Portal de Periódicos da CAPES

External Defibrillation: The Need for Uniformity in Analyzing and Reporting Results

1998; Elsevier BV; Volume: 32; Issue: 2 Linguagem: Inglês

10.1016/s0196-0644(98)70141-6

1097-6760

Roger D. White,

Cardiac electrophysiology and arrhythmias

See related article, p 191.External Defibrillation: The Need for Uniformity in Analyzing and Reporting Results[White RD: External defibrillation: The need for uniformity in analyzing and reporting results. Ann Emerg Med July 1998;32: 234-236.]The rapidly growing initiative to make early defibrillation readily available is best exemplified by the American Heart Association's proposal to conduct a multisite, prospective, randomized controlled clinical trial of public access defibrillation.1Nichol G Hallstrom AP Kerber R et al.American Heart Association report on the second public access defibrillation conference, April 17-19, 1997.Circulation. 1998; 97: 1309-1314Crossref PubMed Scopus (105) Google Scholar The prospect of widespread dissemination of automated external defibrillators (AEDs) in both traditional and nontraditional settings provokes the compelling need to identify and quantitate variables that potentially can enhance the efficacy of deployment of these devices. EMS systems issues immediately come to mind in this context, but so also do AED performance considerations. Optimum waveform configuration, shock energy, safe and effective current, and the effect of impedance to current flow on shock success are the major components that require ongoing evaluation in the setting of prehospital cardiac arrest, where these devices will be used most commonly.In this issue of Annals, Heavens and colleagues report their observations on the effects of transthoracic impedance (TTI) and calculated peak current (Ip) on the response to defibrillation shocks during prehospital cardiac arrest with ventricular fibrillation (VF) as the presenting rhythm.2Heavens JP Cleland MJ Maloney JP et al.The effects of transthoracic impedance and peak current flow on defibrillation success in a prehospital setting.Ann Emerg Med. 1998; 32: 191-199Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar They delivered 717 shocks to 310 patients using AEDs with a monophasic damped sine (MDS) waveform and assessed the effect of TTI and calculated Ip on shock success and patient outcome. To calculate Ip it was necessary to derive peak voltage measurements "on the bench" by discharging representative AEDs into an adjustable resistor with resistances ranging from 40 ohms (Ω) to 200 Ω in 20-Ω intervals. Impedance measurements were obtained from the AED impedance oscillator, which passes a 100-microampere (μA) 32-kHz current between the electrode pads, giving rise to a voltage dependent on patient impedance. The range of impedance accepted for shock delivery with this AED is 30 to 200 Ω. The impedance measurement is made just before delivery of the shock, but this method has been reported to correlate closely with impedance during actual shock delivery.3Cunningham SR Magee H Anderson J et al.Patients in ventricular fibrillation—predicted and actual transthoracic impedance and peak current at defibrillation [abstract].PACE. 1987; 10: 975Google Scholar, 4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Using Ohm's law and inserting the voltage derived from the bench test and the predicted impedance measurement, a calculated Ip was derived. The validity of the methodology used to obtain TTI and calculated Ip, although lacking the precision of direct measurements, is sufficiently accurate in this context. From these extrapolations, measurements, and calculations it was concluded that neither Ip nor TTI was a determinant of shock success, nor was either variable associated with patient survival. Age, witnessed arrest, and bystander CPR did not influence survival significantly; however, only 33 of the 310 patients (11%) survived and 30 of these 33 survivors had sustained a witnessed arrest. The effect of witnessed arrest and bystander CPR may have been obscured by the very small number of survivors. The relatively long call-to-shock time (10.9±4.4 minutes for all 310 patients) probably contributed to the low survival rate, although without details on synchronization of times the accuracy of this time interval is uncertain.What can we conclude from this study? First, the definition of shock success was the termination of VF and restoration of an organized rhythm. By this definition the first shock resulted in conversion in 25.5% (79/310) of patients. Conversion with shocks two and three was 24% and 22.5%, respectively. This is a low shock success rate, but part of the explanation for this likely resides in the definition of shock success chosen by the authors. We and others have defined defibrillation as the conversion of VF to any other rhythm or to asystole,5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar, 6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar, 8White RD Early out-of-hospital experience with an impedance-compensating low-energy biphasic waveform automatic external defibrillator.J Int Cardiol Electrophysiol. 