Select FALSE statement about time constants for volatile anesthetics after three time constants.

Select FALSE statement about time constants for volatile anesthetics after three time constants.

• A. 6 to 12 minutes have elapsed for modern anesthetics
• B. The A-VD for the brain is very small
• C. The expired volatile concentration will rise much less slowly than the preceding 12 minutes
• D. The venous blood will contain 95% of volatile content of arterial blood

Answer: (D)

Time constants describe how fast different compartments equilibrate with the alveolar concentration of a volatile anesthetic. After three time constants, about 95% of the final steady-state concentration is reached in that compartment. For modern agents with low blood–gas solubility, this overall equilibration tends to take roughly 6 to 12 minutes. The brain is highly perfused, so the arterial–venous difference in brain concentration is small; brain tissue reaches near-equilibrium quickly, and venous blood leaving the brain reflects that rapid equilibration. As uptake into tissues continues and tissues with higher reservoirs (like fat) gradually saturate, the expired (alveolar) concentration rises toward the new steady state, but the rate of rise slows as the system nears that state. The statement about venous blood containing 95% of the volatile content of arterial blood is not true for the whole body after three time constants. Because substantial uptake occurs into various tissues, especially lipid-rich ones, the venous return does not track arterial concentration at 95%; it remains lower due to ongoing tissue uptake and storage.

Time constants describe how fast different compartments equilibrate with the alveolar concentration of a volatile anesthetic. After three time constants, about 95% of the final steady-state concentration is reached in that compartment. For modern agents with low blood–gas solubility, this overall equilibration tends to take roughly 6 to 12 minutes.

The brain is highly perfused, so the arterial–venous difference in brain concentration is small; brain tissue reaches near-equilibrium quickly, and venous blood leaving the brain reflects that rapid equilibration. As uptake into tissues continues and tissues with higher reservoirs (like fat) gradually saturate, the expired (alveolar) concentration rises toward the new steady state, but the rate of rise slows as the system nears that state.

The statement about venous blood containing 95% of the volatile content of arterial blood is not true for the whole body after three time constants. Because substantial uptake occurs into various tissues, especially lipid-rich ones, the venous return does not track arterial concentration at 95%; it remains lower due to ongoing tissue uptake and storage.