Feasibility of compression depth estimation from the acceleration signal during cardiopulmonary resuscitation in long-distance trains

Early cardiopulmonary resuscitation (CPR) and early defibrillation improve survival from out-of-hospital cardiac arrest. Long distance trains are increasingly being equipped with defibrillators. CPR feedback devices help rescuers to deliver chest compressions with an adequate depth. Most of them are based on accelerometers placed beneath the rescuer´s hands. However, in a moving train the measured acceleration is a combination of the acceleration of the chest and that of the train. We wanted to evaluate the accuracy of two accelerometer-based systems in this scenario. Chest compressions were delivered on a resuscitation manikin during the Zaragoza-Bilbao (Spain) ALVIA train route. A tri-axial accelerometer was placed between the manikin s chest and the rescuer s hands. We acquired 3min records between consecutive stations with compressions delivered at target depths of 35 and 50 mm. Records corresponding to intervals with diferent average train velocities were selected. We applied a time-domain (TD) method and a frequency-domain method (FD) to the acceleration records for estimating the compression depth. We analysed the implications of using a single axis (al) or composing the three axis (a3) of the acceleration. The median (IQR) unsigned error in mm was 6.4 (3.710.1), 5.9 (2.9-10.1), 1.8 (0.8-3.1), and 2.0 (1.0-3.6), for TDa1, TDa3, FDa1, and FDa3, respectively. Chest compression depth could be accurately estimated from the spectral analysis of the acceleration in a moving train. The accuracy of the time-domain method was severely compromised, with median errors above 10% of the target depth.