Vasopressor and inotrope CRIs for intraoperative hypotension

Intraoperative hypotension is one of the most common things you will manage during small animal anesthesia, and one of the most consequential. Sustained MAPs below 60 to 65 mmHg correlate with postoperative AKI, prolonged recoveries, and increased perioperative mortality, even when the intraoperative episode resolves on its own. The response is usually straightforward and follows a predictable sequence: check the anesthetic plane and adjust depth if indicated, give fluid, and only then reach for a vasoactive drug. The harder part is choosing the right drug, and that decision depends on the mechanism driving the pressure drop rather than on how low the number is.

Two clinical pictures are covered here. The first is inhalant-induced vasodilation in an otherwise healthy patient under elective anesthesia, where the patient has adequate cardiac and renal reserve and the intervention ladder runs from depth check to fluid bolus to vasoactive drug. The second is intraoperative hypotension in a patient with cardiac disease, advanced renal disease, or other comorbidities that limit fluid tolerance, where the fluid bolus is skipped or attenuated and the threshold for starting a vasoactive infusion is lower. Cases in the second group should be approached with input from a cardiologist or criticalist where possible, but they are a real part of clinical practice and you will encounter them at the table.

This article covers when to start a vasopressor or inotrope, how to choose between norepinephrine, dopamine, and dobutamine, the bag-preparation math, titration to a hemodynamic target, monitoring, complications, and weaning.

When to start a vasopressor or inotrope

The working definition of intraoperative hypotension is a MAP below 60 to 65 mmHg or a systolic of less than 90 mmHg, sustained for more than 5 to 10 minutes. Sustained is the key word. A brief dip during a stimulating part of the surgery is not the same as a steady downward drift, and the two situations call for different responses.

The intervention ladder runs in this order.

1. Check anesthetic depth and adjust accordingly. The first move is not to drop the vaporizer setting reflexively, it is to actually assess where the patient is on the depth spectrum. Useful indicators are jaw tone, palpebral reflex, eye position (rotated ventromedially in light-to-moderate planes, central in deep planes), pupil size, response to surgical stimulus, and the ETCO₂ trend. If the patient is genuinely too deep (no jaw tone, central eye position, flat response to surgical stimulus), then reducing the vaporizer setting is appropriate, and the MAP should be reassessed in 2 to 3 minutes. If the patient is already at an adequate or light plane, dropping the vaporizer further will not fix the hypotension and may produce movement, awareness, or a sympathetic surge that complicates the picture. In that situation the cause of hypotension is something other than excess depth, and you should move to the next step. A related lever to consider here is reducing the inhalant requirement by adding supplemental analgesia or sedation, or by using a regional or local block. Inhalants are the dominant driver of dose-dependent vasodilation under anesthesia, and a patient kept at a high vaporizer setting because the analgesic plan is incomplete will be hypotensive for the duration. Adding an opioid CRI, a low-dose ketamine or lidocaine CRI, or a regional block lets you bring the vaporizer down without compromising the depth of anesthesia, which treats the cause of vasodilatory hypotension rather than just the number.
2. Check the heart rate. Bradycardia can be the cause of hypotension rather than a consequence of it. If the heart rate is below normal for the patient and the MAP is low, an anticholinergic is often the right next move and can resolve the hypotension without any vasoactive support. Atropine 0.02 to 0.04 mg/kg IV or glycopyrrolate 0.005 to 0.01 mg/kg IV are both reasonable. One caution applies when the patient has received an α₂ agonist such as dexmedetomidine or medetomidine: these drugs produce a reflex bradycardia in response to their peripheral α₁-mediated vasoconstriction, and that bradycardia is an expected physiologic response rather than a pathologic one. Adding an anticholinergic while α₁-mediated vasoconstriction is still active can produce a hypertensive surge and a sharp increase in myocardial workload, so the combination warrants deliberation rather than reflex. Practice varies: some anesthesiologists will use an anticholinergic with an α₂-treated patient when the rate is severely low and the case demands it, while others prefer to reverse the α₂ with atipamezole and address the rate that way. The right call depends on the cause, the timing relative to the vasoconstriction phase, and the patient. If MAP is adequate despite the bradycardia, treatment is usually not necessary.
3. Check body temperature. Hypothermia is a common and frequently overlooked contributor to intraoperative hypotension. Open body cavities and the continuous flow of cool, dry inhalant gases both pull heat out of the patient over the course of an anesthetic. As core temperature falls, vascular tone drops and contractility decreases, and MAP follows. Active warming with forced-air, conductive, or circulating-water systems will often improve the MAP on its own without any drug intervention. Check the temperature early and reassess it regularly during a long case; small, sustained drops accumulate.
4. Give a fluid bolus. A 5 to 10 mL/kg crystalloid bolus over 10 to 15 minutes addresses volume-responsive hypotension. If the MAP responds but does not normalize, a second bolus is reasonable. This step does not apply to patients with cardiac disease, advanced renal disease, or known volume overload. Those patients need a tailored plan and often skip this step entirely, moving from depth assessment directly to a vasoactive drug.
5. Reassess the mechanism. If the MAP is still inadequate after depth assessment, rate and temperature correction, and fluid resuscitation, the patient is in a fluid-refractory state and you need to ask why. Vasodilation? Poor contractility? A combination? The answer drives the next decision.
6. Start an infusion. Bolus dosing of phenylephrine or ephedrine can serve as a bridge while you set up. If the patient needs repeat bolus dosing to maintain MAP above 65 mmHg, it is time to commit to a continuous rate infusion.

