Introduction: End-tidal carbon dioxide (ETCO2) depends on pulmonary blood flow therefore provides a surrogate indicator of cardiac output generated by cardiopulmonary resuscitation (CPR). Advanced life support resuscitation guidelines recommend using capnography for CPR quality assessment. Chest compressions and ventilations have opposing effects on ETCO2 during CPR: compressions generate blood flow causing a rise in pulmonary CO2 concentration while ventilations exchange high CO2 concentrated volumes with air or oxygen, decreasing CO2 concentration in the lungs.
Purpose of the study: To quantify ETCO2 decrease per each administered ventilation during out-of-hospital CPR.
Materials and methods: Fifty out-of-hospital cardiac arrest episodes containing the continuous capnogram, the compression depth and transthoracic impedance signals, and the ECG acquired from defibrillation pads were randomly selected from a larger database [1]. To isolate the effect of ventilations on ETCO2 we located chest compression pauses between consecutive compression series, where more than three ventilations were provided, and with a non-pulsatile rhythm. In each selected pause, we annotated the ETCO2 values corresponding to each ventilation. We analysed the relationship xn+1 = K∙xn, where xn and xn+1 were the ETCO2 levels of two consecutive ventilations. Factor K was calculated using least-square curve fitting. Adjusted R2 was used to examine the goodness of fit.
Results: Twenty-two (22) pauses were included in the study. Median (IQR) adjusted K factor was 0.91 (0.89-0.93). Adjusted R2 was 0.98 (0.95-0.99). This means that each ventilation produces an ETCO2 decrease of about 9%.
Conclusions: Knowledge of ETCO2 variation with each assisted ventilation could be used to help better assess the influence of chest compression metrics such as depth and rate on ETCO2 measurements during CPR.
[1] Leturiondo et al. Resuscitation 2018;124:63-68.

7 - A model for quantifying the influence of ventilations on end-tidal carbon dioxide variation during out-of-hospital cardiac arrest