1997; 1: 203-208Crossref PubMed Scopus (46) Google Scholar believing that electrophysiologically the term "defibrillation" means termination of ventricular (or atrial) fibrillation, and does not specify the rhythm that ensues after termination of fibrillation. By this definition in one study the first 200-joule (J) MDS shock was successful in 60% (42/70) of patients, and the cumulative success rate with two 200-J shocks was 74.3% (52/70).5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar Of the 70 patients, 17 had had a prehospital arrest. In another study conducted with prehospital arrest patients only and using self-adhesive pad electrodes, first shock success with the MDS waveform was 70% (40% to asystole and 30% to an organized rhythm).6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar In a study by Weaver et al7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar using handheld paddle electrodes, both 175-J and 320-J MDS shocks were successful in 61%, with conversions to asystole (15%) and organized rhythm (46%). In addition to different definitions of successful defibrillation, postshock rhythms evolve in a dynamic manner in time to other rhythms, necessitating inclusion of time points in the definition of shock success. None of these considerations has ever been standardized, as is evident in these studies and in the widely disparate reported shock success rates. It is mandatory that uniform definitions of shock outcomes and postshock rhythms be developed, accepted, and applied by all investigators. Until that occurs it will be difficult to interpret or to compare shock outcomes among studies and among different AEDs. A recently published American Heart Association Scientific Statement on biphasic waveform defibrillation also acknowledges this dilemma.9Cummins RO Hazinski MF Kerber RE et al.Low-energy biphasic waveform defibrillation: Evidence-based review applied to emergency cardiovascular care guidelines.Circulation. 1998; 97: 1654-1667Crossref PubMed Scopus (85) Google Scholar Survival to hospital discharge and restoration of spontaneous circulation are erroneous measures of defibrillation shock efficacy10Cummins RO Eisenberg MS Hallstrom AP et al.What is a "save"?: Outcome measures in clinical evaluations of automatic external defibrillations.Am Heart J. 1985; 110: 1133-1138Abstract Full Text PDF PubMed Scopus (20) Google Scholar because both are determined by several variables unrelated to the defibrillation shock, most importantly patient pathophysiologic substrate and EMS system performance. Without doubt the patient population is the weakest link in the chain of survival.11Stratton S Niemann JT Effects of adding links to "the chain of survival" for prehospital cardiac arrest: A contrast in outcomes in 1975 and 1995 at a single institution.Ann Emerg Med. 1998; 31: 471-477Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar Thus the critical measure of shock efficacy is whether or not VF is terminated, but it is essential that uniform definitions of termination of VF and therefore of shock success be developed and applied by all investigators. The urgent need for such uniformity of definitions of postshock rhythms is obvious in light of expanding implementation of early defibrillation programs and the evolution of new defibrillation waveforms.Second, one must be aware that all of these observations by Heavens et al, and those reported and reviewed here, were made with AEDs delivering MDS waveform shocks and with 175 to 200 J as the initial dose of energy. It is likely that this specific waveform at these energy settings provides sufficient current to defibrillate most patients in the presence of relatively high TTI. Kerber and associates4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar observed that although shocks of 100 J or less did not defibrillate satisfactorily in patients with high TTI (>97 Ω), shocks of 200 J or more did so.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar The higher energy thus provided sufficient current to defibrillate even in the face of high impedance. In the study by Heavens, we are not told how many patients had a high TTI, in whom shock success might have been decreased. Dalzell and Adgey5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar observed that shock success for the first episode of VF using 200 J MDS shocks was similar in patients with high TTI (>115 Ω) and those with TTI less than 115 Ω.5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar This is consistent with the findings of Kerber and colleagues.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Much larger studies likely would identify a small subset of patients with very high impedance for whom 200-J or even higher energy shocks would not achieve the current threshold required for defibrillation. Again, all of these observations have been made in patients receiving MDS shocks. Early experience with an impedance-compensating biphasic truncated exponential (BTE) waveform suggests that this waveform provides sufficient current to achieve defibrillation in patients with high TTI.12White RD Gliner BE Transthoracic impedance does not affect defibrillation efficacy in cardiac arrest victims when shocks are delivered with an impedance-compensating biphasic waveform.Circulation. 