By Freeman, Kimberly Hendey, Gregory W; Shalit, Marc; Stroh, Geoff
ABSTRACT Study Objective. To compare the outcomes of out-of hospital cardiac arrest (OHCA) victims treated with monophasic truncated exponential (MTE) versus biphasic truncated exponential (BTE) defibrillation in an urban EMS system. Methods. We conducted a retrospective review of electronic prehospital and hospital records for victims of OHCA between August 2000 and July 2004, including two years before and after implementation of biphasic defibrillators by the Fresno County EMS agency. Main outcome measures included: return of spontaneous circulation (ROSC), number of defibrillations required for ROSC, survival to hospital discharge, and discharge to home versus an extended care facility. Results. There were 485 cases of cardiac arrest included. Baseline characteristics between the monophasic and biphasic groups were similar. ROSC was achieved in 77 (30.6%, 95% CI 25.2-36.5%) of 252 patients in the monophasic group, and in 70 (30.0% 95% CI 24.5-36.2%) of 233 in the biphasic group (p = .92). Survival to hospital discharge was 12.3% (95% CI 8.8-17%) for monophasic and 10.3% (95% CI 7.0-14.9%) for biphasic (p = .57). Discharge to home was accomplished in 20 (7.9%, 95% CI 5.1-12.0%) of the monophasic, and in 15 (6.4%, 95% CI 3.9-10.4%) of the biphasic group (p = .60). More defibrillations were required to achieve ROSC (3.5 vs. 2.6, p = .015) in the monophasic group. Conclusions. We found no difference in ROSC or survival to hospital discharge between MTE and BTE defibrillation in the treatment of OHCA, although fewer defibrillations were required to achieve ROSC in those treated with biphasic defibrillation. Key words: cardiac arrest; defibrillation; emergency medical services; Utstein template
PREHOSPITAL EMERGENCY CARE 2008;12:152-156
Electrical defibrillation is one of the most important medical interventions in the care of patients with cardiac dysrhythmias. It is one of the only prehospital interventions that has been shown to improve survival after cardiac arrest. The standard form of defibrillation for many years has been the delivery of electrical energy with a monophasic waveform. More recently, the biphasic waveform has been touted as achieving similar if not better rates of conversion, while using less energy.1-7 Most human studies comparing the two modes have been in the controlled setting of a cardiac catheterization lab, during electrophysiologic studies, or defibrillator implantation. A meta-analysis of such trials found that biphasic waveforms at 150-200 joules were as effective as monophasic waveforms at 200-360 joules at terminating episodes of ventricular tachycardia or fibrillation.8 In one prospective study of prehospital cardiac arrest, ventricular fibrillation was terminated with similar efficacy with lower energy biphasic defibrillation as compared to escalating energy monophasic defibrillation. However, survival to hospital arrival or discharge was not evaluated.9 In a recent randomized, controlled trial analyzing the two waveforms delivered at the same energy of 200-360 joules for out-of-hospital cardiac arrest (OHCA) ventricular fibrillation (VF), no statistically significant difference was found in the endpoints of admission alive to the hospital, return of spontaneous circulation (ROSC), survival, and neurologic outcome.10
Biphasic defibrillators have become the industry standard for most manufacturers, and emergency medical service (EMS) providers and hospitals are in the process of replacing monophasic units with newer biphasic models. However, outcome studies for this newer mode of defibrillation have not been widely performed, particularly in the prehospital arena, where it is being increasingly utilized.6
Our goal was to use a “before and after” methodology to compare the success rate of biphasic defibrillation to that of monophasic defibrillation, by: 1) determining the percentage of patients who experienced ROSC, 2) determining the percentage of patients who survived to hospital discharge, 3) evaluating hospital discharge destination, whether to home or extended-care facility, as a surrogate indicator of neurological outcome, and 4) assessing number of attempts required for successful defibrillation.
MATERIALS AND METHODS
This investigation was a retrospective case review study of out- of-hospital cardiac arrest victims who were treated with defibrillation by American Ambulance Advanced Life Support (ALS) providers in Fresno County between August 1,2000 and July 31,2004. This study was approved by the Community Medical Centers Institutional Review Board with informed consent by subjects waived.
The study setting was a mixed urban and suburban region, with a metropolitan population of 500,000 and county population of approximately 1 million. A single private ambulance company provided the majority of out-of-hospital emergency medical services and generated electronic records of all patient encounters.
Selection of Participants
Patients were included if they were at least 18 years of age, suffered nontraumatic out-of-hospital cardiac arrest, and were treated with defibrillation or cardioversion by out-of-hospital ALS providers.
