Abstract Body: Introduction: Chest compressions during CPR cause oscillations in capnography waveforms. The airway opening index (AOI) was proposed to measure the amplitude of these oscillations. Studies indicate that AOI is inversely correlated with the change in intrathoracic pressure and that lower AOI values may reflect airway closure. However, little has been reported about how different waveform capnography approach may affect the appearance or amplitude of these oscillations.
Aim: Quantify the differences in AOI values obtained from mainstream and sidestream waveform capnography.
Methods: We retrospectively analyzed 25 monitor-defibrillator recordings having both concurrent mainstream and sidestream waveform capnography and spirometry-based tidal volume and inspiratory flow. Ventilations were automatically detected in the volume signal. Each ventilation was characterized by the end-tidal carbon dioxide (ETCO₂) values measured using both tracings and annotated for the presence of chest compression oscillations. If the latter were found, oscillation amplitudes were measured in absolute value (mmHg) and AOI was computed as the average difference between each pair of peaks and troughs normalized to the ETCO₂ value of that ventilation.
Results: A total of 3,275 ventilations were annotated with concurrent mainstream and sidestream capnography signal. From these, 695 had oscillations (21.2%) captured by either technology or both. All the oscillations were captured by the mainstream technology, but only 660 (95%) appeared in the sidestream recordings. The median (IQR) oscillation amplitude was 21.8 (7.5, 33.5) mmHg and 14.0 (5.0, 20.0) mmHg (p < 0.001), for mainstream and sidestream, respectively; and the AOI was 45.7% (31.3%, 62.8%) and 29.5% (12.9%, 41.8%), (p < 0.001).
Conclusion: Oscillations in the capnogram generated by chest compressions during CPR were almost always larger when measured with mainstream technology, and 5% were missed entirely when measured by sidestream capnography. Likewise, calculated AOI values were also greater most of the time. Therefore, in the context of monitoring the quality of CPR through oscillations present or not in the capnogram, as well as regarding their amplitudes, waveform capnography approach should be considered.