Urocystoliths are commonly voided into the urethra of male dogs where they often become lodged adjacent to the caudal aspect of the os penis.
If uroliths cause persistent and complete obstruction of the urethral lumen, post-renal azotemia will result. If appropriate treatment is not provided, death will occur within three to four days. Older patients with co-morbid acute or chronic renal disease, heart disease or other debilitating disorders may die in agony even sooner.
Complete obstruction to urine outflow caused by urethroliths in patients with pre-existing bacterial urinary-tract infections may result in an episode of acute generalized, nephritis and septicemia.
However, life-threatening post-renal azotemia often is reversible if urethral patency is restored and hyperkalemia, acidemia and dehydration are corrected in a timely manner.
Once uroliths become lodged in the urethra of male dogs, it is very unlikely that they can be removed by the technique of voiding urohydropropulsion. Although small smooth surfaced urethroliths may sometimes be pushed back into the bladder with the aid of a catheter, this technique often is unsuccessful and is associated with an extremely high risk of urethral trauma and secondary infection. If the uroliths can be moved easily by inserting a catheter into the urethral lumen, our experience has been that they also can be moved readily by retro- grade urohydropulsion with far less chance of iatrogenic trauma.
Urethral patency can be restored by flushing urethroliths back into the bladder lumen by retrograde urohydropropulsion. The efficacy of this technique is related to dilation of a portion of the urethra containing urethroliths with fluid under pressure. However, to be consistently successful, one must understand all aspects of the technique. Lack of familiarity with the details of this procedure often is the cause of failure to remove all of the urethroliths.
This two-part objective review is to emphasize aspects of the technique that are frequently overlooked by colleagues who consulted with us after being unsuccessful in restoring urethral patency with this procedure.
Major steps for retrograde urohydropropulsion
1. Verify and localize the urethro-liths by physical examination and with the aid of appropriate imaging procedure(s).
2. Decompress the urinary bladder by cystocentesis. Save representative aliquots of pretreatment urine for urina-lysis and urine culture.
3. Lubricate the urethral lumen and urethroliths to facilitate retropropulsion of the stones into the bladder lumen.
4. Develop an anesthetic plan based on the status of each patient.
5. Perform retrograde urohydropropulsion.
6. Minimize catheter-induced trauma to various components of the urinary tract, and use appropriate techniques to minimize iatrogenic urinary-tract infection.
7. Consider an appropriate technique to manage the urocystoliths.
Large-capacity syringes, 22-gauge hypodermic needles and intravenous extension sets will facilitate atraumatic cystocentesis. The length of the 22-gauge needle should be adequate to easily reach a point mid-way between the bladder vertex and the junction of the urethra and the bladder neck (Figure 1). Depending on the size of the patient and the distance of the ventral bladder wall from the ventral abdominal wall, 1.5-inch hypodermic or 3-inch spinal needles may be selected.
Figure 1: This illustrates the correct and incorrect sites of insertion of a needle into the bladder for the purpose of decompressing the over-distended lumen. The needle should be inserted in the ventral or ventrolateral surface of the wall cranial to the junction of the bladder neck with the urethra. This will permit removal of a large volume of urine and decompression of the lumen without need for reinsertion of the needle into the bladder.
Two large-capacity syringes, a three-way connecting valve, a flexible urethral catheter, a tube of sterilized aqueous lubricant and sterilized isotonic saline (or its equivalent) are needed to inject a lubricant around the uroliths.
Figure 2: A viscous sterilized aqueous lubricant is placed into the barrel of a large-capacity syringe.
We recommend a lubricant mixture be prepared by connecting the tips of two large-capacity syringes using a three-way valve (Figures 2 & 3).
One syringe can be partly filled by aseptically removing the syringe plunger and adding a sterilized aqueous lubricant (such as K-Y Jelly). The other syringe should be partially filled with sterilized saline solution. After closing the outflow port of the three-way valve, inject these solutions back and forth between the syringes until they combine to form a sterilized solution of reduced viscosity that can be injected through a urethral catheter (Figure 3).
Figure 3: This diagram illustrates the technique for mixing a sterilized aqueous lubricant with physiologic saline solution in two syringes connected by a three-way valve.
Selection of syringe size for urohydropropulsion may substantially affect the volume and pressure of saline in the system. The Law of LaPlace applied to fluid mechanics indicates that the pressure of a liquid (or gas) within a sphere, or in this setting a syringe cylinder, is inversely proportional to the radius of the cylinder. Therefore, a 60-mL syringe with a larger radius would be capable of creating less pressure than could be created in a system with a smaller capacity syringe (e.g., 35-mL or 20-mL capacity syringe). Using this logic, use of a 6- or 12-mL syringe with a smaller radius could potentially generate more pressure than a 20- or 35-mL syringe would provide if sufficient saline were available to fill the lumen of the catheter and to fill and distend the urethra with fluid under pressure. Although adequate pressure may be generated, for most dogs the volume in 6- or 12-mL syringes is insufficient to prime the system. Because the 60-mL syringe likely would hold enough saline to fill the lumens of the catheter and urethra but be incapable of generating as much pressure as the 20- or 35-mL system, and while the 6- and 12-mL syringes likely would generate enough pressure but contain an insufficient volume of saline to distend the urethra and fill the lumen of the catheter, the 20- or 35-mL syringe system is likely to be most effective (Figure 4). Compared to the larger and smaller syringes, 20- to 35-mL syringes can generate enough pressure and contain an adequate volume of liquid to propel urethroliths back into the bladder lumen.
Figure 4: Various syringe sizes illustrating application of the Law of LaPlace.
Either rigid or flexible urethral catheters may be used. The advantage of using a flexible urethral catheter is that it permits placement of the top of the plunger (a piston) on the table so that the barrel of the syringe can be pushed down on the plunger (Figure 5). Advancing the syringe barrel over the plunger allows greater pressure to be generated within the isolated segment of the urethral lumen containing the uroliths compared to conventional use of the syringe.
Figure 5: This shows the correct use of a flexible urethral catheter to increase pressure generated with a syringe plunger. Arrowheads point to digital pressure applied to the junction of the catheter, with syringe and digital pressure applied to the external urethral orifice. The large orange arrow points to the syringe barrel being advanced over the syringe plunger.
Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.