Literature Review: ITDs, Rhythm Analysis for Cardiac Arrest
Aufderheide TP, Nichol G, Rea TD, et al. A trial of an impedance threshold device in out-of-hospital cardiac arrest. N Engl J Med 365(9): 798–806, Sep 1, 2011.
Abstract
The impedance threshold device (ITD) is designed to enhance venous return and cardiac output during CPR by increasing the degree of negative intrathoracic pressure. Methods—[Authors] compared the use of an active ITD with that of a sham ITD in patients with out-of-hospital cardiac arrest who underwent standard CPR at 10 sites in the United States and Canada. Patients, investigators, study coordinators and all care providers were unaware of the treatment assignments. The primary outcome was survival to hospital discharge with satisfactory function—a score of less than or equal to 3 on the modified Rankin scale, which ranges from 0 to 6, with higher scores indicating greater disability.
Results—Of 8,718 patients included in the analysis, 4,345 were randomly assigned to treatment with a sham ITD, and 4,373 to treatment with an active device. A total of 260 patients (6.0%) in the sham-ITD group and 254 patients (5.8%) in the active-ITD group met the primary outcome. There were also no significant differences in the secondary outcomes, including rates of return of spontaneous circulation on arrival at the emergency department, survival to hospital admission and survival to hospital discharge. Conclusions—Use of the ITD did not significantly improve survival with satisfactory function among patients with out-of-hospital cardiac arrest receiving standard CPR.
Stiell IG, Nichol G, Leroux BG, et al. Early versus later rhythm analysis in patients with out-of-hospital cardiac arrest. N Engl J Med 365(9): 787–97, Sep 1, 2011.
Abstract
CPR before defibrillation may perfuse the myocardium and increase the likelihood of successful defibrillation. [Authors] 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—[Authors] conducted a cluster-randomized trial involving adults with out-of-hospital cardiac arrest at 10 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, [both] before the initial ECG analysis. The primary outcome was survival to hospital discharge with satisfactory functional status—a modified Rankin scale score of less than or equal to 3.
Results—[Authors] included 9,933 patients, of whom 5,290 were assigned to early analysis of cardiac rhythm, and 4,643 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. 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, [authors] 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.
Comment
These two large studies, done concurrently by the Resuscitation Outcomes Consortium over 2½ years, involving nearly 10,000 patients and published in the same issue of the New England Journal of Medicine, are disappointing in not supporting two practices common in many of our EMS systems. Out-of-hospital cardiac arrest patients on whom an ITD was used were no more likely to have ROSC, survive or have good neurological outcome that those who received a sham ITD (an identical-looking device that does not create negative intrathoracic pressure). And 3 minutes of predefibrillation EMS CPR was no better than 30 seconds.
There were limitations in both studies. For the ITD, delay in application, airway leaks, inadequate CPR or failure to remove after ROSC, none of which were well assessed, can eliminate any advantage. In both studies, up to 40% of the patients had the procedure (ITD or defibrillation) done outside the protocol-specified time interval. However, the fact that there were nearly identical outcomes on every measure makes it less likely that future studies will reach a different conclusion. And in the early-vs.-late defibrillation study, all patients had some CPR before analysis—leaving open the question of whether defibrillation before CPR might be better yet.
Although these studies did not show positive results, they are still useful to guide our practice. First, evidence for use of the ITD is not strong, and EMS systems that use it should evaluate whether to continue, especially if those resources can be used elsewhere. Second, it would be reasonable to defibrillate early, after a short period of CPR—about the time it takes to prepare, attach, analyze and charge the defibrillator. And third, we need to continue to focus on the treatments we know improve outcomes in cardiac arrest: immediate, continuous high-quality CPR and prompt defibrillation. Further research will help clarify the roles of CPR timing, medications and airway adjuncts.
