Low Urine Output (Case 30)

Chapter 45 Low Urine Output (Case 30)

Gregory Peck, DO, Umber Burhan, MD, Robert L. Benz, MD

Postoperative context: A 58-year-old male with a past medical hx of coronary artery disease has a 1-day hx of low urine output after open cholecystectomy for acute cholecystitis.

Differential Diagnosis

Prerenal (Hypoperfusion) Renal (Intrinsic) Postrenal (Obstructive)
Hypovolemia Acute tubular necrosis (ATN) Urinary retention
Shock syndromes Acute interstitial nephritis (AIN) Benign prostatic hypertrophy (BPH) and ureteral ligature

Speaking Intelligently

My first step in assessing postoperative oliguria is to acquire a relevant hx. This includes knowing the patient’s baseline health levels (preoperative creatinine) and any recent medication exposure (nephrotoxins). I check VS for hypotension, tachycardia, and any imbalance in “ins” and “outs” (including insensible losses). I check operative reports for the procedure, complications, intraoperative fluids, blood loss, and blood products administered. I compare the patient’s urine output to an adequate urine output of 0.5 mL/kg/hr. Next, I make sure that the patient is receiving adequate fluids based on deficit and clinical setting. If I have a suspicion of a postrenal cause, I obtain a bladder scan or insert a Foley catheter to check for obstruction and to monitor urinary response to my interventions. Renal ultrasonography will help r/o hydronephrosis.

PATIENT CARE

Clinical Thinking

• Since the kidney receives 20% of cardiac output, urine output is in part a reflection of that cardiac output. Similarly, it is an indirect measure of intravascular volume status. Decreased urine output is not a dx, but rather a symptom of hypoperfusion, intrinsic renal injury, or postrenal obstruction.
• The causes of decreased urine output fall into one of three categories: prerenal, intrarenal, and postrenal pathology.
• The most common prerenal etiology is hypovolemia, diagnosed clinically in the setting of oliguria, hypotension, tachycardia, and dry mucous membranes. Oliguria due to hypovolemia should correct with appropriate volume replacement. Typically creatinine increases less than 0.1 to 0.3 mg/dL/day.
• The most common intrarenal etiology is ATN, usually due to ischemia or toxin exposure. ATN is usually sudden in onset and worrisome because of its severity and potential for irreversible damage. Typically creatinine increases greater than 0.5 mg/dL/day.
• The most common postrenal etiology is obstruction, either mechanical or physiologic, and is usually due to urinary retention, prostate disease, and/or nephrolithiasis.
• If the patient is already catheterized, verify that the catheter is working properly.
• Check that the patient isn’t voiding in the bathroom without record, or if the patient isn’t already known to have anuria from end-stage renal disease.

History

• Ask about orthostatic syncope, dizziness, and thirst.
• Inquire about suprapubic fullness, which may suggest a bladder etiology such as retention, and dysuria or difficulty voiding, which may suggest prostate disease or UTI.
• A relevant PMH searches for coronary disease, previous infarction, hypertension, previous renal disease, autoimmune disease, prostate and bladder problems, allergies to medications, and recent dye studies in preoperative and intraoperative radiographic studies.
• Review the medications received prior to the oliguric state. Determine exposures to medications such as NSAIDs, angiotensin-converting enzyme (ACE) inhibitors, angiotensin II receptor blockers, and general anesthetics.
• Assess intake and output. Regarding intake: Is the patient NPO? Poor appetite? Has volume been inefficiently maintained? Regarding output: Did the patient undergo a bowel prep? Has there been vomiting, diarrhea, sweating? Is the patient on diuretics?
• Remember that preoperative issues impact on volume status in the postoperative setting.

Physical Examination

• VS and intake/output: Temperature, pulse, BP, central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP), if available, for signs of hypovolemia.
• Appearance: Suspect hypovolemia if pale, sunken eyes, dry mucous membranes, decreased skin turgor, and dry axilla.
• Chest: Jugular venous distention (JVD), crackles and peripheral edema for congestive heart failure.
• Abdomen: Bladder distention and abdominal bruits.
• Surgical: Look for drainage output, incisional losses, possible sources for sepsis.
• Rectal examination: Prostatic hypertrophy.
• Miscellaneous: Check if Foley catheter is properly inserted and not kinked or blocked. Is the IV catheter functioning?

Tests for Consideration

• Serum and urine creatinine levels: This ratio determines whether the etiology is likely prerenal (U/P creatinine > 60) or ATN (ratio < 40) and is a factor in formulating the FENa (fractional excretion of sodium).
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• Urine electrolytes and U/A: Specific gravity and urinary sediment (i.e., casts); differentiate between prerenal and intrinsic renal failure.

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Mar 20, 2017 | Posted by in GENERAL SURGERY | Comments Off on Low Urine Output (Case 30)

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