Summary

In this chapter we have seen that inducing anesthesia with a fresh gas flow of 6 L/min rather than 2 L/min reduces induction time from 9 minutes to 5 minutes. We have also seen that Gas Man is an excellent tool to explore clinical issues and discover exciting new insights.

Theory

In Chapter 15 we saw that with or without intravenous induction, anesthetic level in VRG (vessel-rich tissue group including brain and cardiovascular system) can be brought to 1 MAC quickly. We did this with FGF = 6 L/min and Vaporizer set to 6%.

If that anesthetic were continued with the same settings, anesthetic depth would deepen rapidly. This would happen because despite VRG being at 1 MAC, CKT and ALV, tensions had been elevated to allow VRG to change rapidly. Our job as anesthetists is to change anesthetic depth and then keep it where we want it.

We shall simulate the phenomenon of overdose and then explore methods to assure it does not happen. To do so, we will learn how to achieve the desired anesthetic level quickly and avoid overdose. This is easily achievable once we understand the theory and can apply it.

Discussion

At the completion of Exercise 15-1, Inspired had reached 4.78% and Alveolar had reached 3.37%. See Fig 16-1.

Figure 16-1: Anesthetic level in VRG rapidly achieved 1 MAC.

If the anesthetic levels in DEL, CKT, and ALV were maintained at these values, VRG would soon rise to reach 3.37%. The patient would then experience the effects of very deep anesthesia including cardiovascular depression.

To avoid this problem, the anesthetist must bring the anesthetic level in the brain back to a more suitable value, somewhere near 1.0 MAC.

Depth of anesthesia with inhaled agents can be dramatically and rapidly reduced by using inspired agent underpressure, sometimes caused a negative bolus. This is accomplished by reducing the inspired concentration or tension of anesthetic to a level low enough to rapidly bring the brain and cardiovascular system back down to 1 MAC or less. To achieve this, we set the vaporizer to a low setting or turn it off.

Copy and rename Exercise 15-1 “Negative Bolus”. Run the simulation until it pauses at 4’ 53”. Do this rapidly by holding down the shift key while clicking Run.

Note that tensions displayed are CKT = 4.78, ALV = 3.27, VRG = 2.05. If you were to click Run at this time, VRG would head toward ALV and ALV would head toward CKT and CKT would head toward DEL.

If you try this now, you will see the overdose. Now, simply rewind and fast run back to 4’53”

Here we introduce a negative bolus of sevoflurane. We set the vaporizer to zero and leave the FGF at 6 L/min. At the end of one minute, at time = 5’ 53”, we will pause the Gas Man simulation either manually or by setting a bookmark at 5’ 53”.

When the simulation stops, we see CKT, ALV, and VRG all displaying levels near 1 MAC. Because CKT, ALV, and VRG have achieved these values, the patient is in a new equilibrium and we can set FGF and DEL to maintain 1 MAC levels. We do so by setting FGF = 2 L/min and DEL = 3%. Had we left the vaporizer off longer, we could have reduced anesthetic depth still further. This is the power of the negative bolus

Figure 15-2: FGF = 2 L/min and DEL = 3% after CKT, ALV, and VRG reach 1 MAC.

We now continue the aesthetic to the end of a 10 minute Gas Man Graph view. Change the graph drop-down to ‘10 Min’ and continue.

Figure 15-3: FGF = 2 L/min and DEL = 3% after 10 minutes.