Original Article
Early versus Later Rhythm Analysis in Patients with Out-of-Hospital Cardiac Arrest
N Engl J Med 2011; 365:787-797September 1, 2011
Comments
open through September 7, 2011
- Abstract
Background
In a departure from the previous strategy of immediate defibrillation, the 2005 resuscitation guidelines from the American Heart Association–International Liaison Committee on Resuscitation suggested that emergency medical service (EMS) personnel could provide 2 minutes of cardiopulmonary resuscitation (CPR) before the first analysis of cardiac rhythm. We compared the strategy of a brief period of CPR with early analysis of rhythm with the strategy of a longer period of CPR with delayed analysis of rhythm.
Methods
We conducted a cluster-randomized trial involving adults with out-of-hospital cardiac arrest at 10 Resuscitation Outcomes Consortium sites in the United States and Canada. Patients in the early-analysis group were assigned to receive 30 to 60 seconds of EMS-administered CPR and those in the later-analysis group were assigned to receive 180 seconds of CPR, before the initial electrocardiographic analysis. The primary outcome was survival to hospital discharge with satisfactory functional status (a modified Rankin scale score of ≤3, on a scale of 0 to 6, with higher scores indicating greater disability).
Results
We included 9933 patients, of whom 5290 were assigned to early analysis of cardiac rhythm and 4643 to later analysis. A total of 273 patients (5.9%) in the later-analysis group and 310 patients (5.9%) in the early-analysis group met the criteria for the primary outcome, with a cluster-adjusted difference of −0.2 percentage points (95% confidence interval, −1.1 to 0.7; P=0.59). Analyses of the data with adjustment for confounding factors, as well as subgroup analyses, also showed no survival benefit for either study group.
Conclusions
Among patients who had an out-of-hospital cardiac arrest, we found no difference in the outcomes with a brief period, as compared with a longer period, of EMS-administered CPR before the first analysis of cardiac rhythm. (Funded by the National Heart, Lung, and Blood Institute and others; ROC PRIMED ClinicalTrials.gov number, NCT00394706.)
Media in This Article
Figure 1
Subgroup Analyses of the Primary Outcome.Figure 2
Rate of the Primary Outcome, According to Actual Time to Analysis of Cardiac Rhythm.Article Activity
- Article
Out-of-hospital cardiac arrest is a common and lethal problem, leading to an estimated 330,000 deaths each year in the United States and Canada.1 Overall, the rate of survival to hospital discharge among patients with an out-of-hospital cardiac arrest who are treated by emergency medical services (EMS) personnel is low but varies greatly, with rates ranging from 3.0% to 16.3%.1 This variation in the rate of survival can be attributed partly to local variations in the five key links in the chain of survival: rapid EMS access, early cardiopulmonary resuscitation (CPR), early defibrillation, early advanced cardiac life support, and effective care after resuscitation. 2-6 Concerted efforts by EMS personnel to strengthen these links have led to only a slight increase in survival rates in recent years.
The traditional approach to out-of-hospital cardiac arrest has been to emphasize early analysis of cardiac rhythm, with delivery of defibrillatory shocks, if indicated, as quickly as possible. It has been suggested, however, that many patients may benefit from a period of CPR before the first analysis of rhythm.7 The 2005 resuscitation guidelines from the American Heart Association–International Liaison Committee on Resuscitation (AHA–ILCOR) departed from its previous “shock first” strategy by suggesting that responders could provide 2 minutes of CPR before analysis of cardiac rhythm.3 These changes in the guidelines are supported by the findings of three clinical studies8-10 but are not supported by two others,11,12 and in the 2010 guidelines, the recommendation was modified to say that “there is inconsistent evidence to support or refute” such a delay in the analysis of cardiac rhythm.13 Therefore, the preferred initial approach remains uncertain.14 Our objective was to compare two approaches to the timing of CPR by EMS personnel — a brief period of manual chest compressions and ventilations with prompt initiation of rhythm analysis and defibrillation (early analysis) versus a longer period of compressions and ventilations before the first analysis of cardiac rhythm (later analysis).
