Chapt 19 - Closed Circuit Anesthesia

Define closed-circuit anesthesia.

An anesthetic technique that maintains a constant anesthetic state by adding gases and vapors to the breathing circuit at the same rate the patient’s body removes them.

What happens to exhaled gases in closed-circuit anesthesia?

CO₂ is removed and the remaining warmed, humidified exhaled gases and vapors are mixed with fresh gas to form the next inspired breath.

How are “tension” and “partial pressure” used in this chapter?

They are used interchangeably and represent the effective pressure exerted by a gas in gas, blood, or tissue.

Why is partial pressure often expressed as a percent of 1 atmosphere?

This makes the numeric value of partial pressure equal to concentration in vol%, and works at any barometric pressure because physiologic effect depends on partial pressure.

What does a tissue anesthetic value of “1%” represent?

A partial pressure of 1% of 1 atm, equivalent to 7.6 mm Hg, not a 1% volume concentration in the tissue.

What does Henry’s law state in the context of inhaled anesthetics?

The concentration of anesthetic in liquid or tissue equals its partial pressure × tissue/gas solubility × atmospheric pressure.

What key idea from Dalton’s law applies to inhaled anesthetics?

Each gas in a mixture behaves as if it were alone; its partial pressure determines its behavior independently.

What is assumed about venous blood leaving each tissue compartment?

It is in equilibrium with that tissue, so venous partial pressure equals tissue partial pressure.

Under ideal conditions, how do end-tidal and alveolar partial pressures relate?

End‑tidal partial pressure equals alveolar partial pressure when there is no alveolar dead space.

What happens to anesthetic partial pressures after a very long time at constant inspired concentration?

Partial pressure becomes equal in inspired gas, alveoli, arterial blood, all tissues, and venous blood; net uptake ceases and inspired and exhaled tensions are equal.

Give the general form of the uptake equation for a single tissue perfused with constant vapor tension.

Uptake = K × e
−
t
/
τ
−t/τ
, where K is a constant and τ (tau) is the time constant.

How are time constant (τ) and half-time (t½) related?

τ = 0.69 × t½ and t½ = 1.44 × τ.

What is total body uptake mathematically?

The sum of multiple tissue exponentials, each with its own K and τ, which together approximate a power function K × t
−
0.5
−0.5
.

What is the “square-root-of-time” relationship in closed-circuit anesthesia?

An empiric rule where anesthetic administration rate is inversely proportional to the square root of time (t
−
0.5
−0.5
).

Who first described the t
−
0.5
−0.5
pattern of nitrous oxide uptake and when?

Severinghaus, in 1954, showed nitrous oxide uptake follows a t
−
0.5
−0.5
power function.

What is the classic Lowe technique for closed-circuit anesthesia?

A method using an initial loading dose to set circuit tension, then unit liquid anesthetic doses at times based on the square‑root‑of‑time sequence (0, 1, 4, 9, 16, 25 minutes).

How is the “unit dose” in the Lowe technique determined?

It is proportional to body mass raised to the three‑quarter power (kg
3
/
4
3/4
), following Kleiber’s law.

What does Kleiber’s law state in this context?

Many physiologic processes—oxygen consumption, CO₂ production, cardiac output, fluid requirement—scale with body mass to the three‑quarter power (kg
3
/
4
3/4
).

What are the classic Lowe closed-circuit injection times?

0, 1, 4, 9, 16, and 25 minutes (times whose square roots are consecutive integers).

Why did Lowe advise modifying the square-root-of-time schedule after about 25 minutes?

After 25 minutes, uptake becomes nearly constant as vessel‑rich tissues equilibrate and muscle/fat uptake declines more slowly than t
−
0.5
−0.5
.

What is the basic principle of closed-circuit anesthesia regarding FGF after induction?

Start with high FGF to establish the desired state, then reduce FGF and add O₂, N₂O, and vapor at rates matching uptake so circuit and patient tensions remain constant.

What approximate oxygen consumption is used in classic closed-circuit calculations for a 70‑kg adult?

About 243 mL/min.

What approximate nitrous oxide uptake rate is used after the first 30 minutes?

About 100 mL/min.

In closed/low-flow anesthesia, what are the two sources of inhaled gas?

CO₂‑free exhaled gas and fresh gas added in the correct composition and quantity.

From the patient’s perspective, how do open, semiclosed, low-flow, and closed circuits compare if alveolar tension is the same?

They are equivalent; the patient “feels” the same anesthetic depth because inspired and alveolar tensions are the same.

What is minimum alveolar concentration (MAC)?

The alveolar anesthetic concentration at 1 atm that prevents movement in 50% of patients to a surgical stimulus.

What was Eger’s key contribution related to constant alveolar concentration?

He showed that constant alveolar tension can be produced and maintained if inspired concentration is adjusted appropriately over time based on uptake into tissue groups.

In Eger’s classic example, what inspired halothane concentrations maintain 0.8% alveolar halothane?

Start at 3.3%, then about 2% at 5 minutes, 1.5% at 20 minutes, and 1% at 3 hours.

How does a semiclosed circle system differ from an open system in controlling inspired concentration?

In a semiclosed system, inspired tension depends on FGF, vaporizer setting, exhaled flow, and exhaled agent tension, whereas in an open system inspired equals delivered concentration.

