Animal models of CPR have shown that the optimal blood flows are achieved when chest compressions are performed at 80–100 times per minute and the chest is compressed 1.5 to 2 inches (3–5 cm). The new Guidelines for Cardiopulmonary Resuscitation published by the American Heart Association in 2000 recommend a chest compression rate of 100 times per minute. The proportion of time spent during the compression phase should be 50% of the relaxation phase.