Measurement of end-tidal carbon dioxide (ETCO2) is an established method for monitoring circulation during CPR. ETCO2 values depend on cardiac output and blood flow through venous return to the right side of the heart and therefore reflect CPR quality and predict resuscitation success. A clinical trial9 and a prospective cohort study showed that an ETCO2 value of 10 mmHg during out-of-hospital ALS accurately discriminated between survivors and non-survivors to hospital admission. In a study from Pokorna et al ROSC was associated with a sudden and sustained increase of ETCO2 ≥ 10 mmHg. The latest ALS Guidelines from the European Resuscitation Council5 suggested using an increase in ETCO2 to detect ROSC during CPR. However, the relevant evidence in support is still limited.