Prehospital blood gas analyses in acute patients treated by a ground-based physician-manned emergency unit: a cohort study

The prehospital patients treated by ambulances and mobile emergency care units (MECU) are potentially critically ill or injured. Knowing the risks of serious outcomes in these patients is important for guiding their treatment. Some settings allow for prehospital arterial blood gas analyses. This study aimed to assess the outcomes of prehospital patients in relation to their prehospitally measured lactate, pH, and CO2 levels. The primary outcome was 7-day mortality. This register-based cohort study included patients with one or more prehospital blood gas analyses during their prehospital treatment by a physician-manned MECU, from January 2015 to December 2018. The blood samples were analyzed on an ABL90 Flex analyzer. Absolute values with percentages and odds ratios (OR) with 95% confidence intervals (CI) were calculated for the primary and secondary outcomes within prespecified subgroups. The study included 745 patients, with an overall 7-day mortality rate of 20.0%. The 7-day mortality rates were 11.5% in patients with normal lactate levels (< 2.0 mmol/L), 14.4% with intermediate lactate levels (2.0–3.9 mmol/L), and 33.0% with high lactate levels (≥ 4.0 mmol/L). This corresponded to an OR of 1.30 (95% CI: 0.75–2.24) in the intermediate lactate group (2.0–3.9 mmol/L) and an OR of 3.77 (95% CI: 2.44–5.85) in the high lactate group (≥ 4.0 mmol/L), compared to the reference group with normal lactate. The ORs of 7-day mortality rates were 4.82 (95% CI: 3.00–7.75) in patients with blood pH of < 7.35 and 1.33 (95% CI: 0.65–2.72) in patients with blood pH > 7.45, compared to the reference group with normal pH (7.35–7.45). The ORs of 7-day mortality rates were 2.54 (95% CI: 1.45–4.46) in patients with blood CO2 of < 4.3 kPa and 2.62 (95% CI: 1.70–4.03) in patients with blood CO2 > 6.0 kPa, compared to the reference group with normal CO2 (4.3–6.0 kPa). This study found a strong correlation between increasing 7-day mortality rates and high blood lactate levels, low levels of pH, and abnormal CO2 blood levels, in prehospital patients undergoing prehospital blood analysis.