What is the recommended next step for a 69-year-old man with postoperative acute kidney injury (Impaired renal function), hyperkalemia, and metabolic acidosis, who has been experiencing severe pain and somnolence following surgical repair of a tibial plateau fracture, and has received Patient-Controlled Analgesia (PCA) hydromorphone, anti-nausea medications, and a bowel regimen, with a history of hypertension, obesity, and sleep apnea, currently presenting with oliguria and mild abdominal tenderness?

From the Guidelines

I strongly recommend obtaining urine sodium and creatinine measurements as the next step for this patient with postoperative acute kidney injury. This diagnostic test will help differentiate between prerenal azotemia and intrinsic renal injury, which is crucial for determining appropriate management. The patient shows several signs of volume depletion despite receiving maintenance fluids, including poor oral intake, oliguria (only 175 mL output in 24 hours), and elevated creatinine. The fractional excretion of sodium (FENa) calculated from urine sodium and creatinine will indicate whether the kidneys are appropriately conserving sodium (suggesting prerenal causes) or inappropriately excreting it (suggesting acute tubular necrosis). This patient's clinical picture suggests prerenal azotemia from volume depletion, medication effects (opioids), and possibly the hypotensive episode during surgery. The hyperkalemia, metabolic acidosis (low bicarbonate), and hypertension further support the need to clarify the kidney injury's nature before proceeding with specific interventions like fluid boluses or medications. Once the etiology is confirmed, targeted therapy can be initiated to prevent further kidney damage, as suggested by the guidelines for acute kidney injury management 1.

Some key points to consider in the management of this patient include:

• The patient's intake/output has been 2000/175 mL during the past 24 hours, indicating oliguria and possible volume depletion.
• The patient's creatinine is 1.9, potassium is 5.4, and chloride is 110, suggesting acute kidney injury and possible electrolyte imbalances.
• The patient's clinical presentation and laboratory results are consistent with prerenal azotemia, but intrinsic renal injury cannot be ruled out without further testing.
• The use of urine sodium and creatinine measurements can help differentiate between prerenal and intrinsic renal causes of acute kidney injury, as suggested by the guidelines 1.
• The management of hyperkalemia is crucial in this patient, and the use of potassium-lowering agents may be necessary, as suggested by the expert consensus document on the management of hyperkalaemia in patients with cardiovascular disease treated with renin angiotensin aldosterone system inhibitors 1.

Overall, obtaining urine sodium and creatinine measurements is a crucial step in the management of this patient with postoperative acute kidney injury, and will help guide further treatment and management.

From the Research

Patient Assessment

The patient is a 69-year-old man who was admitted to the hospital for surgical repair of a tibial plateau fracture and is now experiencing postoperative acute kidney injury. He has been receiving lactated Ringer's solution (75 mL/hr) since the surgery and has been eating and drinking little. The patient's intake/output has been 2000/175 mL during the past 24 hours, and his creatinine level is 1.9, indicating impaired kidney function.

Recommended Course of Action

Given the patient's condition, the following options should be considered:

• Bolus of crystalloid fluid: The patient's low urine output and high creatinine level suggest hypovolemia, which may be contributing to his acute kidney injury. Administering a bolus of crystalloid fluid, such as lactated Ringer's solution, may help to improve his volume status and renal function 2, 3.
• Urine sodium and creatinine measurement: Measuring the patient's urine sodium and creatinine levels can help to determine the cause of his acute kidney injury and guide further management. A low urine sodium level may indicate prerenal azotemia, while a high level may suggest intrinsic renal damage 4, 5.
• Bedside bladder ultrasonography: Performing a bedside bladder ultrasonography can help to assess the patient's bladder function and rule out post-renal causes of his acute kidney injury, such as urinary retention.

Rationale

The use of balanced crystalloids, such as lactated Ringer's solution, is preferred over normal saline in critically ill adults, as they can help to prevent acute kidney injury and improve outcomes 5, 6. However, the patient's individual needs and response to treatment should be closely monitored, and adjustments made as necessary.

Key Considerations

• The patient's underlying medical conditions, such as hypertension, obesity, and sleep apnea, should be taken into account when managing his fluid therapy and acute kidney injury.
• The patient's medication regimen, including his PCA hydromorphone and anti-nausea medications, should be reviewed to ensure that they are not contributing to his acute kidney injury.
• The patient's fluid status and renal function should be closely monitored, and adjustments made to his fluid therapy as necessary to prevent further deterioration.