Depending on the length of urethral obstruction, cats may be quite ill or relatively normal when brought to the clinic. Severely affected cats often have elevated blood urea nitrogen, creatinine, potassium, phosphorus, and magnesium levels. They may have metabolic acidosis and ionized hypocalcemia. This prospective study evaluated arterial blood pressure of 28 cats presenting with urethral obstruction and having a wide spectrum of disease severity. Twenty cats had normal blood pressure, and 8 had high blood pressure. In general, less severely affected cats tended to be hypertensive. Cats with severe electrolyte abnormalities, which have been predicted to be hypotensive, may have been able to maintain a normal blood pressure by activation of the renin-angiotensin system. Severe biochemical and physiologic derangements can occur in cats with urethral obstruction and normal blood pressure.

COMMENTARY: Despite the investigators' hypothesis, hypotension does not seem to be associated with metabolic changes seen in postrenal azotemia in cats with urethral outflow obstruction. This is a bit surprising because cardiac effects of hyperkalemia, acidemia, and perhaps hypothermia might be expected to induce hypotension. Cats with less severe azotemia tended to be hypertensive. If cats with urethral obstruction are indeed proven to be hypertensive, it may result from pain and excitement. However, pain and excitement diminish as bladder distension becomes prolonged and the metabolic changes increase lethargy and stupor. Based on the findings of this study, measurement of blood pressure cannot evaluate the degree of metabolic disturbance in cats with urethral obstruction.

Assessment of blood pressure in cats presented with urethral obstruction. Malouin A, Milliagan JA, Drobatz KJ. J VET EMERG CRIT CARE 17:15-21, 2007.