Abstract Body: Introduction: End-tidal CO₂ (ETCO₂) has long been proposed as a trustworthy non-invasive indicator of the effectiveness of cardiopulmonary resuscitation (CPR). In fact, ETCO₂ values and trends have been associated to return of spontaneous circulation (ROSC), likelihood of survival or possibility of death. Two different technologies are available to measure ETCO₂, mainstream and sidestream. In mainstream, the sensor is directly placed on the patient’s airway and samples CO₂ tensions within the airway. Conversely, in sidestream, gas travels through a tubing connected to the patient’s airway, with the CO₂ values being measured from a sampling port on this circuit distant from the airway.
Aim: Quantitatively analyze the differences in ETCO₂ values between mainstream and sidestream technologies measured concurrently in patients undergoing out-of-hospital CPR.
Methods: Manual CPR out-of-hospital cardiac arrest cases with defibrillator, spirometry-based tidal volume and inspiratory flow, and concurrent capnography data recorded by both mainstream and sidestream approaches after intubation. For each episode, ventilations were automatically detected in the volume signal and characterized by the ETCO₂ values measured using the mainstream and sidestream waveform capnography tracings. Differences in the measurements obtained by both technologies were quantified in absolute (mmHg) and relative values (%).
Results: A total of 25 cases were analyzed, comprising 3,275 ventilations with concurrent mainstream and sidestream waveform capnography. Median (IQR) ventilation rate was 12.4 (9.6, 15.6) per minute. Median (IQR) tidal volumes were 380 (318, 478) mL. ETCO₂ values for mainstream presented a median (IQR) of 45.7 (37.6, 62.2) mmHg, while for sidestream were 43.0 (35.0, 59.0) mmHg (p < 0.001). The distribution of absolute differences was 2.6 (1.4, 4.3) mmHg, and of relative differences 6.0% (3.3%, 9.8%).
Conclusion: Sidestream yielded ETCO₂ measurements that were almost always lower than those recorded by mainstream, with median differences close to 3mmHg, or more than 6%. Even if registered values are similar, precise measurements are vital in the context of monitoring the quality of CPR. Because both technologies are widely used, sensors may need to be calibrated to account for technical differences in order to yield accurate measurements.