Types of heart rhythms:

• Ventricular Tachycardia (VT, V-Tach)
  Cardiac conduction orginates from reentrant circuits in the ventricles. The heart rate is usually fast and the rhythm is regular. The heart may or may not (pulseless VT) be able to eject blood to the body. An AED cannot tell the difference between the two. Shock may be advised, but should only be given if the victim does not have a pulse (pulseless VT).

• Ventricular Fibrillatio (VF, V-Fib)
  Cardiac conduction orginates from chaotic foci in the ventricles. The heart rate is fast and the rhythm is chaotic. The heart is not able to eject blood to the body (always pulseless). An AED may confuse fine VF and asystole because both can be somewhat flat. In the case of VF, shock should be advised and given.

• Asystole ("Flat-Line")
  There is no electrical activity and the electrocardiogram shows a "flat-line". The heart is not able to eject blood to the body (always pulseless). Shock is not advised. CPR and advanced cardiac life support are required.

• Bradycardia ("Brady")
  The heart rate is slow and the rhythm may be regular or irregular. The heart's ability to eject blood to the body may or may not be impaired. Advanced cardiac life support is required as well as CPR if there is no pulse.

• Pulseless Electrical Activity (PEA)
  Formerly known as electro-mechanical dissociation (EMD). Cardiac conduction is present and the electrocardiogram may be normal. However, the heart is not able to eject blood to the body for other reasons. CPR and advanced cardiac life support are required.

85% of pulseless victims are either in VF or pulseless VT. Only 15% of pulseless victims are in asystole, brachycardia, or PEA.

Why is it important to defibrillate early? When defibrillation is done within the first minute, survival is close to 90%. After the first minute, survival decreases by 2-10% per minute. After 10 minutes, chances of survival are almost nil. Performing CPR while waiting for defibrillation improves these odds. However, early defibrillation remains the critical step to successful resuscitation.

In 2005, the American Heart Association modified their guidelines regarding defibrillation. The old guidelines suggested performing up to 3 shocks in succession in patients with VF or pulseless VT. The new guidelines have reduced this to 1 shock to minimize the time wasted during shocking sequences and maximize the time performing CPR. Resuming CPR immediately after the first shock circulates blood to the heart and potentiates the effect of the shock.

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