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Biomedical Case Study 1
Physiologic Scenario: "How Long Do I Have?"

N. Ty Smith, M.D.

This scenario demonstrates the value and necessity of anesthetic preoxygenation. The procedure used in this scenario is not particularly realistic, but it demonstrates the principles without having the added confusion from actual OR procedures and details.

This scenario is designed to allow the user to study the impact of denitrogenation on the maintenance of adequate oxygenation during apnea. Although there are three specific subscenarios - no denitrogenation, about one minute of denitrogenation, and about four minutes of denitrogenation - the user may modify this and other scenarios in many ways. For example, one may modify the duration of denitrogenation, the fresh gas flow, the initial pulmonary state of the patient, and the duration of the apnea. To modify the initial state of the patient, one may change lung parameters or shunt. The airway obstruction scenario will certainly make the scenario more interesting, but you must use thiopental to implement this incident, and this will make any comparison with other variations less accurate.

Procedure for All Tests:

Open the Dynamic Gas Display (DGD) and the Dynamic Time Plots. Select the plot variables (SpO2, SvO2, neuromuscular block level, tracheal gas flow, PaO2, PvO2, Partial Pressure O2 Brain, Partial Pressure O2 Myocardium, Volume % O2 in Lung), or just open the pre-selected plotset called "HowLong".

Detailed steps for each section of this test are available in the Body Simulation for Anesthesia users' manual.

Procedure for Case 1: No Preoxygenation:

Start a rapid sequence like induction. Set the anesthesia machine to deliver 4 liters/min of 100% O2, but do not place the mask on the patient. Now give the drugs: Start with a bit of fentanyl, then thiopental and succinylcholine as usual. After the patient has stopped breathing, give a full dose of vecuronium. When the SpO2 drops below 40%, place the mask and ventilate the patient.

The following are static image captures from the Body Simulation software:

Figure 1

Note that on the Dynamic Gas Displays there are several "Dynamic Pie Charts". These pie charts show the magnitude of the gas in the compartment (proportional to the radius of the pie chart) and the percentages of gases that make up the contents of the compartment.

O2 : Green N2 : Yellow CO2 : Gray N2O : Blue

Inhaled agents are shown as their typical color distributed with their bottles (not seen in this example).

Figure 2

During Body Simulation sessions it is possible to easily see the scale of the Y axes of the graphs. Because these are static images, it is difficult to tell. The scales are:

SpO2: 0-1 SvO2: 0-1 Neuromuscular Block Level: 0-2 Tracheal Gas Flow: -1.6 - +1.6 L/min

PaO2: 0-760 mmHg PvO2: 0-760 mmHg

PP O2 Brain: 0-760 mmHg PP O2 Myocardium: 0-760 mmHg Vol % O2 in Lung: 0-1

Comments for Case 1:

Several points are to be made for this case. However, first note the approximate % volume of O2 in the lungs -- about 15% (at the beginning of the Time Plots, Figure 2, or on the DGD, Figure 1). At this time the SpO2 is 97.0% (see the text grid on the right side of the DGD). The moment when the respiration stops is determined from the time plots -- the tracheal gas flow stops oscillating about zero -- this corresponds to when the neuromuscular block level reaches about 0.5. The SpO2 and PaO2 start their crash shortly after respiration ceases. It seems as if it takes about 3 minutes for the SpO2 to hit 40%. Partial pressure of O2 in the brain starts a marked drop once the PaO2 has reached 40 mmHg.

Procedure for Case 2: One Minute Preoxygenation:

Start a rapid sequence like induction. Set the anesthesia machine to deliver 4 liters/min of 100% O2 and place the mask on the patient. Preoxygenate for 1 minute (normal respiration). Now give the drugs: Start with a bit of fentanyl, then thiopental and succinylcholine as usual. After the patient has stopped breathing, give a full dose of vecuronium. When the SpO2 drops below 40%, place the mask and ventilate the patient.

Figure 3

Figure 4

Note the time scale on this plot : 12 minutes

Comments for Case 2:

Now note the approximate % volume of O2 in the lungs when the SpO2 seems to have stopped its increase -- about 50% (at about 1 minute on the Time Plots, Figure 4, or on the DGD, Figure 3). At this time the SpO2 is 99.1% (see the text grid on the right side of the DGD). The SpO2 and PaO2 do not start their crash shortly after respiration ceases -- but start to crash almost 6 minutes after respiration ceases. Once the SpO2 starts to crash, similar characteristics of the other parameters of interest are seen.

Procedure for Case 3: Four Minutes Preoxygenation:

Start a rapid sequence like induction. Set the anesthesia machine to deliver 4 liters/min of 100% O2 and place the mask on the patient. Preoxygenate for 4 minutes (normal respiration). Now give the drugs: Start with a bit of fentanyl, then thiopental and succinylcholine as usual. After the patient has stopped breathing, give a full dose of vecuronium. When the SpO2 drops below 40%, place the mask and ventilate the patient.

Figure 5

Figure 6

Note the time scale on this plot : 20 minutes

Comments for Case 3:

Note the approximate % volume of O2 in the lungs at the end of the preoxygenation period -- about 75%. At this time the SpO2 is 99.3% (see the text grid on the right side of the DGD, Figure 5). In this case the SpO2 and PaO2 start their crash about 8 minutes after respiration ceases. Once the SpO2 starts to crash, similar characteristics of the other parameters of interest are seen.

Conclusion, Lessons, Questions:

Please discuss the following questions with your peers.

Is denitrogenation really useful?
In this healthy patient, is five minutes of denitrogenation significantly better than one minute?
What is the longest you have ever required to complete an intubation?
How often has that happened?
Is it worth the extra time on all patients for this occasional patient?
Does the presence of 100 % O 2 in the circuit make any difference? During the apneic period, change the flow from 100% O 2 to 20 % O 2 and observe the Dynamic Gas System screen or the Dynamic Time Plots.
Why doesn't the SpO2 reach 100% when the patient is breathing 100% O2 as set on the anesthesia machine? See Figure 7 (below).

Figure 7

Figure 7 shows a time plot of SpO2 and O2 percentage in the lungs while the patient is receiving 100% O2 as set on the anesthesia machine. The patient is breathing spontaneously for the duration of this plot (i.e. this is not part of the apnea cases in the first 6 figures). Notice the value of the lung O2 percentage when the SpO2 starts to level off (about 50%).