A common mistake is starting a vasopressor before the earlier steps have been fully worked through. Vasoconstriction can mask inadequate intravascular volume, untreated bradycardia, or progressive hypothermia, and the patient pays for that postoperatively in renal injury and a longer recovery. Confirm that the patient is volume-replete, that heart rate and temperature are reasonable, and that the patient is not running too deep before adding a drip.

Choosing between norepinephrine, dopamine, and dobutamine

The three drugs covered here act through different combinations of three adrenergic receptors. α₁ activation constricts vascular smooth muscle, raising systemic vascular resistance and therefore mean arterial pressure. β₁ activation increases cardiac contractility and heart rate, raising cardiac output. β₂ activation relaxes vascular smooth muscle in skeletal muscle beds, producing mild vasodilation. The hemodynamic problem in front of you (low SVR, low contractility, or both) should determine which receptor profile you reach for, and that determines which drug.

Norepinephrine is α₁-predominant with some β₁ activity. It raises systemic vascular resistance, and therefore MAP, with minimal change in cardiac output at routine infusion rates. It is the first-line drug for vasodilatory hypotension, which is by far the most common scenario under inhalant anesthesia. Start at 0.05 to 0.1 µg/kg/min and titrate to the MAP target.

Dopamine has a dose-dependent receptor profile. At clinically useful starting doses (3 to 10 µg/kg/min) the effect is β₁-predominant, producing positive inotropy and chronotropy. Above 10 µg/kg/min, α₁-mediated vasoconstriction emerges and dopamine begins to behave more like a mixed inotrope-vasopressor, raising both cardiac output and systemic vascular resistance. The clinical niche for dopamine is hypotension accompanied by bradycardia, because the β₁ effect addresses both abnormalities at once. The trade-off is that the same chronotropy is dose-dependent and is the source of dopamine’s main complication: ventricular ectopy and ventricular tachycardia. Start at 5 µg/kg/min and titrate.

Dobutamine is β₁-predominant with mild β₂-mediated vasodilation. Its job is to improve cardiac output through increased contractility, not to restore vascular tone. It is the right choice when poor contractility is the dominant problem, which is the case in dilated cardiomyopathy, in any cause of myocardial dysfunction, and sometimes after a long anesthetic in an otherwise stable patient who has slowly developed reduced contractility. Dobutamine is not appropriate for pure vasodilatory hypotension. The β₂ effect adds to the vasodilation already produced by the inhalant, and MAP can paradoxically fall. Start at 2 to 5 µg/kg/min in dogs, 1 to 3 µg/kg/min in cats. Cats are more sensitive to β-mediated tachyarrhythmias, particularly with concurrent hyperthyroidism.

The decision usually comes down to a short checklist. Vasodilation with adequate cardiac output: norepinephrine. Hypotension with bradycardia and good contractility: dopamine. Poor contractility as the dominant abnormality: dobutamine. A mixed picture of low SVR and low cardiac output: norepinephrine plus dobutamine, each titrated separately to its own target.

The math, two ways

Every weight-based CRI is governed by the same relationship between dose, weight, bag concentration, and pump rate. Which variable you treat as the unknown depends on which workflow you are in. Two workflows exist and both are clinically valid. The dominant one in small animal anesthesia practice is described first, followed by a less common variant used when carrier fluid volume matters.