Methods
Study Design and Oversight
A detailed description of the methods has been published previously.15 The Resuscitation Outcomes Consortium (ROC) is a clinical trial consortium comprising 10 U.S. and Canadian universities and their regional EMS systems.16 The ROC investigators designed the Prehospital Resuscitation Impedance Valve and Early Versus Delayed Analysis (ROC PRIMED) trial to study two randomized comparisons.15,17 The first comparison, in which early analysis of cardiac rhythm was compared with later rhythm analysis, is the subject of this article. The second, concurrent comparison, in which the use of an impedance threshold device (ITD) was compared with the use of a sham ITD, is reported elsewhere in this issue of the Journal.18 Most patients were enrolled simultaneously in both the early-analysis-versus-later-analysis component and the active-ITD-versus-sham-ITD component of the ROC PRIMED trial, although the two components had slightly different eligibility criteria. Additional details are provided in the Supplementary Appendix, available with the full text of this article at NEJM.org.
The protocol was approved by the institutional review or research ethics boards at each participating site. The trial protocol, including the statistical analysis plan, is available at NEJM.org. All the authors vouch for the completeness and accuracy of the data and the analyses and for the fidelity of the study to the trial protocol.
Study Setting and Population
The trial was conducted at 150 of the 260 EMS agencies participating in the ROC. The trial agencies were selected because they had the capability to provide advanced cardiac life-support interventions and to record CPR process measures and because they met prespecified quality criteria during an initial run-in phase.
We included all persons 18 years of age or older who had an out-of-hospital cardiac arrest that was not the result of trauma and who were treated with defibrillation, delivery of chest compressions, or both by EMS providers. Persons were excluded if the arrest was witnessed by EMS personnel; if they had a blunt, penetrating, or burn-related injury; if the arrest was due to exsanguination; if they were pregnant; if they were prisoners; if they had an “opt-out” bracelet, indicating that they wished to opt out of the study; if they had “do not attempt resuscitation” orders; if the rhythm analysis was performed by police or a lay responder; or if they received initial treatment by an EMS agency that was not in the ROC. Patients were not required to provide informed consent; according to the regulations of the Food and Drug Administration and the Canadian Tri-Council agreement, this study qualified for exception from the requirements for informed consent because it involved research conducted during an emergency situation.
Randomization
Each of the 10 participating ROC centers (or sites) was divided into approximately 20 subunits, designated as “clusters,” according to EMS agency or geographic boundaries or according to defibrillator device, ambulance, station, or battalion. Randomization of clusters was stratified according to site. All episodes of cardiac arrest in a cluster were randomly assigned to one CPR strategy; after a set period of time, ranging from 3 to 12 months, all episodes in that cluster were then assigned to the other strategy. All the clusters were assigned to cross over to the other strategy one or more times during the study at fixed intervals; we estimated that approximately 100 patients would be included during each interval.
Study Intervention
Patients in the early-analysis group were assigned to receive 30 to 60 seconds of chest compressions and ventilations (sufficient time to place defibrillator electrodes) before electrocardiographic (ECG) analysis, and those in the late-analysis group were assigned to receive 3 minutes of chest compressions and ventilations before ECG analysis. The assigned intervention was implemented by the first qualified EMS provider to arrive at the scene (defibrillation-capable firefighter, emergency medical technician, or paramedic). The start and stop times for CPR were recorded by the responders, and the information was supplemented by the recording of defibrillator time.
The training of participating EMS providers emphasized uninterrupted chest compressions except for required ventilations, with compressions and ventilations applied in a 30:2 ratio, and specified that advanced airway devices were to be placed with minimal interruptions to compressions. Every 6 months, the EMS providers underwent some retraining that included written reminders, slide presentations, and Web-based modules. All ROC sites implemented high-quality electronic monitoring of the CPR process with the use of defibrillator hardware and software. Adherence to the protocol-specified performance targets and to the requirements for data submission was monitored throughout the study by a study monitoring committee, which provided regular feedback to sites.
Outcome Measures
The primary outcome was survival to hospital discharge with satisfactory functional status, defined as a score of 3 or less on the modified Rankin scale.19-21 This is a validated scale, ranging from 0 to 6, that is commonly used for measuring the performance of daily activities by people who have had a stroke. Lower scores represent better performance; scores of 4 or higher represent severe disability or death. Secondary outcomes were survival to discharge, survival to hospital admission, and return of spontaneous circulation at the time of arrival at the emergency department.
