Norepinephrine CRI

Strong α₁ and α₂ adrenergic agonist with moderate β₁ activity. Acts at α-adrenergic receptors to cause peripheral vasoconstriction and at β receptors to cause positive inotropy and coronary artery vasodilation. Total peripheral resistance is increased, raising systolic and diastolic blood pressure. Perfusion to abdominal organs, skin, and skeletal muscle can be reduced (especially at higher doses), while coronary blood flow increases. Onset 1–2 min IV; duration 1–2 min after stop. Rapidly metabolized by COMT and MAO.

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Clinical background

Norepinephrine is the first-line vasopressor for fluid-refractory hypotension in most veterinary critical-care contexts. It produces the blood-pressure response of a pressor (α₁-mediated arteriolar constriction) without the marked tachycardia or arrhythmogenicity of epinephrine, which is why most authors recommend reaching for norepinephrine before alternatives when the goal is to raise mean arterial pressure in a patient who has already had adequate fluid resuscitation.

Pharmacology

Endogenous catecholamine with strong α₁ and modest β₁ activity, and minimal β₂ activity. This receptor profile produces:

• Arteriolar vasoconstriction (α₁), raising systemic vascular resistance and mean arterial pressure
• Modest inotropy (β₁), which partly offsets the increased afterload so cardiac output is usually maintained or mildly increased
• Minimal chronotropy at typical doses, because the β₂-mediated reflex slowing from rising MAP is roughly balanced by the direct β₁ effect on rate. Heart rate often does not change much at infusion rates ≤1 µg/kg/min

This balance is what makes norepinephrine clinically attractive: blood pressure rises, cardiac output is preserved, and the heart rate stays relatively unchanged. Compare with epinephrine, which raises rate and contractility more aggressively and has been associated with more arrhythmias and higher tissue oxygen demand.

Onset after starting an IV infusion is within 1–2 minutes; the effect dissipates within 1–2 minutes after stopping. The drug is rapidly metabolized by COMT and MAO and does not cross the blood–brain barrier.

Indications

Primary use cases:

• Fluid-refractory hypotension, septic or distributive shock once volume status is corrected
• Anesthesia-induced (most often inhalant-induced) hypotension that has not responded to fluid bolus and lightening the anesthetic plane
• Maintenance of perfusion pressure during procedures or in critical illness when the goal is a specific MAP target

The Surviving Sepsis Campaign positions norepinephrine as the first-line vasopressor for septic shock; veterinary practice has converged on the same recommendation. The veterinary indication set extends to any cause of significant vasodilation that has not responded to fluids: septic shock, anaphylaxis, inhalant-induced anesthetic hypotension, and as a component of multi-pressor management in refractory shock.

Norepinephrine is not a substitute for fluid resuscitation. Starting a vasopressor in a hypovolemic patient raises blood pressure by squeezing an under-filled tank; perfusion does not improve. Volume status must be assessed and corrected first.

Dosing

• Dogs and cats, hypotension: 0.05–2 µg/kg/min CRI, titrated to MAP target
• Initial rate: 0.05–0.1 µg/kg/min
• Most patients respond at: <0.5 µg/kg/min; one isoflurane-anesthetized canine study reported an average effective dose of 0.44 µg/kg/min
• Caution above: 1 µg/kg/min. At higher doses the heart rate response increases on top of the existing inotropy, and the marginal clinical benefit of going higher is uncertain. Reassess indication, fluid status, and concurrent anesthetic depth before exceeding this range
• Hard maximum: 2 µg/kg/min in standard practice; doses above this are reported in refractory cases but should prompt consideration of adding a second pressor (vasopressin) or epinephrine rather than continuing to escalate norepinephrine alone

Cat doses use the same range as dogs per Plumb’s, although cats may be anecdotally more susceptible to adverse effects and the published feline literature is thinner. Start at the lower end and titrate slowly.

Start at 0.05–0.1 µg/kg/min and increase in 0.05 µg/kg/min steps every 5–10 minutes against the MAP target. Most patients who are going to respond do so at <0.5 µg/kg/min; if you are above 1 µg/kg/min and still chasing a MAP target, the question is usually “what else is going on” rather than “should I go higher.”

Administration

Norepinephrine is supplied in the US as a 1 mg/mL (1000 µg/mL) concentrate, typically a 4 mL vial (4 mg total). Standard preparation is one vial diluted into a 250 mL, 500 mL, or 1 L bag, giving 16, 8, or 4 µg/mL respectively. For short anesthesia and surgical infusions, either 5% dextrose or 0.9% NaCl is acceptable as the carrier fluid. For prolonged infusions in sepsis or ICU care, prefer a dextrose-containing diluent because the mildly acidic pH slows oxidative degradation of the catecholamine.

Commercial premixed bags (4 mg, 8 mg, or 16 mg in 250 mL) are also available and avoid the dilution step entirely.

A central line is strongly preferred for any sustained norepinephrine infusion because of extravasation risk. Peripheral administration is acceptable in emergencies: get the patient stabilized, then place central access. When you must infuse peripherally, use the largest vein and the largest-gauge catheter that’s practical (a 20-gauge has been recommended for canines), check the site frequently, and have a plan ready in case extravasation is suspected.

