Hydromorphone CRI
Semisynthetic full mu-opioid agonist, 5–7× more potent than morphine. No histamine release (safe for IV bolus). Caution: hyperthermia in cats. DEA C-II.
Clinical background
Hydromorphone is a semisynthetic full mu-opioid agonist and the most commonly used parenteral opioid for hospitalized dogs and cats in North American small animal practice. It is approximately 5–7 times more potent than morphine on a mg/kg basis, has a predictable pharmacokinetic profile, and is well-tolerated as both a bolus and a CRI. Its wide use reflects a favorable balance of efficacy, cost, availability, and clinical familiarity.
Pharmacology
Full mu-opioid agonism at central and peripheral receptors mediates analgesia, sedation, respiratory depression, decreased GI motility, and antitussive effects. Unlike morphine, hydromorphone does not cause significant histamine release, it can be given IV without the risk of histamine-mediated hypotension seen occasionally with morphine.
Hepatic glucuronidation is the primary metabolic route; renal excretion of the glucuronide metabolite follows. Dose reduction should be considered in severe hepatic disease. Renal impairment leads to metabolite accumulation but is less clinically significant than with morphine (whose active M6G metabolite accumulates in renal failure).
Half-life in dogs: approximately 0.5–1.5 hours. Clinical duration after a single IV bolus: 2–4 hours. CRI reaches steady state relatively quickly, making dose titration practical.
DEA Schedule II (C-II) controlled substance.
Indications
- Postoperative pain: first-line parenteral opioid in dogs and cats post-surgery
- Moderate-to-severe acute pain: trauma, pancreatitis, visceral pain
- Premedication: standard component of balanced anesthetic protocols
- ICU analgesia. CRI preferred for sustained effect without accumulation of repeated boluses
- Sedation: at higher doses contributes meaningfully to procedural sedation when combined with a sedative
Dosing
The calculator combines Plumb’s analgesia and anesthesia extra-label dose ranges into a single titration ladder so the clinician can read across the full clinical use case. Rates assume syringe pump delivery of the undiluted stock vial. Pump rates of 0.1–1 mL/hr are normal for opioid CRIs on a syringe pump and are within the syringe pump’s reliable delivery range; they are below the threshold for accurate fluid pump delivery. If a syringe pump isn’t available, dilute hydromorphone into a carrier fluid bag and run at the patient’s maintenance fluid rate.
Dogs: - Loading bolus: 0.025–0.1 mg/kg IV - CRI: 0.02–0.1 mg/kg/hr (combined analgesia + anesthesia) - Pure analgesia is typically dosed around 0.03 mg/kg/hr (highlighted on the ladder as “Typical analgesia”). Doses above 0.05 mg/kg/hr are anesthesia-context infusions and are flagged on the ladder. - Intermittent bolus (alternative protocol): 0.05–0.2 mg/kg IV/IM/SC q 2–4 hr
Cats (per Plumb’s, more conservative than the dog combined range): - Loading bolus: 0.025 mg/kg IV - CRI: 0.01–0.05 mg/kg/hr (start at the low end) - Intermittent bolus (alternative protocol): 0.05–0.1 mg/kg IV/IM/SC q 2–6 hr
Sustained CRIs above 0.05 mg/kg/hr for more than 12 hours may cause sedation and adverse effects severe enough to require reducing the rate. The calculator flags ladder rows above this threshold.
Cats are more sensitive to opioid-induced dysphoria and hyperthermia than dogs. Hyperthermia is a known adverse effect of hydromorphone in cats, monitor temperature and discontinue if rectal temperature exceeds 40°C (104°F). The mechanism is central thermoregulatory disruption rather than true fever.
Adverse effects
Dogs: - Respiratory depression, dose-related; bradypnea most common; apnea possible at high doses - Bradycardia - Vomiting, particularly after IM administration; less common IV - Panting, common in dogs at analgesic doses; not a reliable indicator of pain when on opioids - Decreased GI motility
Cats: - Hyperthermia: unique to cats among domestic species; monitor rectal temperature during CRI - Dysphoria, restlessness, vocalization, mydriasis; more common at higher doses or in cats in pain - Respiratory depression, less pronounced than dogs at equivalent doses but still clinically relevant - Euphoria has been reported and can cause paradoxical agitation
Drug interactions
- CNS depressants: additive sedation and respiratory depression with benzodiazepines, α₂ agonists, acepromazine, and inhalant anesthetics. Reduce doses of co-administered agents in balanced protocols.
- MAO inhibitors (selegiline, amitraz, linezolid): serotonin syndrome and severe CNS depression risk; avoid combination.
- Other opioids: do not stack full mu-agonists; if switching, allow washout or titrate the new agent downward.
- Buprenorphine and butorphanol: partial agonist / agonist-antagonist co-administration will reduce hydromorphone analgesia and can precipitate withdrawal-like agitation in opioid-dependent patients.
Sources
- Plumb’s Veterinary Drugs, hydromorphone monograph.
- Pypendop BH, Ilkiw JE: Pharmacokinetics of hydromorphone hydrochloride in healthy cats. Am J Vet Res 2008.
- KuKanich B et al: Pharmacokinetics of hydromorphone hydrochloride in healthy dogs. Vet Anaesth Analg 2008.