The first phase of CPR is called the electrical phase and extends from the beginning of cardiac arrest to approximately 5 minutes following the arrest. Defibrillation is most effective during the electrical phase. If an AED is available when a cardiac arrest is witnessed, it is imperative that defibrillation pads are applied promptly and the device is allowed to analyze the victim. Approximately 70% of cardiac arrest occurs in the home. For this reason, it is recommended that one person in every household receives CPR/AED training, as this can help increase the number of survivors of cardiac arrest. AED’s are now available for home use and can be purchased without a prescription.
The Circulatory Phase
The second phase is the circulatory phase, also known as the hemodynamic phase. This phase last between 5-15 minutes after cardiac arrest and CPR is the most critical intervention during this time. Adequate myocardial blood flow is related to adequate coronary and cerebral (CPP) perfusion pressure which is the major factor in successful resuscitation efforts and neurologically normal outcomes. Not all resuscitations are successful, and those with poor CPP inevitably result in brain death. CPP is generated during chest compressions and takes time to develop or “build up”. When chest compressions are stopped CPP decreases. This is the reason standards for CPR have changed to include more chest compressions than in the past. When caring for a victim in cardiac arrest it is important that interruptions in chest compressions be kept to minimum.
The Metabolic Phase
The metabolic phase extends beyond 10-15 minute after cardiac arrest. At this point the effectiveness of CPR and defibrillation are minimal and the prognosis for a positive outcome is poor.
Therapeutic Hypothermia
The 2005 resuscitation guidelines recommend that out of hospital cardiac arrest patients who have been resuscitated undergo therapeutic hypothermia being cooled to between 89.6 – 93.2 for 12 to 24 hours if:
They remain unconscious
They have return of spontaneous circulation (ROSC)
The patient’s initial cardiac rhythm was ventricular fibrillation
Therapeutic hypothermia has been associated with improved functional recovery and reduced cerebral histological deficits in various models of animals in cardiac arrest. It may suppress many chemical reactions associated with reperfusion injury and improve oxygen delivery to the brain. Therapeutic hypothermia also appears to maintain stroke volume and arterial blood pressure, as well as increase systemic vascular resistance while decreasing the heart rate.