Statistical Analysis
We estimated that with enrollment of 13,239 patients who could be evaluated, the study would have 99.6% power to detect an improvement in the primary outcome from 5.4% with early analysis of heart rhythm to 7.4% with later analysis, assuming a group-sequential stopping rule at a two-sided alpha level of 0.05 with up to three interim analyses (O'Brien–Fleming boundaries).22 This calculation took into consideration the concurrent ITD portion of the trial, which required the enrollment of 14,154 patients who could be evaluated, in order to have 90% power to detect a 25% difference in the outcome between the two groups in that trial.
Analyses of the primary and secondary effectiveness outcomes were performed on the basis of a modified intention-to-treat principle with data from eligible patients in whom the cardiac arrest was not due to drowning, strangulation, or electrocution and for whom the primary outcome was known. An independent data and safety monitoring board reviewed the data at prespecified intervals and used a group-sequential stopping rule. The primary analysis compared the outcomes between the groups with the use of the Wald statistic for the treatment group in a generalized linear mixed model.23 The model included random effects for each of the clusters, accommodated the binary distribution of the outcome variable, and used a linear-link function to estimate an absolute difference in risk.
The between-group difference in the primary outcome, adjusted for baseline characteristics, was calculated with the use of a multiple linear regression model, with robust standard errors to accommodate clustering and the binary distribution of the outcome. Analyses of binary secondary outcomes and subgroup analyses were performed with the use of generalized-estimating-equation models to estimate differences in risk.24 Mean scores on the modified Rankin scale were compared between the two treatment groups with the use of a linear model.
We conducted further exploratory analyses of the data using kernel density estimators to estimate the distribution of time from the start of CPR to the actual analysis of cardiac rhythm, separately within treatment groups.25 The association between the primary outcome and the time of cardiac-rhythm analysis was explored with the use of smoothing splines, and confidence intervals were computed with the use of the bootstrap method.26,27
Results
Enrollment and Randomization
The first site commenced the run-in phase in June 2007. All the sites stopped enrollment in November 2009, when the data and safety monitoring board recommended that the trial be stopped early because continuing recruitment was unlikely to change the outcome of the study. Of 13,460 patients screened, 10,365 were enrolled, and 10,153 underwent randomization. Of these, 195 were excluded from the data analysis when their cardiac arrest was confirmed to be due to drowning, strangulation, or electrocution, and 25 were excluded because the outcome with respect to the primary end point was unknown. Thus, 9933 patients were included in the primary data analysis (Fig. 1 in the Supplementary Appendix).
Characteristics of the Two Study Groups
The early-analysis group comprised more patients than the later-analysis group (5290 vs. 4643) owing to early termination of the trial. The two study groups were evenly balanced with respect to baseline characteristics except that there were small group imbalances in the distribution of patients across sites (Table 1Table 1
Baseline Characteristics of the Patients Included in the Primary Analysis.); however, these would not have any appreciable effect on the results because of the cluster-crossover design, which yields treatment comparisons within clusters. Not all the scheduled cluster crossovers had occurred at the time of termination, although each cluster had crossed over at least once. The postrandomization characteristics of the patients in each group are provided in Table 2Table 2
Postrandomization Characteristics of the Patients Included in the Primary Analysis.. The median time to the analysis of cardiac rhythm was 42 seconds (interquartile range, 27 to 80) in the early-analysis group and 180 seconds (interquartile range, 151 to 190) in the later-analysis group. A majority of patients in each group received rhythm analysis within the targeted range for that group: 68% of patients in the early-analysis group received analysis of cardiac rhythm within the targeted range of 0 to 60 seconds and 60% of patients in the later-analysis group received analysis of cardiac rhythm within the targeted range of 150 to 210 seconds (Fig. 2 in the Supplementary Appendix).Primary and Secondary Outcomes
A total of 310 patients in the early-analysis group (5.9%) and 273 patients in the later-analysis group (5.9%) survived to hospital discharge with a modified Rankin score of 3 or less, with a cluster-adjusted difference between later cardiac analysis and early cardiac analysis of −0.2 percentage points (95% confidence interval [CI], −1.1 to 0.7; P=0.59) (Table 3Table 3
