InfusionFox
Emergency

Methocarbamol CRI

Skeletal muscle relaxant for refractory continuous spasm or rigidity. Primary indications: tetanus (Clostridium tetani toxin-mediated spasms), permethrin / pyrethrin toxicity in cats (topical exposure tremors), strychnine toxicosis, and tremorogenic mycotoxicosis (penitrem A, roquefortine). Intermittent bolus dosing is the more common Plumb's-listed approach; CRI is reserved for refractory cases where continuous muscle relaxation is needed and intermittent bolus produces unacceptable rebound rigidity between doses. Supportive care for sedated / recumbent patients is essential.

Stock: 100 mg/mL (100 000 µg/mL), 20 mL multi-dose vial (2 000 mg)
How this calculator works

Enter the patient's weight and the desired CRI dose. The calculator picks a concentration (50, 20, or 10 mg/mL) that keeps the pump in its accurate range.

The recommended concentration shows a suggested tag that updates as you change the patient inputs.

Loading dose options shown above the CRI cover three indications: tetanus/strychnine 55–220 mg/kg (label), pyrethrin/pyrethroid 40–80 mg/kg (extra-label), and mycotoxin 11–35 mg/kg (extra-label). The 2 mL of stock per minute administration cap (Plumb's) keeps hypotension manageable.

Typical range: 5.0–15.0 mg/kg/hr (dogs and cats)
20000 µg/mL
10 mL of 100 mg/mL stock (1 000 mg total) into 40 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL
Use a different concentration

All preparations are pharmacologically equivalent. The default (20000 µg/mL) fits the most common clinical use case. The alternatives below cover situational needs.

Awaiting input

Enter a patient weight to see the result.

Reference

How the calculation works

Methocarbamol CRI is dosed in mg/kg/hr. Because the prepared CRI concentration is expressed in µg/mL, the formula needs a × 1000 to convert the mg dose into µg before dividing:

$$\text{mL/hr} = \frac{\text{weight}_{\text{kg}} \times \text{dose}_{mg/kg/hr} \times 1000}{\text{concentration}_{\mu g/mL}}$$

Patient weight (kg) times dose (mg per kg per hour) gives mg per hour. Multiplying by 1,000 converts mg to µg so the units match the bag concentration. Dividing by µg/mL yields mL per hour.

In target-pump-rate mode the pump rate is fixed by the clinician and the bag concentration is the unknown. The preparation runs in three steps.

Step 1: bag concentration

Same numerator as standard-bag mode (the total drug delivered per hour for this patient), but divided by the chosen pump rate instead of by a chosen bag concentration:

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

Step 2: total drug in the bag

The bag concentration times the chosen bag volume gives the total micrograms of drug to add. Convert to milligrams (divide by 1,000) for the size of stock you'll actually be drawing:

$$\text{total drug}_{\mu g} = \text{bag concentration}_{\mu g/mL} \times \text{bag volume}_{\text{mL}}$$
$$\text{total drug}_{mg} = \frac{\text{total drug}_{\mu g}}{1{,}000}$$

Step 3: volume of stock to draw

Total drug divided by the stock vial concentration gives the volume of stock to draw and add to the bag:

$$\text{stock volume}_{\text{mL}} = \frac{\text{total drug}_{mg}}{\text{stock}_{mg/mL}}$$

Worked through together: divide the total drug in the bag by the stock vial's mg/mL to get the mL of stock that contains that much drug. That is the volume to pull into the syringe and add to the bag.

Worked example with current inputs

Enter a patient weight to see the worked example.

Reference

Recommended dilutions

Target concentration Dilution When useful
50000 µg/mL 10 mL of 100 mg/mL stock (1 000 mg total) into 10 mL of 0.9% NaCl or 5% dextrose, final volume 20 mL (1:1 dilution; scale to 50 mL syringe with 25 mL stock + 25 mL diluent for longer runs) Standard 1:1 dilution of stock. Recommended for patients ≥10 kg at typical 10 mg/kg/hr doses.
20000 µg/mL 10 mL of 100 mg/mL stock (1 000 mg total) into 40 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL Recommended for patients 3–10 kg. More dilute than the 1:1 prep; pump rate stays in the precision range for medium patients.
10000 µg/mL 5 mL of 100 mg/mL stock (500 mg total) into 45 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL Dilute preparation for cats and very small dogs (<3 kg) or any patient at very low doses.

Stock: methocarbamol injectable is 100 mg/mL in a 20 mL multi-dose vial (2 000 mg per vial). Dilute in 0.9% NaCl or 5% dextrose; both compatible. For fluid-restricted ICU patients, undiluted stock can be run via syringe pump (high viscosity; verify pump tolerates it). Daily-dose tracking is essential: the 330 mg/kg/day Plumb's ceiling is reached at 13.75 mg/kg/hr × 24 hr. CRIs running more than 24 hours at the upper end of the range require explicit cumulative-dose accounting and may need a planned taper or transition to an alternative. Polyethylene-glycol vehicle: the injectable formulation contains PEG-300 as a solubilizer. Renal failure cases accumulate PEG; avoid prolonged CRI use, or substitute oral dosing once the patient tolerates it.

Dilution helper — Methocarbamol CRI

Work out how many mL of stock drug and diluent to combine for any target concentration. Stock is pre-filled for Methocarbamol CRI; change it if you're using a different vial.

Suggestions from the reference table above
Draw up

Enter stock, target concentration, and final volume.