The most common cause of urinary obstruction in small animal surgery is urinary stones in the urethra. Other causes of obstruction
are neoplasia in the urethra, and neoplasia compressing the urethra. Generally, small cystic calculi migrate to the neck of
the bladder during micturition and pass into the urethra. In the male, urethral calculi most commonly lodge caudal to the
os penis. In the female, calculi may lodge at any location along the length of the urethra. Urethral obstruction is more common
in the male than female.
Urinary obstruction requires medical treatment to stabilize the patient to surgery and surgical treatment to release the obstruction.
Dogs and cats with urinary obstruction are presented with hematuria, stranguria, and pollakiuria. Plain radiographs or radiographs
with contrast material (excretory radiographs, cystourethrograms) will confirm the presence of a urinary stones or mass in
Blood work with complete blood count, and biochemistry is required to evaluate kidney function of the patient. Urinary obstruction
is associated with severe post renal azotemia and severe electrolytes imbalance. Hyperkalemia is the most common electrolyte
abnormality. Hyperkalemia can induce severe bradycardia if the concentration is over 5 mEq/dl. Therefore, it is important
to reduce the hyperkalemia before anesthetizing patients with urinary obstruction.
The most efficient technique to lower the potassium concentration is to diaries the patient. If the patient can still urinate
on its own intravenous fluid therapy will induce a diuresis. If the patient cannot urinate cystocenthesis can be performed
to empty the bladder. The concentration of potassium is monitored very closely to be able to anesthetize the patient as soon
as the potassium is back to normal limits. If diuresis is not efficient insuline/glucose and bicarbonate can be used. While
the glusoce moves intracellular it brings the potassium in the intracellular space. Potassium and hydrogen particles are
exchanged which results in metabolic acidemia. Therefore bicarbonate is required to control the acidosis. This technique
lowers the potassium within 30 minutes. If the potassium concentration is not reduced then calcium gluconate is used to protect
the myocardium form the effect of potassium.