Characterization of mechanical properties of adult chests during pre-hospital manual chest compressions through a simple viscoelastic model

Aim: The purpose of this study was to develop a simple viscoelastic model to characterize the mechanical properties of chests during manual chest compressions in pre-hospital cardiopulmonary resuscitation (CPR). Methods: Force and acceleration signals were extracted from CPR monitors used during pre-hospital resuscitation attempts on adult patients. Individual chest compressions were identified and segmented from the chest displacement computed using the force and acceleration. Each compression-recoil cycle was characterized by its elastic coefficient k (a measure of stiffness) and its compression and recoil damping coefficients, d_c and d_r, respectively (measures of viscosity). We compared the estimated and the calculated chest displacement to assess the goodness of fit of the model. We characterized the chest of patients at the beginning of CPR in relation to sex and age, and their variation as CPR progressed. Results: A total of 1,156,608 chest compressions from 615 patients were analysed. Mean (95% CI) coefficient of determination R² for the viscoelastic model was 97.9% (97.8–98.1). At the beginning of CPR, k was 104.9 N⋅cm^-1 (102.0–107.8), d_c was 2.868 N⋅s⋅cm^-1 (2.751–2.984) and d_r was 4.889 N⋅s⋅cm^-1 (4.648–5.129). Damping during recoil was significantly higher than during compression. Stiffness was lower in women than in men. There were no differences in damping coefficients with sex but a higher d_r with increasing age. All model coefficients decreased with compression count, with an overall decrease after 3,000 chest compressions of 34.6%, 48.8% and 37.2%, respectively. Conclusion: The model accurately described adult chest mechanical properties during CPR, highlighting differences between compression and recoil, sex and age, and a progressive reduction in chest stiffness and viscosity along resuscitation. Our findings may merit further investigation into whether patient-tailored and time-sensitive chest compression technique may be appropriate.