Progress in Resuscitation: Title and subTitle BreakAn Evolution, Not a Revolution

There was a time not long ago when the quality and quantity of chest compressions did not seem that important during resuscitation—at least not in comparison with early defibrillation. A ventricular tachyarrhythmia is the initiating event in more than 80% of patients who develop out-of-hospital, primary cardiac arrest during ambulatory electrocardiographic monitoring.¹ Survival declines rapidly if defibrillation is not performed in the first few minutes (defibrillation phase) because myocardial adenosine triphosphate (ATP) levels begin to decrease as fibrillating myocardial cells continue to consume ATP at a nearly normal rate.^{2 - 3}

As a result, ventricular fibrillation (VF) is the presenting rhythm in only 22% to 35% of out-of-hospital cardiac arrest cases when emergency medical services (EMS) personnel arrive on scene.^{4 - 5} By this time, myocardial ATP stores have often declined to critical levels and a defibrillation shock will usually terminate VF but frequently results in either asystole or pulseless electrical activity as cells run out of high-energy phosphate “fuel.” During this circulatory phase, a brief period of effective chest compression before defibrillation can boost myocardial ATP stores and increase the likelihood that a perfusing rhythm will follow defibrillation.^{6 - 9} Thus, the strategy for resuscitation from cardiac arrest has evolved from “shock first and often” to a time-critical, orchestrated approach of high-quality cardiopulmonary resuscitation (CPR), defibrillation, and postresuscitation care.

This new approach highlights the importance of high-quality, minimally interrupted chest compressions to maximize tissue oxygen delivery and intracellular high-energy phosphate levels. Conventional, closed-chest CPR is at best imperfect, producing hemodynamic change similar to that observed in cardiogenic shock. Minimal interruption of chest compression helps to maximize tissue oxygen delivery and, hence, myocardial high-energy phosphate levels. Aufderheide et al¹⁰ showed that both high-quality chest compression (adequate depth, force, and duration) and complete chest wall decompression are needed to maximize stroke volume and improve venous filling during the upstroke, respectively.

The odds of survival from cardiac arrest increase when continuous, or nearly continuous, high-quality chest compressions are performed during resuscitation.¹¹ Abella et al¹² used a monitor/defibrillator with additional sensing capabilities to measure CPR quality (chest compression rate and depth, ventilation rate, and the fraction of cardiac arrest time without chest compressions) during resuscitation in 67 patients who experienced in-hospital cardiac arrest. The authors found that the quality of chest compression was inconsistent and often did not meet published guideline recommendations, even when performed by well-trained hospital staff. In a follow-up study,¹³ the same authors used a handheld recording device to measure chest compression rate as a surrogate for CPR quality in 3 different hospitals and found that higher chest compression rates correlated significantly with initial return of spontaneous circulation in 97 in-hospital patients with cardiac arrest. Thus, chest compression quantity and quality are critical determinants of survival from cardiac arrest.

Positive pressure ventilation may not be necessary during the first few minutes of CPR because spontaneous agonal respirations often contribute to respiratory exchange of gases. Chest compression alone with a patent airway generates significant minute ventilation and the quantity of oxygen needed is reduced significantly during low blood flow states.^{14 - 15} Chandra et al¹⁶ assessed the time course of change in arterial blood gases during resuscitation in a canine experimental VF model and found that chest compression alone during CPR can maintain adequate gas exchange to sustain the oxygen saturation level above 90% for at least 4 minutes. Survival from out-of-hospital cardiac arrest is as good in humans when bystanders perform only chest compressions compared with when bystanders perform both chest compressions and mouth-to-mouth ventilation.¹⁷

In this issue of JAMA, Bobrow and colleagues¹⁸ provide an analysis based on a before and after comparison as well as a protocol compliance analysis of out-of-hospital cardiac arrest survival outcomes in which the standard EMS response, based on the American Heart Association's 2000 Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care,¹⁹ was replaced with an approach that attempted to deliver minimally interrupted chest compressions, early administration of a vasoconstrictor, and delayed invasive airway management. These changes are similar but not identical to CPR recommendations in the 2005 American Heart Association guidelines.^{20 - 21}

