How the Peri-Shock Pause in Automated External Defibrillators Affects CPR

You and your EMT partner are dispatched in your BLS ambulance to a quiet residential neighborhood for an adult male in cardiac arrest. A paramedic unit has also been sent, but it’s several minutes behind you. Dispatch reports that they are coaching the patient’s wife on how to perform hands-only CPR. On your arrival, a police officer directs you into the living room where you find that another officer has taken over chest compressions.

Your partner switches with the officer while you prepare the automated external defibrillator (AED), turning the device on and then exposing your patient’s chest. Without interrupting your partner’s chest compressions you place the pads on the patient. The AED chirps, “Stand clear of patient; analyzing rhythm.” The room is quiet. You trade places with your partner to minimize hands-off time after the AED finishes analysis.

The AED continues, “Shock advised, charging.” You and your partner exchange glances, wondering why it takes so long for an AED to charge. “Stand clear of patient; press shock now.” Your partner presses the shock button and the patient’s body jerks as the shock is delivered. It then says, “It is safe to touch the patient.”

After immediately resuming chest compressions you think about how long it took to analyze, charge and shock the patient. You’re concerned because you are aware that your patient wasn’t receiving life-saving compressions during those valuable seconds of delay.

Sudden cardiac arrest (SCA) is a leading cause of death among adults in the U.S. and elsewhere. For people suffering SCA who present with a shockable rhythm presumably due to a cardiac etiology, survival to hospital discharge rates vary widely across the country–from close to zero in some areas up to better than 50% in others.(1)

Although high-quality CPR is the cornerstone of resuscitation efforts, unnecessary pauses and delays in CPR play a critical role in reducing rates of survival. One specific area of focus involves the pauses during the rhythm analysis and charging of the AED, also known as the peri-shock pause.


AED Operation
Automated external defibrillators, regardless of manufacturer, operate under the same general principles.(2) After the unit is turned on, the rescuer is given prompts to perform actions such as application of electrode pads or pressing a button to analyze the rhythm. Once this analysis period begins, the devices ask that the patient not be touched or moved so the computer algorithm can analyze the ECG. The AED will then classify the rhythm as either shockable or non-shockable.

Each manufacturer has different strategies for the analysis and classification of the rhythm. However, all of the devices exhibit high sensitivity and specificity for shockable and non-shockable rhythms. Modern analysis strategies even attempt to classify shockable rhythms by their likelihood for successful defibrillation.

Once analysis has determined a rhythm is shockable, an AED will direct the user to stand clear and deliver a shock to the patient after the unit has charged. Modern, biphasic devices will determine the patient’s transthoracic impedance and adjust the energy level and duration of application prior to the shock. This strategy enables a higher first chance success over older monophasic devices which delivered fixed energy levels. For subsequent shocks, some biphasic devices even feature escalating energy levels.

This high level overview represents how most devices work, with little variation. How and when an AED charges, however, differs from device to device. The “peri-shock pause” that occurs while an AED charges plays a large role in the hands-off time.(3) More importantly, the less hands-off time, the higher the compression fraction.

Compression fraction is the proportion of time during which chest compressions are performed in each minute of CPR. This takes into account the pauses for breaths, the pauses for AED analysis and charging, and all other unnecessary pauses. This proportion can be measured by machine or by hand and is considered a modifiable factor of quality.

How Important Is the Compression Fraction?
Minimally interrupted CPR is a relatively new, but critically important component of resuscitation. Animal studies in 2002 and 2003 that focused on hands-off time found that pauses greater than 15 seconds drastically reduced rates of return of circulation.(4.5) Any increase in the number or duration of these interruptions also decreased shock success.(6) Researchers then began evaluating each component of resuscitation in terms of its impact on hands-off time.

Interruptions for ventilations, pulse checks, advanced airway placement and rhythm analysis all were featured in numerous studies.(7) The animal trials that found associations with lower survival rates and poor outcomes paved the way for clinical trials in humans.(8)

With a critical eye on our standard of care, studies were designed to evaluate protocols that minimized hands-off and no-flow time.(9) The quality of CPR was benchmarked not only in terms of rate and depth, but also in terms of its interruptions. A high compression fraction quickly became the standard of care.

