(Circulation. 2002;105:2270.) © 2002 American Heart Association, Inc. Clinical Investigation and Reports Effects of Interrupting Precordial Compressions on the Calculated Probability of Defibrillation Success During Out-of-Hospital Cardiac Arrest Trygve Eftestøl, PhD; Kjetil Sunde, MD, PhD; Petter Andreas Steen, MD, PhD From Stavanger University College, Department of Electrical and Computer Engineering, Stavanger, Norway (T.E.), and Ulleval University Hospital, Department of Anesthesiology, Oslo, Norway (K.S., P.A.S.). Correspondence to Trygve Eftestøl, Stavanger University College, Department of Electrical and Computer Engineering, PO Box 2557, Ullandhaug, 4091 Stavanger, Norway. E-mail trygve.eftestol@tn.his.no Background— Cardiopulmonary resuscitation (CPR) creates artifacts on the ECG and, with automated defibrillators, a pause in CPR is mandatory during rhythm analysis. The rate of return of spontaneous circulation (ROSC) is reduced with increased duration of this hands-off interval in rats. We analyzed whether similar hands-off intervals in humans with ventricular fibrillation causes changes in the ECG predicting a lower probability of ROSC. Methods and Results— The probability of ROSC after a shock was continually determined from ECG signal characteristics for up to 20 seconds of 634 such hands-off intervals in patients with ventricular fibrillation. In hands-off intervals with an initially high (40% to 100%) or median (25% to 40%) probability for ROSC, the probability was gradually reduced with time to a median of 8% to 11% after 20 seconds (P<0.001). In episodes with a low initial probability (0% to 25%; median, 5%), there was no further reduction with time. Conclusions— The interval between discontinuation of chest compressions and delivery of a shock should be kept as short as possible. Key Words: cardiopulmonary resuscitation • defibrillation • electrocardiography • heart arrest • Fourier analysis