Method and apparatus for delivering atrial defibrillaton therapy with improved effectiveness

A method for defibrillating a patient's atria with an implantable defibrillator, comprising:

• (a) detecting atrial fibrillation in the patient's heart;
• (b) monitoring cycle length stability of at least one atrial fibrillation signal across first and second implanted electrodes during a first monitoring time period following atrial fibrillation detection;
• (c) after said first monitoring time period, evaluating cycle length stability of said signal relative to said cycle length stability monitored during said first monitoring period during a second monitoring time period;
• and
• (d) delivering an atrial defibrillation shock during said second monitoring period when cycle length stability of said atrial fibrillation signal is within a desired measure of cycle length stability monitored during said first monitoring time period.

An implantable defibrillator, comprising:

• a pulse generator;
• atrial fibrillation sensing means;
• sensing means for monitoring fibrillation cycle lengths during a monitoring time period following fibrillation detection;
• electrode means for delivering a defibrillation shock during a period of atrial fibrillation when said fibrillation cycle lengths are within a desired measure of each other.