['Performing high-quality cardiopulmonary resuscitation immediately before electrical defibrillation serves as an important predictor of shock success. Long preshock pauses in cardiopulmonary resuscitation frequently occur, as noted by recent clinical investigations. We sought to determine whether these long pauses were due to difficulties in identifying shockable rhythms or rather due to local factors during resuscitation attempts.', 'Prospective in-hospital study of cardiac arrest resuscitation attempts coupled with a retrospective review of preshock pause rhythms by 12 trained providers. Reviewers scored rhythms by ease of identification using a discrete Likert scale from 1 (most difficult to identify) to 5 (easiest to identify). The resuscitation cohort was organized into preshock pause-duration quartiles for statistical analysis. Resident physicians were then surveyed regarding human factors affecting preshock pauses.', 'A total of 118 preshock pauses from 45 resuscitation episodes were collected. When evaluated by quartiles of preshock pause duration, difficulty of rhythm identification did not correlate with increasing pause time. In fact, the opposite was found (longest preshock pause quartile of 23.8-60.2 secs vs. shortest pause quartile of 1.1-7.9 secs; rhythm difficulty scores, 3.2 vs. 3.0; p = .20). When 29 resident physicians who recently served on resuscitation teams were surveyed, 18 of 29 (62.1%) attributed long pauses to lack of time sense during resuscitation, and 16 of 29 (55.2%) thought that room crowding prevented rapid defibrillation.'] Does difficulty of cardiac arrest rhythm identification correlate with length of chest compression pause before defibrillation?