The 2000 Guidelines instructed rescuers to give 15 chest compressions followed by 2 ventilations and also called for the delivery of 3 “stacked” shocks for shockable rhythms without performing chest compressions in between defibrillation attempts. This practice resulted in a prolonged period of time without any chest compressions and subsequently resulted in large fluctuations in cerebral and cardiac blood flow. The introduction of what Bobrow et al termed minimally interrupted cardiac resuscitation (MICR) requires a dramatic change in behavior for EMS personnel by discouraging early bag valve mask ventilation and endotracheal intubation, and requiring 2 minutes of uninterrupted CPR before rhythm assessment, pulse checks, or defibrillation.

EMS rescue personnel who provided MICR were permitted to deliver bag valve mask ventilation at their discretion but were discouraged from hyperventilating patients. However, the authors cannot quantify how much ventilation was actually performed on individual cases and whether rescuers always performed the initial 200 continuous compressions. They also do not report the time differences to initial epinephrine administration in the 2 groups despite identifying more rapid administration of a vasoconstrictor as one of the goals of the new intervention. Because of these shortcomings, it is unclear exactly how much of the effect observed was due to the intervention and what part or parts of the intervention may be responsible for the beneficial effects. To the authors' defense, the detailed tracking of chest compression and ventilation quality remains elusive to most EMS systems. Although technology exists to track these parameters, it has not been available long enough to be adopted by most EMS systems and the tracking of ventilation is the least reliable function even when the equipment is used.

The fact that the authors were able to implement a dramatic change in a long-standing clinical practice by training more than 2000 EMS personnel on a novel, time-sequenced method for delivering resuscitation, implement a standardized data collection method for cardiac arrest in 62 EMS agencies, and develop a reliable mechanism for obtaining outcomes data in patients delivered to several hospitals throughout the state is a tremendous accomplishment and clearly raises the bar for other EMS agencies for the reporting of observational data. Only 61% of patients in the before and after analysis received complete compliance with the performance of MICR. This is not surprising given what is known about learning behavior and the prolonged amount of time typically required for widespread adoption of new treatments and therapies.

The authors demonstrate that the survival benefit associated with the MICR intervention decays over time. It is unclear whether the markedly improved survival observed immediately after the implementation of the MICR protocol represents a Hawthorne effect or an actual benefit from 1 or more of the clinically altered parameters in the study population. Another possibility is that a single training session without periodic refresher training or skills testing may be insufficient to maintain the skill-set necessary to achieve improved outcomes.

Despite the limitations of this study (which the authors acknowledge), there is an important take-away message: outcomes for resuscitating patients in cardiac arrest remain dismal, yet significant improvements are possible. Although this study cannot determine definitively which part or parts of this described intervention may be associated with the increase in survival, it does appear the take-home message is that survival can be improved considerably. Both the before and after analysis groups as well as the protocol compliance analysis groups demonstrated a substantial improvement in survival-to-hospital discharge without sacrificing good neurological status. No differences were observed in return of spontaneous circulation rates or survival-to-hospital admission after the implementation of the MICR protocol. Reasons could include that MICR provides no immediate benefit for restarting the heart or that the benefit is most evident in the postresuscitation reperfusion injury time frame. Although changes in local hospital postresuscitation care cannot be excluded as the reason for improved survival-to-hospital discharge, the fact that the single postresuscitation intervention known to significantly improve survival (hypothermia) was not implemented in any of these patients goes against this theory.

Although the concept of MICR needs further scientific evaluation, perhaps in the form of a randomized, controlled, clinical trial with precise documentation of protocol compliance, these details are likely not important factors to the numerous additional survivors who are back home with their families after the implementation of this new protocol. Progress in improving survival after cardiac arrest is most commonly made by a gradual evolution of science and its translation into clinical medicine rather than single, earth-shattering revolutions. This study, along with the study by Rea et al,²² represents confirmation that the quality of CPR, particularly the need for minimally interrupted chest compression and the lesser importance of positive pressure ventilation, is a meaningful development in the evolution of resuscitation science.