Reply to: Blood gas measurements during arrest and after return of spontaneous circulation: Apples and oranges

To the Editor, We thank Dr. Wittig et al[1]cite.Google Scholar for their interest in our study, and we welcome the opportunity to better clarify our work. [2]Hernández-Tejedor A. González Puebla V. Corral Torres E. Benito Sánchez A. Pinilla López R. Galán Calategui M.D. Ventilatory improvement with mechanical ventilator versus bag in non-traumatic out-of hospital cardiac arrest: SYMEVECA study, phase 1.Resuscitation. 2023; 192109965https://doi.org/10.1016/j.resuscitation.2023.109965Google Scholar As well underlined in their letter, the situation of patients in cardiac arrest (intra-CPR) and patients who have already got the recovery of spontaneous circulation (ROSC) is obviously different. In fact, an elevation in end-tidal CO2 (EtCO2) is an indicator to consider the possibility of ROSC. 3Soar J. Böttiger B.W. Carli P. et al.European Resuscitation Council Guidelines 2021: Adult advanced life support.Resuscitation. 2021; 161: 115-151Google Scholar, 4Lui C.T. Poon K.M. Tsui K.L. Abrupt rise of end tidal carbon dioxide level was a specific but non-sensitive marker of return of spontaneous circulation in patient with out-of-hospital cardiac arrest.Resuscitation. 2016; 104: 53-58Google Scholar In our study we decided to make a simplification by analyzing the data of these patients in aggregate assuming a random distribution of ROSC/intra-CPR in the study groups, as was the case, to avoid the reducing effect on the number of patients in each subgroup by subdividing the groups based on this. In fact, the initial approach was to carry out this subdivision and also another one on the initial rhythm, but it would have been impossible to include enough patients in each subgroup in a reasonable time frame with the expected recruitment rate. However, given the concerns arisen by our colleagues, we have carried out this subanalysis. Out of the 39 cases with arterial blood gas (ABG) analysis, in 18 of them the sample was taken immediately after confirming ROSC, that is, after the cessation of chest compressions (9 of them in patients with initial shockable rhythm and 9 with non-shockable rhythm). PaCO2 in IPPV-mechanically ventilated patients (11) was 66.57 ± 17.28 mmHg vs 92.93 ± 22.11 mmHg in manually ventilated patients (7), p = 0.01. In those with intra-CPR sampling, 15 minutes after intubation as described in methods (21 patients, 9 with initial shockable rhythm and 12 with non-shockable rhythm), PaCO2 in mechanically ventilated patients (13) was 68.86 ± 24.49 mmHg vs 98.50 ± 51.00 mmHg in those ventilated with a resuscitator bag (8), p = 0.08. We can see that the original results persist when performing the subanalysis independent of the patient's condition at the time of blood sampling. We disagree with the statement that paCO2 significantly rise sharply within the first minutes of ROSC. In fact, it is EtCO2 the measurement that rises fast after ROSC as blood flow improves and more blood volume with a high CO2 concentration is exposed to the alveolar-capillary membrane. In the study by Spindelboeck et al[5]Spindelboeck W. Gemes G. Strasser C. et al.Arterial blood gases during and their dynamic changes after cardiopulmonary resuscitation: A prospective clinical study.Resuscitation. 2016; 106: 24-29Google Scholar 1) each patient had just one ABG measurement, so it was not possible to know the evolution of blood gases in a single patient from ongoing CPR to ROSC; 2) the median time from ROSC to ABG sampling was 15 minutes; and 3) for intra-CPR vs ROSC comparison only patients that subsequently got ROSC and arrived to hospital could be selected. Nonetheless, in post-ROSC cases median PaCO2 was only 9 mmHg lower and EtCO2 was 6.5 mmHg higher than in intra-CPR cases. That fact evidenced in their study and the small PaCO2 difference regardless of the condition (intra-CPR or immediately after ROSC) when the sample is obtained also evidenced in ours is due to the longer time needed for blood CO2 clearance. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.