What happens to required vaporizer settings as FGF is reduced in a semiclosed circuit?

Vaporizer settings must increase because a greater fraction of lower‑tension exhaled gas is rebreathed, diluting fresh gas.

Why can modern vaporizers not provide the theoretical halothane setting needed at 1 L/min FGF for constant 0.8% alveolar?

The calculated initial setting exceeds 10%, higher than modern vaporizer limits and near the agent’s vapor pressure.

In a fully closed circuit at FGF 0.25 L/min, what theoretical halothane setting is needed to match an open 3.3% inspired, 0.8% alveolar condition?

Approximately 40% halothane, which is impossible due to vaporizer and vapor pressure limits.

What is liquid injection anesthesia?

A closed‑circuit technique where liquid volatile anesthetic is injected directly into the breathing circuit in boluses or by infusion instead of using a vaporizer.

What is the main advantage of liquid injection anesthesia?

It overcomes the maximum concentration limitation of vaporizers, allowing very high effective circuit concentrations at low FGF.

Why are inspired and expired tensions less smooth with liquid bolus injection compared with vaporizer delivery?

Bolus dosing produces peaks and troughs in circuit concentration, though the brain’s time constant smooths depth.

What is Gas Man?

An educational computer simulation that models inhaled anesthetic pharmacokinetics using compartments (circuit, alveoli, vessel‑rich group, muscle, fat) and their flows.

Which body compartments are represented in Gas Man?

Breathing circuit (CKT), alveoli (ALV), vessel‑rich group (VRG), muscle (MUS), and fat (FAT).

How are compartment anesthetic tensions illustrated in Gas Man?

Each compartment is shown as a container filled to a “height” representing its partial pressure; at equilibrium all heights are equal.

What percentage of cardiac output goes to VRG, muscle, and fat in the model?

About 75% to VRG, 20% to muscle, and 5% to fat.

What determines mixed venous anesthetic tension returning to the lungs?

It is dominated by blood from the VRG because that group receives most cardiac output.

In a simulation with high FGF and 5% isoflurane, what proportion of delivered agent may be wasted?

Example: 0.9 L uptake vs 6.0 L delivered, so about 5.1 L (85%) is discarded or remains in the circuit (15% efficiency).

How is efficiency defined in Gas Man simulations?

Efficiency = patient uptake volume ÷ delivered volume, expressed as a percentage.

What is alveolar overpressure?

A transient increase in alveolar anesthetic tension above the eventual target level to speed the rise in brain and VRG tension.

What is the approximate delay between alveolar/end‑tidal tension and brain anesthetic tension?

About 3–5 minutes under ideal conditions.

How can the ET value at a single moment be interpreted with respect to brain tension?

Current ET predicts brain tension in a few minutes; ET from a few minutes earlier reflects current brain tension.

What is a clinically realistic key target for inhaled anesthetic depth?

Constant anesthetic tension in the VRG (brain and spinal cord), often at 1 or 1.3 MAC.

What are common titration endpoints in closed-circuit anesthesia?

Maintenance of inspired tension, expired (end‑tidal) tension, and/or estimated anesthetic depth from clinical and monitoring signs.

What qualitative signs are integrated to assess anesthetic depth?

Pupil size, blood pressure, heart rate, and sometimes processed EEG indices, plus ET agent concentration.

What limits true closed-circuit anesthesia in some setups?

Technical constraints such as mass spectrometer monitors that cannot return sampled gas and vaporizer output limits.

Why did older multiplexed mass spectrometers prevent true closed-circuit practice?

Sampled gas from each OR could not be returned to the same patient’s circuit, so the system was not fully closed.

How have modern anesthesia machines improved feasibility of closed or near-closed circuits?

Stand‑alone or integrated gas monitors can return sampled gas to the breathing circuit, reducing waste and maintaining closed conditions.

What is meant by “continuum from high-flow to low-flow anesthesia”?

A spectrum where FGF is progressively reduced from high (open-like) to low or closed, changing efficiency and rebreathing but potentially keeping the same alveolar tension.

In low-flow anesthesia, why must vaporizer settings be higher than in open circuits for the same inspired tension?

Because a larger proportion of lower‑tension exhaled gas is rebreathed and dilutes the fresh gas.

What safety concern exists with very low FGF in closed-circuit anesthesia?

Risk of hypoxia or unintended gas composition if O₂ delivery does not meet metabolic needs or monitoring is inadequate.

What is an “almost–closed‑circuit” anesthesia machine?

A commercial system designed to operate at very low flows, approaching closed-circuit behavior, available in some countries.

What is the role of respiratory gas sampling in closed-circuit anesthesia?

To measure inspired and expired gas tensions and, when gas is returned to the circuit, to allow accurate monitoring without breaking the closed system.

What is the concern about buildup of toxic substances in closed circuits?

Prolonged closed use could theoretically allow accumulation of impurities or metabolites, so machine design and practice must minimize this risk.

What is the main economic and environmental advantage of closed/low-flow anesthesia?

Dramatic reduction in anesthetic agent consumption and waste gas pollution.

Why is there no single reliable quantitative measure of anesthesia depth despite multiple monitors?

Measures like BP, HR, and processed EEG provide useful information but do not perfectly correlate with anesthetic “depth” in all patients and situations.