1997; 96: 1-561Google Scholar, 13Poole JE White RD Kanz K-G et al.Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest.J Cardiovasc Electrophysiol. 1997; 8: 1373-1385Crossref PubMed Scopus (116) Google Scholar More experience is needed to confirm these findings in the clinical setting.For most patients with prehospital VF arrest system factors, particularly delays to defibrillation, determine patient outcome.14Valenzuela TD Roe DJ Cretin S et al.Estimating effectiveness of cardiac arrest interventions.Circulation. 1997; 96: 3308-3313Crossref PubMed Scopus (655) Google Scholar Although there likely are patients with very high TTI who cannot be defibrillated with MDS waveforms at 200 J or higher, available evidence supports the conclusion that for the apparent majority of patients, TTI does not contribute significantly to shock success. It is of paramount importance that implementation of early defibrillation programs be accompanied by a commitment to collect accurate shock data, including impedance measurements, to define with more certainty the role that TTI may play in shock outcomes as new waveforms evolve and take their place in AEDs. It is also essential that uniform descriptors of defibrillation and postshock rhythms be developed by a consensus group and then applied uniformly by all clinical investigators as more data are collected and reported defining the effect of early defibrillation and new waveforms in a variety of settings. These are challenges that necessitate resolution if defibrillation is to develop in a controlled and scientific context. See related article, p 191. External Defibrillation: The Need for Uniformity in Analyzing and Reporting Results[White RD: External defibrillation: The need for uniformity in analyzing and reporting results. Ann Emerg Med July 1998;32: 234-236.]The rapidly growing initiative to make early defibrillation readily available is best exemplified by the American Heart Association's proposal to conduct a multisite, prospective, randomized controlled clinical trial of public access defibrillation.1Nichol G Hallstrom AP Kerber R et al.American Heart Association report on the second public access defibrillation conference, April 17-19, 1997.Circulation. 1998; 97: 1309-1314Crossref PubMed Scopus (105) Google Scholar The prospect of widespread dissemination of automated external defibrillators (AEDs) in both traditional and nontraditional settings provokes the compelling need to identify and quantitate variables that potentially can enhance the efficacy of deployment of these devices. EMS systems issues immediately come to mind in this context, but so also do AED performance considerations. Optimum waveform configuration, shock energy, safe and effective current, and the effect of impedance to current flow on shock success are the major components that require ongoing evaluation in the setting of prehospital cardiac arrest, where these devices will be used most commonly.In this issue of Annals, Heavens and colleagues report their observations on the effects of transthoracic impedance (TTI) and calculated peak current (Ip) on the response to defibrillation shocks during prehospital cardiac arrest with ventricular fibrillation (VF) as the presenting rhythm.2Heavens JP Cleland MJ Maloney JP et al.The effects of transthoracic impedance and peak current flow on defibrillation success in a prehospital setting.Ann Emerg Med. 1998; 32: 191-199Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar They delivered 717 shocks to 310 patients using AEDs with a monophasic damped sine (MDS) waveform and assessed the effect of TTI and calculated Ip on shock success and patient outcome. To calculate Ip it was necessary to derive peak voltage measurements "on the bench" by discharging representative AEDs into an adjustable resistor with resistances ranging from 40 ohms (Ω) to 200 Ω in 20-Ω intervals. Impedance measurements were obtained from the AED impedance oscillator, which passes a 100-microampere (μA) 32-kHz current between the electrode pads, giving rise to a voltage dependent on patient impedance. The range of impedance accepted for shock delivery with this AED is 30 to 200 Ω. The impedance measurement is made just before delivery of the shock, but this method has been reported to correlate closely with impedance during actual shock delivery.3Cunningham SR Magee H Anderson J et al.Patients in ventricular fibrillation—predicted and actual transthoracic impedance and peak current at defibrillation [abstract].PACE. 1987; 10: 975Google Scholar, 4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Using Ohm's law and inserting the voltage derived from the bench test and the predicted impedance measurement, a calculated Ip was derived. The validity of the methodology used to obtain TTI and calculated Ip, although lacking the precision of direct measurements, is sufficiently accurate in this context. From these extrapolations, measurements, and calculations it was concluded that neither Ip nor TTI was a determinant