Purpose of the study: Manual chest compressions (CC) alter chest wall compliance in patients under cardiac arrest. Several studies have reported a decrease in chest wall stiffness with cardiopulmonary resuscitation (CPR) time. The aim of this study was to analyse if changes in chest compliance over CPR time are reflected by changes in the CC waveform metrics.
Materials and methods: We retrospectively analysed CC 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). Five metrics were measured for each CC (see figure): compression rate, compression depth (dc), duty cycle (DC), compression area index (CAId), and recoil area index (RAId). Values were averaged every 30 s and compared between three categories according to CPR time: <10 min, 10-20 min, and >20-30 min.
Results: Median (IQR) episode duration was 25 (19-33) min. A total of 254,103 CC were annotated, 2,327 (1,714-2,954) per episode. Compression depth and DC did not significantly change over time (P=0.73, P=0.1, respectively), varying from a median of 49 (42-56) mm in the <10 min group to 49 (43-56) mm in the >20-30 min group, and from 44 (41-47) % to 45 (42-47) %, respectively. Compression rate, CAId and RAId were statistically different between groups (P<0.001), but changes were small: 110 (104-116) min-1 to 113 (107-120) min-1; 42 (39-45) % to 41 (37-43) %; and 46 (43-48) % to 44 (42-47) %.
Conclusion: Changes in chest wall compliance during manual CPR did not importantly influence CC waveform. CPR guidelines have been developed based on the initial 10-minute period. Understanding the evolution of compression waveforms may help shape insight into whether adjustment of performance guidelines over the longer course typical of rescues may be warranted.

6 - Evolution of chest compression waveform metrics over time during manual cardiopulmonary resuscitation