The course of acute pancreatitis depends heavily on the severity of the condition, and it runs in two phases. About 75% of cases are mild or interstitial and resolve within one week; when the patient is not mild, the disease extends beyond one week, and the two phases become distinguishable.
The first phase usually lasts one week and is characterised by systemic symptoms that may result in organ failure. That organ failure is caused by the systemic inflammatory response syndrome, or SIRS, a cytokine storm originating from the pancreatic foci, whose common manifestations are respiratory failure, fever, tachycardia, hypotension and leukocytosis. Most organ failure appears on the first day of admission, and these patients should be treated as severe acute pancreatitis. The duration of the organ failure is what sets the grade: if it persists for more than 48 hours, severe acute pancreatitis is confirmed, and if it resolves within 48 hours, the episode is classified as moderately severe. Organ failure in the first week occurs, in more than 95% of cases of acute pancreatitis, in patients classified as moderately severe or severe; it is very rare in mild acute pancreatitis.
The second phase starts after one week and is characterised by local complications and the infections that follow them. By definition, mild acute pancreatitis does not enter the second phase and resolves within the first.