Association between serum bicarbonate and 28-day mortality in septic shock patients: A cohort study

Septic shock, a high-mortality subtype of sepsis, is associated with significantly elevated mortality rates compared to sepsis without shock. Although metabolic acidosis (reflected by serum bicarbonate levels) is a common complication in septic shock, robust evidence regarding the prognostic value of initial serum bicarbonate specifically in intensive care unit (ICU)-admitted septic shock patients remains limited. This study therefore aimed to evaluate the association between initial serum bicarbonate and 28-day mortality in a large cohort of septic shock patients. This retrospective cohort study utilized data from the Medical Information Mart for Intensive Care IV (v2.2) database, which contains de-identified records of 73,181 adult ICU admissions (2008-2019). Adult patients with septic shock were identified using international classification of diseases -9 (785.52) and international classification of diseases-10 (R6521, T8112XA) codes (n = 7216). Inclusion required an available first recorded serum bicarbonate level during the index ICU admission; patients with missing bicarbonate or mortality data were excluded. The primary exposure was serum bicarbonate, and the primary outcome was 28-day all-cause mortality. Multivariable logistic regression was used to assess the association, expressed as odds ratios (ORs) with 95% confidence intervals (CIs). Nonlinearity was explored using generalized additive models. Among 5287 ICU patients (median age 67.7 years), 1662 (31.4%) died within 28 days. Generalized additive modeling revealed a significant U-shaped association between serum bicarbonate and 28-day mortality (edf = 2.96, χ² = 62.33, P < .001). Below 20 mEq/L, each 1 mEq/L increase was associated with a significantly reduced 28-day mortality risk (OR = 0.91, 95% CI 0.88-0.93, P < .0001). Above 27 mEq/L, each 1 mEq/L increase was associated with a significantly increased mortality risk (OR = 1.11, 95% CI 1.02-1.20, P = .0185). In patients with septic shock, the association between serum bicarbonate and 28-day mortality risk followed a U-shaped curve. Both lower and higher serum bicarbonate levels were associated with an increased risk of 28-day mortality. These findings highlight the need for personalized acid-base management strategies.