Residency · Residency · Anesthesiology
Uptake and Distribution of Inhaled Anesthetics
Overview
The speed at which an inhaled anesthetic reaches a brain partial pressure sufficient for surgical anesthesia depends on the interplay between delivery to the alveoli and removal by the pulmonary blood. The ratio of alveolar concentration to inspired concentration (FA/FI) is the key index of uptake kinetics, and understanding partition coefficients is essential for predicting onset, offset, and potency differences among agents.
Partition Coefficients
Blood-Gas Partition Coefficient
The blood-gas partition coefficient defines the solubility of an agent in blood relative to the gas phase. Agents with a low blood-gas coefficient — such as desflurane (0.42), sevoflurane (0.65), and nitrous oxide (0.47) — show a rapid rise in FA/FI and therefore produce fast induction and emergence. Agents with a high blood-gas coefficient, like isoflurane (1.46) and halothane (2.54), are absorbed more readily into the blood, which slows the rise of alveolar partial pressure and delays induction.
| Agent | Blood-Gas Partition Coefficient | Oil-Gas Partition Coefficient | Relative Speed of Induction | |
|---|---|---|---|---|
| Nitrous Oxide | 0.47 | 1.4 | Fast | |
| Desflurane | 0.42 | 18.7 | Fast | |
| Sevoflurane | 0.65 | 47.2 | Moderate-Fast | |
| Isoflurane | 1.46 | 90.8 | Moderate | |
| Halothane | 2.54 | 224 | Slow | The more soluble the agent, the more it is pulled into the bloodstream before the alveolar concentration can climb. |
Oil-Gas Partition Coefficient
The oil-gas partition coefficient correlates with anesthetic potency and has an inverse relationship with MAC. Desflurane has an oil-gas coefficient of 18.7, sevoflurane 47.2, and isoflurane 90.8. Higher oil-gas solubility means a lower MAC and a more potent agent.
Tissue-Blood Partition Coefficients
Tissue-blood partition coefficients determine the rate of equilibration with different tissue groups. The vessel-rich group (brain, heart, kidneys, liver) equilibrates rapidly due to high perfusion. The muscle group, with moderate perfusion, acts as a reservoir. The fat group has very high solubility for volatile agents but very low perfusion, making it significant only during prolonged anesthetics.
FA/FI Curve and Factors Affecting Rise
Factors That Increase the Rate of Rise (Speed Induction)
Several factors accelerate the rise of FA/FI and thus speed induction. Increasing the inspired concentration (FI) through overpressurization — turning the vaporizer above the target — delivers more agent to the alveoli. Increased alveolar ventilation through hyperventilation has a similar effect. Decreased cardiac output means less agent is removed from the alveoli per unit time, so FA rises faster (though tissue delivery is slower). Low blood-gas solubility results in less uptake into the blood, and decreased ventilation-perfusion mismatch allows more efficient gas exchange.
Factors That Decrease the Rate of Rise
Conversely, the rate of rise is slowed by low FI, hypoventilation, high cardiac output (which increases uptake from the alveoli), high blood-gas solubility, and increased V/Q mismatch. Right-to-left shunting is particularly relevant because shunted blood dilutes the arterial concentration of the anesthetic.
The Concentration Effect
At high inspired concentrations — classically described for nitrous oxide at 60-70% — the uptake of large volumes of gas from the alveolus produces two phenomena. The concentrating effect means that the remaining gas is concentrated in a smaller volume. Augmented inspired ventilation means fresh gas is drawn in to replace the absorbed volume. The net result is that FA/FI rises faster at higher FI. This effect is clinically significant primarily for nitrous oxide due to the high concentrations used.
The Second Gas Effect
When a high-volume first gas (nitrous oxide) is taken up rapidly, it accelerates the rise in concentration of a co-administered "second gas" such as sevoflurane. The concentrating effect and augmented ventilation from nitrous oxide uptake increase the alveolar partial pressure of the second gas. The clinical significance of this effect is debated and is modest with modern low-solubility agents.
Tissue Uptake
Tissue Groups
| Group | % Body Weight | % Cardiac Output | Equilibration Time |
|---|---|---|---|
| Vessel-rich (brain, heart, kidneys, liver) | 10% | 75% | 5-15 min |
| Muscle | 50% | 19% | 1-4 hours |
| Fat | 20% | 6% | Days |
| Vessel-poor (bone, ligaments, cartilage) | 20% | ~0% | Negligible uptake |
Implications
Induction reflects equilibration of the vessel-rich group. Prolonged anesthesia loads muscle and fat stores, which delays emergence — a context-sensitive effect for inhalational agents. Fat stores are particularly important for highly lipid-soluble agents, with significance decreasing in the order halothane, isoflurane, sevoflurane, and desflurane.
