HOME

What's New

Anesthesiology
Links

Educational Info
Airway
Cardiothoracic
Inhalational Agents
IV Anesthesia
Malig Hyper
Neuroanesthesia
Obstetric
Pain
Pediatric Anes
SICU

Faculty

MH Mission

Non-physician
Providers

Patient Information

Research

Residency Program

Residents

Inhalational Anesthetic Agents
Pharmacokinetics

The depth of general anesthesia depends on the partial pressure (or gas fraction) exerted by the inhalational agent in the patient's brain (b). This brain partial pressure depends on arterial (a) blood partial pressure which depends on alveolar (A) partial pressure which depends on partial pressure of agent in the inspired gas (I):

pI -> pA -> pa -> pb, or

FI -> FA -> Fa -> Fb

Recall that:
partial pressure of a component gas = (fraction of total gas that is component gas) x (total pressure), or

pc = Fc x P

To change Fb in our patient's brain, we monitor and control FI and, indirectly, FA and Fa.
Delivering Inhalational Agents

Controlling Inspired Fraction of Agent, FI

FI depends on total fresh gas flow (FGF in L/min), total volume of the breathing system or circuit, and any absorption of agent by the anesthesia machine and breathing system.

Factors Affecting Alveolar Fraction of Agent, FA

FA depends on three variables: uptake of agent from the lung, pulmonary ventilation and inspired concentration of agent.
  1. Uptake of agent from the lung alveoli into the alveolar capillary blood depends on
    • solubility of agent in blood,
    • alveolar blood flow, and
    • alveolar-venous partial pressure difference.
    For relatively the insoluble agents
    • N2O and desflurane,
    • slow uptake leads to
    • fast rise in FA.
    Degree of solubility is expressed in the blood:gas solubility ratio, lambdab/g:
    Agent Blood/Gas Brain/Blood Muscle/Blood Fat/Blood
    Nitrous oxide 0.47 1.1 1.2 2.3
    Halothane 2.4 2.9 3.5 60
    Enflurane 1.9 1.5 1.7 36
    Isoflurane 1.4 2.6 4.0 45
    Desflurane 0.42 1.3 2.0 2.7
    Sevoflurane 0.65 1.7 3.1 48