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.