Discard any solution that has turned pink, brown, or developed a precipitate, and discard unused solution after the standard hang time. Do not co-administer norepinephrine in a line containing sodium bicarbonate or other alkalinizing solutions, and do not mix it with iron-containing fluids or oxidizing agents; the catecholamine is rapidly destroyed in those conditions.

Name confusion with epinephrine

NOREPInephrine and EPInephrine are repeatedly cited as among the most commonly confused drug pairs in human and veterinary medicine. They are pharmacologically related but clinically distinct: norepinephrine is a vasopressor with modest inotropy, epinephrine is a powerful inotrope and chronotrope with vasopressor effects at higher doses, and the typical doses for the two drugs differ. Use redundant verification: read the vial label, confirm with a second person where possible, label the bag clearly with the drug name and concentration, and ensure pump rate and dose are double-checked at handoff. ISMP lists both as high-alert medications.

Drug interactions

• Tricyclic antidepressants and MAO inhibitors potentiate norepinephrine’s vasopressor effect. Use lower doses and titrate carefully if these are on board.
• α-adrenergic blockers (acepromazine, phenoxybenzamine) blunt the vasopressor response. Acepromazine is the more clinically relevant case in veterinary anesthesia: in an aceprom-premedicated patient, norepinephrine may be a better choice than epinephrine (which can produce paradoxical hypotension via “epinephrine reversal”), but the response will be partially blocked by the α-antagonism. Phenylephrine is another reasonable alternative in this setting.
• Halogenated inhalant anesthetics sensitize the myocardium and increase arrhythmia risk during catecholamine infusions. The risk with norepinephrine is lower than with epinephrine or dopamine but is not zero, especially at higher infusion rates.
• Beta-blockers can produce unopposed α-mediated vasoconstriction with severe hypertension if they are concurrent.

Adverse effects

• Extravasation injury and tissue necrosis: the most clinically important risk. Local α₁-mediated vasoconstriction at the infusion site can cause severe tissue ischemia and necrosis if the catheter dislodges or leaks. Suspected extravasation should prompt immediate cessation of the infusion, aspiration through the catheter before removal, and consideration of local infiltration with phentolamine.
• Excessive vasoconstriction and end-organ ischemia at higher doses, particularly splanchnic and renal beds. Hyperlactatemia in a patient on a high-dose norepinephrine infusion may reflect this rather than the underlying shock.
• Reflex bradycardia at higher doses, the result of strong α-mediated MAP rise triggering baroreflex slowing
• Tachyarrhythmias are less common than with epinephrine or dopamine but reported. Risk rises with dose, concurrent inhalant anesthesia, hypokalemia, and hypomagnesemia.
• Hypertension if the dose is not titrated against a specific MAP target

Monitoring

• Continuous or frequent blood pressure, ideally MAP measured directly (arterial line) for ICU use, or oscillometric q5–10 min for anesthesia. Set a specific MAP target (commonly 65–80 mmHg in dogs, 60–75 mmHg in cats, adjusted for context) before starting and titrate to it.
• Continuous ECG for rhythm and rate
• Lactate trend in shock patients, response to the pressor and changes in tissue perfusion
• Urine output as a perfusion marker
• Mucous membrane color, CRT, extremity temperature: clinical indicators of peripheral perfusion. Cold extremities at high MAP suggest excessive vasoconstriction.
• Serum potassium and magnesium: correct derangements before or during infusion to reduce arrhythmia risk
• The infusion site at every check, with documented inspection. The combination of “MAP rising on norepinephrine” and “infusion site looks fine” is what you want; either alone is not enough.

Extravasation management

Extravasation is the dominant local-injury risk. If it occurs:

1. Stop the infusion immediately. Do not pull the catheter yet.
2. Aspirate as much of the leaked drug as possible through the catheter
3. Then remove the catheter
4. Consider local infiltration with phentolamine (5–10 mg in 10 mL saline, infiltrated SC into the affected area) as soon as possible. Phentolamine is an α-blocker that reverses the local vasoconstriction
5. Warm compresses, elevation, and ongoing assessment for tissue compromise
6. Switch the infusion to a new IV access (preferably central) and resume

Topical nitroglycerin has also been described in human medicine as an alternative to phentolamine. Document the event and the area of involvement.

Weaning

Reduce the rate in 0.05 µg/kg/min increments every 10–15 minutes against MAP. Most patients tolerate gradual weaning well; abrupt cessation can produce rebound hypotension, particularly after prolonged infusions. Do not stop the infusion abruptly unless transitioning to another pressor or unless the indication has clearly resolved.

Sources

• Plumb’s Veterinary Drugs, norepinephrine monograph (current edition).
• Hart S, Silverstein DC. Catecholamines. In: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. 3rd ed. Elsevier; 2023:855–859. Chapter 147; Table 147.1.
• Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock. (Human guideline; the vasopressor recommendations have informed veterinary critical-care practice.)
• ISMP List of High-Alert Medications in Acute Care Settings.