of shock success, nor was either variable associated with patient survival. Age, witnessed arrest, and bystander CPR did not influence survival significantly; however, only 33 of the 310 patients (11%) survived and 30 of these 33 survivors had sustained a witnessed arrest. The effect of witnessed arrest and bystander CPR may have been obscured by the very small number of survivors. The relatively long call-to-shock time (10.9±4.4 minutes for all 310 patients) probably contributed to the low survival rate, although without details on synchronization of times the accuracy of this time interval is uncertain.What can we conclude from this study? First, the definition of shock success was the termination of VF and restoration of an organized rhythm. By this definition the first shock resulted in conversion in 25.5% (79/310) of patients. Conversion with shocks two and three was 24% and 22.5%, respectively. This is a low shock success rate, but part of the explanation for this likely resides in the definition of shock success chosen by the authors. We and others have defined defibrillation as the conversion of VF to any other rhythm or to asystole,5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar, 6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar, 8White RD Early out-of-hospital experience with an impedance-compensating low-energy biphasic waveform automatic external defibrillator.J Int Cardiol Electrophysiol. 1997; 1: 203-208Crossref PubMed Scopus (46) Google Scholar believing that electrophysiologically the term "defibrillation" means termination of ventricular (or atrial) fibrillation, and does not specify the rhythm that ensues after termination of fibrillation. By this definition in one study the first 200-joule (J) MDS shock was successful in 60% (42/70) of patients, and the cumulative success rate with two 200-J shocks was 74.3% (52/70).5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar Of the 70 patients, 17 had had a prehospital arrest. In another study conducted with prehospital arrest patients only and using self-adhesive pad electrodes, first shock success with the MDS waveform was 70% (40% to asystole and 30% to an organized rhythm).6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar In a study by Weaver et al7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar using handheld paddle electrodes, both 175-J and 320-J MDS shocks were successful in 61%, with conversions to asystole (15%) and organized rhythm (46%). In addition to different definitions of successful defibrillation, postshock rhythms evolve in a dynamic manner in time to other rhythms, necessitating inclusion of time points in the definition of shock success. None of these considerations has ever been standardized, as is evident in these studies and in the widely disparate reported shock success rates. It is mandatory that uniform definitions of shock outcomes and postshock rhythms be developed, accepted, and applied by all investigators. Until that occurs it will be difficult to interpret or to compare shock outcomes among studies and among different AEDs. A recently published American Heart Association Scientific Statement on biphasic waveform defibrillation also acknowledges this dilemma.9Cummins RO Hazinski MF Kerber RE et al.Low-energy biphasic waveform defibrillation: Evidence-based review applied to emergency cardiovascular care guidelines.Circulation. 1998; 97: 1654-1667Crossref PubMed Scopus (85) Google Scholar Survival to hospital discharge and restoration of spontaneous circulation are erroneous measures of defibrillation shock efficacy10Cummins RO Eisenberg MS Hallstrom AP et al.What is a "save"?: Outcome measures in clinical evaluations of automatic external defibrillations.Am Heart J. 1985; 110: 1133-1138Abstract Full Text PDF PubMed Scopus (20) Google Scholar because both are determined by several variables unrelated to the defibrillation shock, most importantly patient pathophysiologic substrate and EMS system performance. Without doubt the patient population is the weakest link in the chain of survival.11Stratton S Niemann JT Effects of adding links to "the chain of survival" for prehospital cardiac arrest: A contrast in outcomes in 1975 and 1995 at a single institution.Ann Emerg Med. 1998; 31: 471-477Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar Thus the critical measure of shock efficacy is whether or not VF is terminated, but it is essential that uniform definitions of termination of VF and therefore of shock success be developed and applied by all investigators. The urgent need for such uniformity of definitions of postshock rhythms is obvious in light of expanding implementation of early defibrillation programs and the evolution of new defibrillation waveforms.Second, one must be aware that all of these observations by Heavens et al, and those reported and reviewed here, were made with AEDs delivering MDS waveform shocks and with 175 to 200 J as the initial dose of energy. It is likely that this specific waveform at these energy settings provides sufficient current to defibrillate most patients in the presence of relatively high TTI. Kerber and associates4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar observed that although shocks of 100 J or less did not defibrillate satisfactorily in patients with high TTI (>97 Ω), shocks of 200 J or more did so.