrode positions for defibrillation: anterior-apex, anterior-posterior and apex-posterior [3]. However, there are only limited data available comparing TTI associated with these various electrode positions [4]. If there are systematic differences in TTI between these positions then the position with the lowest TTI might be preferred, as more current would be generated by equal shock energies. Furthermore, there are no data available comparing TTI of these various positions when using self-adhesive monitor-defibrillator electrode pads, an increasingly popular electrode technology which will become even more
0300-9572/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0300-9572(98)00050-1
140
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
common as automatic external defibrillators become widely available [5,6]. This study sought to determine any differences among TTI of the various electrode positions using self adhesive monitor-defibrillator pads.
2. Methods This study was approved by the Human Research Committee of the University of Iowa Hospitals and Clinics. Informed written consent was obtained from each subject. In Study 1, we applied large (78 cm2) Hewlett-Packard (HP M1749A) self-adhesive electrode defibrillation pads at the three AHA-recommended positions to 20 volunteer subjects (ten male and ten female; ages 21–79; median 42.19 18.2). Body surface area (BSA) ranged from 1.4 to 3.0 (median 1.8, mean 2.290.9) m2. Ten men and women over age 42 (median age) were patients seen in our cardiology clinic. The ten men and women under age 42 were volunteers obtained from fellow physicians and medical students. In Study 2 we used the same electrode pads in six subjects under age 42 (three new, three from the Study 1 patients).
2.1. Study 1 Three positions were compared for TTI in all 20 subjects. The anterior-apex position was defined as the anterior electrode positioned in the right parasternal position at the second intercostal space and the apex electrode positioned in the left mid-axillary line, 5th intercostal space. The anterior-posterior position was defined as the anterior electrode positioned in the left parasternal position, 2nd intercostal space and the posterior electrode placed in the right infra-scapular position at the 5th intercostal space. The apex-posterior position was defined as the apex electrode placed in the left mid-axillary line, at the 5th intercostal space, and the posterior electrode placed in the right infra-scapular position at the 5th intercostal space.
2.2. Study 2 Six subjects were studied to evaluate whether placing the posterior electrode in the right infrascapular (recommended by the AHA) [3] versus the left infrascapular position altered TTI. Four electrode pad positions were evaluated in Study 2: apex-right infrascapular, apex-left infrascapular, anterior-right infrascapular, anterior-left infrascapular. We verified that repeated applications of the same pads to the same position would not affect TTI by repeating the TTI measurements in the anterior-posterior position at the end of all data collection for each subject.
We measured TTI in each electrode position by using a previously described and validated test pulse method [7,8]. Briefly, this process predicts TTI by applying a 5-V, 3.25-kHz square wave current through the chest. The low level current flows between the self adhesive pads and is monitored by a microprocessor. TTI is predicted by comparison of the current achieved to known references. No actual shocks are delivered, and the low-level current is painless. In each position TTI was measured at end expiration for three measurements and the average was taken as a single data point and BSA was calculated. Standard univariate and multivariate analyses were used to compare TTI and the various positions. Data are presented as mean9 standard deviation. Statistics were derived through the SPSS (Chicago, IL) statistical program. PB 0.05 is considered significant.
3. Results The subjects ranged in age from 21 to 79 years (median 42.19 18.2).
3.1. Study 1 The TTI’s of the various positions are shown in Fig. 1. TTI was not statistically different among the various positions: anterior-apex, 82.0924.6 V; anterior-posterior (right infrascapular) 71.29 23.6 V; apex-posterior (right infrascapular) 77.0924.7 V; and for the repeated anterior-posterior measurement, 72.49 22.5 V (P= NS) (Fig. 1). There were no differences in TTI based on gender: male, 77.49 20.5 V; female, 74.2926.8 V (P=NS). Correlation of TTI with BSA revealed the following linear relationship, TTI= 15.9 (BSA)+ 46.7, r=0.60, P B0.01 for the anterior-apex electrode position. The other two electrode positions showed similar relationship of TTI and BSA (Table 1). The overall relationship of TTI with BSA was TTI =14.3 (BSA) + 43.8, r= 0.55, PB 0.001 (Table 1). Correlation of TTI with age revealed the following relationship, TTI=0.9 (age)+45.6, r= 0.61, PB0.005 for the anterior-apex electrode position. The other two positions showed similar relationship of TTI to age (Table 2). The overall relationship of TTI with age was TTI= 0.8 (age)+ 39.5, r=0.61, PB 0.001 (Table 2). When accounting for BSA there was a difference in TTI with age. Based on the median age of 42, TTI was 55.39 9.0 V for subjects under 42 years of age and 86.6913.7 V for subjects over 42 years of age, both with BSAB 1.8, PB 0.001. TTI was 64.29 19.6 V for subjects under 42 years of age and 93.19 21.9 V for subjects over 42 years of age, both with BSA\1.8, PB 0.001 without interaction between BSA and age (Fig. 2).
