Bladder stones becoming lodged in the urethra are the most common cause of obstruction. This occurs mostly in the male dog because they have a long urethra which narrows distally.
A urethrotomy is a surgical procedure that entails entering the urethra at the level of the obstruction to remove it. It is more difficult to remove stones through a urethrotomy than a cystotomy. The complication rate tends to also be higher with the urethrotomy. This is why it is better to retropulse the stones back into the bladder and remove them from there.
Retropulsing stones from the urethra is another LIFE SAVING procedure every veterinarian will need to perform someday.
If the obstruction is ACUTE, the dog will quickly become uncomfortable, anxious, and unable to pass urine despite frequent attempts. This is an emergency situation; the onset of azotemia is an indicator of severity. In acute cases retropulsion should be successful 100% of the time. Lack of success is usually due to inappropriate catheter selection and flushing technique. Follow the steps described here and ensure that your team (yes, you need a team for this) understands their role.
A CHRONIC obstruction usually results from a smaller stone being lodged in the urethra, typically at the level of or within the os penis. The stone is too large to be passed during micturition but is not so large that it fully obstructs the flow of urine. These patients present with a history of taking a long time to urinate, often with a very narrow stream. They may also display pollakiuria, dysuria and hematuria. These are by far the more difficult stones to dislodge and retropulse back into the bladder. Failure to successfully retropulse a stone from the urethra is more likely to occur with this patient.