A 45-year-old man presents to ED with fever, rigors and hypotension after a week of dysuria and flank pain. BP is 84/50 mmHg, pulse 122 bpm, RR 28 breaths/min, SpO₂ 95% on room air, lactate 4.5 mmol/L. You suspect septic shock from pyelonephritis. After drawing blood cultures, what is the most appropriate immediate management priority?