AimPrehospital cardiopulmonary resuscitation is performed from scene arrival to hospital arrival. The diverse prehospital resuscitation phases can affect the quality of chest compressions. This study aimed to evaluate the dynamic changes in chest compression quality during prehospital resuscitation. MethodsAdult out-of-hospital cardiac arrest patients treated without prehospital return of spontaneous circulation were included in Seoul between July 2020 and September 2021. The chest compressions quality was assessed using a real-time chest compression feedback device. The prehospital phase was divided by key events during the prehospital resuscitation timeline (phase 1: first 2 min after initiation of chest compression, phase 2: from the end of phase 1 to 1 min prior to ambulance departure; phase 3: from 1 min before to 1 min after ambulance departure; phase 4: from the end of phase 3 to hospital arrival). The main outcome was no-flow fraction. The no-flow fraction between prehospital phases was compared using repeated-measure analysis of variance. ResultsIn total, 788 patients were included. Mean no-flow fraction was the highest in phase 3 (phase 1: 11.3% ± 13.8, phase 2: 19.3% ± 12.3, phase 3: 33.0% ± 34.9, phase 4: 18.7% ± 23.7, p < 0.001). The mean number of total no-flow events per minute was also the highest in phase 3. The minute-by-minute analysis showed that the no-flow fraction rapidly increased before ambulance departure and decreased during ambulance transport. ConclusionDynamic changes in chest compression quality were observed during prehospital resuscitation phase. The no-flow fraction was the highest from 1 min before to 1 min after ambulance departure.