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
141
Fig. 1. TTI of the three AHA-recommended electrode positions. There were no significant differences between the three positions.
3.2. Study 2 The TTI of the four positions were not significantly different: apex-right infrascapular, 76.89 18.4 V; apexleft infrascapular, 72.1918.7 V; anterior right infrascapular 72.59 19.4, V; anterior-left infrascapular 71.6 918.6, V (P =NS).
4. Discussion The major finding of this study was that there were no significant differences in transthoracic impedance among the three electrode defibrillation positions recommended by the AHA anterior-apex, anterior-posterior and apex-posterior. Furthermore, placing the posterior electrode in either the left or right infrascapular position did not significantly affect transthoracic impedance. Achieving cardiac defibrillation is dependent upon adequate current flow, which in turn is dependent on the energy selected and the TTI. The TTI is determined by many factors including: chest size, electrode chest Table 1 Relationship of transthoracic impedance (V) to body surface area Electrode position
Regression equation
Correlation
Anterior-apex
TTI=15.9 (BSA)+46.7 TTI =15.0 (BSA)+37.8 TTI = 13.0 (BSA)+48.0 TTI = 14.3 (BSA)+43.8
r= 0.60
PB0.01
r= 0.60
PB0.01
r= 0.50
PB0.01
r= 0.55
PB0.001
Anterior-posterior Apex-posterior All positions
wall interface (i.e. contact pressure), electrode area and edge length, number of time between shocks and others [1,2,9–12]. Previous studies on TTI and electrode position used hand-held electrode paddles [4]. Using this older system in patients undergoing elective cardioversion of atrial arrhythmias, Kerber et al. [4] found no significant difference between TTI of anterior-apex versis anteriorposterior electrode placement. They noted a decline in TTI with larger electrode paddles. Our present study differs from this previous study in the use of the new self-adhesive pads rather than electrode paddles. The use of the self-adhesive pads eliminates the variable of hand-held electrode pressure as a TTI determinant [1]. Further, our electrode size was constant, unlike the previous study [8] which evaluated electrode paddles of varying size. Despite these differences we found no difference in TTI among the three commonly used defibrillation positions. This is consistent with the previous investigation’s conclusions. Most previous studies have placed posterior electrodes in the left infrascapular position [4,13,14]. The American Heart Association recommendations have been inconsistent; the 1992 AHA Guidelines for CPR Table 2 Relationship of transthoracic impedance to age
P value Electrode position
Regression equation
Correlation
P value
Anterior-apex
TTI=0.9 45.6 TTI =0.7 38.0 TTI =0.9 35.2 TTI=0.8 39.5
(age)+
r =0.61
PB0.005
(age)+
r =0.59
PB0.01
(age)+
r= 0.70
PB0.001
(age)+
r= 0.61
PB0.001
Anterior-posteior Apex-posterior
TTI, transthoracic impedance (V); BSA, body surface area (m2).
All positions
142
L.A. Garcia, R.E. Kerber / Resuscitation 37 (1998) 139–143
Fig. 2. TTI versus age subdivided by BSA. ‘A’ and ‘C’ refer to the first and third columns.
and Emergency Cardiac Care [3] recommend a right infrascapular position, while the 1994 AHA Textbook of Advanced Cardiac Life Support [15] describes a left infrascapular position. Does it matter? We compared left versus right infrascapular positions with the other electrode placed in anterior or apex locations, and found no significant difference. The posterior electrode may be placed in either the left or the right infrascapular location. Our study supports previous work that TTI is dependent on BSA [1,7]. Kerber et al. [1] showed TTI was related to BSA (using hand-held paddles), but the relationship was stronger with chest width; increased inter-electrode distance increased TTI. This study also shows an effect of age on TTI. This was not simply due to an increase in BSA with age: accounting for BSA, TTI was higher in older individuals. This finding may be due to age-related changes in body habitus (kyphoscoliosis) with an increase in anterior-posterior chest diameter, thus increasing TTI with age. Two significant limitations of this study are: (1) although equivalent TTI will result in equivalent transthoracic current flow among different electrode positions, transcardiac current flow may differ. The proportion of the transthoracic current which traverses the heart is not necessarily constant. Lerman [16] has demonstrated transcardiac current to be as low as 4% of the transthoracic current using hand-held electrode paddles placed in the anterior-apex position. (2) No actual shocks were given in this study; the effectiveness in terminating ventricular fibrillation of electrodes placed in different positions was not assessed. In summary, the various recommended and commonly used electrode positions for defibrillation yield equivalent TTI when self-adhesive monitor-defibrillator
electrode pads are used. Automatic external defibrillators use such electrode pads; any of the AHA-recommended electrode placement positions are appropriate for these devices.
Acknowledgements Supported in part by NHLBI grants HL07121 (LAG) and HL53284 (REK).
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