Conversion Tables

Anesthesia practice requires constant and precise conversion of units to ensure patient safety. Errors in conversion can lead to significant morbidity and mortality. Here is a comprehensive list of conversion tables and formulas routinely used by anesthesia providers.

1. Weights and Measures (The Foundation)

These are the most fundamental conversions, performed for virtually every patient.

To Convert
From
To
Formula / Method
Clinical Context
Weight Pounds (lbs) Kilograms (kg) Divide by 2.2 Essential for all drug dosing. All pediatric and most adult drug doses are calculated in mcg/kg or mg/kg.
Weight Kilograms (kg) Pounds (lbs) Multiply by 2.2 Communicating with patients or in settings where lbs are the standard.
Height Inches (in) Centimeters (cm) Multiply by 2.54 Used for pediatric dosing (e.g., Broselow tape), estimating endotracheal tube size, and equipment selection.
Temperature Fahrenheit (°F) Celsius (°C) Subtract 32, then multiply by 5/9 Monitoring core body temperature for hypothermia or malignant hyperthermia.
Temperature Celsius (°C) Fahrenheit (°F) Multiply by 9/5, then add 32 Communicating with patients or staff unfamiliar with Celsius.

2. Drug Dosing and Concentrations

Critical for preparing and administering medications safely.

To Convert
From
To
Formula / Method
Clinical Context
Drug Mass Milligrams (mg) Micrograms (mcg) Multiply by 1,000 For potent drugs like fentanyl, epinephrine, and phenylephrine where doses are in the mcg range.
Drug Mass Micrograms (mcg) Milligrams (mg) Divide by 1,000 Reversing the above, e.g., when documenting total dose given.
Concentration Percent (%) mg/ml Multiply by 10 Crucial for drawing up drugs. Propofol 1% = 10 mg/ml. Lidocaine 2% = 20 mg/ml. Phenylephrine 1% = 10 mg/ml.
Dose mg/kg Total Dose (mg) Weight (kg) × Dose (mg/kg) Calculating a one-time dose, such as for an antibiotic or induction agent.

3. Infusion Rates (Vasoactive Drugs)

This is one of the most frequent and high-stakes calculations in anesthesia, especially in the ICU and for major cases.

To Convert
From
To
Formula / Method
Clinical Context
Infusion Rate mcg/kg/min ml/hr (Dose × Weight × 60) / (Concentration in mcg/ml) The cornerstone of vasoactive infusion management. See detailed formula below.
Dopamine/Dobutamine mcg/kg/min ml/hr (Weight in kg) / (Concentration in mg/ml) The "Rule of 4s": If you mix 400mg in 250ml (1.6 mg/ml), the ml/hr is roughly the wt in kg. More common is the "Rule of 6s" (600mg in 100ml, 6 mg/ml).
Norepinephrine/Epinephrine/Phenylephrine mcg/min ml/hr (Dose × 60) / (Concentration in mcg/ml) The "Rule of Threes": Mix 3mg in 50ml (60 mcg/ml) or 6mg in 100ml. Then, ml/hr = mcg/min.

Detailed Formula: mcg/kg/min to ml/hr

This is the universal formula for any weight-based infusion.

Example: A 70 kg patient needs a norepinephrine infusion starting at 0.05 mcg/kg/min. You have a standard mix of 4 mg norepinephrine in 250 ml D5W.

  1. Identify Variables:

    • Dose = 0.05 mcg/kg/min
    • Weight = 70 kg
    • Concentration = 4 mg in 250 ml

  2. Unify Units: Convert concentration to mcg/ml.

    • 4 mg = 4,000 mcg
    • Concentration = 4,000 mcg / 250 ml = 16 mcg/ml

  3. Apply the Formula:

    • Infusion Rate (ml/hr) = (Dose × Weight × 60) / (Concentration)
    • Infusion Rate = (0.05 mcg/kg/min × 70 kg × 60 min/hr) / (16 mcg/ml)
    • Infusion Rate = (210) / 16 = 13.125 ml/hr

4. Physiological Pressures

Used for interpreting hemodynamic and respiratory monitoring.

To Convert
From
To
Formula / Method
Clinical Context
Airway/CVP Pressure Centimeters of Water (cmH₂O) Millimeters of Mercury (mmHg) Divide by 1.36 Converting central venous pressure (CVP) or airway pressures to a unit comparable to blood pressure.
Airway/CVP Pressure Millimeters of Mercury (mmHg) Centimeters of Water (cmH₂O) Multiply by 1.36 Standard for airway pressures on ventilators (Peak, Plateau, PEEP).
Blood Gas Pressure Kilopascals (kPa) Millimeters of Mercury (mmHg) Multiply by 7.5 Converting ABG values (PaO₂, PaCO₂) from kPa (used outside the US) to the familiar mmHg.
Blood Gas Pressure Millimeters of Mercury (mmHg) Kilopascals (kPa) Divide by 7.5 Converting ABG values to kPa.

5. Laboratory Values

Essential for managing metabolic and electrolyte disturbances.

To Convert
From
To
Formula / Method
Clinical Context
Glucose mg/dl mmol/L Divide by 18 Managing intraoperative and ICU blood glucose.
Creatinine mg/dl µmol/L Multiply by 88.4 Assessing renal function for drug clearance.
Hemoglobin g/dl g/L Multiply by 10 Assessing oxygen-carrying capacity and transfusion triggers.
Electrolytes (Na⁺, K⁺, Cl⁻) mEq/L mmol/L Often 1:1 For monovalent ions, the values are numerically identical. Important for managing electrolyte replacement.

6. Volatile Anesthetics

Used for calculating and delivering inhalational anesthetics.

To Convert
From
To
Formula / Method
Clinical Context
MAC Age-adjusted MAC % on Vaporizer MAC_age = MAC_40yr × (1 - (Age-40) × 0.01) Calculating the appropriate MAC for an elderly patient.
MAC Fraction (Fi) % Multiply by 100 Converting the gas monitor display (e.g., 0.015) to a percentage (1.5%).

Common MAC Values (for a 40-year-old)

  • Sevoflurane: 2.0%
  • Desflurane: 6.0%
  • Isoflurane: 1.2%

Safety and Best Practices

  1. Double-Check Everything: Always have a colleague verify high-risk drug infusions and calculations, especially for pediatric patients or potent vasoactive drugs.
  2. Use Smart Pumps: Modern infusion pumps have drug libraries that allow you to enter the dose in mcg/kg/min, and the pump calculates the ml/hr. This dramatically reduces errors but requires you to enter the patient's weight and drug concentration correctly.
  3. Label Everything: Clearly label all syringes and infusion bags with the drug name, concentration, and time of preparation.
  4. Know Your "Rules of Thumb": Rules like the "Rule of Threes" for norepinephrine are excellent for quick mental checks to ensure a pump rate is in the correct ballpark. If a rate seems wrong, it probably is. Stop and re-calculate.
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