Prove it: Does a delay between AED shock and compressions reduce survival rates?
Survival was highest when the pause was less than 20 seconds, and research supports the AHA recommendation to minimize interruptions in chest compressions
Rescue 18 and Engine 12 respond to an office building where bystanders are reported to be doing CPR. Engine 12 arrives first, verifies no pulse and takes over CPR. An AED applied by the firefighters recommends a shock. After the shock, the firefighters resume CPR.
Rescue 18 arrives and transfers the patient to a manual defibrillator, while medic Williams verifies the presence of ventricular fibrillation and begins charging the device. When he does, the firefighters stop CPR as one of them yells, "Clear!"
Williams asks the crew to resume chest compressions while the machine is charging; however, the firefighters seem reluctant to do so. After a short period of no compressions, Williams delivers the shock and the firefighters resume CPR.
Over the next 15 minutes, they establish an IV and administer epinephrine, amiodarone, and two additional countershocks. Each time Williams charged the defibrillator, the firefighters clear the patient’s chest.
At the 20-minute mark, the patient is asystolic. Since the arrest occurred in a public place, the local protocol does not allow the medics to terminate resuscitation efforts on scene. After the 10 minute ride to the hospital, the emergency department physician terminates the resuscitation effort.
In 2011, researchers from the Resuscitation Outcomes Consortium found the odds of survival significantly decreased when out-of-hospital rescuers stopped CPR for more than 20 seconds before delivering a shock.  However, the authors of that study identified a number of limitations that affected the final interpretation of the data, not the least of which was the small sample size of patients (n = 815).
In 2007, the ROC began enrolling patients for a randomized controlled trial called ROC PRIMED, which sought to examine what effects several different resuscitation strategies had on survival.  The authors of the current study used the more robust patient database from the ROC PRIMED study to examine the effects that peri-shock pauses in chest compressions has on outcome. 
From that database, the current researchers selected all adult patients who suffered an out-of-hospital cardiac arrest and presented in a shockable rhythm to the first arriving EMS crews. All patients received at least one shock from either a manual or automated defibrillator.
The research team excluded patients who either received their first shock from non-EMS personnel using a public access defibrillator or arrested in the presence of EMS crews.
Including these patients in the analysis could make the intervention appear more effective than it really is since survival in both of these patient groups is generally higher than when EMS arrives to find patients in cardiac arrest or when EMS delivers the first shock. The researchers also excluded the patients if the CPR process data was incomplete.
All of the participating EMS agencies used monitor/defibrillators equipped with impedance sensors. These devices recorded changes in electrical resistance across the chest that occurs when rescuers perform various interventions, such as chest compression or artificial ventilation.
As resistance changed, reviewers could determine the exact moment when rescuers delivered each chest compression. This allowed exact measurements of the time when no one was performing compressions before and after a shock, or the hands-off period.
The interval between the moment the rescue team stops compressions and delivers the shock is the pre-shock pause. The interval between delivery of the shock and the moment when the rescue team resumes chest compressions is the post-shock pause. Adding these intervals together gives the peri-shock pause. 
The primary outcome measure for this study was survival to hospital discharge. A secondary outcome measure was the neurological status of the patient. Researchers considered survival to be neurologically intact if upon discharge from the hospital the patient scored less than or equal to three on a Modified Rankin Score.
About 3,500 patients in the ROC PRIMED study presented to EMS in a shockable rhythm, or about 10 percent of all the patients in the ROC PRIMED study database. After excluding patients who did not meet the inclusion criteria or had missing data, the final patient population for this study was 2,006.
A comparison between the group with complete CPR process records and the excluded group found no significant differences with respect to gender, witnessed vs. unwitnessed arrest status, the presence of bystander CPR, or the location of the arrest. This suggests that excluding those patients likely had little effect on the outcome.
An unadjusted data analysis showed that the highest survival to hospital discharge rates occurred in patients with a pre-shock pause of 10.1 to 15.0 seconds. Survival was highest in patients with a post shock pause of 5 seconds or less.
Overall, survival was highest when the peri-shock pause was less than 20 seconds. However, unadjusted data includes the simultaneous effects of many variables, some of which could blur the true effect of the pauses.
Using a mathematical technique known as multivariate logistic regression analysis, the researchers isolated the effects that each of these variables had on the outcome.
After adjusting for the Utstein predictors of survival, chest compression fraction, compression rate and ROC site, the researchers found both the odds of survival and being neurologically intact were about 50 percent higher if rescuers keep the pre-shock interval to less than 10 seconds when compared to a pre-shock pause of greater than 20 seconds. These adjusted results represent the true effects of the intervention.
If rescuers kept the peri-shock interval under 20 seconds, both the odds of survival and the odds of being neurologically intact at survival almost doubled compared with peri-shock intervals over 40 seconds.
The researchers could not find any survival benefits associated with the post shock pause.
What this means for you
Animal studies published over a decade ago demonstrate that prolonged or frequent interruptions in chest compression caused by AED analysis or rescue breathing resulted in worsened myocardial perfusion and neurological impairment. [5, 6, 7]
Another study conducted in the out-of-hospital environment found an association between frequent interruptions in chest compression and a decreased probability of successful conversion from ventricular fibrillation to a perfusing rhythm.  Shortening the pre-shock pause by even a few seconds can improve the probability of a successful conversion following delivery of a shock. [9, 8]
EMS providers can have a positive influence on survival from out-of-hospital cardiac arrest by adopting any strategy that reduces the pre-shock and subsequent peri-shock pauses in chest compressions. This includes coordinating (with practice) the actions of the team so that the defibrillator operator can deliver the shock immediately after the chest compressor (and other team members) "clears" the patient’s chest.
To accomplish this, the defibrillator must be ready to deliver the shock even before the chest compressor has finished pushing on the patient’s chest. Rescuers who perform compressions while the AED is charging can reduce the pre-shock pause to less than three seconds.  Rescuers who charge a manual defibrillator during chest compressions can achieve similar results. 
Although the current findings support earlier work, EMS agencies must recognize what the research does not say. Although the data was collected as part of a randomized controlled trial for other interventions, this analysis was observational only.
Thus, it is only possible to state there is an association between the variables. Associations do not imply causation; one cannot state the reductions in the pre- and peri-shock intervals caused the increase in survival.
The researchers also report that many of the participating ROC sites did not collect data on chest compression depth. Thus, more than half of the compression depth data in the study group was missing. It is therefore not possible to know whether there was a difference in chest compression depth between patients who received the shortest or the longest hands-off intervals.
Since chest compression depth is known to influence survival , it is possible the current results reflect the compression depth effects rather than the effects of the hands-off interval.
Similarly, the researchers excluded 11 percent of the eligible patients because of missing shock pause data. The analysts could not find any significant differences between those with complete data and those with missing data suggesting the exclusion likely did not influence the outcome.
However, without the data, one can never be sure.
This is the largest study to examine the relationship between peri-shock pauses and survival following out of hospital cardiac arrest presenting in a shockable rhythm. The researchers replicated the findings from their previous work, and their results further support the American Heart Association recommendation to minimize interruptions in chest compressions, especially in the peri-shock interval. 
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