A 35-year-old man presents to the emergency department with severe abdominal pain and hypotension (BP 80/50 mmHg). He has a 2-day history of acute pancreatitis confirmed by elevated serum amylase and lipase. Clinical examination reveals signs of peritonitis. His heart rate is 120/min, respiratory rate 28/min, and urine output has dropped to 0.3 mL/kg/hr. Which investigation is most appropriate to confirm the type of shock and guide immediate fluid resuscitation?