Cardiac Resuscitation Research Review

Chest compressions have been the center of attention during cardiac resuscitation for the past few years, and for good reason. In its 2010 guidelines revision, the American Heart Association (AHA) rewrote the ABCs to the now established CAB approach based on increasing data linking improved outcomes to effective compressions.¹

Since then the science of chest compressions has developed rapidly. This article reviews two recently published papers on two of the most controversial subtopics: continuous chest compressions and automated chest compression devices.

Continuous Chest Compressions

Graham N, et al. Trial of continuous or interrupted chest compressions during CPR. N Engl J Med, December 3, 2015.

This is a large study performed across eight of the Resuscitation Outcomes Consortium (ROC) sites in North America and 114 EMS systems.

A total of 23,711 adult patients with non-traumatic out-of-hospital cardiac arrest were enrolled and assigned to either the continuous compressions or interrupted compressions groups. The outcomes measured were survival to hospital discharge and favorable neurologic function at discharge. The results showed no significant difference in survival or favorable neurologic outcome between the two methods.

Table 1 lays out the numbers in the trial. Table 2 lists the patients who were excluded, which is important to be aware of as these findings do not necessarily apply to them. Table 3 explains the protocol used in each arm of the trial.

There has been a lot of discussion about whether compression-only CPR is superior to conventional CPR. Previous studies showed an improvement in outcomes for both bystander-provided compression-only CPR and EMS-provided minimally interrupted compressions.^2,3 However, these studies were observational, which is a lower-quality standard of evidence than this newer randomized trial. So what does this mean for EMS? Before drawing conclusions, let’s discuss some interesting findings in the data.

The chest-compression fraction is a measure of the proportion of each minute that compressions were actually being delivered. The 2015 AHA guidelines call for a minimum of 0.6, or 60%. Conventional CPR reduces the fraction by definition due to pauses for ventilations, while other pauses could occur in any setting. So one of the advantages that continuous compressions should have over conventional CPR is a better chest-compression fraction.

In this study, both arms had very high chest-compression fractions, 0.77 and 0.83. Why is this? This is likely the effect of well-trained and effective EMS teams providing high-quality CPR at baseline. CPR training has strongly advocated minimizing interruptions in compressions for several years now, and it seems to be paying off. However, this already high fraction may have reduced much of the possible improvement to be gained from continuous compressions.

Another important point to note is that the continuous compressions protocol was not the same as the established cardio-cerebral CPR (CCR) that involves passive oxygenation and delayed advanced airway insertion while three cycles of 200 chest compressions are given. Of note, that protocol was given a “reasonable alternative” recommendation in the 2015 AHA guidelines.⁴ This study does not support or refute the usefulness of that study, but there is a separate ongoing study evaluating this approach.

Bottom Line

This new study does not dramatically change our everyday practice. It finds that continuous chest compressions with PPV do not improve outcomes when compared to conventional CPR, but it doesn’t worsen them either. For now, EMS providers should follow the 2015 AHA guidelines as built into their local protocols.

Automated Chest Compression Devices

Bonnes J, et al. Manual cardiopulmonary resuscitation versus CPR including a mechanical chest compression device in out-of-hospital cardiac arrest: A comprehensive meta-analysis from randomized and observational studies. Ann Emerg Med, Nov 19, 2015 (Epub ahead of print).

The use of mechanical CPR devices has grown considerably over the past decade and continues to be implemented in an increasing number of EMS systems. Despite the promising early observational studies, several randomized trials have shown mechanical CPR devices to be at best equivalent, and at worst harmful. In fact, the first randomized trial had to be halted early due to safety concerns.⁵

This new meta-analysis looks at 20 published studies on mechanical chest compression devices, five of which are randomized (high quality of evidence) and 15 that are observational (lower quality of evidence). Overall, the five randomized trials included 12,206 patients and showed no improvement in survival or neurologic outcome when mechanical compressions were compared to manual compressions.

Interestingly, the observational studies did show an improvement in survival to hospital admission when mechanical compressions were used. However these studies also showed no improvement for the other two outcome measures of survival to discharge and positive neurologic outcome. Why is this? In observational studies, responders had the ability to use their discretion in using the device. This creates risk of a phenomenon called “selection bias,” in which patients who were more likely to survive were preferentially given the device, which led to better outcomes. Using a randomized trial format mitigates this and other biases, which is why it is considered the gold standard for evidence-based medicine.

So why don’t the mechanical CPR devices work as well as we hoped? It depends on your perspective. Previous individual randomized trials suggested that devices may be equivalent, which in a resource-limited setting such as a scene or the back of an ambulance could actually be a plus. Using a device that is shown to be as effective as manual CPR can free up crew members to pursue other tasks and prevent standing in the back of a moving ambulance.

This article brings up some other important issues about mechanical CPR devices that are yet to be addressed, such as efficacy of incorporated defibrillation algorithms. Most notably, they draw attention to the lack of data on the safety of the devices and suggest that differing injury patterns between manual and mechanical CPR could affect survival.

Bottom Line

The authors conclude that “the cumulative evidence of high-quality randomized data does not support a routine strategy of mechanical CPR to improve clinical outcomes.” However, they do not seem to worsen outcomes either, so for now, it is reasonable to continue to use them in the field as an alternative to manual CPR.

References

1. Berg RA, Hemphill R, Abella BS, et al. Part 5: adult basic life support: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2010; 122: Suppl 3: S685–705.

2. Bobrow BJ, Clark LL, Ewy GA, et al. Minimally interrupted cardiac resuscitation by emergency medical services for out-of hospital cardiac arrest. JAMA, 2008; 299: 1158–65.

3. Bobrow BJ, Spaite DW, Berg RA, et al. Chest compression-only CPR by lay rescuers and survival from out-of-hospital cardiac arrest. JAMA, 2010; 304: 1447–54.

4. Kleinman ME, Brennan EE, Goldberger ZD, et al. Part 5: adult basic life support and cardiopulmonary resuscitation quality: 2015 American Heart Association guidelines update for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation, 2015; 132: Suppl 2: S414–35.

5. Hallstrom A, Rea TD, Sayre MR, et al. Manual chest compression vs use of an automated chest compression device during resuscitation following out-of-hospital cardiac arrest: a randomized trial. JAMA, 2006;295:2620–2628.

Sean M. Kivlehan, MD, MPH, NREMT-P, is an international emergency medicine fellow at Brigham & Women’s Hospital, Harvard Medical School. He is also a  member of the EMS World Editorial Advisory Board. E-mail sean.kivlehan@gmail.com.