Emergence and Recovery
Washout Kinetics
Recovery depends on the same factors as induction but operates in reverse. Low blood-gas solubility agents (desflurane, sevoflurane) produce the fastest emergence. Duration of anesthesia matters because longer cases mean more tissue loading and slower washout. Desflurane and sevoflurane show minimal prolongation of emergence even after long cases compared to isoflurane.
Diffusion Hypoxia
Diffusion hypoxia occurs at the end of nitrous oxide administration when large volumes of N2O diffuse back into the alveolus and dilute alveolar oxygen. Prevention is straightforward: administer 100% O2 for 5-10 minutes after discontinuing N2O.
Special Considerations
Closed-Circuit and Low-Flow Anesthesia
At low fresh gas flows, uptake by the patient creates a discrepancy between dialed and delivered concentration, requiring an understanding of agent uptake over time to avoid under- or overdosing. The advantages of low-flow anesthesia include cost savings, reduced environmental pollution, and conservation of heat and humidity.
Pediatric Considerations
Children have a higher alveolar ventilation relative to FRC, which produces a faster rise of FA/FI. Their higher cardiac output partially offsets this effect, but the net result is still faster induction compared to adults.
Effect of Cardiac Shunts
A right-to-left intracardiac shunt slows induction because shunted blood dilutes the arterial concentration. A left-to-right shunt has minimal clinical effect on induction speed and may slightly speed the FA/FI rise by delivering more agent-laden blood to the lungs.
<image>A graph showing FA/FI curves over time (x-axis: minutes, y-axis: FA/FI ratio from 0 to 1.0) for nitrous oxide, desflurane, sevoflurane, isoflurane, and halothane. Each curve labeled with the agent name and its blood-gas partition coefficient. Nitrous oxide and desflurane rise fastest toward 1.0, while halothane rises slowest. Annotations show that lower blood-gas solubility correlates with faster rise.</image>
<image>A schematic diagram of the alveolus showing the three phases of inhaled anesthetic transfer: (1) delivery from the breathing circuit to the alveolus via ventilation, (2) uptake from the alveolus into pulmonary capillary blood determined by blood-gas solubility, cardiac output, and the alveolar-venous partial pressure gradient, and (3) distribution via arterial blood to tissue groups (vessel-rich, muscle, fat, vessel-poor) with arrows of different sizes indicating relative blood flow to each group.</image>
<image>A comparative bar chart showing blood-gas and oil-gas partition coefficients for desflurane, sevoflurane, isoflurane, and halothane. Two sets of bars per agent, one for blood-gas (small values) and one for oil-gas (larger values), illustrating the inverse relationship between oil-gas coefficient and MAC value.</image>
Clinical Pearls
The blood-gas partition coefficient is the single most important factor determining the speed of induction and recovery with inhaled agents. Overpressurization — dialing up the vaporizer above the desired maintenance concentration — is the clinical application of the concentration effect for faster induction. A patient with low cardiac output (from cardiomyopathy, hemorrhagic shock, or similar conditions) will have a faster rise in FA/FI but is also at higher risk of anesthetic overdose, demanding careful titration. Context-sensitive recovery applies to volatile agents just as it does to intravenous drugs: after prolonged cases, agents with higher solubility show greater delays in emergence. Right-to-left cardiac shunts slow inhalational induction, so IV induction should be considered in patients with significant shunts. Finally, supplemental O2 should always be administered after discontinuing N2O to prevent diffusion hypoxia.
References
- Eger EI II. Anesthetic Uptake and Action. Baltimore: Williams & Wilkins, 1974. (Foundational text)
- Eger EI II, Saidman LJ. Illustrations of inhaled anesthetic uptake, including intertissue diffusion to and from fat. Anesth Analg. 2005;100(4):1020-1033.
- Hendrickx JFA, et al. The pharmacokinetics of inhaled anesthetics and carrier gases. Best Pract Res Clin Anaesthesiol. 2022.
- Miller RD, et al. Miller's Anesthesia, 9th edition. Chapters on Inhaled Anesthetic Pharmacokinetics.