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar The higher energy thus provided sufficient current to defibrillate even in the face of high impedance. In the study by Heavens, we are not told how many patients had a high TTI, in whom shock success might have been decreased. Dalzell and Adgey5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar observed that shock success for the first episode of VF using 200 J MDS shocks was similar in patients with high TTI (>115 Ω) and those with TTI less than 115 Ω.5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar This is consistent with the findings of Kerber and colleagues.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Much larger studies likely would identify a small subset of patients with very high impedance for whom 200-J or even higher energy shocks would not achieve the current threshold required for defibrillation. Again, all of these observations have been made in patients receiving MDS shocks. Early experience with an impedance-compensating biphasic truncated exponential (BTE) waveform suggests that this waveform provides sufficient current to achieve defibrillation in patients with high TTI.12White RD Gliner BE Transthoracic impedance does not affect defibrillation efficacy in cardiac arrest victims when shocks are delivered with an impedance-compensating biphasic waveform.Circulation. 1997; 96: 1-561Google Scholar, 13Poole JE White RD Kanz K-G et al.Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest.J Cardiovasc Electrophysiol. 1997; 8: 1373-1385Crossref PubMed Scopus (116) Google Scholar More experience is needed to confirm these findings in the clinical setting.For most patients with prehospital VF arrest system factors, particularly delays to defibrillation, determine patient outcome.14Valenzuela TD Roe DJ Cretin S et al.Estimating effectiveness of cardiac arrest interventions.Circulation. 1997; 96: 3308-3313Crossref PubMed Scopus (655) Google Scholar Although there likely are patients with very high TTI who cannot be defibrillated with MDS waveforms at 200 J or higher, available evidence supports the conclusion that for the apparent majority of patients, TTI does not contribute significantly to shock success. It is of paramount importance that implementation of early defibrillation programs be accompanied by a commitment to collect accurate shock data, including impedance measurements, to define with more certainty the role that TTI may play in shock outcomes as new waveforms evolve and take their place in AEDs. It is also essential that uniform descriptors of defibrillation and postshock rhythms be developed by a consensus group and then applied uniformly by all clinical investigators as more data are collected and reported defining the effect of early defibrillation and new waveforms in a variety of settings. These are challenges that necessitate resolution if defibrillation is to develop in a controlled and scientific context. [White RD: External defibrillation: The need for uniformity in analyzing and reporting results. Ann Emerg Med July 1998;32: 234-236.] The rapidly growing initiative to make early defibrillation readily available is best exemplified by the American Heart Association's proposal to conduct a multisite, prospective, randomized controlled clinical trial of public access defibrillation.1Nichol G Hallstrom AP Kerber R et al.American Heart Association report on the second public access defibrillation conference, April 17-19, 1997.Circulation. 1998; 97: 1309-1314Crossref PubMed Scopus (105) Google Scholar The prospect of widespread dissemination of automated external defibrillators (AEDs) in both traditional and nontraditional settings provokes the compelling need to identify and quantitate variables that potentially can enhance the efficacy of deployment of these devices. EMS systems issues immediately come to mind in this context, but so also do AED performance considerations. Optimum waveform configuration, shock energy, safe and effective current, and the effect of impedance to current flow on shock success are the major components that require ongoing evaluation in the setting of prehospital cardiac arrest, where these devices will be used most commonly. In this issue of Annals, Heavens and colleagues report their observations on the effects of transthoracic impedance (TTI) and calculated peak current (Ip) on the response to defibrillation shocks during prehospital cardiac arrest with ventricular fibrillation (VF) as the presenting rhythm.2Heavens JP Cleland MJ Maloney JP et al.The effects of transthoracic impedance and peak current flow on defibrillation success in a prehospital setting.Ann Emerg Med. 1998; 32: 191-199Abstract Full Text Full Text PDF PubMed Scopus (18) Google Scholar They delivered 717 shocks to 310 patients using AEDs with a monophasic damped sine (MDS) waveform and assessed the effect of TTI and calculated Ip on shock success and patient outcome. To