A 72-year-old man is admitted with an acute exacerbation of heart failure. He is breathless at rest with bilateral basal crackles up to mid-zones and peripheral oedema. His blood pressure is 110/70 mmHg, heart rate 95 bpm. His usual medications include ramipril, bisoprolol, and spironolactone. An IV infusion of furosemide is commenced. Which parameter is most crucial to monitor closely within the first 24-48 hours of diuretic therapy?