Chest stiffness dynamics in extended continuous compressions cardiopulmonary resuscitation

Aim of the study: To characterize the effects of extended duration continuous compressions cardiopulmonary resuscitation (CPR) on chest stiffness, and its association with adherence to CPR guidelines. Methods: Records of force and acceleration were extracted from CPR monitors used during attempts of resuscitation from out-of-hospital cardiac arrest. Cases of patients receiving at least 1000 compressions were selected for analysis to focus on extended CPR efforts. Stiffness was normalized per patient to their initial stiffness. Force remaining at the end of compression was used to identify complete release. Non-parametric statistical methods were used throughout as underlying distributions of all types of measurements were non-Gaussian. Averages are reported as median (interquartile range). Results: More than 1000 chest compressions were delivered in 471 of 703 cases. Rate of change in normalized stiffness (S-n) was unrelated to patient age, sex or initial ECG rhythm, and did not predict survival. Most (76%) chests became less stiff over the course of resuscitation efforts. While the remainder (24%) exhibited increased stiffness, overall S-n decreased monotonically, declining by 31% through 3500 compressions. Rate adherence did not show a consistent trend with S-n. Depth adherence and complete release improved modestly with decreasing S-n. Conclusion: Chest compressions during extended CPR reduced the stiffness of most patients' chests, in the aggregate by 31% after 3500 compressions. This softening was associated with modestly improved adherence to depth and release guidelines, with inconsistent relation to rate adherence to guidelines.