Researchers from the Resuscitation Outcomes Consortium concluded that, “increasing chest compression fraction (hands-on time) during out-of-hospital resuscitation of patients with ventricular fibrillation is an independent determinant of survival to hospital discharge. Devising CPR protocols that take advantage of this simple fact can save thousands of lives each year and are extremely inexpensive to implement.”(10) Therefore, the effect that a high compression fraction has on survival to discharge shouldn’t be underestimated.

Simply put, the more time we have our hands on the chest doing compressions, the more people we will send home from the hospital neurologically intact. Seattle-King County, Wash., is recognized internationally for having survival rates for witnessed cardiac arrest due to ventricular fibrillation exceeding 50%.

Peter Kudenchuk, MD, a professor of medicine at the University of Washington and associate medical director for King County EMS, relates the effect of peri-shock pauses on compression fractions: “Both animal and clinical studies have shown the adverse effects on outcome from even brief pauses in CPR immediately before defibrillation. If usin­g a manual defibrillator, one way to minimize such pauses is to pre-charge the device during the final seconds of CPR immediately before the next rhythm analysis. A “˜quick look’ to confirm [ventricular fibrillation] VF, followed immediately by shock from a pre-charged manual defibrillator can save precious wasted seconds of “˜no flow’ that would otherwise be taken up by charging.

“Even seemingly brief seconds of interrupted CPR can add up quickly during a resuscitation. I’m convinced that keeping those interruptions to a minimum is among the ingredients that contributes to resuscitation success in Seattle-King County, as it can anywhere.”


What About Our AED?
As we related in our scenario, providers who have used an AED during a cardiac arrest often feel like they have waited an eternity to resume chest compressions after the unit advises to “stand clear.” Seconds tick away as the machine analyzes the rhythm. If a shockable rhythm is detected, more seconds tick away as the AED charges. Finally, the shock is delivered, but yet more seconds go by before the unit advises to, “resume CPR.” During the peri-shock pause, which can approach 20 seconds or more, no chest compressions are delivered to the patient.

We know that interruptions of compressions for any reason have negative effects on survival.(11) Coronary perfusion pressure takes time to build up, and drops off swiftly and dramatically when chest compressions are stopped.(12) Forward flow remains inadequate during these interruptions, and perfusion pressures must be built up again after each pause in CPR.(13) When this happens just before defibrillation, the heart is no longer “primed” and in the best possible state to receive the shock.(14)

When studied in AEDs, peri-shock pauses have a definite effect on survival. A 2011 article in Circulation stated: “In patients with cardiac arrest presenting in a shockable rhythm, longer peri-shock and pre-shock pauses were independently associated with a decrease in survival to hospital discharge. The impact of pre-shock pause on survival suggests that refinement of automatic defibrillator software and paramedic education to minimize pre-shock pause delays may have a significant impact on survival.”(15)

Peri-shock pauses are an area that must be targeted for improvement in automated external defibrillator design. Wide variability exists in the length of time for both pre- and post-shock pauses in AEDs.(16) And,  of the seven commercially available designs studied, only one was capable of delivering a shock within 10 seconds from the end of CPR. One study, which compared automated with manual defibrillation, found a decrease in survival rates with AEDs.(5)

“˜Manufacturers Corner’
We asked three of the major manufacturers of AEDs–Philips, Physio-Control and ZOLL–to explain when and how their devices charge and how laypersons and rescuers can minimize the peri-shock pause.

Philips Healthcare AEDs begin charging during the analysis period using a technology they call Quick Shock. Specifically, their FR3, HS1 and FRx devices use the hands-off time for analysis and to also charge the device. Philips reports that these AEDs are typically capable of delivering a shock as soon as analysis determines whether the rhythm is shockable.

