The Postoperative Patient with Bowel Obstruction
Patients with bowel obstruction are often older and have limited
reserves in several organ systems. Estimating the degree of loss of fluids is difficult
because fluid is retained in the bowel lumen, where it cannot be measured. The slowly
developing volume depletion allows time for full expression of compensatory mechanisms
that mask the degree of the deficit. Patients have often ingested no fluids for
many hours before entering the hospital and are often vomiting. The patient may
have ischemic bowel injury with severe bowel wall edema and continued sequestration
of luminal fluid and formation of ascites. Perioperative fluid intake is much greater
than measured output, leading consultants to suggest diuresis for fluid overload.
Nutrition is impaired preoperatively, and protein losses into the bowel are increased,
leading to hypoalbuminemia and exacerbating the loss of fluid from the vascular space.
The clinical picture is one of ongoing fluid requirement in the absence of external
fluid loss, commonly referred to as third spacing.
The goals of fluid management in this group are similar to those
for other patients and include restoration of the vascular volume and interstitial
volume, correction of electrolyte depletion, correction of acidosis, reduction of
systemic vascular resistance into the normal range, and optimization of oxygen delivery
and use. Initial fluid infusion can restore intravascular volume sufficiently for
blood pressure and heart rate to improve. However, intravascular volume may still
be depleted, with a low
cardiac output and severe arterial and venous vasoconstriction. The ECF volume (except
for the bowel) remains dehydrated and continues to accept fluids from the vascular
space. The vasoconstricted patient fails to perfuse all tissues, limiting the rate
at which the ECF volume can be replenished. If fluids are infused more rapidly than
the rate at which they can enter the ECF volume, increased filling pressures may
result in pulmonary edema. It is crucial to estimate filling pressures, cardiac
output, and systemic vascular resistance while aggressive intravascular fluid with
balanced salt solution and colloid is pursued. It may be necessary to administer
arterial vasodilators such as nitroprusside to facilitate correction.
Management of a typical patient includes frequent monitoring of
arterial blood pressure, heart rate, CVP, pulse pressure and respiratory variation,
urine output, and electrolytes. If these are stable, the hemoglobin level and COP
should be monitored every 2 to 4 hours until stable. A rising hemoglobin level indicates
ongoing loss of plasma water, with or without protein loss. An increasing COP indicates
continued loss of plasma water in excess of protein loss. Because maintenance fluid
requirements persist, dextrose 5% is infused in 0.45% NaCl with 20 to 40 mEq of KCl/L
at maintenance rate.
The fluid lost to the bowel and ECF volume is similar to plasma
water in electrolyte composition, and a balanced salt solution is a reasonable first
choice for the fluid boluses required to sustain plasma volume. Because the fluid
lost also contains protein, albumin or colloid osmotic pressure should be monitored
and colloid infused. If a low COP (<15 to 18 mm Hg) coexists with hemodynamic
instability, replacement should be started at 3 mL/kg/hour if the CVP (and pulmonary
artery occlusion pressure [PAOP] if pulmonary artery catheter has been placed) is
within the patient's usual range. If it is high, replacement should start at 1 to
2 mL/kg/hour. If the urine output rises above 1.5 mL/kg/hour, the physician should
check for glucosuria. If glucosuria is absent, the fluid infusion rate is reduced
by 0.5 mL/kg/hour. If the CVP or PAOP value rises above the patient's usual values
and urine output is 0.5 to 1.5 mL/kg/hour, the fluid infusion rate is reduced by
0.5 mL/kg/hour. If the values for CVP or PAOP, or both, decrease below the patient's
baseline and the urine flow rate is low, balanced salt or colloid solution is administered
rapidly (0.5 to 2 mL/kg/min) with close monitoring of filling pressures.
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