Definition

Restricted flow of urine from the kidneys through any point of the urinary tract to the external urethral orifice.

Pathophysiology

• Physical or functional obstruction of the urinary tract resulting in partial or complete cessation in renal excretory function. As luminal pressure builds it is transmitted to the level of the kidney and its functional units (nephrons). As tubular pressure exceeds filtration pressure, GFR ceases. Ensuing pathophysiologic consequences depend on site, degree, and duration of obstruction. Complete obstruction produces a pathophysiologic state characterized by uremia, acidemia and hyperkalemia.
• Perforation of the excretory pathway with extravasation of urine (urethral tear or bladder rupture) is a functional equivalent.

Systems Affected

• Renal/Urologic.
• Cardiovascular, gastrointestinal, nervous, and respiratory systems also affected relative to duration of obstruction and severity of metabolic derangement.

Signalment

• Dog and cat
• More common in males than females

Signs

Causes

Risk Factors

• Urolithiasis, particularly in males
• Feline lower urinary tract disease, particularly in males
• Prostatic disease in male dogs

Differential Diagnosis

• Owners may have difficulty distinguishing urinary obstruction from constipation.
• Signs of feline idiopathic cystitis can also be difficult to distinguish from obstruction, especially if there are no signs of systemic illness. Determination of bladder size (large/firm with obstruction, small with cystitis) can help distinguish.
• Animals whose urinations are not routinely observed by owners can present for signs referable to systemic illness (uremia) rather than concern for obstruction.
• Evaluation of any azotemic patient, in conjunction with history and physical exam, should include consideration of possible post-renal causes (e.g., urinary obstruction).
• Once recognized, diagnostic efforts focus on assessing the degree of systemic impact and metabolic derangement, as well as identifying the location and causeof obstruction.

CBC/Biochemistry/Urinalysis

• Results of a hemogram are usually normal, but a stress leukogram may be seen.
• Biochemical analysis reveals azotemia, hyperphosphatemia, metabolic acidosis, hyperkalemia, and decreased ionized calcium proportional to the duration of complete obstruction. Cats may also have stress hyperglycemia.
• Hematuria and proteinuria are common. Crystalluria may be associated with urolithiasis, though can be present in the absence of stones. Atypical epithelial cells may be seen in patients with neoplasia.

Other Laboratory Tests

• Given the potential for concurrent urinary tract infection in obstructed dogs (especially secondary to uroliths), urine culture may be beneficial. However, cats with obstruction are unlikely to have bacterial cystitis, and presenting urine culture is not recommended.
• Uroliths passed or retrieved should be sent for crystallographic analysis to determine their mineral composition.

Imaging

Additional Diagnostic Procedures

• Electrocardiography may detect abnormalities secondary to hyperkalemia, including tall T waves, prolonged PR interval, widened QRS complexes, loss of P waves, bradycardia, and atrial standstill.
• Transurethral catheterization has diagnostic and therapeutic value. With attempted passage of the urinary catheter, the location and nature of obstructing material may be determined. Some or all of the obstructing material (e.g., small uroliths and feline urethral plugs) may dislodge and pass antegrade out of the urethra or retrograde into the bladder. Animals that cannot urinate despite being readily catheterized likely either have intramural lesions or functional urinary retention.
• Cytologic evaluation of specimens obtained with the assistance of catheters may be diagnostic, particularly carcinomas of the urethra or bladder and some prostatic diseases.
• Cystoscopy can be helpful, particularly in female dogs with intramural lesions of the bladder neck or urethra.

Emergency Management

• Complete obstruction is a medical emergency that can be life-threatening; treatment should be started immediately.
• Initial goals are combating metabolic derangements of post-renal uremia, especially significant hyperkalemia, and establishing urinary patency.
• Intravenous fluids administration of isotonic crystalloid based on degree of cardiovascular compromise, dehydration, and potential for post-obstructive diuresis.
• For severe hyperkalemia (K⁺ >8 mmol/L, significant bradycardia/ECG changes); administer calcium gluconate (1 mL/kg over 3–5 minutes, titrated based on resolution of ECG changes), regular insulin (0.1–0.2 U/kg IV once) and dextrose bolus (1 mL/kg IV, diluted, over 5 minutes). May need 50% dextrose to continue on dextrose infusion (2.5–5%) for 4–6 hours to avoid hypoglycemia. For more severe hyperkalemia or acidemia (K+ >10 mmol/L, pH <7.1) consider sodium bicarbonate (1 mL/kg IV over 5–10 minutes).
• Decompresive cystocentesis may allow more immediate relief of intravesicular pressure, resumption of GFR, and decreased back-pressure for catheterization efforts.
• Urethral catheterization under heavy sedation or general anesthesia (see below) to relieve physical obstruction and establish urethral patency.
• Flushing urinary bladder until clear effluent, attach to sterile collection system.

