Managing the common comorbidities of feline urethral obstruction - Veterinary Medicine
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Managing the common comorbidities of feline urethral obstruction
Urethral obstruction is a common condition in male cats. True excellence in the handling of these cases comes not from placing and managing the urinary catheter but in anticipating and treating the other conditions that result from obstruction.


VETERINARY MEDICINE


COMPLICATIONS THAT OCCUR DURING TREATMENT

Postobstructive diuresis

Postobstructive diuresis (POD) is a long-recognized complication that is most common in animals that have had a urethral obstruction for a long period.6 When it occurs, the patient produces a large amount of urine that greatly exceeds the volume of fluids being administered. In some cases, POD can lead to severe dehydration, hypovolemia, and shock.6 In a recent study, 46% of obstructed cats had POD during the first six hours after the obstruction was removed.7 Although which cats will develop POD cannot be predicted, a pH below 7.35 (i.e. acidosis) correlates with the presence of POD.7 POD also occurs in animals that have been obstructed longer and are more clinically affected.7

Three reasons for POD have been reported: urea diuresis, salt diuresis, and water diuresis. Urea diuresis occurs when BUN concentrations increase secondary to decreased GFR. When the patient has been catheterized and urine flow has been reestablished, the kidneys filter and excrete the urea load, leading to massive losses of water along with the urea.6,8,9 Salt diuresis is similar except that excess sodium rather than urea is excreted along with massive amounts of water when the obstruction is relieved.6,9,10 Water diuresis is a short-term and transient nephrogenic diabetes insipidus that occurs when the collecting tubule in the kidney becomes temporarily unresponsive to antidiuretic hormone and cannot reabsorb water.6

In all cases of POD, the kidneys lose their ability to reabsorb sodium and water secondary to the obstruction and loss of the medullary gradient occurs, which promotes water loss from the kidneys.8 Other factors contributing to POD include increases in total body water due to the inability to urinate8,9 and a buildup of urea and other solutes within the renal tubules, which induces an osmotic diuresis.9,10

Managing POD. Management of POD requires monitoring urine production and matching it to the rate of intravenous fluid administration. This is accomplished by using a closed urinary collection system and measuring the volume of urine produced. Urine is typically measured every four to six hours and the total volume of urine is divided by the time over which it was produced to determine the total milliliters of urine per hour (ml/hr). The fluid pump should then be set to that same ml/hr until the next urine measurement time point. Thus, the fluid rate is always changed in reaction to the previous amount of urine production. It is important not to be afraid of extremely high fluid rates in these patients (in some cases upwards of 100 ml/hr for a standard sized cat). Not matching the urine output is more detrimental to these patients than unwarranted concerns about fluid-overloading because insufficient fluid replacement can lead to dehydration and worsening prerenal azotemia.

The duration of POD varies, but anecdotally seems to last 24 to 48 hours in most cases. Evaluate several factors to determine whether the POD has resolved. First, the measured urine production will match the administered intravenous fluids for several consecutive measurement periods. Second, the urine will appear grossly normal and dilute without blood contamination. Third, when the intravenous fluid rate is reduced, the next urine measurement will also be reduced to match the amount of fluid administered. If the measured urine amount is higher than the administered fluids, the POD is still ongoing and the fluid rate should be increased again to match the urine production. In a recent study, some cats still displayed POD 84 hours after the obstruction was removed.7

Urethral tear

Urethral tears are rare in cats, estimated to occur in 0.6% of the total number of cats with lower urinary tract disease (including both obstructed and nonobstructed cats).11 Tearing is most often iatrogenic, caused by manipulating a urethral catheter.11 Once the urethra is torn, urine leaks into the subcutaneous tissues causing severe inflammation.

Clinical signs can include pain in the perineal and preputial regions, swelling in the preputial and surrounding areas, or severely inflamed and even necrotic skin at the site of the subcutaneous urine leakage (typically the perineal or scrotal and preputial region).12 Other signs reported include hematuria, anorexia, and depression.12 A urethral tear can be difficult to diagnose when it occurs because cats may display only vague clinical signs. It is important to recognize that cats may not show any changes in urination and may be able to urinate normally.12

Managing tears. A torn urethra can be managed in several ways, all centered on the basic concept of preventing contact of urine with the urethra while it heals. This eliminates the complications of skin necrosis and inflammation secondary to the caustic effects of the urine and reduces periurethral inflammation secondary to urine leakage.13 One method of urine diversion is to simply keep a urinary catheter in place until the urethra heals.11,13 This catheter can be carefully placed through the urethra in the normal retrograde fashion or can be placed using a guidewire that is introduced (via cystotomy) from the bladder, directed normograde out the urethra, and used to guide a catheter from the urethra into the bladder ('inside to outside technique").12

Other methods of urinary diversion involve placing a cystostomy tube through the skin into the bladder. The traditional method of tube cystostomy placement involves a caudal ventral midline approach where the entire bladder is visualized and a tube is placed through a purse-string suture on the ventral bladder surface and exited through a paramedian incision on the cat's ventral abdomen.14 The tube used for this technique is usually a mushroom-tipped urinary catheter that is emptied at least four times a day to keep the bladder small so that urine does not travel down the urethra.

Newer published techniques for cystostomy tube placement involve a minimally invasive approach in which the bladder is entered through the lateral aspect of the flank.15 The entire bladder is not visualized with this technique; only a portion of the bladder wall is seen through the incision. A mushroom-tipped catheter is placed through a purse-string suture into the portion of the bladder wall seen through the incision, and the tube exits dorsal to the incision site on the lateral abdomen.

Laparoscopy has also been used to place a percutaneous Foley catheter in dogs16 and a transcutaneous pigtail catheter (silicone balloon catheter with a pigtail tip) in sheep17 to allow for urine diversion. In people, similar pigtail catheters have been placed percutaneously into the bladder with ultrasound guidance.18

Healing time of the urethra can range from several days to a week or more (reported as five to 14 days in a recent study).12 The patency of the urethra can be periodically evaluated by a contrast urethrogram (i.e. injecting contrast medium from a catheter that is pulled out to the most distal portion of the urethra and looking for leakage into the tissues surrounding the urethra).11 Typical contrast agents used are noniodinated agents such as iohexol or diatrizoate meglumine and diatrizoate sodium solution (MD-76R—Mallinckrodt) administered at a maximal dosage of 5 ml/cat. Once a contrast agent can be successfully injected from the tip of the penis into the bladder without any leakage, the urethra is healed and the catheter or cystotomy tube can be removed.


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Source: VETERINARY MEDICINE,
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