CPR SCIENCE

One fact about CPR and other post–cardiac 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.

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.

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.

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.

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 2005 GUIDELINES for CPR AND ECC

The American Heart Association 2005 Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care⁵ emphasizes the critical importance of effectively performed CPR to resuscitation.

The most significant change to CPR recommended by the Guidelines is a change in the ratio of chest compressions to rescue breaths. The old standard, from the 2000 Guidelines, was 15 compressions for every two rescue breaths. This was changed to 30 compressions for every two rescue breaths. The change was based on studies showing that blood circulation decreased when compressions were interrupted and that it takes several compressions to build up enough pressure to begin re-circulating blood. This is the most significant change since CPR's inception in the early 1960s.

The Guidelines also recommend:

• Performing two minutes of CPR between shocks. Shocks should not be stacked one after another, suggesting that after a single shock, the time is better used to build up the CPP, since without building the CPP to 15 mm Hg there will be no ROSC.
• After an unwitnessed arrest, it is not useful to waste time trying to analyze a non-perfusing rhythm. CPR should be begun immediately.
• The number of ventilations should be reduced to 8-12 per minute. Hyperventilation raises intrathoracic pressure and can decrease the efficacy of compressions.

Further specific guidelines about the performance of CPR include:

• The person performing CPR should press hard enough so that a venous pulse may be felt during CPR in the absence of effective arterial blood flow.
• During compressions of an adult, there should be 1.5-2 inches of chest displacement.
• Compressions should be given rapidly, at about 80-100 per minute.
• Interruptions should be minimized. Ventilations should be given at 8-10 per minute.