Myth #1: Morphine cannot be used in cats because of central nervous system excitement.
Morphine metabolism is slightly slower in cats than in dogs (the plasma half-life in cats is about 1.3 hours vs. 1 hour in dogs). But the key reason cats require a decreased dosage is that their volume of distribution is smaller than in dogs. The lower volume of distribution results in higher plasma concentrations being achieved with a given dose. Since the half-life is similar in the two species, the dosing interval is the same.
Myth #2: The most common adverse effects of opioid administration are cardiovascular and respiratory depression.
Contrary to popular belief, the most common adverse effects of opioids don't involve cardiovascular or respiratory depression. The most common adverse effects include hypothermia, sedation, nausea, vomiting, panting, and defecation.
Although opioids can cause pronounced bradycardia, cardiac output remains stable because of increased stroke volume in animals administered clinically recommended dosages. Remember that cardiac output is the product of stroke volume and heart rate, so there is no net change in blood pressure or tissue perfusion in these cases.
A dose-dependent respiratory depression does occur with opioid administration, but with clinically recommended dosages, the respiratory depressant effects are clinically insignificant. Keep in mind that mild respiratory depression can be a concern in animals with head trauma or pre-existing pulmonary diseases, or in combination with other respiratory depressants.
Butch KuKanich, DVM, PhD, DACVCP
Department of Anatomy and Physiology
College of Veterinary Medicine
Kansas State University
Manhattan, KS 66506