Workflow 1: standard bag, calculate the pump rate

In the great majority of small animal anesthesia cases, the bag is prepared to a standard concentration ahead of time, and the calculation returns the pump rate that delivers the desired dose. The bag is the anchor, the pump rate is the output, and that pump rate floats with patient weight and dose. This is the workflow most references and textbooks describe, and it is the default behavior of the InfusionFox calculator.

$$\text{pump rate}_{\text{mL/hr}} = \frac{\text{dose}_{\mu g/kg/min} \times \text{weight}_{\text{kg}} \times 60_{\text{min/hr}}}{\text{bag concentration}_{\mu g/mL}}$$

Example: A 15 kg, 8-year-old intact female Australian Cattle Dog mix presents for emergency ovariohysterectomy for pyometra. She was appropriately fluid-resuscitated preoperatively and her MAP was acceptable on induction. Twenty minutes into the procedure her MAP dropped to 55 mmHg, depth is appropriate, and a 10 mL/kg crystalloid bolus has not restored it. The hypotension is fluid-refractory and vasodilatory in mechanism. Norepinephrine is the right drug for this patient. The bag is the standard 16 µg/mL norepinephrine preparation. Norepinephrine stock is 1 mg/mL. The starting dose is 0.1 µg/kg/min. What pump rate delivers this dose?

Step 1: prepare the bag

The standard concentration for a norepinephrine CRI bag is 16 µg/mL. The first task is to put the right amount of drug into a 250 mL bag of 0.9% NaCl to reach that concentration, then work out how many milliliters of the 1 mg/mL stock vial that requires.

Multiply the target concentration by the bag volume to get the total drug amount.

$$\frac{16\,\mu g}{\cancel{mL}} \times 250\,\cancel{mL} = 4{,}000\,\mu g = 4\,mg$$

Then divide by the stock concentration to get the volume of stock to draw.

$$\frac{4\,\cancel{mg}}{1} \times \frac{1\,mL}{1\,\cancel{mg}} = 4\,mL$$

The recipe is to remove 4 mL from a 250 mL bag of 0.9% NaCl, draw 4 mL of 1 mg/mL norepinephrine stock, and add it to the bag. The final concentration is 16 µg/mL.

Step 2: convert the dose into a per-hour quantity

The dose is written per kilogram and per minute, but pumps run per hour. The next step is to turn the per-minute, per-kg dose into the total drug amount to be delivered each hour for this specific patient.

Two conversions happen at once. Multiplying by 15 (the patient’s weight in kg) removes the per-kilogram component of the dose. Multiplying by 60 (the minutes in an hour) removes the per-minute component. The 60 is not a pharmacology constant, it is just how many minutes are in an hour.

$$\frac{0.1\,\mu g}{\cancel{kg}\cdot\cancel{min}} \times 15\,\cancel{kg} \times \frac{60\,\cancel{min}}{hr} = \frac{90\,\mu g}{hr}$$

Kilograms cancel against kilograms, minutes cancel against minutes, and the surviving units are micrograms per hour. This dog needs 90 µg of norepinephrine each hour.

Step 3: divide by the bag concentration to get the pump rate

The bag is 16 µg/mL. To deliver 90 µg per hour from a bag at that concentration, divide the per-hour drug requirement by the concentration.

$$\frac{90\,\mu g}{hr} \times \frac{1\,mL}{16\,\mu g} = 5.6\,\frac{mL}{hr}$$

Micrograms cancel, and the surviving units are milliliters per hour. The pump runs at approximately 5.6 mL/hr to deliver 0.1 µg/kg/min. To titrate up later, change the pump rate; the bag stays the same. The titration ladder on the calculator shows what the pump rate would be at every standard dose increment.

Workflow 2: target pump rate, calculate the bag concentration

A second workflow, more common in critical-care and ICU settings, inverts the question. Instead of fixing the bag concentration and solving for the pump rate, you fix the pump rate (typically a low rate like 1 to 3 mL/hr to minimize carrier fluid) and solve for the bag concentration. This is useful when the carrier fluid volume matters in itself, for example in patients with congestive heart failure, dilated cardiomyopathy, or valvular disease who cannot tolerate the higher mL/hr that a standard bag would require, or in very small patients where any added fluid load is significant.

$$\text{bag concentration}_{\mu g/mL} = \frac{\text{dose}_{\mu g/kg/min} \times \text{weight}_{\text{kg}} \times 60_{\text{min/hr}}}{\text{pump rate}_{\text{mL/hr}}}$$

$$\text{total drug amount}_{\mu g} = \text{bag concentration}_{\mu g/mL} \times \text{bag volume}_{\text{mL}}$$

