Fever and hypothermia are important risk factors for mortality in critically ill patients. We examined the association between day-one (D1) core body temperature and in-hospital mortality in ICU patients with spontaneous subarachnoid haemorrhage (SAH).

We performed a multicentre retrospective cohort study using the Australian and New Zealand Intensive Care Society Adult Patient Database. Adults (≥18 years) with SAH admitted to ICU between January 2018 and June 2024 were included (n = 7,567). Primary exposures were core temperature used for the Australian and New Zealand Risk of Death (ANZROD) score (T _ANZ ), and the highest (T _HI ) and lowest (T _LO ) core temperatures recorded during the first 24 ICU hours. The primary outcome was in-hospital mortality. Associations were modelled using generalised linear models with natural cubic splines and covariate adjustment.

Mean T _ANZ was 36.0 °C (SD 0.97), mean T _HI 37.3 °C (SD 0.68), and mean T _LO 36.0 °C (SD 0.96). Overall in-hospital mortality was 17% (n = 1,304). T _ANZ , T _HI and T _LO showed non-linear U-shaped associations with mortality. Lowest observed mortality occurred at T _ANZ 35.8–38.2 °C, T _HI 36.8–38.2 °C and T _LO 36.1–37.8 °C. Mortality increased below these ranges for T _ANZ (OR:1.55, 95% CI:1.34−1.79), T _HI (OR:1.42, 95% CI:1.20−1.69) and T _LO (OR:1.38, 95% CI:1.19−1.60), and above the optimal range of T _HI (OR: 1.47, 95% CI:1.19−1.80).

In critically ill SAH patients, D1 core temperature had a U-shaped association with in-hospital mortality. Further research should determine whether temperature variation is a modifiable risk factor in this population.