Purpose: High temporal resolution brain haemoglobin concentration signals obtained by near infrared spectroscopy show chest compression activity during cardiopulmonary resuscitation. The purpose of this study was to develop an algorithm to measure chest compression frequency in out-of-hospital cardiac arrest (OHCA) using cerebral oximetry signals.
Materials and methods: Concurrent recordings of cerebral oximetry (NIRO, Hammamatsu) and thoracic impedance (LifePak15, Stryker) were obtained by an advanced life support ambulance of Emergentziak-Osakidetza, the Basque emergency system. Chest compressions were annotated in the impedance and used as ground truth. High time-resolution (20Hz) changes of oxy (ΔO2Hb), deoxy (ΔHHb) and total (ΔcHb) haemoglobin changes were obtained in the left frontal lobe. The three haemoglobin signals were filtered in the 80-220 compressions per minute (cpm) range, and the signal spectrum computed applying the Fast Fourier Transform to 10s intervals. The chest compression frequency was estimated as the frequency with largest spectral amplitude.
Results: 29 OHCA patients were included, and the median (1–3 quartile) delay to connect the oximeter was 8.3 (5.8–12.3) minutes. In total 284 chest compression sequences were analysed with median duration 1.1 (0.7–1.6) minutes, and 133 (90– 179) compressions per sequence. The median (10–90 percentile) absolute error for compression frequency for the ΔO2Hb, ΔHHb and ΔcHb signals were 0.67 (0.11–5.45) cpm, 0.83 (0.13–19.09) cpm and 0.96 (0.15–28.90) cpm, respectively. The median relative errors were 0.60 (0.10–4.48)%, 0.75 (0.12–14.77)% and 0.86 (0.13– 23.41)%, respectively. The Bland-Altman analysis (see Figure) for compression frequency using the ΔcHb signal had 90% and 95% levels of agreement (LOA) of -2.94–4.38 cpm and -6.40–7.79 cpm, respectively.
Conclusions: Chest compression frequency can be accurately measured using haemoglobin concentration signals measured by high time-resolution cerebral oximetry. Total haemoglobin gives a more accurate measure than oxy or deoxy haemoglobin concentration.