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Introduction: According to resuscitation guidelines and research literature, chest compression (CC) depth, rate and impulsiveness are related to CPR quality. However, physiological changes of the chest and rescuer fatigue, following extended periods of CCs, may compromise the quality of CC administration. Aim: Retrospectively evaluate adaptions of rescuers to varying chests within series of CCs. Methods: Monitor-defibrillator recordings of adult out-of-hospital cardiac arrest cases were processed to identify continuous series of CCs between pauses. For each CC, its compression impulsiveness index (CII) and stiffness ( k ) were calculated. Each series was characterized by the mean k of the first 10 CCs. Chests were categorized by high, medium or low initial stiffness, into 3 equal-sized groups. CII and k were averaged every 10 CCs and normalized to the first set of each series, in order to study their dynamics within CC series. Results: 5,688 series of median (IQR) length 185 (114–216) CCs were used. Chests were characterized by k of 129.66 (114.89–154.69), 87.03 (80.27–94.94) and 58.41 (49.74–66.04) for high, medium and low stiffness. CII was 5.57 (5.08–6.17), 5.34 (4.90–5.90) and 5.10 (4.74–5.57), respectively. CII and k decreased within series ( p trend < 0.05). After 20 sets of 10 CCs, the median of CII declined 2.89%, 2.01% and 1.62% for high, medium and low stiffness chests; whereas the median of k dropped 15.88%, 10.66% and 2.33%. Conclusion: Chest stiffness decreases over the course of CC series, with greater declines observed in initially stiffer chests ( p < 0.05). Rescuers accommodate their efforts, but only slightly reduce impulsiveness; such reduction possibly being related to having to deal with a less stiff chest. Rescuers appear to adjust their efforts in response to changes in chest stiffness and not to fatigue.