The impact of ventilation rate on end-tidal carbon dioxide level during manual cardiopulmonary resuscitation

Aim: Ventilation rate is a confounding factor for interpretation of end-tidal carbon dioxide (ETCO₂) during cardiopulmonary resuscitation (CPR). The aim of our study was to model the effect of ventilation rate on ETCO₂ during manual CPR in adult out-of-hospital cardiac arrest (OHCA). Methods: We conducted a retrospective analysis of OHCA monitor-defibrillator files with concurrent capnogram, compression depth, transthoracic impedance and ECG. We annotated pairs of capnogram segments presenting differences in average ventilation rate and average ETCO₂ value but with other influencing factors (e.g. compression rate and depth) presenting similar values within the pair. ETCO₂ variation as a function of ventilation rate was adjusted through curve fitting using non-linear least squares as a measure of goodness of fit. Results: A total of 141 pairs of segments from 102 patients were annotated. Each pair provided a single data point for curve fitting. The best goodness of fit yielded a coefficient of determination R² of 0.93. Our model described that ETCO₂ decays exponentially with increasing ventilation rate. The model showed no differences attributable to the airway type (endotracheal tube or supraglottic King-LT-D). Conclusion: Capnogram interpretation during CPR is challenging since many factors influence ETCO₂. For adequate interpretation, we need to know the effect of each factor on ETCO₂. Our model allows quantifying the effect of ventilation rate on ETCO₂ variation. Our findings could contribute to better interpretation of ETCO₂ during CPR.