Calculating mcg/kg/min for Continuous Infusions

Calculating mcg/kg/min is used for continuous intravenous infusions where the dose is based on the patient’s weight and delivered per minute. This is common for medications such as vasoactive drugs, where precise titration is required. The calculation determines the infusion rate that must be set on a pump, usually in mL/hr.

What You Need to Know

mcg/kg/min calculations involve combining weight-based dosing with time. The prescribed rate is given per kilogram per minute, so the calculation must account for both the patient’s weight and the time component before converting to mL/hr.

Key points:

• the prescribed dose is in micrograms per kilogram per minute

• patient weight must be in kilograms

• the final answer is usually required in mL/hr

• the calculation involves multiple steps

A quick check:

• higher weight = higher dose

• higher prescribed rate = higher infusion rate

• final answer must be in mL/hr for pump settings

The Formula

Required dose (mcg/min) = Prescribed rate × Weight (kg)
mL/hr = (Required dose × 60 ÷ Stock strength) × Volume

Where:
Required dose = dose per minute based on weight
60 = converts minutes to hours
Stock strength = amount of drug available
Volume = mL the stock is contained in

Worked Examples

Example 1
Order: 5 mcg/kg/min
Weight: 70 kg
Stock: 200 mg in 50 mL

Step 1: Calculate required dose
5 × 70 = 350 mcg/min

Step 2: Convert to hourly dose
350 × 60 = 21000 mcg/hr

Step 3: Convert stock to same unit
200 mg = 200000 mcg

Step 4: Calculate mL/hr
(21000 ÷ 200000) × 50
= 0.105 × 50
= 5.25

Answer: 5.25 mL/hr

Example 2
Order: 10 mcg/kg/min
Weight: 60 kg
Stock: 100 mg in 50 mL

10 × 60 = 600 mcg/min
600 × 60 = 36000 mcg/hr
100 mg = 100000 mcg

(36000 ÷ 100000) × 50
= 0.36 × 50
= 18

Answer: 18 mL/hr

Practice Questions

1. Order: 4 mcg/kg/min
   Weight: 80 kg
   Stock: 200 mg in 50 mL

2. Order: 8 mcg/kg/min
   Weight: 75 kg
   Stock: 150 mg in 50 mL

3. Order: 6 mcg/kg/min
   Weight: 65 kg
   Stock: 120 mg in 60 mL

4. Order: 12 mcg/kg/min
   Weight: 50 kg
   Stock: 100 mg in 50 mL

5. Order: 5 mcg/kg/min
   Weight: 90 kg
   Stock: 200 mg in 100 mL

Answers

1. 4 × 80 = 320 mcg/min
   320 × 60 = 19200 mcg/hr
   200 mg = 200000 mcg
   (19200 ÷ 200000) × 50 = 4.8 mL/hr

2. 8 × 75 = 600 mcg/min
   600 × 60 = 36000 mcg/hr
   150 mg = 150000 mcg
   (36000 ÷ 150000) × 50 = 12 mL/hr

3. 6 × 65 = 390 mcg/min
   390 × 60 = 23400 mcg/hr
   120 mg = 120000 mcg
   (23400 ÷ 120000) × 60 = 11.7 mL/hr

4. 12 × 50 = 600 mcg/min
   600 × 60 = 36000 mcg/hr
   100 mg = 100000 mcg
   (36000 ÷ 100000) × 50 = 18 mL/hr

5. 5 × 90 = 450 mcg/min
   450 × 60 = 27000 mcg/hr
   200 mg = 200000 mcg
   (27000 ÷ 200000) × 100 = 13.5 mL/hr