They note that during this hands-off period, it’s important that rescuers don’t touch the patient because Philips Healthcare devices continually reassess the rhythm for changes and will disarm their device if they detect a non-shockable rhythm. Their latest AED–the FR3–boasts hands-off times of eight seconds, below the recommended window of 10 seconds for interruptions in compressions.

Physio-Control takes a different approach, with their LIFEPAK 500 and 1000 AED, and have specifically engineered their defibrillators to allow pre-shock CPR. This technology, called cprMAX, allows services to configure the AED to allow up to 30 seconds of chest compressions after analysis has completed and while the device charges. Physio-Control reports a charge to shock time of less than seven seconds. 

With this approach, rescuers stop compressions to allow analysis and can then resume compressions while the device charges. Once the pre-shock CPR interval is completed, they’re directed to stop compressions and to deliver the shock.  Studies have shown that the sooner a defibrillation is delivered after the termination of CPR, the higher the chance for success.

ZOLL devices take a conservative approach and begin charging after the first three-second analysis period determines the rhythm is shockable. While the ZOLL AED is charging, it continues to analyze the rhythm to ensure it remains shockable. Rescuers are prompted to stand clear of the patient during this time period.

ZOLL reports that its devices are capable of delivering a shock within six seconds of analysis. ZOLL AEDs are also the first to feature a rich set of real-time CPR feedback including rate, depth and release benchmarking. Previously, this technology was available only on full cardiac monitors.


Improving AED Design
AED manufacturers each take different approaches to minimizing hands-off time while charging their devices. Philips and ZOLL AEDs charge quickly during analysis, while Physio-Control AEDs are programmed to provide for a period of CPR during charging. Neither of these strategies has yet to be proven superior; however, current resuscitation literature can drive the design of future AEDs.

A period of chest compressions while charging a manual defibrillator is strongly supported in the literature and common in resuscitative efforts. AHA guidelines initially suggested this in 2000 and reiterated its importance in 2005 and again in 2010.(17—19) A high compression fraction with a minimization of pre- and post-shock pauses is a cornerstone of successful resuscitation.

Pre-charging a manual defibrillator prior to analysis minimizes hands-off time and is becoming popular during team-based resuscitation. In this “pit crew” approach to CPR, which is being used by many EMS systems, personnel do not stop compressions for analysis until the manual defibrillator has finished charging. If the rhythm check is shockable, a shock is rapidly delivered and CPR continues, otherwise a rescuer disarms the device.”

When we asked the device manufacturers about pre-charging their manual devices during compressions, Philips and ZOLL expressed perceived safety concerns associated with an unattended charged device during CPR. However, a review of the literature doesn’t present the same concerns.(20) And  the typical semi-automated design of AEDs prevents the operation of the therapy button until the device prompts for rescuers to clear the patient.

Physio-Control’s design, which programs in a period for CPR during charging, is closest to the approach taken during pit crew CPR. However, given that there are two periods of interruptions, training and familiarity with the device are required to ensure each of these interruptions do not unnecessarily add to the peri-shock pause.

In Seattle-King County, Wash., Medic One has adopted a protocol of 30 uninterrupted compressions during AED charging, even if the device can deliver a shock sooner. After the shock is delivered, many of the major AED devices have added features to ensure high quality CPR continues between analysis periods. Philips devices can be configured to prompt users with metronomes and CPR instructions. ZOLL has integrated CPR feedback devices into their electrode pads. A metronome guides rate and the ZOLL AED displays depth and release metrics to ensure compression quality meets the standard.

Even with these features, it’s the opinion of the authors that device manufacturers could continue to improve the hands-off time currently required with their AEDs. Pre-charging during CPR has proven to be safe and feasible with manual defibrillators and could be integrated into automated devices as well. Use of CPR feedback devices that can filter CPR artifact would allow the AED charging systems to use predictive behavior to determine if charging prior to analysis is indicated. Without this capability,  AEDs will withhold charging until analysis starts. This approach balances battery life with the likelihood of shock success and no-flow time.

