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A common clinical scenario that junior doctors encounter is oliguria in postoperative patients. It is not unusual to find junior doctors prescribing a fluid challenge hastily, rather than first systemically assessing the patient.
Oliguria may be defined as urine output that is inadequate to maintain physiological homoeostasis. In practice this equates to a urine output of less than 0.5 ml/kg/hr for at least two consecutive hours or a daily urine output of less than 400 ml/day (about 15 ml/hr).w1w2
Oliguria is not a clinical diagnosis but a sign that indicates an underlying disorder. So, management of oliguria must not only be aimed at restoring urine output but also at identifying and treating this underlying disorder.w1w2 Whatever the underlying cause, if left untreated oliguria may lead to acute renal failure and its sequelae, including hyperkalaemia, acidosis, and fluid overload. The key to successful management of postoperative oliguria lies in having a systematic and targeted approach to assessing patients.
Understand the physiology
Causes of oliguria in the postoperative period are conveniently subdivided into prerenal, renal, and postrenal causes.w2 Prerenal hypovolaemia is the most common cause of postoperative oliguria. It usually stems from inadequate management of fluids perioperatively because perioperative fluid losses are often underestimated.
For example, preoperative fluid loss with bowel preparation is easily overlooked, as is bleeding and exudation from the operative site. Although hypovolaemia can to some degree be compensated for by fluid shift from the interstitium into the circulation, large fluid losses will ultimately drain the circulation. This in turn leads to hypoperfusion of the kidneys and oliguria, which, if prolonged, will lead to acute renal failure.
It is important to seek the underlying disorder causing the oliguria. Hypovolaemia may be caused by dehydration. But why is the patient dehydrated? Did it result from inadequate perioperative fluid management, vomiting, or diarrhoea? Is there overt or occult haemorrhage from the operative site?
In the postoperative period, later on, sepsis may be the cause of hypovolaemia, which depletes the circulation through a fluid shift from the intravascular space into the interstitium through "leaky" capillaries-that is, capillaries with increased permeability caused by inflammatory mediators and bacterial factors.
Another prerenal cause of postoperative oliguria is acute heart failure, when perfusion of the kidneys...