Outcomes for the Patients Included in the Primary Analysis.). There was also no significant difference between the study groups with respect to any of the secondary outcomes. An analysis adjusted for potential confounders evaluated the effect of study group on survival and showed a difference of −0.3 percentage points (95% CI, −1.3 to 0.7) between later cardiac analysis and early cardiac analysis (P=0.61).Additional Analyses
We conducted a number of prespecified and post hoc subgroup analyses (Figure 1Figure 1
Subgroup Analyses of the Primary Outcome.) and found that the absence of significant differences in the rate of survival between the two study groups was consistent across subgroups. The relationship between the site-specific treatment effect and the site-specific probability of survival overall is shown in Figure 3 in the Supplementary Appendix.When the outcomes were analyzed on an as-treated basis, the rates of survival with satisfactory functional status were 6.0% among the 3982 patients in whom the analysis of cardiac rhythm was performed between 0 and 60 seconds and 5.9% among the 3115 patients in whom the analysis of cardiac rhythm was performed between 150 and 210 seconds (P=0.97). In an additional exploratory analysis, we evaluated the rate of survival as a function of the actual time to the first rhythm analysis, regardless of the study group (Figure 2Figure 2
Rate of the Primary Outcome, According to Actual Time to Analysis of Cardiac Rhythm.). The chance of survival with satisfactory functional status did not improve with increasing time to the first analysis of cardiac rhythm, and among patients with an initial rhythm of ventricular tachycardia or ventricular fibrillation who received CPR from a bystander, the rate of survival tended to decline with increasing time to the first rhythm analysis.Discussion
In this randomized trial, we tested the hypothesis that patients with an out-of-hospital cardiac arrest might benefit from the administration of CPR by EMS personnel for approximately 3 minutes before the first analysis of cardiac rhythm (with delivery of a defibrillator shock as appropriate). We found that there was no significant difference in the rate of survival with satisfactory functional status between the two EMS strategies of a brief period of CPR with early analysis of cardiac rhythm and a longer period of CPR with delayed analysis of rhythm. Subgroup and adjusted analyses also did not show any significant differences in the outcomes between the two study groups. We further explored the relationship between the rate of survival and the actual time to rhythm analysis and found that outcomes did not improve with increasing time to analysis. This finding suggests that there is no advantage of delaying the analysis of cardiac rhythm during EMS-administered CPR. Indeed, the data suggest that there may be a disadvantage of delaying the rhythm analysis in the subgroup of patients with a first rhythm of either ventricular tachycardia or ventricular fibrillation who have received CPR from a bystander. Overall, our data suggest that the administration of 2 minutes of CPR by EMS personnel before the first analysis of rhythm, which was suggested in the 2005 guidelines of the AHA–ILCOR, is unlikely to provide a greater benefit than CPR of shorter duration.
The hypothesis that a brief period of initial CPR before analysis of cardiac rhythm could be beneficial is based primarily on the concept that a few minutes of chest compressions may increase myocardial perfusion, thus improving the metabolic state of the cardiac myocytes and enhancing the likelihood of successful defibrillation.7 Several studies in animals with experimentally induced ventricular fibrillation showed that the outcomes with delayed countershock after a period of chest compressions were better than the outcomes with earlier countershock,21,28,29 whereas other studies failed to show a benefit of CPR before shock.30,31 Five previous clinical studies also attempted to evaluate this issue, but all five had limitations involving the design or sample size, and none had findings that were definitive.8-12 Cobb et al.,8 in a before-and-after study, showed that the rate of survival increased after the implementation of a policy that required 90 seconds of CPR before analysis of cardiac rhythm when an automated external defibrillator was used. Wik et al.9 conducted a randomized trial and found no significant difference between the outcomes after immediate defibrillation and those after 3 minutes of basic CPR before defibrillation, but the outcomes in a subgroup with response times exceeding 5 minutes were better after initial CPR than after immediate defibrillation. Randomized trials reported by Jacobs et al.11 and Baker et al.12 showed no significant difference in outcomes with early as compared with late defibrillation. Bradley et al.10 performed an observational analysis and found that CPR by EMS personnel for 46 to 195 seconds before defibrillation was weakly associated with an improved rate of survival.
