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The Science of CPR


Cardiopulmonary resuscitation (CPR) is celebrating its 50th anniversary this year. Although used for many years in conjunction with other resuscitation interventions, recent studies demonstrating the reality of its use in clinical settings and linking its quality to patient outcomes have revolutionized attitudes surrounding the importance of properly performed chest compressions. The key factors important to quality chest compressions include chest compression rate, depth and fraction, which will be described separately below.

Compression Depth
Chest compression depth is one of the most important considerations in CPR quality but one of the hardest to adequately achieve. Current AHA Guidelines advocate a compression depth of 1.5 to 2 inches.(1) However, clinical studies have shown that inadequate depth is common for both in-hospital (IHCA) and out-of-hospital cardiac arrest (OHCA) populations, with compressions shallower than 1.5 inches being delivered 37.4% to 72% of the time during actual resuscitation efforts.(2-4) This is important because deeper chest compressions have been correlated with improved perfusion and key clinical outcomes.(4,5)

Chest compression depth is one of the main determinants of coronary perfusion pressure (CPP), which in turn is a primary predictor of survival.

Ristagno and colleagues demonstrated that optimal compression depth in pigs of 25% of the anterior-posterior chest diameter (6 cm) vs. 70% of that (4.2 cm) resulted in significantly higher CPP and end-tidal carbon dioxide, a marker of perfusion, which correlated with markedly improved 72-hour survival rates.(6)

The importance of CPP during cardiac arrest in humans was corroborated by Paradis and colleagues in their clinical study of 100 witnessed OHCAs.(7) In that study, CPP was found to be the best single predictor of ROSC, with maximal values less than 15 mmHg perfectly predicting a failure to achieve ROSC.

In more recent clinical work, our group measured CPR quality using an accelerometer-equipped defibrillator in both the in-hospital and out-of hospital settings. This study showed a dose-response correlation between increasing compression depth and shock success with a twofold increase in shock success associated with every additional 5 mm of depth.(5)

Using the same technology in the OHCA setting, Kramer-Johansen et al showed a similar dose-response associ­ation between compression depth and survival to hospital admission with an adjusted 5% increase in the odds of survival for every 1 mm of compression depth (p=0.01) (see Figure 1, above).(4)

Compression Rate
As with compression depth, the number of compressions delivered per unit of time is also highly correlated with outcomes in both animal and clinical studies, and the reality is often different than the recommendations. Studies in both the IHCA and OHCA setting have shown that compression rates are lower than the AHA-recommended rate of 100/minute in as much as half the time in clinical practice and that this, too, is associated with poor outcomes.(1-3,8)

In an animal study, decreasing compression rates correlated linearly with decreased CPP and rates of ROSC.9 This finding was supported by a multicenter IHCA study, in which patients achieving ROSC received a higher number of compressions per minute when compared with those who did not (90+17/minute for initial survivors vs. 79+18/minute for non-survivors [p=0.003]).(8)

An important consideration in compression rate calculations is the total pause time in compressions, which significantly affects the actual compressions/minute the patient receives. For example, Wik and colleagues showed a mean rate of 121/minute when compressions were ongoing, but when combined with a compression fraction of 52% (time with compressions/total time without a pulse), the mean compression rate was only 64/minute, far below the 2005 standard.(3)

Pauses in Compressions
The 2005 Guidelines advocate pauses of no more than 10 seconds during each two-minute cycle of CPR, which would result in a compression fraction of over 90%.(1) However, it is common for chest compressions to be paused for a variety of reasons, which include drug administration, line and airway management, pulse and rhythm checks or patient transport.

Even brief (<15 second) pauses in chest compressions have been shown to correlate with myocardial dysfunctional, abrupt and profound drops in CPP and carotid artery blood flow, and increases in right atrial pressures with concomitant decreases in left heart pressures, which result in decreased rates of ROSC.(9,10) Interestingly, although these changes occur within a few seconds of pausing compressions, it takes more than one minute of good-quality chest compressions to restore them to pre-pause levels.(10)

Clinical studies have confirmed that pauses in compressions are detrimental to cardiac arrest victims. Our group showed that the median pause immediately preceding defibrillation (pre-shock pause) was 15.3 seconds and that every five-second decrease in that pause was associated with an almost doubling of the odds of shock success.(5)

Similarly, a Resuscitation Outcomes Consortium study demonstrated that only 14% of the 506 patients studied had a compression fraction above 80%. In addition, compression fraction was found to be an independent predictor of survival to discharge for patients with an initially shockable rhythm.(11)

Given this understanding, new emphasis has been placed on minimizing interruptions in chest compressions. In fact, the AHA issued a statement in 2008 advising compression-only CPR for untrained bystanders. Those who have been previously trained in CPR are now given the choice of conducting 30:2 compressions with rescue breaths or continuous, compression-only CPR.(12)

In addition, studies looking at hands-only CPR for untrained rescuers and “CAB” (i.e., circulation, airway, breathing) strategies for trained rescuers have shown significant promise. Bobrow and colleagues introduced a protocol of minimally interrupted cardiac resuscitation (MICR) for OHCAs in two cities.(13) The protocol consisted of an initial 200 compressions at a rate of 100/minute, rhythm analysis with a single shock if indicated, and immediate delivery of 200 compressions prior to pulse check and rhythm analysis. In lieu of rapid endotracheal intubation, an oral-pharyngeal airway and a nonrebreather face mask were placed and passive oxygen insufflation was allowed for the first three cycles of compressions and rhythm analysis. For all rhythms, survival to hospital discharge increased from 1.8% to 5.4% for the MICR group vs. historical controls (OR 3.0; 95% CI, 1.1–8.9). For witnessed arrest with shockable rhythms, survival to discharge increased from 4.7% to 17.6% in the MICR group vs. historical control (OR 8.6; 95% CI, 1.8–42.0).