calculate Ip it was necessary to derive peak voltage measurements "on the bench" by discharging representative AEDs into an adjustable resistor with resistances ranging from 40 ohms (Ω) to 200 Ω in 20-Ω intervals. Impedance measurements were obtained from the AED impedance oscillator, which passes a 100-microampere (μA) 32-kHz current between the electrode pads, giving rise to a voltage dependent on patient impedance. The range of impedance accepted for shock delivery with this AED is 30 to 200 Ω. The impedance measurement is made just before delivery of the shock, but this method has been reported to correlate closely with impedance during actual shock delivery.3Cunningham SR Magee H Anderson J et al.Patients in ventricular fibrillation—predicted and actual transthoracic impedance and peak current at defibrillation [abstract].PACE. 1987; 10: 975Google Scholar, 4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Using Ohm's law and inserting the voltage derived from the bench test and the predicted impedance measurement, a calculated Ip was derived. The validity of the methodology used to obtain TTI and calculated Ip, although lacking the precision of direct measurements, is sufficiently accurate in this context. From these extrapolations, measurements, and calculations it was concluded that neither Ip nor TTI was a determinant of shock success, nor was either variable associated with patient survival. Age, witnessed arrest, and bystander CPR did not influence survival significantly; however, only 33 of the 310 patients (11%) survived and 30 of these 33 survivors had sustained a witnessed arrest. The effect of witnessed arrest and bystander CPR may have been obscured by the very small number of survivors. The relatively long call-to-shock time (10.9±4.4 minutes for all 310 patients) probably contributed to the low survival rate, although without details on synchronization of times the accuracy of this time interval is uncertain. What can we conclude from this study? First, the definition of shock success was the termination of VF and restoration of an organized rhythm. By this definition the first shock resulted in conversion in 25.5% (79/310) of patients. Conversion with shocks two and three was 24% and 22.5%, respectively. This is a low shock success rate, but part of the explanation for this likely resides in the definition of shock success chosen by the authors. We and others have defined defibrillation as the conversion of VF to any other rhythm or to asystole,5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar, 6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar, 8White RD Early out-of-hospital experience with an impedance-compensating low-energy biphasic waveform automatic external defibrillator.J Int Cardiol Electrophysiol. 1997; 1: 203-208Crossref PubMed Scopus (46) Google Scholar believing that electrophysiologically the term "defibrillation" means termination of ventricular (or atrial) fibrillation, and does not specify the rhythm that ensues after termination of fibrillation. By this definition in one study the first 200-joule (J) MDS shock was successful in 60% (42/70) of patients, and the cumulative success rate with two 200-J shocks was 74.3% (52/70).5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar Of the 70 patients, 17 had had a prehospital arrest. In another study conducted with prehospital arrest patients only and using self-adhesive pad electrodes, first shock success with the MDS waveform was 70% (40% to asystole and 30% to an organized rhythm).6Behr JC Hartley LL York DK et al.Truncated exponential versus damped sinusoidal waveform shocks for transthoracic defibrillation.Am J Cardiol. 1996; 78: 1242-1245Abstract Full Text PDF PubMed Scopus (26) Google Scholar In a study by Weaver et al7Weaver WD Cobb LA Copass MK Ventricular defibrillation—a comparative trial using 175-J and 320-J shocks.N Engl J Med. 1982; 307: 1101-1106Crossref PubMed Scopus (200) Google Scholar using handheld paddle electrodes, both 175-J and 320-J MDS shocks were successful in 61%, with conversions to asystole (15%) and organized rhythm (46%). In addition to different definitions of successful defibrillation, postshock rhythms evolve in a dynamic manner in time to other rhythms, necessitating inclusion of time points in the definition of shock success. None of these considerations has ever been standardized, as is evident in these studies and in the widely disparate reported shock success rates. It is mandatory that uniform definitions of shock outcomes and postshock rhythms be developed, accepted, and applied by all investigators. Until that occurs it will be difficult to interpret or to compare shock outcomes among studies and among different AEDs. A recently published American Heart Association Scientific Statement on biphasic waveform defibrillation also acknowledges this dilemma.9Cummins RO Hazinski MF Kerber RE et al.Low-energy biphasic waveform defibrillation: Evidence-based review applied to emergency cardiovascular care guidelines.Circulation. 