004 Fluid Choice in Tomcat Urethral Obstruction (Blocked Cats)

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Not all blocked cats are hypoperfused on presentation but those that are should be resuscitated with fluid therapy given as boluses to restore perfusion with the approach being tailored to the individual patient. This should be done BEFORE you consider sedating or indeed anaesthetising to attempt catheterisation.

Will I rupture the bladder?

“If I give blocked cats lots of fluid therapy while their bladder is obstructed, won’t I rupture the bladder?"

NO! The risk is very very very small and has to be weighed up against the undoubted benefit of fluid resuscitation in these cases. 

Why won't I rupture the bladder?

For the kidney to make urine it needs to have an adequate blood supply. The blood supply to the kidneys of a blocked cat in shock will be moderately or even severely compromised and this means that the impetus to make urine will be significantly decreased. At the same time as the bladder is obstructed and under high pressure this pressure is transmitted via the ureters to the kidneys; this back pressure on the kidneys will also act to oppose urine production. So on the one hand there is little forward impetus to make urine and on the other hand there is back pressure not to make urine; hence blocked cats in shock are unlikely to be making much urine at all. Obviously as you perform intravenous fluid resuscitation and the blood supply to the kidneys increases you will promote the formation of urine but you will also be going on to deal with the obstruction very soon and so this is not a concern.


Does it matter which fluid we choose for the initial resuscitation phase?

A replacement isotonic crystalloid is used. Typical choice is between Hartmann’s (buffered lactated Ringer’s solution, compound sodium lactate) and  0.9% sodium chloride ('normal' saline).

Blocked cats have:

  • Hyperkalaemia: clinically significant life-threatening hyperkalaemia is not uncommon in the more severe cases
  • Acidaemia due to metabolic acidosis: typically less of a concern but can have significant adverse physiological consequences and can be life-threatening if sufficiently severe

0.9% sodium chloride:

Potassium-free; blocked cats are hyperkalaemic so why would you want to give them more potassium?

But...promotes hyperchloraemic acidosis and can exacerbate existing acidaemia.

Hartmann's solution:

Contains 5 mmol/l potassium - as do Normosol-R and Plasmalyte 148; this is nevertheless lower than clinically significant hyperkalaemia and use will decrease potassium concentration through dilution.

But...solution that promotes alkalosis, alkalinising solution; may help resolve metabolic acidosis.

(Also contains small amount of calcium which is no bad thing given that these cats are often hypocalcaemic.)

Bottom line: choice of fluid between these two types has not thus far been shown to be clinically significant....

Drobatz KJ, Cole SG. The influence of crystalloid type on acid-base and electrolyte status of cats with urethral obstruction. J Vet Emerg Crit Care 2008. 18(4):355-361.

Prospective randomised non-blinded study

Cats that presented to the Matthew J. Ryan Veterinary Hospital of the University of Pennsylvania with urethral obstruction between September 2000 and November 2004 were eligible for inclusion.

Inclusion criteria:

  • Baseline blood pH and potassium measured initially
  • At least one subsequent measurement of these parameters within the following 12 hours

Urethral obstruction diagnosed based on compatible clinical signs + large, firm, non-expressible urinary bladder

Exclusion criteria:

  • Owners elected to not pursue treatment and elected euthanasia
  • Baseline blood pH and potassium as well as one subsequent measurement of those variables within 12 hours of admission not obtained

Coin-flip randomisation to receive either Normosol-R or 0.9% sodium chloride.

Blood samples collected before any therapy for analysis of blood pH, bicarbonate, pCO2, base excess, sodium, chloride, potassium, ionised calcium, ionised magnesium, and glucose.
Biochemical analysis of serum urea nitrogen (SUN), serum creatinine, and serum phosphorus performed on some cats.

Further therapy and monitoring at discretion of attending clinician; non-blinded


  • 68 cats: 39 Normosol-R, 29 0.9% sodium chloride
  • Very similar comparable groups including with respect to initial pH and potassium concentrations
  • While there were no statistically significant differences between groups at baseline, cats in the Normosol- R group had a significantly higher blood bicarbonate concentration at 12 hours and significantly higher blood pH at 6 hours and 12 hours.
  • There were no statistically significant differences in blood potassium concentration or the amount of fluid administered between groups at any time.
  • In comparison with the absolute values, the increase in blood pH from baseline was significantly greater in the Normosol-R group than the 0.9% sodium chloride group at 6 hours and 12 hours following admission and this difference also approached significance at 2 hours.
  • Blood potassium levels declined steadily in both groups and no statistically significant differences were found in the rate of decline from baseline between groups.
  • Cats in the 0.9% sodium chloride group had a significantly higher blood sodium concentration at 2 hours and 6 hours, as well as a significantly higher blood chloride concentration at 6 hours and 12 hours. The increase in blood chloride from baseline was significantly higher in the 0.9% sodium chloride group than in the Normosol-R group at 2 hours, 6 hours, and 12 hours.

Use of Normosol-R was associated with a greater increase from baseline in blood pH and bicarbonate values than was the use of 0.9% sodium chloride.

Although Normosol-R contains 5mmol/l of potassium, the absolute value and rate of decline in blood potassium levels were nearly identical between the groups in this study.

Authors say: “While statistically significant, the differences identified between groups in the present study may not have been clinically significant in this patient population. From both prior research and widespread clinical experience, it is clear that both balanced isotonic crystalloid solutions and 0.9% sodium chloride are acceptable fluid choices for the treatment of most cases of urethral obstruction in cats. However, possi-ble implications of the current study pertain to severely affected cats with profound metabolic acidosis. In these cases, any acidifying effect of 0.9% sodium chloride could be deleterious, as severe metabolic acidosis impairs cardiovascular function, compounding the untoward effects of hyperkalemia and hypocalcemia. The use of a balanced isotonic crystalloid would achieve similar results in the treatment of per-fusion abnormalities and hyperkalemia, along with more rapid improvement in acid–base status.”

Study limitations:

  • Not blinded
  • Treatment not standardised across groups
  • No mention of power calculation with respect to sample sizes needed to show significant different between the groups
  • As always...larger sample size if possible!

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