Example: An 8 kg Cavalier King Charles Spaniel with stage C myxomatous mitral valve disease and documented reduced systolic function is stable on pimobendan, furosemide, and a low-dose ACE inhibitor. The patient presents for an urgent laceration repair after a fall, a procedure that cannot be deferred for medical optimization. Ten minutes into the case the MAP dropped to 55 mmHg, the heart rate is mildly elevated, and the pulse quality is weak. This is a low-output picture from poor contractility, not a vasodilatory picture, so the right drug is an inotrope rather than a vasopressor. Dobutamine at 5 µg/kg/min is the starting dose. Depth is appropriate and the fluid bolus step is skipped because volume expansion in this patient pushes toward decompensation.

Because the patient is volume-restricted, the carrier fluid that comes along with the drug needs to be minimal. A target pump rate of 3 mL/hr is chosen so the carrier contribution stays under 5 mL/kg/hr (below a maintenance rate for this patient). The bag is 250 mL of 0.9% NaCl, dobutamine stock is 12.5 mg/mL.

Step 1: convert the dose into a per-hour quantity

Same per-hour conversion as in Workflow 1. The weight is different (8 kg) and the dose is higher (5 µg/kg/min, the standard starting dose for dobutamine) but the structure is the same.

$$\frac{5\,\mu g}{\cancel{kg}\cdot\cancel{min}} \times 8\,\cancel{kg} \times \frac{60\,\cancel{min}}{hr} = \frac{2{,}400\,\mu g}{hr}$$

This dog needs 2,400 µg (2.4 mg) of dobutamine each hour.

Step 2: determine the required bag concentration

The pump is set to deliver 3 mL per hour, and that 3 mL has to contain the 2,400 µg of drug calculated in Step 1. The bag concentration is therefore 2,400 µg distributed in 3 mL.

$$\frac{2{,}400\,\mu g}{\cancel{hr}} \times \frac{1\,\cancel{hr}}{3\,mL} = \frac{800\,\mu g}{mL}$$

Hours cancel, and the surviving units are micrograms per milliliter. The bag needs to be 800 µg/mL.

Quick check: 800 µg/mL multiplied by 3 mL/hr returns 2,400 µg/hr, which is the value from Step 1. The arithmetic is internally consistent.

Step 3: calculate the total drug amount to add to the bag

The bag is 250 mL. To make every mL of it 800 µg/mL, multiply the concentration by the bag volume.

$$\frac{800\,\mu g}{\cancel{mL}} \times 250\,\cancel{mL} = 200{,}000\,\mu g = 200\,mg$$

Milliliters cancel, and the surviving unit is micrograms. Converting to milligrams (1,000 µg per mg) gives 200 mg, which is the more practical unit for syringe selection.

Step 4: calculate the volume of stock to draw

Dobutamine stock is 12.5 mg/mL. To draw 200 mg from a 12.5 mg/mL solution requires 16 mL.

$$\frac{200\,\cancel{mg}}{1} \times \frac{1\,mL}{12.5\,\cancel{mg}} = 16\,mL$$

Milligrams cancel, and the surviving unit is milliliters. The final preparation is 16 mL of 12.5 mg/mL dobutamine stock added to 250 mL of 0.9% NaCl, run at 3 mL/hr, delivering 5 µg/kg/min to an 8 kg dog. To titrate up later, change the pump rate using the calculator’s titration ladder; the bag stays the same. Compared to a standard 250 µg/mL dobutamine bag, which would have run at roughly 9.6 mL/hr for this patient, the target-pump-rate prep saves about 6.6 mL/hr of carrier fluid, which matters cumulatively over the procedure and recovery period in a CHF patient.

The Norepinephrine, Dobutamine, and Dopamine CRI calculators on the Calculators tab each do this math automatically in either workflow, display the formula, and include a worked example using the patient’s actual weight. The same four-step sequence in Workflow 2 applies across all three drugs. Only the starting dose range and the stock concentration change.

Titration

Titration is to the MAP, not to a target dose. Adjust the infusion rate in 25 to 50% increments every 5 to 10 minutes until the MAP reaches 65 mmHg. After each adjustment hold the rate for 10 minutes before changing it again, because the response to the previous change is still developing during that window.

Each drug has a practical ceiling worth respecting.

