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Biomedical Case Study 3
Desflurane Kinetics Demonstration - A Very Long Case

N. Ty Smith , MD

K. Starko

This is a test case to demonstrate the kinetics of desflurane in muscle and fat. The procedure is simple, and not very realistic, but should not detract from the point of the demonstration. Note that for both the Dynamic Time Plots and the Drug Bars , the graphics change in real time (or accelerated time if selected). This dynamic graphing technique imparts a unique feel for the qualitative properties of the kinetics of the agent distribution.

Procedure:

Open the Anesthesia Machine window and select desflurane as the inhaled agent. Set up the Drug Bars to display desflurane and the Time Plots to plot concentration of deslflurane in the muscle and the fat and mass of deslflurane in the muscle and the fat. Make sure the Y axis scales for the masses are set to 100,000 (ml), and to 0.10 for the concentrations. Optionally plot concentrations and masses of the myocardium, the brain gray matter, and the arteries.

Perform an inhaled agent induction using a high concentration of desflurane. When the patient's respiration is diminished, initiate mechanical ventilation. Proceed with the case maintaining the end tidal desflurane concentration at 1 MAC (6.5%).

Results:

Figure 1 shows the first hour of the desflurane uptake. The Y axis scale for the concentrations is 0.13 (13%), and 650 (ml) for the masses. You can see from the arterial concentration of desflurane that the initial fresh gas flow was higher than 1 MAC, but was turned down to 1 MAC at about 3 or 4 minutes. The arterial concentration after the initial induction is ½ of the scale, or 6.5% (1 MAC).

Notice also that concentrations and masses of desflurane in the myocardium, brain gray matter, and arteries equilibrate within minutes (3 ½ - 7 minutes) of the initial induction. This is not the case for the muscle or the fat compartments. It is plain to see that the masses of desflurane in the muscle and fat continue skyrocketing off the chosen scale, while the smaller compartments are long since saturated.

The muscle and fat will have to take up a large mass of desflurane in order for the concentrations to match the other compartments due to the relative masses being inequitable. Note also that the muscle and fat partition coefficients for desflurane are approximately 2.0 and 27 respectively. The desflurane partition coefficients for myocardium and gray matter are both 1.3. The large desflurane partition coefficient for fat and low fat blood flow will be the largest contributing factors for the very long equilibration time for inhaled desflurane in fat.

Figure 1

(Note the subtle difference between the concentration indicators ( [ ) and the mass indicators ( | ) in the legend. A more clear distinction is available when the plots are being selected by the user. Also, the abbreviations are: Myo = myocardium, BG = brain gray matter, Mus = muscle, Fat = fat, Art = arteries.)

Figure 2 shows the Drug Bars window. Here again we can see the gray matter concentration of desflurane is 1 MAC. All the tissue compartments have equilibrated at 1 MAC, however, the muscle compartment is still not at 1 MAC, and the fat compartment hasn't even registered on the scale!

It is apparent from the time plots and the Drug Bars that the muscle and the fat are gobbling up huge quantities of desflurane compared to the other compartments.

Figure 2

Figure 3 shows 96 hours of desflurane administration (!). It is clear that the muscle compartment has effectively saturated at about 12 hours, however by 96 hours the fat compartment is only just nearing saturation.

Figure 3

(Note the subtle difference between the concentration indicators ( [ ) and the mass indicators ( | ) in the legend. A more clear distinction is available when the plots are being selected by the user. Also, the abbreviations are: Myo = myocardium, BG = brain gray matter, Mus = muscle, Fat = fat, Art = arteries.) The end tidal agent concentration curve was started at about 86 hours - hence it is not visible for the duration of the plot.

Figure 4, the corresponding Drug Bars , show the relative concentrations and masses after 96 hours of simulated time. It is easy to see that the tissue concentrations are almost all equilibrated, and the fat compartment is the last to equilibrate in accordance with time plots.

Figure 4