Go
       
 

CPR SCIENCE

One fact about CPR and other postcardiac arrest treatments is certain: no amount of defibrillation will impact outcomes if there is no perfusion to the heart and brain. Re-establishing blood flow to the vital organs is the single most important factor for successful resuscitation, especially when the duration of cardiac arrest is prolonged beyond four minutes.

Tang et al., doing a study with an opioid receptor agonist, noted that after induced cardiac arrest, myocardial PO2 did not go to zero until after four minutes of non-perfusion. That was totally contradictory to traditional textbook physiology, which projects that when cardiac output ceases, the heart becomes ischemic immediately. The group was later able to show, using orthogonal polarization imaging, that the reason for this window was the time it takes until the arterial and venous pressures balance. Until they do, the red cells continue to bring oxygen to the myocardium. Beyond this window, the only means to resuscitate patients is to re-establish circulation to perfuse the myocardium.

Additional evidence for the four-minute window is found in energy stores. In order for the heart to contract, membrane potentials must be maintained, and calcium must remain sequestered in the cells. These processes are dependent upon adenosine triphosphate (ATP). As the oxygen supply is depleted, the myocardium becomes depleted of ATP. Membrane potentials degrade, myocardial function decreases, and the heart gradually becomes no longer receptive to a shock after about four minutes of VF. 

Coronary Perfusion Pressure

According to the 2000 American Heart Association Guidelines for CPR and ECC, “The important pressure for perfusion of the myocardium is coronary perfusion pressure….”

The driving force for coronary blood flow is aortic pressure less the pressure resisting flow (right atrial pressure), and this yields the blood pressure gradient for the vascular bed. Coronary perfusion pressure is therefore the difference between right arterial pressure (RAP) and aortic pressure (AoP) during diastole.

Note: Obtaining coronary perfusion pressure (CPP) is an invasive technique that measures the pressure in the coronary arteries immediately upon diastole and is used primarily for research purposes. It is neither routinely available nor practical in the resuscitation setting.

Nonetheless, CPP has tremendous clinical significance, and Paradis, who measured the coronary perfusion pressure of patients undergoing CPR in an ICU, demonstrated this.

circ1

In Paradis' study, no patient achieved return of spontaneous circulation (ROSC) with coronary perfusion pressures less than 15 mm Hg, while ROSC was achieved in 79% of those patients with a CPP greater than 25 mm Hg.

The following show the progression of ventricular fibrillation (VF) over time and the impact of CPR on reconstituting the waveform. Initially, VF is coarse and generally still shockable. Myocytes are still contracting uniformly along a few wave fronts.

circ2

After five minutes, myocyte contraction is more independent, more wave fronts develop, and the ability of the heart to respond to a shock declines dramatically. This critical time can be observed by the smoothing and decreasing amplitude of the waveform.

circ3

Shown below is the waveform after three minutes of effective CPR. By providing perfusion to the heart, CPR reconstitutes the VF into the shockable coarse form.

circ4

VF After Three Minutes of Effective CPR

Establishing circulation with CPR at a level fast enough and deep enough to achieve an effective CPP is therefore the goal of CPR compressions. This was clearly the goal of the changes to the 2005 AHA guidelines for CPR.

AHA 2010 GUIDELINES for CPR AND ECC

The American Heart Association 2010 Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care1 emphasizes the critical importance of effectively performed CPR to resuscitation.

2010 Guideline Changes

  • Refinements have been made to recommendations for immediate recognition and activation of the emergency response system based on signs of unresponsiveness, as well as initiation of CPR if the victim is unresponsive with no breathing or no normal breathing (ie, victim is only gasping).
  • “Look, listen, and feel for breathing” has been removed from the algorithm.   
  • Continued emphasis has been placed on high-quality CPR (with chest compressions of adequate rate and depth, allowing complete chest recoil after each compression, minimizing interruptions in compressions, and avoiding excessive ventilation).
  • There has been a change in the recommended sequence for the lone rescuer to initiate chest compressions before giving rescue breaths (C-A-B rather than A-B-C). The lone rescuer should begin CPR with 30 compressions rather than 2 ventilations to reduce delay to first compression.
  • Compression rate should be at least 100/min (rather than “approximately” 100/min).
  • Compression depth for adults has been changed from the range of 1½ to 2 inches to at least 2 inches (5 cm).

The Guidelines also recommend:

  • Allowing for complete chest recoil after each compression
  • Minimizing interruptions in chest compressions
  • Avoiding excessive ventilation

References

  1. 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2010;122(suppl 3):S640-S656.