Study on the linear relation between chest compression depth and the fluctuation caused in the thoracic impedance acquired by defibrillation pads

The thoracic impedance (TI) signal, available in current automated external defibrillators, has been proposed as an indicator of the compression depth (CD) in animal models of cardiac arrest. This study analysed the linear relationship between the maximum CD and the fluctuation caused in the TI in 19 out-of-hospital cardiac arrest episodes. The mean of the CD maxima, D_max, and the mean peak-to-peak fluctuation, Z_pp, were computed for every 5 s from the CD and TI signals, respectively. Three analyses were performed: distributions of D_max and Z_pp in all episodes, linear relation between D_max and Z_pp (correlation coefficient for every episode, R_e, and for the complete dataset, R_c) and time evolution of the correlation coefficient, R_i, for three consecutive intervals along every episode. Median (25^th - 75^th percentiles) for Z_pp were 1.12 (0.78 - 1.48), 1.35 (0.94 - 1.89) and 1.67 (1.09 - 2.33) Ω for D_max <;38, 38≤ D_max ≤51 (optimal compressions), and D_max >51 mm respectively. High overlap between the three distributions was observed. R_e varied between 0.04-0.83 (median=0.34), and R_c was 0.27. Time evolution of R_i did not show any tendency. R_i varied between 0.06-0.94 (median=0.52). Linearity between D_max and Z_pp showed high variability between episodes in humans. The correlation coefficient for the complete dataset was low.