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Physiological background

The failing heart

During sudden cardiac arrest, the failing heart goes into a life-threatening arrhythmia. The arrhythmia can be very rapid or appear as unorganised, chaotic electrical activity (called Ventricular Fibrillation; VF). Approximately 30% of the out-of-hospital patients have VF as first rhythm. This arrhythmia can be cured by an electrical shock delivered by a defibrillator.

The rest of the out-of-hospital patients either have a normal but non-perfusing rhythm (called Pulseless Electrical Activity) or no electrical activity at all (called Asystole – shown as a straight line on the electrocardiogram). These cardiac arrests are “non-shockable” , in which cardiopulmonary resuscitation together with treating the underlying cause are the most important rescue activities.

When every second counts

In the event of a sudden cardiac arrest, every second is crucial. If a defibrillator is readily available and the patient’s heart is in VF, the arrest can be successfully converted into a pulse-producing rhythm via an electrical shock.

However, after more than four minutes of cardiac arrest, a defibrillation alone is less likely to succeed, in which chest compressions performed first is key for survival.

What makes compressions so important?

First of all, compressions are needed to provide blood flow to the brain. Cardiac arrest means that there is no blood circulation and no delivery of oxygen. The brain starts to deteriorate after only a few minutes without oxygen, and after ten minutes severe brain damage is likely to have already occurred. To minimise this risk, compressions should be effectively performed and without interruption.

Secondly, effective chest compressions have shown to increase the coronary artery perfusion pressure, which keeps the heart muscle from suffering ischemia. Research (1) has shown that a coronary perfusion pressure over 15 mm Hg is necessary to restore spontaneous circulation.

Finally, if the heart is in VF, and if more than four minutes have lapsed since the arrest occurred, the heart needs to be “primed” before it is ready to respond to a shock that is to be delivered by a defibrillator. Experimental work (2) has shown that during the first five minutes of VF, blood moves from the arteries to the veins, in turn filling up the right side of the heart. The right side of the heart becomes expanded and the left part empties. To make a defibrillation successful the volume needs to be re-routed.

Physiology of CPR

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References with link to Pubmed

1. Paradis et al, J Am Med Assoc; 263:1106-13

2. Steen et al. Resuscitation 58 (2003); 249-258

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