If assisted reperfusion prior to defibrillation (and subsequent to failed shock) is important, it is reasonable to suppose that the benefit is related to quality of chest compressions. Observations of resuscitative efforts in the field indicate that maintaining compressions is one of the more difficult tasks, for a variety of reasons. Observations in the laboratory with trained paramedics show that the depth of compression and the compression rate diminish rapidly with time from the onset of CPR, without the participant being aware that his/her effort is actually diminished.