“Doctor, your postop colectomy patient isn’t putting out much urine.”
Mr. O’Flaherty is a 70-year-old gentleman who is two hours post left colectomy. His vitals are: BP 110/70, HR 110 (regular), respiration 18, and temperature 37.5°C. In the last hour he has made 15 mL of urine.
1. List at least three clinical indicators of adequate peripheral perfusion.
2. What does a spot urine sodium of 12 mEq/L in a postoperative patient mean?
There are multiple possible causes of oliguria in the postoperative patient, although inadequate renal perfusion is the most common. Stress, acute kidney injury, and obstruction are other common causes (see Table 35-1). We will address each of these in turn.
Table 35-1. The Most Common Causes of Oliguria |Favorite Table|Download (.pdf)
Table 35-1. The Most Common Causes of Oliguria
|Prerenal||Inadequate perfusion Stress|
|Intrarenal||Acute kidney injury|
As surgeons, we use multiple reassuring indicators of adequate cardiac output and hemodynamic stability, which include:
- Invasive indicators:
Blood lactic acid
Blood metabolic acid (pH >7.35 without respiratory compensation; so, PaCO2 >35 mm Hg)
Mixed venous O2 sat >65% (requires a central venous or pulmonary artery catheter)
- Less invasive indicators:
Stable BP (relative to preoperative)
Stable heart rate
Comfortable respiratory rate (when you are sick, or running up stairs, you breathe faster)
Big toe temperature (short of sepsis, if your big toe is warm, you’re doing OK)
The focus of this chapter is the last item on this list: urine output. Because the kidneys are exquisitely sensitive to changes in cardiac output, oliguria can indicate that your patient has compromised renal perfusion—usually but not always due to hypovolemia. Note that using urine output as a measure of end-organ perfusion is very similar to using goal-directed therapy (GDT) principles as described in the chapter on Shock.
Hypovolemia in the acute postoperative period is usually due to one of two causes. Most commonly it is the result of so-called third spacing, where leakage of plasma into the surgical field (and beyond!) results in depleted intravascular volume. However, hypovolemia can also be due to hemorrhage, which must always be in the back of your mind.
When treating an acutely postoperative patient who you believe is oliguric due to hypovolemia, a standard approach is to give a 500 mL to 1 L bolus of crystalloid. If the patient does not respond as expected within 30 minutes, then you should begin looking more carefully for other potential causes of inadequate cardiac output. This should include a CBC to evaluate for hemorrhage as well as a spot urine sodium (see below).
While hypovolemia is the usual focus of surgeons, it is important to remember that oliguria is ...
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