Given the complex clinical circumstances of out-of-hospital cardiac arrest, precise control of the time to the first analysis of cardiac rhythm is difficult to achieve. In our trial, the duration of CPR before the first analysis of rhythm did not fall within the assigned target for 36% of the patients. Although this observation raises the question of quality control in training and trial supervision, the participating EMS agencies were high-functioning services with advanced-level paramedics; in addition, they had collected high-quality patient data before the start of the trial, and they made continuous efforts to reinforce performance targets. Thus, although implementation of the protocol was imperfect, it nonetheless represents the degree of precision with which such therapies are likely to be practiced in the clinical setting of out-of-hospital cardiac arrest. Furthermore, despite this limitation, there was very good separation between the two study groups in the duration of CPR, and a variety of data analyses confirmed the primary finding of no significant difference in the outcome between patients who had early rhythm analysis and those who had later rhythm analysis.
Our results indicate that in most cases, the outcome is similar with as few as 30 seconds and as many as 180 seconds of EMS-administered CPR before the analysis of cardiac rhythm. The exception is the case of cardiac arrest witnessed by EMS responders, which was not evaluated in this study and for which rapid defibrillation remains the standard of care.13 Our results also do not address the strategy of immediate analysis of cardiac rhythm without any preceding CPR, since we deliberately insisted on some CPR for the early-analysis group, in the belief that good patient care required cardiopulmonary support while the defibrillator was being prepared.
Exploratory examination of our data suggests that a strategy of brief CPR and early analysis may be more appropriate than longer CPR and later analysis for patients who have received CPR from a bystander before the arrival of professional responders. Conversely, for patients who have not received CPR from a bystander, there is no approach that is clearly advantageous with respect to the time to analysis of rhythm. The 2010 guidelines of the AHA–ILCOR give little direction as to the preferred period of CPR before analysis of cardiac rhythm.13 Each EMS system should consider its operational situation when deciding on its strategy for initial EMS-administered CPR. We believe that it is important to administer CPR for some period while the defibrillator pads are being applied and that compressions should be of high quality with minimal interruptions.
In conclusion, in a large clinical trial, we evaluated the timing of the analysis of cardiac rhythm during CPR in patients who had an out-of-hospital cardiac arrest that was not witnessed by EMS personnel. We found no difference in the outcome between the EMS strategy of a brief period of CPR before early rhythm analysis and that of a longer period of CPR before delayed rhythm analysis.
Supported by grants from the National Heart, Lung, and Blood Institute, the National Institute of Neurological Disorders and Stroke, the Canadian Institutes of Health Research–Institute of Circulatory and Respiratory Health, and the Heart and Stroke Foundation of Canada. The Resuscitation Outcome Consortium is supported by a series of cooperative agreements with 10 regional clinical centers and one data coordinating center (5U01 HL077863, HL077881, HL077871 HL077872, HL077866, HL077908, HL077867, HL077887, HL077873, HL077865) from the National Heart, Lung, and Blood Institute in partnership with the National Institute of Neurological Disorders and Stroke, U.S. Army Medical Research and Material Command, the Institute of Circulatory and Respiratory Health of the Canadian Institutes of Health Research, Defence Research and Development Canada, the Heart and Stroke Foundation of Canada, and the American Heart Association.
Dr. Nichol reports receiving grant support from the Laerdal Foundation for Acute Medicine and Medtronic Foundation and travel fees from Sotera Wireless, being a board member of Medic One Foundation, and participating in research collaborations with Gambro Renal, Lifebridge Medizintechnik, and Sotera Wireless; Dr. Ornato, serving on an advisory board for ZOLL Circulation; Dr. Callaway, that he and his institution receive royalties for patents from Medtronic related to the timing of defibrillation; Dr. Aufderheide, consulting fees from Jolife and Medtronic; Dr. Daya, consulting and lecture fees from Philips Healthcare and owning stock in Amgen, Johnson & Johnson, and Roche; Dr. Morrison, grant support from the Laerdal Foundation for Acute Medicine; Dr. Pirrallo, consulting fees from ZOLL Medical; and Dr. Weisfeldt, that he and his institution receive royalties for patents from Imricor Medical Systems related to an MRI Insensitive Pacemaker.
Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.
No other potential conflict of interest relevant to this article was reported.
We thank the EMS providers and first responders for their efforts in making this logistically challenging trial possible; and Alfred P. Hallstrom, Ph.D., Scott S. Emerson, M.D., Ph.D., and Gerald van Belle, Ph.D., for their leadership.
Source Information
The authors' affiliations are listed in the Appendix.
