Ringer’s lactate administered at 15 °C leads to a greater and more prolonged increase in blood pressure compared to 37 °C

18 Participants were randomized to receive 30 ml/kg bodyweight Ringer's Lactate at 37° or 15 °C over 30 min. In a second session, participants were crossed over. Over a 120 min period after starting the fluid bolus we measured mean arterial pressure (MAP), cardiac output, systemic vascular resistance, and catecholamine levels. After infusion with cold fluids, the absolute increase in MAP at 45 min was significantly higher at + 6.5 mmHg (95% CI 4.8-8.2) compared with warm fluids (+ 0.6 mmHg, 95% CI, - 1.6 to 2.8; p < 0.001). This increase in MAP was longer-lasting after cold fluids (81.7 min, 95% CI 62.5-100.9) than after warm fluids (19.2, 95% CI 3.4-35; p < 0.001). While cardiac output was similar, systemic vascular resistance increase was greater after cold fluids (159 dyn s/cm⁵, 95% CI 9.5-309) compared to warm fluids (- 66 dyn s/cm⁵, 95% CI - 191 to 57; p = 0.012). Moreover, noradrenaline increased by up to 246% during cold fluids, and decreased with warm fluids (p < 0.001). Fluid bolus given at 15 °C, compared to 37 °C, leads to a greater and more prolonged increase in MAP accompanied by release of intrinsic noradrenaline and vasoconstriction. These results suggest that fluid temperature rather than volume is predominantly responsible for any increase in MAP.Trial Registration: EudraCT-nummer 2022-002137-34 and clinicaltrials.gov NCT05610254 (first registration 09/11/2022).