The impact of arterial blood pH on short-term mortality in adult medical emergency department visits: a population-based, multicentre study

Background
The acid-base balance in arterial blood is critical in determining patient outcomes during emergency care. Although acid-base imbalances are common, the impact of pH levels on short-term mortality in emergency department (ED) settings is not well studied. Most evidence comes from intensive care unit settings where interventions can affect pH levels and outcomes, raising concerns about these findings' generalisability to the ED setting.
The objectives were to examine the correlation between arterial blood pH and 0–2- and 3–7-day mortality in adult medical ED visits.

Methods
This population-based, multicentre cohort study included all adult medical ED visits of 18 years or older from seven departments in the Region of Southern Denmark from January 1, 2016, to March 19, 2018. The inclusion criteria required an arterial blood gas (ABG) analysis within four hours of arrival. The outcomes were 0–2- and 3–7-day mortality rates stratified by pH levels and adjusted for confounders with hazard ratios (HRs) compared to normal pH levels.

Results
The study included 64,725 adult medical ED visits and 31,650 unique individuals with ABG testing. Most patients had normal pH (54.2%), followed by alkalosis (31.8%), acidosis (11.3%), and severe acidosis (2.7%). Dyspnoea was the most common symptom (29.5%), and 64% of patients were aged ≥65. The largest group of patients had no comorbidities. The most frequent discharge diagnoses were respiratory diseases (29.2%), unspecific findings (18.6%), and circulatory diseases (12.4%).
The mortality rates for patients within 0–2 days and 3–7 days of the index visits were 2.4% and 2.1%, respectively. Patients with severe acidosis (pH < 7.20) had the highest risk, with mortality rates of 20.8% and 8.9% (HR 9.6 and 5.2, respectively). Acidosis (pH 7.20–7.34) was associated with mortality rates of 7.4% and 5.2% (HR 4.1 and 2.7, respectively). Within two days, nearly 60% of patients with pH < 6.90 died. In the adjusted analysis, lower pH levels, increased burden of comorbidities, and advanced age were associated with increased risk of short-term mortality.

Discussion and conclusions
Our study highlights that lower pH, increased comorbidities, and older age are linked to higher short-term mortality risk in the ED. Notably, 40% survival among patients with pH < 6.90 challenges the prevailing medical consensus, suggesting a critical re-evaluation of prognostic values and treatment approaches for acute patients may be necessary.

Trial Registration
Permission to obtain and store data was obtained from the regional authorities (identifier 3-3013-2272/1 and identifier 17/24904, amendment identifier 20/24502).

Funding
This study did not receive any specific funding.

Ethical approval and informed consent
Not needed.