In August 2002, all defibrillators were switched from the monophasic truncated exponential (MTE) waveform MRL 360 SLX Monitor and DC Defibrillator (Medical Research Laboratories, Buffalo Grove, IL) to the biphasic truncated exponential (BTE) waveform Zoll M Series Monitor (Zoll Medical Corporation, Chelmsford, MA). Protocols for the treatment of cardiac arrest remained unchanged, except that escalating energies from 200-360 joules were used with the MTE defibrillators, while escalating energies from 150-200 joules were used with BTE defibrillators.
Data Collection and Processing
Data collected from out-of-hospital electronic records included 911 call-response interval, call-shock interval, patient age, sex, presenting rhythm, presence of bystander cardiopulmonary resuscitation (CPR), type of defibrillator, number of defibrillations, defibrillation energy in joules, ROSC, and transport destination. The call-response interval is defined as the time from 911 call receipt to time of arrival on scene, and call- shock interval is defined as the time from call receipt to time of first defibrillatory shock. This format is consistent with Utstein reporting guidelines.11 Hospital records were reviewed to determine survival to discharge and discharge destination. We also obtained records of deaths reported to the county coroner’s office.
The primary outcome measures were ROSC after treatment with defibrillation, number of defibrillatory shocks required to achieve ROSC, and survival to hospital discharge. Since formal cognitive and functional testing was not available for most patients, we used the discharge disposition, whether to home or to an extended-care facility, as a surrogate marker of neurologic outcome.
Primary Data Analysis
Between-group comparisons of times, treatments, and outcomes were conducted using f-tests and Fisher’s exact tests available online (http://graphpad.com/ quickcalcs), with an alpha of 0.05, and statistical comparisons were reviewed by a statistician. Because our sample size was determined by the number of cases that occurred during a specified time interval, we did not perform an a priori power calculation. However, we did perform a post-hoc power calculation to address the possibility of type II error (http:// www.dssresearch.com/toolkit/spcalc/power _p2.asp).
Characteristics of Study Subjects
A total of 485 cases of out-of-hospital cardiac arrest met the inclusion criteria and had electronic out-of-hospital records available during the study period. Two hundred fifty-two of these patients were treated with MTE defibrillation and 233 of these patients were treated with BTE defibrillation. The age, gender, and presence of bystander CPR were similar between the MTE and BTE groups (Table 1). The call-response intervals and call-shock intervals were also similar between the two groups, but there was a trend toward shorter biphasic call-response and call-shock intervals when only those patients presenting in ventricular tachycardia (VT) or VF were considered (Table 1). The proportion of patients whose initial arrest rhythm was VT or VF was also similar between groups (Table 1).
Seventy-seven (30.1%, 95% CI 25.2-36.5%) of the 252 patients who were treated with MTE defibrillation achieved ROSC. Similarly, 70 (30.0% 95% CI 24.536.2%) of the 233 patients who were treated with BTE defibrillation achieved ROSC (p = .92) (Fig. 1). A similar number of patients, 31 (12.3%, 95% CI 8.8-17%) of the MTE group and 24 (10.3%, 95% CI 7.0-14.9%) of the BTE group, were discharged alive from the hospital (p = .57). Discharge to home was also unchanged and was accomplished in 20 (7.9%, 95% CI 5.1-12.0%) of the MTE patients and in 15 (6.4%, 95% CI 3.9-10.4%) of the BTE group (p = .60). The remainder of the surviving patients were discharged to extended care facilities (Fig. 1). We were unable to locate hospital or coroner data for 18 patients.
The mean number of total shocks delivered was significantly higher in those treated with MTE defibrillation (4.3 shocks, 95% CI 3.9-4.7) than BTE defibrillation (2.9 shocks, 95% CI 2.6-3.1, p = 0.001). Patients who achieved ROSC also required more shocks in the monophasic group with a mean total of 3.5 (95% CI 2.94.1) than in the biphasic group with a mean total of 2.6 (95% CI 2.1-3.0, p = 0.015) (Table 2). A post-hoc power calculation revealed that with our sample size and an alpha of 0.05, the study had a power of 74.7% to detect an absolute increase of 10% in ROSC, 89.9% to detect a 10% increase in survival to discharge, and 46.4% to detect a 5% increase in survival to discharge.
