Clinical paperArterial carbon dioxide tension has a non-linear association with survival after out-of-hospital cardiac arrest: A multicentre observational study
Introduction
Despite advances in post-resuscitation care, hypoxic-ischemic encephalopathy remains a common cause of death after out-of-hospital cardiac arrest (OHCA).1 Ischemic-reperfusion brain injury involves a number of mechanisms, including inadequate cerebral oxygen delivery and impaired autoregulation of cerebral blood flow.2 Cerebral oxygen delivery is determined by cerebral blood flow, which in turn can be affected by numerous factors including arterial carbon dioxide tension (PaCO2).3
A recent meta-analysis of eight observational studies4, 5, 6, 7, 8, 9, 10, 11 found that both high and low PaCO2 levels were associated with worse survival outcomes.12 These findings are consistent with international guidelines that a normal PaCO2 should be targeted during post-resuscitation care.13, 14 Data from existing randomized-controlled-trials targeting different PaCO2 levels in adult patients after cardiac arrest are sparse.15, 16 Most of the observational studies are limited by their use of arbitrary cut-points to define an optimal PaCO2, analysis of PaCO2 using data from one single time point or over a limited period of time, and assuming PaCO2 is linearly associated with survival.12 Analysis assuming a linear association contradicts the results of a meta-analysis and two multi-centre cohort studies in which a non-linear association between PaCO2 and patient outcome was demonstrated.5, 17
We hypothesised that normocapnia within the first 24 h after OHCA is associated with a better chance of survival compared to hypocapnia or hypercapnia, and aimed to determine the optimal PaCO2 cut-points for survival after OHCA.
Section snippets
Study design and setting
This multicentre retrospective cohort study included all patients with OHCA of non-traumatic aetiology18 transported to one of four adult tertiary intensive care units (ICUs)19 in Perth, Western Australia, between January 2012 and December 2017. In 2017 the Perth metropolitan area had a population 2.05 million.20 St John Western Australia (SJWA) is the sole provider of emergency medical services in Western Australia. SJWA delivers a single tier of road-based paramedics who provide advanced life
Selection of the study population and its characteristics
Between January 2012 and December 2017, SJWA paramedics attended 8016 patients with non-traumatic OHCA. Of these, 3997 (49.9%) had resuscitation commenced by paramedics and 2812 (70.4%) were transported to a hospital emergency department (954; 33.9% with ROSC). Of the 887 (31.5%) patients who survived to emergency department discharge, 728 (82.1%) were admitted (directly or indirectly) to a tertiary hospital with 502 (69.0%) to the ICU. Nine patients (1.8%) were excluded because two were not
Discussion
This multicentre cohort study found that normocapnia during the first 24-h of ICU admission was associated with a significantly higher odd of survival compared to hypocapnia (<35 mmHg) or hypercapnia (>45 mmHg), and was the third most important predictor of OHCA survival. These results are clinically relevant and require careful consideration.
Our findings are consistent with a recent meta-analysis of eight observational studies including 23,434 patients, in which the relationship between PaCO2
Conclusions
Compared to hypocapnia (<35 mmHg) and hypercapnia (>45 mmHg), normocapnia (35−45 mmHg) within the first 24-h of ICU admission after OHCA was associated with a significantly greater chance of survival to hospital discharge and at 12 months. Our results support the existing international guidelines, and have important implications on how OHCA patients should be managed within the first 24 h after ICU admission.
Conflict of interest
None.
Funding
Nicole Mckenzie received PhD funding from the Australian Resuscitation Outcomes Consortium – NHMRC Centre of Research Excellence (#1029983) and an Australian Government Research Training Scholarship.
The WA OHCA Registry is funded by St John WA.
Judith Finn is a NHMRC Leadership Fellow, receiving salary and research funding from a NHMRC Investigator grant (#1174838).
Janet Bray received a Heart Foundation Fellowship.
CRediT authorship contribution statement
Nicole Mckenzie: Conceptualization, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Judith Finn: Conceptualization, Supervision, Writing - review & editing. Geoffrey Dobb: Conceptualization, Writing - review & editing. Paul Bailey: Writing - review & editing. Glenn Arendts: Writing - review & editing. Antonio Celenza: Writing - review & editing. Daniel Fatovich: Writing - review & editing. Ian Jenkins: Writing - review & editing. Stephen Ball: Data
Acknowledgments
KMH acknowledges the support of the WA Department of Health and Raine Medical Research Foundation through the Raine Clinical Research Fellowship.
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