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Chapter: Clinical Anesthesiology: Anesthetic Equipment & Monitors : Non cardiovascular Monitoring

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Urinary Output Monitors

Urinary bladder catheterization is the only reliable method of monitoring urinary output. Insertion of a urinary catheter is indicated in patients with con-gestive heart failure, renal failure, advanced hepatic disease, or shock.

URINARY OUTPUT

Indications

Urinary bladder catheterization is the only reliable method of monitoring urinary output. Insertion of a urinary catheter is indicated in patients with con-gestive heart failure, renal failure, advanced hepatic disease, or shock. Catheterization is routine in some surgical procedures such as cardiac surgery, aortic or renal vascular surgery, craniotomy, major abdomi-nal surgery, or procedures in which large fluid shifts are expected. Lengthy surgeries and intraop-erative diuretic administration are other possible indications. Occasionally, postoperative bladder catheterization is indicated in patients having diffi-culty voiding in the recovery room after general or regional anesthesia.

Contraindications

Bladder catheterization should be done with utmost care in patients at high risk for infection.

Techniques & Complications

Bladder catheterization is usually performed by surgical or nursing personnel. To avoid unneces-sary trauma, a urologist should catheterize patients suspected of having abnormal urethral anatomy. A soft rubber Foley catheter is inserted into the blad-der transurethrally and connected to a disposable calibrated collection chamber. To avoid urine reflux and minimize the risk of infection, the cham-ber should remain at a level below the bladder. Complications of catheterization include urethral trauma and urinary tract infections. Rapid decom-pression of a distended bladder can cause hypoten-sion. Suprapubic catheterization of the bladder with tubing inserted through a large-bore needle is an uncommon alternative.

Clinical Considerations

An additional advantage of placing a Foley catheter is the ability to include a thermistor in the catheter tip so that bladder temperature can be monitored. As long as urinary output is high, bladder tempera-ture accurately reflects core temperature. An added value with more widespread use of urometers is the ability to electronically monitor and record urinary output and temperature.

Urinary output is a reflection of kidney perfu-sion and function and an indicator of renal, car-diovascular, and fluid volume status. Inadequate urinary output (oliguria) is often arbitrarily defined as urinary output of less than 0.5 mL/kg/hr, but actually is a function of the patient’s concentrating ability and osmotic load. Urine electrolyte composi-tion, osmolality, and specific gravity aid in the dif-ferential diagnosis of oliguria.

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