1998; 97: 1654-1667Crossref PubMed Scopus (85) Google Scholar Survival to hospital discharge and restoration of spontaneous circulation are erroneous measures of defibrillation shock efficacy10Cummins RO Eisenberg MS Hallstrom AP et al.What is a "save"?: Outcome measures in clinical evaluations of automatic external defibrillations.Am Heart J. 1985; 110: 1133-1138Abstract Full Text PDF PubMed Scopus (20) Google Scholar because both are determined by several variables unrelated to the defibrillation shock, most importantly patient pathophysiologic substrate and EMS system performance. Without doubt the patient population is the weakest link in the chain of survival.11Stratton S Niemann JT Effects of adding links to "the chain of survival" for prehospital cardiac arrest: A contrast in outcomes in 1975 and 1995 at a single institution.Ann Emerg Med. 1998; 31: 471-477Abstract Full Text Full Text PDF PubMed Scopus (39) Google Scholar Thus the critical measure of shock efficacy is whether or not VF is terminated, but it is essential that uniform definitions of termination of VF and therefore of shock success be developed and applied by all investigators. The urgent need for such uniformity of definitions of postshock rhythms is obvious in light of expanding implementation of early defibrillation programs and the evolution of new defibrillation waveforms. Second, one must be aware that all of these observations by Heavens et al, and those reported and reviewed here, were made with AEDs delivering MDS waveform shocks and with 175 to 200 J as the initial dose of energy. It is likely that this specific waveform at these energy settings provides sufficient current to defibrillate most patients in the presence of relatively high TTI. Kerber and associates4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar observed that although shocks of 100 J or less did not defibrillate satisfactorily in patients with high TTI (>97 Ω), shocks of 200 J or more did so.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar The higher energy thus provided sufficient current to defibrillate even in the face of high impedance. In the study by Heavens, we are not told how many patients had a high TTI, in whom shock success might have been decreased. Dalzell and Adgey5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar observed that shock success for the first episode of VF using 200 J MDS shocks was similar in patients with high TTI (>115 Ω) and those with TTI less than 115 Ω.5Dalzell GWN Adgey AAJ Determinants of successful transthoracic defibrillation and outcome in ventrcular fibrillation.Br Heart J. 1991; 65: 311-316Crossref PubMed Scopus (60) Google Scholar This is consistent with the findings of Kerber and colleagues.4Kerber RE Kouba C Martins J et al.Advance prediction of transthoracic impedance in human defibrillation and cardioversion: Importance of impedance in determining the success of low-energy shocks.Circulation. 1984; 70: 303-308Crossref PubMed Scopus (116) Google Scholar Much larger studies likely would identify a small subset of patients with very high impedance for whom 200-J or even higher energy shocks would not achieve the current threshold required for defibrillation. Again, all of these observations have been made in patients receiving MDS shocks. Early experience with an impedance-compensating biphasic truncated exponential (BTE) waveform suggests that this waveform provides sufficient current to achieve defibrillation in patients with high TTI.12White RD Gliner BE Transthoracic impedance does not affect defibrillation efficacy in cardiac arrest victims when shocks are delivered with an impedance-compensating biphasic waveform.Circulation. 1997; 96: 1-561Google Scholar, 13Poole JE White RD Kanz K-G et al.Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest.J Cardiovasc Electrophysiol. 1997; 8: 1373-1385Crossref PubMed Scopus (116) Google Scholar More experience is needed to confirm these findings in the clinical setting. For most patients with prehospital VF arrest system factors, particularly delays to defibrillation, determine patient outcome.14Valenzuela TD Roe DJ Cretin S et al.Estimating effectiveness of cardiac arrest interventions.Circulation. 1997; 96: 3308-3313Crossref PubMed Scopus (655) Google Scholar Although there likely are patients with very high TTI who cannot be defibrillated with MDS waveforms at 200 J or higher, available evidence supports the conclusion that for the apparent majority of patients, TTI does not contribute significantly to shock success. It is of paramount importance that implementation of early defibrillation programs be accompanied by a commitment to collect accurate shock data, including impedance measurements, to define with more certainty the role that TTI may play in shock outcomes as new waveforms evolve and take their place in AEDs. It is also essential that uniform descriptors of defibrillation and postshock rhythms be developed by a consensus group and then applied uniformly by all clinical investigators as more data are collected and reported defining the effect of early defibrillation and new waveforms in a variety of settings. These are challenges that necessitate resolution if defibrillation is to develop in a controlled and scientific context.