Address reprint requests to Dr. Stiell at istiell@ohri.ca.
The investigators in the Resuscitation Outcomes Consortium (ROC) are listed in the Supplementary Appendix, available at NEJM.org.
Appendix
The authors' affiliations are as follows: the Department of Emergency Medicine and Ottawa Hospital Research Institute, University of Ottawa, Ottawa (I.G.S., C.V.); the Clinical Trials Center, Department of Biostatistics (G.N., B.G.L., J.P., R.H.S.), and the Department of Medicine (G.N., T.D.R., P.J.K.), University of Washington; and the University of Washington–Harborview Center for Prehospital Emergency Care (G.N.) — both in Seattle; the Department of Emergency Medicine, Virginia Commonwealth University, Richmond (J.P.O.); the Department of Emergency Medicine (J.C.) and the School of Population and Public Health (D.A.), University of British Columbia, and British Columbia Emergency and Health Services Commission (J.C., D.A.) — both in Vancouver, Canada; the Department of Emergency Medicine, University of Pittsburgh, Pittsburgh (C.W.C., D.H.); the Department of Emergency Medicine, Medical College of Wisconsin, Milwaukee (T.P.A., R.G.P.); the Department of Surgery (Emergency Medicine), University of Texas Southwestern Medical Center at Dallas, Dallas (A.H.I., R.F.); Center for Policy and Research in Emergency Medicine, Department of Emergency Medicine, Oregon Health and Science University, Portland (M.R.D., D.Z.); the Department of Emergency Medicine, University of Alabama at Birmingham, Birmingham (H.E.W., S.S.); the Division of Emergency Medicine, Department of Medicine (L.J.M.) and Department of Family and Community Medicine (S.C.), University of Toronto; and Rescu Keenan Research Centre, Li Ka Shing Knowledge Institute, St. Michael's Hospital (L.J.M., S.C.) — both in Toronto; the Department of Emergency Medicine, University of California, San Diego, San Diego (D.D., G.M.V.); the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD (G.S.); and the Department of Medicine, Johns Hopkins Medical Institutions, Baltimore (M.W.).
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Douglas Chamberlain, David Fletcher, Malcolm Woollard, Anthony Handley. (2012) Protocol C. Current Opinion in Critical Care1
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Matthew Huei-Ming Ma, Wen-Chu Chiang, Patrick Chow-In Ko, Chi-Wei Yang, Hui-Chi Wang, Shey-Ying Chen, Wei-Tien Chang, Chien-Hwa Huang, Hao-Chang Chou, Mei-Shu Lai, Kuo-Long Chien, Bin-Chou Lee, Chien-Hwa Hwang, Yao-Cheng Wang, Guan-Hwa Hsiung, Ying-Wen Hsiao, Anna Marie Chang, Wen-Jone Chen, Shyr-Chyr Chen. (2012) A randomized trial of compression first or analyze first strategies in patients with out-of-hospital cardiac arrest: Results from an Asian community. Resuscitation
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Alexandra King. (2011) Cardiac resuscitation: Optimum CPR strategies clarified. Nature Reviews Cardiology 8:11, 611-611
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Aufderheide, Tom P., Nichol, Graham, Rea, Thomas D., Brown, Siobhan P., Leroux, Brian G., Pepe, Paul E., Kudenchuk, Peter J., Christenson, Jim, Daya, Mohamud R., Dorian, Paul, Callaway, Clifton W., Idris, Ahamed H., Andrusiek, Douglas, Stephens, Shannon W., Hostler, David, Davis, Daniel P., Dunford, James V., Pirrallo, Ronald G., Stiell, Ian G., Clement, Catherine M., Craig, Alan, Van Ottingham, Lois, Schmidt, Terri A., Wang, Henry E., Weisfeldt, Myron L., Ornato, Joseph P., Sopko, George, . (2011) A Trial of an Impedance Threshold Device in Out-of-Hospital Cardiac Arrest. New England Journal of Medicine 365:9, 798-806
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- Comments (1)
Do Not Disturb?
Compression brings a fibrillating heart glucose and possibly longer survival. From the brain, at rest, initially well oxygenated and unable to glycolyse, the incoming blood removes oxygen and perhaps the chance of full recovery. Figure 2's fascinating curves suggest that if full ventilation and defibrillation are likely within 5 minutes it may be wise to wait.