Matt Harmon

169 Chapter eight Study strengths and limitations There is no consensus how to report carbon dioxide levels in relation to outcome in cardiac arrest patients and previous studies have employed methods suited to the nature of their data (single lowest/highest values versus serial measurements, within a defined time period versus not, using a pre-specified sampling plan or not). In this study we have employed many different analytic approaches and used different outcomes (functional outcome and biomarkers) in order to provide an as robust as possible investigation. It is important to emphasize that the study was conceived post-hoc and with a definite exploratory approach. All results must be regarded as hypothesis generating, and due to the observational design, we cannot make causality statements. Blood gases represent the PaCO 2 at a certain point in time and we assumed that the PaCO 2 in between blood samples was linear. It is also important to point out that patients not surviving the analysis period were excluded from the analysis to allow a defined exposure period of carbon dioxide. There are however considerable strengths in our analysis as data were derived from a large, well-controlled cohort of OHCA patients with availability of important confounders. Physiological and biochemical data were collected prospectively at specified time points according to a pre-defined protocol and blood gasses were analyzed in a uniform way. Measurements were serial and therefore likely to demonstrate the association of PaCO 2 with outcome in the post cardiac arrest phase more accurately than single measurements. Follow-up data were acquired with face-to-face interviews using a structured protocol and the loss of patients in the follow-up period was minimal. 24 We performed sensitivity analyses of patients with all data registered at all sampling points and including all patients, also those not surviving the full analysis period, and obtained similar results. Conclusion Dyscarbia after ROSC was common in OHCA patients, but measured as extreme values and over time not associated with neurological outcome at 6-month follow- up. Mild hypercapnia was not associated with adverse outcome and there was no interaction with temperature group affiliation.