Purpose of the study: Chest wall stiffness decreases with increasing number of chest compressions (CC) during cardiopulmonary resuscitation (CPR). The aim of this study was to quantify the evolution over time of the compression force that is needed to achieve a certain depth during manual CPR.
Materials and methods: We retrospectively analysed CC force and depth waveforms extracted from 102 out-of-hospital cardiac arrest episodes (TVF&R, OR, USA). CPR was manual and guided with Q-CPR technology (Philips Medical Systems, MA, USA). For each CC, we measured compression depth and three features extracted from the force signal (see figure): maximum compression force (Fc), compression area index (CAIf) and release area index (RAIf). CCs presenting leaning (release force >2.5kg) were discarded. Values were averaged every 30 s and compared between three depth categories: 40-45, >45-50, >50-55mm, and according to CPR time: <10min, 10-20min, and >20-30min.
Results: Median (IQR) episode duration was 25 (19-33) min, and a total of 254,103 CC were annotated, 2,327 (1,714-2,954) per episode. Fc significantly decreased over time (P<0.001): for 40-45mm, Fc decreased from a median of 45 (35-57) kg in the <10min group to 32 (27-38) kg in the >20-30min group. For >50-55mm, Fc decreased from a median of 47 (39-57) kg to 39 (30-51) kg. CAIf and RAIf were statistically different between groups (P=0.0017, P<0.001, respectively), but changes were small: CAIf varied from 32 (30-36) % in the <10 min group to 34 (30-36) % in the 20-30 min group, and RAIf from 34 (30-38) % to 35 (31-39) %.
Conclusion: Rescuer’s force required to achieve a certain depth decreased with increasing CPR time, confirming that chest resistance decreases over time. The compression and recoil phases of the force waveform changed slightly over time. These CAIf and RAIf changes may be related to rescuer fatigue.

7 - Time-varying relationship between chest compression force and depth during manual cardiopulmonary resuscitation