About the Propofol Calculator

This guide provides supporting information for the Propofol Infusion Calculator. It details the tool's outputs, use cases, and the clinical principles behind propofol administration. The information is intended for educational purposes and should not replace clinical judgment, institutional protocols, or official prescribing information.

Outputs Explained

The calculator provides four distinct modes, each with specific outputs:

• Rate (ml/hr) Mode: Calculates the volumetric infusion pump rate required to deliver a specific weight-based dose (e.g., in mcg/kg/min). The primary output is ml/hr.
• Dose (mcg/kg/min) Mode: Determines the weight-based dose a patient is receiving based on a known infusion rate. This is useful for evaluating current therapy. The primary output is mcg/kg/min.
• Bolus (ml) Mode: Calculates the volume (in ml) needed for a single intravenous bolus dose based on patient weight and a desired dose in mg/kg.
• TCI Model Mode: Estimates the initial bolus and infusion rates needed to achieve a target plasma concentration (Ce) using pharmacokinetic models (Schnider or Marsh). This is an advanced mode for Target-Controlled Infusion systems.

How to Use the Calculator

For Infusion Rate or Dose Calculation:

1. Select the desired mode: Rate to find ml/hr, or Dose to find mcg/kg/min.
2. Enter the patient's weight. You can use kilograms (kg) or pounds (lbs). An Ideal Body Weight (IBW) calculator is available for reference.
3. Enter the known variable: either the desired dose (for Rate mode) or the current infusion rate (for Dose mode).
4. Select the propofol concentration being used (typically 1% or 10 mg/ml).
5. The calculated result will be displayed automatically.

For Target-Controlled Infusion (TCI) Mode:

1. Select the TCI Model tab.
2. Enter the patient's weight, height, age, and biological sex. These are required for pharmacokinetic calculations, especially for the Schnider model.
3. Enter the desired target effect-site concentration (Ce) in mcg/ml.
4. Choose the appropriate pharmacokinetic model (Schnider or Marsh).
5. The tool will output an initial loading dose (bolus) and an initial infusion rate to achieve and maintain the target concentration.

Dosing Overview

Propofol dosing is highly individualized based on the clinical scenario, patient comorbidities, and concurrent medications. The following are general ranges for adults and are not prescriptive.

• General Anesthesia Induction: Typically 1.5–2.5 mg/kg administered as a slow IV bolus. Elderly or debilitated patients may require less.
• General Anesthesia Maintenance: Continuous infusion of 100–200 mcg/kg/min (6–12 mg/kg/hr).
• ICU Sedation: Initiated with a slow bolus (if needed) followed by a maintenance infusion of 5–50 mcg/kg/min (0.3–3 mg/kg/hr). Dosing should be titrated to the minimum effective rate using a validated sedation scale.

Switching and Titration

Propofol has a rapid onset and offset, allowing for precise control. Titration, rather than switching, is the primary method of dose adjustment.

• Titration: Adjust infusion rates in small increments (e.g., 5-10 mcg/kg/min) and allow 3-5 minutes to assess the clinical effect before making further changes.
• Weaning: For prolonged sedation, reduce the infusion rate gradually to avoid abrupt awakening and ensure a smooth transition to an alternative sedative or extubation.

Infusion Interruption

A "missed dose" concept does not apply to continuous infusions like propofol. Any interruption will lead to a rapid decrease in plasma concentration and loss of effect.

• Brief Interruption: If the infusion is stopped for a short period (e.g., a few minutes), it can typically be restarted at the previous rate. The patient should be closely monitored for signs of inadequate sedation or anesthesia.
• Prolonged Interruption: If the infusion is stopped for a longer duration, re-initiation may require a small bolus or a temporary increase in the infusion rate to re-establish the desired level of sedation, followed by titration back to the maintenance rate.

Safety Alerts

• Hemodynamic Instability: Propofol can cause significant hypotension and bradycardia, particularly during induction and in hypovolemic or cardiac-compromised patients. Continuous hemodynamic monitoring is essential.
• Respiratory Depression: Propofol is a potent respiratory depressant and can cause apnea. Airway management equipment and trained personnel must be immediately available.
• Propofol-Related Infusion Syndrome (PRIS): A rare but life-threatening complication associated with high-dose (>4 mg/kg/hr or >67 mcg/kg/min) and prolonged (>48 hours) infusions. Signs include metabolic acidosis, rhabdomyolysis, hyperkalemia, and cardiac failure.
• Pain on Injection: Common. Can be minimized by using a larger vein or pre-administering lidocaine.

Frequently Asked Questions (FAQ)

What is the difference between the Marsh and Schnider TCI models?

The Marsh model is a simpler model based primarily on total body weight. The Schnider model is more complex, incorporating age, height, and sex to calculate Lean Body Mass (LBM) and adjust pharmacokinetic parameters, which can provide more accurate dosing in diverse patient populations.

Why does the Schnider model require age, height, and sex?

These variables are used to estimate the patient's Lean Body Mass (LBM) and adjust for age-related changes in drug distribution and clearance. This aims to provide a more personalized dosing regimen compared to models based solely on total weight.

What is Target Concentration (Ce) in the TCI model?

Ce refers to the "effect-site concentration," which is the theoretical concentration of the drug at its site of action in the central nervous system. The goal of a TCI pump is to achieve and maintain this target Ce to produce a stable level of anesthesia or sedation.

Can I use this calculator for pediatric patients?

No. The pharmacokinetic models (Marsh, Schnider) and standard dosing regimens implemented in this calculator are validated for adults. Pediatric propofol dosing requires different models (e.g., Paedfusor, Kataria) and protocols.

What is the difference between mcg/kg/min and mg/kg/hr?

They are two different units for expressing the same infusion dose rate. To convert from mcg/kg/min to mg/kg/hr, you multiply by 60 (minutes in an hour) and divide by 1000 (mcg in a mg). The calculator provides a toggle for convenience.

How is Ideal Body Weight (IBW) calculated in the tool?

The tool uses the Devine formula: For males, 50 kg + 2.3 kg for each inch over 5 feet. For females, 45.5 kg + 2.3 kg for each inch over 5 feet. This is a common estimation method.

Should I use actual, ideal, or adjusted body weight for obese patients?

This is a complex clinical question. For induction, some evidence suggests dosing based on LBM or adjusted body weight may be more appropriate. For maintenance infusions, dosing based on actual body weight is common but may risk overdose. The TCI models, particularly Schnider, attempt to account for this by using LBM. Always consult institutional guidelines.

Does the calculator account for renal or hepatic impairment?

No. The calculator does not adjust for organ dysfunction. While propofol's primary clearance is not heavily dependent on renal function, severe hepatic impairment may affect its metabolism. Dose adjustments should be made based on clinical assessment and response.

References

• 1. U.S. Food and Drug Administration (FDA). (2017). DIPRIVAN® (propofol) injectable emulsion, USP Prescribing Information. View FDA Label
• 2. National Institute for Health and Care Excellence (NICE). (2010). Sedation in over 16s in critical care. Guideline NG57. View NICE Guideline
• 3. Sahinovic, M. M., Struys, M., & Absalom, A. R. (2018). Clinical Pharmacokinetics and Pharmacodynamics of Propofol. Clinical pharmacokinetics, 57(12), 1539–1558. View on PubMed/NCBI
• 4. Glen, J. B. (1998). The development of 'Diprifusor': a TCI system for propofol. Anaesthesia, 53 Suppl 1, 13–21. View Article