Nursing Care

• Post-obstructive management geared toward maintaining fluid balance, continued correction of metabolic derangements, and sedation/analgesia (see below).
• Rate of fluid administration should be determined based on maintenance, dehydration and ongoing losses. Some patients can experience a post-obstructive diuresis, with significant urinary losses. It is important to keep up with these losses to avoid dehydration.
• Hyperkalemia should resolve within hours after de-obstruction and may eventually need to be supplemented. Azotemia should also significantly decrease within 12–24 hours.
• Patients with urolithiasis will often require cystotomy for stone removal with associated postoperative care.

Drug(s)

• Sedation/analgesia/anesthesia for urethral catheter placement should be selected based on patient stability, and potential concern for decreased renal clearance. Acepromazine plus an opioid (e.g., methadone, buprenorphine) if stable, opioid plus benzodiazepine if unstable. Ketamine and a benzodiazepine is a commonly used injectable combination. General anesthesia, after premedication and induction, may be indicated as it provides the most urethral relaxation, especially for obstructed dogs.
• In the post-obstructive period, continued sedation and analgesia is beneficial.
• Urethral relaxants, such as acepromazine and/or prazosin, may be beneficial for post-obstructive cats or patients with neurogenic urine retention.
• Antibiotics may be indicated with infection (e.g., UTI or prostatitis), ideally only when strongly suspected or with documented evidence (such as cytology or positive culture). Antibiotics should not be administered to prevent UTI while a urinary catheter in place as this is not effective and can promote bacterial resistance.

Contraindications

• Corticosteroids are contraindicated while a urinary catheter is in place. This can predispose to the development of urinary tract infection.
• Given their potential impact on renal blood flow, nonsteroidal anti-inflammatory medications should be initially avoided in more metabolically compromised patients.

Precautions

Avoid drugs that reduce blood pressure (e.g., acepromazine) or induce cardiac dysrhythmia (e.g., ketamine) until dehydration and hyperkalemia are resolved.

Patient Monitoring

• If the initial electrocardiogram indicates life-threatening changes, continuous monitoring to guide treatment and evaluate response is warranted.
• Assess urine production and hydration status frequently, and adjust fluid administration rate accordingly (as described above).
• Monitor renal values and electrolytes. Sicker patients may require more frequent monitoring (q6–12), whereas once daily may be sufficient for more stable patients.
• The urinary catheter can be removed once metabolic derangements and post-obstructive diuresis has resolved, and/or the urine appears to be clear of gross debris, clots, etc.
• After urinary catheter removal, close monitoring to verify the ability to urinate adequately for at least 12–24 hours.
• Cats may benefit from continued pain medication and urethral relaxation for 5–7 days at home, as well as recommendations for increased water intake and environmental enrichment to help decrease risk of re-obstruction.

Possible Complications

• Death.
• Injury to the excretory pathway (e.g., urethral tear) while trying to relieve obstruction.
• Urine leakage or bladder rupture if decompressive cystocentesis performed.
• Hypokalemia during post-obstructive diuresis.
• Recurrence of obstruction.

Associated Conditions

• Bradycardia secondary to hyperkalemia
• Azotemia, hyperphosphatemia, and metabolic acidosis

Age-Related Factors

In older dogs, the underlying cause of obstruction (e.g., neoplasia and prostate disease) often is difficult to treat.

Synonyms

Urethral obstruction

See Also

• Azotemia and Uremia
• Feline Idiopathic Lower Urinary Tract Disease
• Hyperkalemia
• Urinary Retention, Functional

Client Education Handout Available Online