Summary

In this chapter, the response of two sequential compartments to a step input to the first compartment has been investigated. The measure of the delay of each compartment is the mean transit time, MTT. For each simple, fully-mixed compartment, MTT = . The effects of the two compartments are additive in delaying the response to a step input; the mean transit time is the sum of the individual mean transit times. With some difficulty, delivered tension can be adjusted to optimize the response in the alveolar compartment. This is done by the use of overpressure.

Theory

In this chapter you will study the response of the alveolar compartment to a step change in inspired anesthetic tension. The resulting curve is termed the alveolar tension curve. Analogous to the one compartment example, this could be termed the wash-in of the patient including lungs and tissues, or simply, patient wash-in. But it is far from a simple exponential.

To demonstrate and dissect the alveolar tension curve into its component parts, use the Gas Man Program to simulate anesthetic administration at constant inspired concentration (tension) through an open circuit. Open circuit is the term used to signify that the clinician has perfect control over inspired tension without the patient rebreathing exhaled gases and diluting inspired tension. The terms open and non-rebreathing are used interchangeably.

The alveolar tension curve shape has several important components, each of which must be understood. In this chapter, effects of alveolar ventilation, cardiac output, blood/gas solubility, tissue uptake, and venous return of anesthetic will be demonstrated and explained.

The alveolar tension curve is extremely important; it is the key to understanding inhalation anesthesia uptake and distribution and the process of anesthetic tension equilibration from the vaporizer to the patient's brain and anesthesia depth.

Exercise 5-1: The Open Circuit Alveolar Tension Curve

The open circuit alveolar tension curve is the alveolar response to a step change in inspired tension, representing patient wash-in.

SETUP

Parameter Selection Agent Isoflurane DEL (%) 5 Circuit Open To observe the response of the alveolar FGF (L/min) 10 compartment to a step change in inspired VA (L/min) 4 tension, adjust the parameters as shown. CO (L/min) 5 View (min) 15 Speed 60x Special None

Observation

Figure 5-1 shows the Picture and Graph of the results of Exercise 5-1 after 15 minutes, displaying anesthetic tension in response to 5 % delivered and inspired isoflurane.

While watching the Gas Man Picture, note the initial rapid rise in ALV (alveolar anesthetic tension). Note also the more sluggish rise in VRG (vessel-rich group) and barely discernible rise in MUS (muscle) and FAT (fat). After about 2 minutes of simulated time, observe the Gas Man Graph. Observe especially the Gas Man Graph and focus on the alveolar curve.