Kellum et al instituted a similar protocol for witnessed arrests with initial shockable rhythms in two rural EMS counties that resulted in a significant improvement in both overall patient survival (47% vs. 20%) and neurologically intact survival (39% vs. 15%, p=0.002).(14)

Improving CPR Quality
Various methods by which to improve compression quality and patient outcomes are currently under investigation. Training strategies that incorporate simulation, frequent educational refreshers, debriefing and real-time feedback are being studied with promising results.(15) Treatment bundles that incorporate combinations of these, as well as changes in resuscitation protocols that delay ventilations early during the resuscitation effort to provide consistent, continuous chest compressions, have also been put into practice in some clinical settings.

Efforts to improve technology are also underway. Defibrillator tech­nology that filters artifact in the ECG tracings produced by CPR may be useful because it allows for rhythm analysis without the need to pause compressions for rhythm checks.

CPR is one of few interventions known to affect survival from cardiac arrest, but is hard to do well in clinical practice. Common errors include shallow and slow chest compressions with frequent interruptions. Focused interventions to address these shortfalls could improve survival from cardiac arrest.


1. 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2005;112(24 Suppl):IV1–203.
2. Abella BS, Alvarado JP, Myklebust H, et al: “Quality of cardiopulmonary resuscitation during in-hospital cardiac arrest.” JAMA. 2005;293(3):305–310.
3. Wik L, Kramer-Johansen J, Myklebust H, et al: “Quality of cardiopulmonary resuscitation during out-of-hospital cardiac arrest.” JAMA. 2005;293(3):299–304.
4. Kramer-Johansen J, Myklebust H, Wik L, et al: “Quality of out-of-hospital cardiopulmonary resuscitation with real time automated feedback: A prospective interventional study.” Resuscitation. 2006;71(3):283–292.
5. Edelson DP, Abella BS, Kramer-Johansen J, et al: “Effects of compression depth and pre-shock pauses predict defibrillation failure during cardiac arrest.” Resuscitation. 2006;71(2):137–145.
6.  Ristagno G, Tang W, Chang Y et al: “The quality of chest compressions during cardiopulmonary resuscitation overrides importance of timing of defibrillation.” Chest. 2007;132:70–75.
7. Paradis NA, Martin GB, Rivers EP, et al: “Coronary perfusion pressure and the return of spontaneous circulation in human cardiopulmonary resuscitation. JAMA. 1990;263(8):1106–1113.
8. Abella BS, Sandbo N, Vassilatos P, et al: “Chest compression rates during cardio­pulmonary resuscitation are suboptimal—A prospective study during in-hospital cardiac arrest.” Circulation. 2005;111(4):428–434.
9. Yu T, Weil MH, Tang W, et al: “Adverse outcomes of interrupted precordial compression during automated defibrillation.” Circulation. 2002;106(3):368–372.
10. Steen S, Liao Q, Pierre L, et al: “The critical importance of minimal delay between chest compressions and subsequent defibrillation: a haemodynamic explanation.” Resuscitation. 2003;58(3):249–258.
11. Christenson J, Andrusiek D, Everson-Stewart S, et al: “Chest compression fraction determines survival in patients with out-of-hospital ventricular fibrillation.” Circulation. 2009;120:1241–1247.
12. Sayre MR, Berg REA Cave DM et al: “Hands-only (compression-only) cardiopulmonary resuscitation: A call to action for bystander response to adults who experience out-of-hospital sudden cardiac arrest: A science advisory for the public from the American Heart Association Emergency Cardiovascular Care Committee.” Circulation. 2008;117:2162–2167.
13. Bobrow BJ, lani CL, Ewy GA, et al: “Minimally interrupted cardiac resuscitation by emergency medical services for out-of-hospital cardiac arrest.” JAMA. 2008;299(10):1158–1165.
14. Kellum MJ, Kennedy KW, Barney R, et al: “Cardiocerebral resuscitation improves neurologically intact survival of patients with out-of-hospital cardiac arrest.” Annals of Emergency Medicine. 2008;52(3):244–252.
15. Seethala RR, Esposito EC, Abella BS: “Approaches to improving cardiac arrest resuscitation performance.” Current Opinions in Critical Care. 2010;16:196–202.

Disclosure: Dr. Edelson reports that she has received a grant and research support from Philips Healthcare and Laerdal Medical (two partners in the CPR Improvement Working Group).

This article originally appeared in an editorial supplement to December 2010 JEMS, FireRescue, Journal of Emergency Nursing and ACEP News as The Science of CPR: Identifying the factors key to improved patient outcomes.


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