There are only two cardiac arrest rhythms to consider: ventricular fibrillation and pulseless ventricular tachycardia (VF/VT).
A. Ventricular Fibrillation/Pulseless Ventricular Tachycardia
VF is defined as a pulseless chaotic disorganized rhythm with an undulating irregular pattern that varies in size and shape and has a ventricular waveform greater than 150 beats per minute. VT is an irregular wide QRS complex tachy¬cardia (see the chapter Arrhythmias/Palpitations). Patients with V Tmay remain stable, alert, and have a pulse. Patients with unstable ventricular tachycardia are hemodynamically unstable with a blood pressure of less than 90 mmHg, chest pain, shortness of breath, clouding of consciousness, or rapid loss of consciousness.
The American Heart Association (AHA) Guidelines 2000 for CPR and emergency cardiovascular care advises that in patients with cardiac arrest it is advisable to always assume that the rhythm is VF or pulseless V T. Because individuals who can be saved from cardiac arrest are usually in VF or pulseless V T, the earliest possible delivery of defibrillation is the single most effective intervention. A recent study in Norway, however, indicates that the use of three minutes of CPR prior to defibrillation, rather than immediate defibrillation, resulted in better outcomes among VF patients who received attention more than five minutes after symptom onset. In patients who received defibrillation immediately, 46% achieved a spontaneous palpable pulse on admission versus 56% of those within three minutes of CPR. Further studies are necessary to clarify this finding.

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