Electrolytes & Fluids

Hypomagnesemia / MgSO4 CRI

IV magnesium sulfate CRI for hypomagnesemia in dogs and cats. Enter serum Mg, body weight, and MgSO4 stock concentration; returns a CRI rate from a 4-tier sliding scale. Also worth considering in workup of refractory hypocalcemia or hypokalemia where occult hypomagnesemia is the missing link.

MgSO4 50%: 4 mEq/mL

Dilute before infusion. Never give MgSO4 by direct IV push. Dilute into D5W or 0.9% NaCl. Rapid administration can cause hypotension, flushing, and cardiovascular collapse.
Watch for hypermagnesemia. Hyporeflexia, weakness, lethargy, hypotension, and prolonged PR interval are early signs. Severe hypermagnesemia causes respiratory depression and cardiac arrest. Reduce or discontinue if these develop. Mg is renally excreted, reduce rate substantially in renal insufficiency.
Refractory K and Ca derangements. Mg deficiency impairs PTH release (limiting Ca correction) and is a cofactor for Na/K-ATPase (limiting K correction). A patient whose Ca or K won't correct despite apparently adequate supplementation should have Mg measured.

Using this in DKA?

Hypomagnesemia commonly worsens after DKA therapy starts (insulin shifts Mg intracellularly alongside K and P). Hoehne notes Mg decline in both dogs and cats during DKA management. Check Mg 1–2 times daily during DKA. See the DKA management hub for the full workflow.

Species

Dog Cat

Patient weight

lb kg

Serum total magnesium (mg/dL)

Reference range typically 1.5–2.5 mg/dL (dogs) or 1.7–2.5 mg/dL (cats); verify against your lab's range. Bands: ≥1.5 not indicated · 1.2–1.5 mild · 0.9–1.2 moderate · <0.9 severe.

MgSO4 stock concentration

50% (4 mEq/mL) 25% (2 mEq/mL)

Most veterinary hospitals stock 50% MgSO4 (4 mEq/mL). Some use the 25% formulation. Read the vial label before drawing.

Awaiting input

Enter a patient weight to see the result.

Reference

How the calculation works

The sliding scale gives a target rate in mEq/kg/day. Total daily mEq × weight, then divide by stock concentration to get volume, then divide by 24 for mL/hr:

$$\text{mL/hr} = \frac{\text{rate}_{\text{mEq/kg/day}} \times \text{weight}_{\text{kg}}}{\text{stock}_{\text{mEq/mL}} \times 24}$$

Worked example with current inputs

Enter a patient weight to see the worked example.

Reference

Hypomagnesemia sliding scale

Serum Mg (mg/dL)	Severity	MgSO4 rate
Mg > 1.5 mg/dL	normomagnesemia	not indicated
Mg 1.2–1.5 mg/dL	mild	0.25 mEq/kg/day
Mg 0.9–1.2 mg/dL	moderate	0.5 mEq/kg/day
Mg < 0.9 mg/dL	severe	1.0 mEq/kg/day

Sources

Sliding scale and dosing range: Hoehne SN. Diabetic Ketoacidosis. In: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. 3rd ed. Elsevier; 2023. Chapter 73, Box 73.1 (MgSO4 rate range 0.25–1 mEq/kg/day IV).
Magnesium disorders overview: DiBartola SP. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 4th ed. Elsevier; 2012. Chapter 8 (Disorders of Magnesium).