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Cardiology

Magnesium Sulfate CRI

Refractory ventricular arrhythmias (VPCs, ventricular tachycardia, torsades de pointes) in dogs and cats, particularly when hypomagnesemia is documented or suspected. Generally chosen after standard antiarrhythmic options (lidocaine for VT, procainamide) have not produced adequate control. Also used for severe documented hypomagnesemia replacement (CRI preferred over bolus for severe deficiency). Adjunct for refractory bronchospasm in severe asthma is described but less common in veterinary practice. Continuous ECG and BP monitoring required.

Stock: 50% magnesium sulfate (500 mg/mL, 4.06 mEq/mL), 50 mL multi-dose vial
How this calculator works

Enter the patient's weight and the desired CRI dose. The calculator picks a concentration (100, 50, or 25 mg/mL) that keeps the pump in its accurate range (≥ 2 mL/hr for most syringe pumps).

The recommended concentration shows a suggested tag that updates as you change the patient inputs. Click any tab to override.

Loading dose for ventricular arrhythmia (25–50 mg/kg IV over 5–10 min) is shown above the CRI maintenance rate. Slow administration minimizes the hypotension risk.

Typical range: 5.0–50.0 mg/kg/hr (dogs and cats)
50000 µg/mL
5 mL of 50% stock (2 500 mg total) into 45 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL
Use a different concentration

All preparations are pharmacologically equivalent. The default (50000 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

Magnesium Sulfate 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
100000 µg/mL 10 mL of 50% stock (5 000 mg total) into 40 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL Concentrated preparation for patients ≥10 kg. Most common ICU prep for medium-to-large dogs at the typical 25 mg/kg/hr dose.
50000 µg/mL 5 mL of 50% stock (2 500 mg total) into 45 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL Standard preparation for patients 3–10 kg. The textbook 1:10 dilution of the 50% stock.
25000 µg/mL 2.5 mL of 50% stock (1 250 mg total) into 47.5 mL of 0.9% NaCl or 5% dextrose, final volume 50 mL Dilute preparation for patients <3 kg or any patient at very low doses where the 50 mg/mL preparation would drop pump rate below 2 mL/hr.

Standard preparation: dilute the 50% magnesium sulfate stock (500 mg/mL) into a 50 mL syringe with 0.9% NaCl or 5% dextrose. Both diluents are compatible. For longer-running infusions, the same concentrations can be prepared in 100 mL or 250 mL bags using proportionally more stock. Compatibility caveats: magnesium sulfate is incompatible with many calcium-containing solutions (precipitation risk) and with sodium bicarbonate. Do not co-administer in the same line with these; flush the line before and after if a Y-site is unavoidable. Monitoring: serum magnesium 4–6 hours after starting the CRI where available, then every 12–24 hours. Patellar reflex check as an objective hypermagnesemia surveillance test; loss of patellar reflex is the earliest sign and prompts immediate dose reduction or hold.

Dilution helper — Magnesium Sulfate CRI

Work out how many mL of stock drug and diluent to combine for any target concentration. Stock is pre-filled for Magnesium Sulfate 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.