We found that BTE defibrillation was no more efficacious than MTE defibrillation in the out-of-hospital setting for achieving ROSC or survival to hospital discharge. Although fewer shocks were necessary with BTE defibrillation, the rate of ROSC, hospital discharge, and discharge to home were not improved. Others have similarly failed to demonstrate a survival benefit for biphasic defibrillation.10,12-15 White et al. found no difference in return of spontaneous circulation (ROSC) or discharge home.9 Morrison et al. enrolled all nontraumatic out-of-hospital cardiac arrest victims presenting in VF or VT treated with defibrillation and found improved biphasic shock efficacy compared to monophasic efficacy, but, again, no improvement in ROSC or survival to hospital discharge.16 Most recently, Kudenchuk et al. prospectively randomized OHCA VF victims to monophasic dampened sine (MDS) or BTE waveform defibrillation and found no difference in hospital admission, hospital discharge, ROSC, and neurologic outcome.10
Fewer defibrillatory shocks and lower energy requirements to achieve ROSC in BTE defibrillation has been consistently reported in other studies.12-16 Schneider and Stothert also found a greater percentage of ROSC in their biphasic groups, compared to monophasic.12,14 However, our study, like those of Van Elam, Morrison, and Kudenchuk, found no difference in ROSC or survival between groups.10,13,16
Biphasic monitors have a practical advantage of being smaller and lighter devices. There is also a theoretical advantage to using less energy and possibly causing less myocardial stunning or damage during resuscitation.17,18 Less myocardial damage should theoretically translate to superior clinical outcomes, although this potential advantage has not yet been demonstrated.10,12-15 Several studies of postresuscitation myocardial injury have indicated that low-energy shocks may produce less injury and dysfunction.8,17,19 However, this finding has not been consistent, and other studies have shown no difference in postresuscitation myocardial dysfunction.1,19,20 Regardless, we did not find any superiority of biphasic defibrillation over monophasic defibrillation with respect to ROSC or survival to hospital discharge. Biphasic defibrillation has prematurely been promoted as an exceptional tool to assist in resuscitation, but evidence of improved patient outcomes has been lacking. In an environment where the costs of medical care are skyrocketing, smaller EMS systems may not have the financial resources to “upgrade” to newer biphasic equipment but may feel pressure to do so. The literature to date suggest that such pressure is unwarranted and premature.
There were several limitations of our study. First, the design was retrospective, which raises the possibility of important baseline differences between the two study groups. This design could have biased our results in either direction. Also, there were missing data points in some EMS and hospital records. Most importantly, we could find no hospital or coroner data on 18 patients. A large difference in survival in this group of patients could have significantly changed the results of our study. However, this subgroup was very similar to the rest of the study group in age (mean, 63 years), gender (56% male), and ROSC (27.8%). If all five patients with ROSC who were lost to follow-up survived to hospital discharge, there still would have been no significant difference in survival between the monophasic and biphasic groups.
A second limitation was that we only included cases from the private ambulance company serving most of the county because this company generated an electronic EMS record. Our design excluded the small percentage of patients in our system transported by small providers using paper records. Although our data include the large majority of urban and rural patients in our area, the excluded patients were all from the rural setting. However, it seems unlikely that any bias introduced by excluding this small subgroup would have altered our results, given previous reports demonstrating poorer outcomes with long prehospital response and transport times.9
Third, the majority of patients who were transported to a hospital were taken to one of five local facilities. Although each of the five hospitals had intensive care capabilities, outcomes may have differed by hospital. Our numbers were too small to allow for meaningful subgroup analysis by various hospitals, but even if there were significant overall survival differences between the hospitals, the differences would likely affect both the biphasic and monophasic groups equally. Also, there were no major changes in the care of cardiac arrest survivors during the study period, with none of the local hospitals adopting the routine use of hypothermia.
Fourth, we did not use a rigorous definition or formal determination of favorable neurologic outcome. Instead, we used discharge to home or to an extended care facility. Most patients discharged home were noted to be alert and ambulatory, and likely had more favorable neurologic outcomes than those discharged to extended care facilities, but no formal assessments were done. Again, this method would have likely biased equally between MTE and BTE defibrillation toward more favorable outcomes overall.
Fifth, only BTE and MTE waveforms were analyzed. Given evidence that rectilinear biphasic and monophasic dampened sine waveforms may produce different results, the results of this study cannot be extrapolated to those waveforms.
Finally, we did not examine whether the number of shocks or amount of energy used conferred a functional advantage to survivors of cardiac arrest in terms of left ventricular dysfunction, congestive failure, or longterm survival.
We found no difference in ROSC or survival to hospital discharge between monophasic and biphasic defibrillation in the treatment of OHCA, although fewer defibrillations were required to achieve ROSC in those treated with biphasic defibrillation.
We thank Brandy Snowden (research coordinator), Luke Wright, Brandi Martin (research assistants), Erik Peterson, Donna Hankins, American Ambulance, and Ronna Mallios (statistician) for their invaluable assistance with this project.
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Kimberly Freeman, MD, Gregory W. Hendey, MD, Marc Shalit, MD, Geoff Stroh, MD
Received July 12,2007 from the Department of Emergency Medicine, UCSF-Fresno, Medical Education Program, Fresno, California. Accepted for publication November 2, 2007.
No external support or outside financial interest contributed to this study.
Address correspondence and reprint requests to: Gregory W. Hendey, MD, FACEP, UCSF-Fresno, Emergency Medicine, Medical Education Program, 155 North Fresno Street, Fresno, CA 93701-2302. e- mail: [email protected]
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