As ECG analysis and battery technology improves, there potentially will be no delay between hands-off and ready-to-shock times. Integration of CPR filtering and CPR feedback is a must as well. Is it feasible to have an AED that features CPR feedback metrics, predictive ECG analysis, instant shock capabilities and fail-safe, no-shock behavior within the next five years.

Case Conclusion
At the next rhythm check of the patient presented at the start of this article, the AED recommends another shock, and it’s delivered while compressors are rotated. The ALS unit arrives and is given a quick hand-off report. They establish intraosseous access and place an oropharyngeal airway device and a non-rebreather mask on the patient.

As the AED chirps, reminding the crews to stand clear of the patient, the paramedics switch the therapy cable over to their monitor.

Prior to asking for an interruption in compressions they pre-charge their defibrillator and ask that a fresh compressor be ready to take over CPR. At the rhythm check the patient remains in ventricular fibrillation. The paramedics shock the patient and immediately ask that compressions resume. A second ALS crew arrives to assist with the arrest and therapeutic hypothermia therapy.

In the next six minutes, the patient receives two more defibrillations, intubation during CPR, which is confirmed with waveform capnography, and three rounds of code medications. A sudden increase is noted in the patient’s end-tidal CO2, prompting the code commander to ask one of the team members to palpate the patient’s femoral and carotid pulses prior to the anticipated next rhythm check.

The defibrillator is again pre-charged but disarmed when sinus tachycardia is noted on the monitor. Simultaneously, a 12-lead ECG is obtained while post-arrest therapeutic hypothermia is initiated. The ECG shows an anterior wall ST-segment elevated myocardial infarction (STEMI) and the code commander steps outside to notify the hospital. The patient is secured onto a backboard and moved to the unit.

The patient receives emergent primary percutaneous coronary intervention (PCI), resulting in two stents placed in their left anterior descending coronary artery. Therapeutic hypothermia is continued in-hospital, and the patient is gradually rewarmed the next day in the coronary care unit. On day six, the patient is discharged neurologically intact and without sequelae.

Ten years ago, this scenario would be the plot for a made-for-TV movie. But today these saves happen across the U.S. A team-based approach to prehospital cardiac arrest care that emphasizes minimally interrupted chest compressions, early appropriate defibrillation and post-arrest care, including therapeutic hypothermia, maximizes your patient’s chance for survival.(21)

As systems work to streamline and improve their ALS care, we need to ensure we don’t lose focus on improving the important BLS care rendered to our patients. Dispatcher-initiated CPR instructions are becoming the standard and a broad push for public involvement in hands-only CPR and use of AEDs is in effect.

The continued enhancement of automated external defibrillators and expanded deployment and location alerting of bystanders are all areas  ripe for continued improvement and present the  opportunity to greatly affect patient outcomes as public access to, and use of, defibrillators becomes more commonplace.

Automated external defibrillators on the market today have a wide variability in peri-shock operation, each with their own strategy to minimize pre-shock and post-shock pauses. Manufacturers should work to ensure the next generation of devices integrates real-time CPR feedback, predictive analysis and instant shock capabilities to minimize the number and duration of interruptions in chest compressions. By leveraging advances made in cardiac arrest care, AED manufacturers have a unique opportunity to lead the way in basic life support care.


David Baumrind, BA, EMT-CC, is a graduate of Northwestern University, and an EMT-critical care at the East Hampton Village Ambulance Association in Suffolk County, New York. He’s an associate editor for the EMS 12-Lead Blog and Podcast. He remains passionate about the science of resuscitation, and has presented on topics such as high-performance CPR and the use of structured and supported debriefings to improve survival from sudden cardiac arrest. Contact him via email at
Christopher Watford, BSc, NREMT-P, is a lieutenant and board member at Leland Volunteer Fire/Rescue Department in southeastern North Carolina. He’s an associate editor for the EMS 12-Lead Blog and Podcast. At his day job, he works as a lead software engineer for Global Nuclear Fuel—Americas and is a captain on their industrial fire brigade. Contact him via e-mail at

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