What Is Target-Controlled Infusion (TCI)?

Target-controlled infusion, commonly called TCI, is a method of intravenous drug delivery that uses pharmacokinetic models to automatically calculate the infusion rate needed to reach and maintain a specific drug concentration in the patient's body.

Instead of manually setting a rate in mL/h and guessing when the drug reaches therapeutic levels, the anesthetist simply sets a target concentration. The TCI pump does the math in real time, adjusting the infusion rate second by second.

The Three-Compartment Model

TCI systems are built on compartmental pharmacokinetics. The most common approach is the three-compartment mammillary model, which divides the body into:

• Central compartment (V1) - represents the blood and highly perfused organs. The drug enters here first during injection.
• Rapid peripheral compartment (V2) - represents well-perfused tissues like muscle. Drug moves between V1 and V2 relatively quickly.
• Slow peripheral compartment (V3) - represents poorly perfused tissues like fat. Drug moves slowly into and out of this compartment.

Each compartment has specific rate constants (k values) that describe how fast the drug moves between them and how fast it is eliminated. These constants come from population studies where researchers measured blood drug levels in groups of patients over time.

Plasma Targeting vs Effect-Site Targeting

TCI pumps offer two targeting modes:

Plasma targeting (Cp)

The pump aims for a specific drug concentration in the blood (central compartment). This is straightforward and predictable, but the brain concentration lags behind plasma, which means the clinical effect takes time to catch up with what the display shows.

Effect-site targeting (Ce)

The pump aims for a specific drug concentration at the effect site, which is the brain for anesthetic agents. To achieve the target Ce faster, the pump initially overshoots the plasma concentration, which drives the drug across the blood-brain barrier more quickly. This results in faster onset of clinical effect but requires a brief period of higher plasma levels.

Effect-site targeting uses an additional rate constant called ke0, which describes the equilibration between plasma and the effect site. Different models use different ke0 values, which is why the same Ce target can produce different clinical results depending on which model you choose.

How TCI Pumps Calculate the Rate

The TCI algorithm works by solving differential equations that describe drug movement between compartments. At each calculation cycle (typically every few seconds), the pump:

1. Measures the current state of all three compartments (based on accumulated calculations)
2. Determines what infusion rate is needed during the next time step to move the concentration toward the target
3. Applies that rate, then recalculates for the next step

This is why TCI is sometimes called "model-driven" infusion. The pump continuously runs a pharmacokinetic simulation of the patient and adjusts the hardware output accordingly.

Why TCI Matters

TCI offers several advantages over manual infusion rate control:

• Predictable drug levels - the anesthetist can think in terms of concentration (mcg/mL) rather than infusion rates (mL/h)
• Smoother anesthesia - continuous adjustment prevents the peaks and troughs common with bolus-based techniques
• Faster titration - effect-site targeting allows quicker response to changing clinical needs
• Wake-up prediction - the model can estimate when the patient will reach consciousness based on current drug levels

Common TCI Models

The most widely used PK models for TCI include:

• Marsh (1991) - propofol model, weight-proportional. Used in the original Diprifusor system.
• Schnider (1998/1999) - propofol model, incorporates age, height, weight, and sex.
• Minto (1997) - remifentanil model, similar to Schnider in using patient demographics.
• Eleveld (2018) - newer propofol model designed to work across a wider patient population including children and the elderly.

Limitations to Keep in Mind

TCI models are population-based estimates. They predict what happens in an "average" patient from the study population. Individual patients may differ significantly due to factors like organ function, genetic variation, concurrent medications, and disease state.

That is why TCI is a tool to assist clinical judgment, not replace it. The anesthetist must always monitor the patient's clinical response and adjust targets accordingly.