• Norepinephrine is usually effective at 0.05 to 1.0 µg/kg/min. Doses above 1.0 µg/kg/min in a patient who is still hypotensive should make you stop and look for an unaddressed cause: undertransfused volume status, anaphylaxis, sepsis, mechanical obstruction (pneumothorax, tamponade), or unrecognized surgical hemorrhage.
• Dopamine is usually effective at 3 to 15 µg/kg/min. Above 15 µg/kg/min the α-predominant profile adds arrhythmogenicity without proportional MAP benefit. The right move at that point is to transition to norepinephrine rather than to continue escalating dopamine.
• Dobutamine is usually effective at 2 to 10 µg/kg/min. Cats can develop ventricular arrhythmias above 5 µg/kg/min, particularly with concurrent hyperthyroidism.

Monitoring

Patients on a vasopressor or inotrope infusion need, at minimum:

• Continuous invasive arterial pressure monitoring is the gold standard. If invasive access is not available, high-frequency oscillometric NIBP (cycling every 1 to 2 minutes) is an acceptable substitute. Cycling at intervals longer than 2 minutes does not give you the temporal resolution to titrate safely.
• Continuous ECG throughout the infusion. The most frequent reason to reduce a rate or change an agent is the appearance of ventricular ectopy or ventricular tachycardia, and you will not catch those reliably without continuous ECG.
• Pulse oximetry and capnography to complete the routine intraoperative panel.
• A dedicated intravenous line for the infusion. Vasoactive drugs should not share a catheter with intermittent bolus medications, because each flush effectively delivers a vasopressor bolus.

For patients in whom the infusion is continued postoperatively, add hourly urine output, serum lactate every 4 to 6 hours, and electrolytes (especially potassium and magnesium) at least twice daily. Vasopressor therapy contributes to hypokalemia and hypomagnesemia, and both are independent risk factors for ventricular arrhythmia.

Complications

Three complications are seen often enough to warrant active surveillance.

Arrhythmia. Ventricular ectopy is most often produced by dopamine and dobutamine. Norepinephrine is less arrhythmogenic than either. If runs of ventricular tachycardia appear, reduce the rate by 50% or switch agents.

Reflex bradycardia can occur with norepinephrine at higher rates, particularly in a patient who started off bradycardic. It is rarely a clinical problem on its own, and adding a low-dose dobutamine arm to the regimen typically resolves it.

Extravasation. All α-agonists produce severe local vasoconstriction and tissue necrosis if the IV catheter infiltrates. Vasopressor infusions should run through a well-secured catheter (ideally a central line for sustained therapy), and the catheter site should be checked at least every 30 minutes. If infiltration is identified, stop the infusion, leave the catheter in place to use as a route for the local antidote, and infiltrate the area with phentolamine 5 to 10 mg diluted in 10 mL of 0.9% NaCl. New IV access is then established elsewhere.

Weaning

Vasopressors and inotropes are weaned, not stopped. Once the patient has been hemodynamically stable at the current rate for 15 to 30 minutes (typically postoperatively, after the surgical stimulus has resolved and the anesthetic plane is lighter), halve the rate and recheck the MAP in 10 minutes. If MAP is stable, halve again. Continue this stepwise reduction until you are below the lowest titration step, and then stop.

Abrupt discontinuation in a still-anesthetized or recently-anesthetized patient risks rebound hypotension, because the endogenous catecholamine response has not yet ramped up to take over. The 10-minute halving steps give that handoff time to happen.

If the patient becomes hypotensive at a reduced rate, return to the previous rate, wait another 15 to 30 minutes, and try again. Patients who fail repeated weaning attempts almost always have an unaddressed reason: residual volume deficit, ongoing sepsis, continuing surgical stimulus, or residual anesthetic effect. Recheck the workup before committing to a longer infusion.

Sources

• Plumb DC. Plumb’s Veterinary Drug Handbook. Monographs for norepinephrine, dopamine, and dobutamine, current edition.
• Grimm KA, Lamont LA, Tranquilli WJ, et al., eds. Veterinary Anesthesia and Analgesia: The Fifth Edition of Lumb and Jones. Wiley Blackwell, 2015. Hypotension management chapter.
• ACVAA position statements on anesthetic monitoring.
• 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats, Section 4 (fluid therapy and anesthesia). aaha.org/fluid-therapy
• Mazzaferro EM, Powell LL. Fluid therapy for the emergent small animal patient: crystalloids, colloids, and albumin products. Veterinary Clinics of North America: Small Animal Practice 2013;43:721-734.