Inhalational Anesthetics



Inhalational Anesthetics

Uptake and Distribution

December 2, 1996

Steven L. Shafer, M.D.

Questions:

1. What is the effect of each of the following drugs on MAC:

ephedrine

methoxamine

amphetamine

fentanyl

MAO inhibitors

tricyclic antidepressants

pancuronium

Should mental retardation reduce MAC?

How would you test the influence of curare on MAC?

The MAC of N2O is higher than predicted by its oil/gas partition coefficient Why?

Why MAC of CO2 is less than predicted by its oil/gas partition coefficient. Why?

What are the anesthetic effects of room air at atmospheric pressure?

2. Why do we care about the rate of alveolar rise of an anesthetic, when the clinical effect results from the anesthetic concentration in the brain?

Which anesthetics would come on most rapidly if given intravenously?

Draw the VRG, MG, FG, and VPG uptake curves for nitrous oxide and halothane.

How are MAC experiments done, if it takes 100 hours for the fat to reach 3 time constants?

Draw the approximate Falveolar/Finspired curves for nitrous oxide, isoflurane, and halothane.

3. Draw approximate hydraulic models for nitrous oxide, halothane, and isoflurane.

4. Explain the difference between the concentration effect and the gas effect.

How can you use the concentration and second gas effects to maximal advantage during a mask induction?

What will happen to the arterial oxygen content on transferring from breathing air to breathing 21% oxygen in nitrous oxide?

5. Why do more soluble anesthetics show more of an effect from changes in Valveolar?

What will happen to the minute alveolar ventilation and the alveolar isoflurane concentration of isoflurane of a patient breathing 5% isoflurane.

6. Explain how an endobronchial intubation will slow the rate of nitrous transport to the brain, but not slow the rate of ether transport to the brain.

The volume of the normal gut is 100 ml (present company excepted). What would be the theoretical maximal volume of a patient breathing 80% nitrous?

What is the rate difference between expansion of the bowel and expansion of a pneumothorax from nitrous oxide?

7. If your anesthesia machine could delivery CO2, how would you use it at the end of your cases?

Why could you get diffusion hypoxia at the end of the case, when the patient is breathing no nitrous oxide, while you won't get it during the case, when the patient is breathing 80% nitrous?

Why don't you get diffusion hypoxia from the nitrogen washout at the beginning of a case?

8. What is the correct O2 flow from a Copper Kettle for delivering 1 MAC under the following conditions:

Drug Fresh Gas Flow Copper Kettle Flow

Halothane 1.0 _______

Forane 2.0 _______

Enflurane 3.0 _______

9. You have inadvertently filled your enflurane vaporizer with halothane. There is no time to change the fluids. Where should you set your enflurane vaporizer to administer 1 MAC of halothane?

10. Isoflurane costs $75 / 50 ml bottle. How much more does it cost, per hour, to run 5 liter flows than 1 liter flows at 1% isoflurane?

11. A new inhalational anesthetic, Brutane®, has a blood/gas partition coefficient of 10 and a MAC of .002%. What is the rate of uptake after 10,000 minutes?

I. Key Concept:

Read Eger's Anesthetic Uptake and Action several times before you take your written boards. Reading this book is the single best preparation for the written boards. Despite its age (copyright 1974), it remains the definitive work on this topic. Except for this paragraph, this handout is entirely based upon Eger's text.

II. MAC

A. The minimum alveolar concentration at 1 atmosphere that produces immobility in 50% of patients or animals exposed to a noxious stimulus.

B. A measure of anesthetic potency.

C. Equally applicable to all inhaled anesthetics.

D. Measured at equilibrium, where the partial pressure at the site of action presumably equals the partial pressure in the alveolus.

1. Anesthetic effect results from the partial pressure of the gas at the site of drug effect, not the concentration.

2. Gases diffuse down their partial pressure gradients, regardless of the total partial pressure of all gases present!

E. Factors not affecting MAC:

1. Eger claims that stimulus intensity (e.g. skin incision, tail clamp, 50 Hz current via needle electrodes) does not affect MAC. Subsequent research and every day clinical experience suggests this may not be true.

2. Small variability between individuals within a species (SD = + 10-20 percent).

3. Varies by about 50% between species.

4. No gender effect.

5. No time effect (e.g. MAC after 10 hours of anesthesia is the same as MAC during the first hour.)

6. No effect from pH.

7. No effect from pCO2, (except when over 90 mm Hg) despite widely held belief that elevated CO2 increases anesthetic requirement.

8. Elevations in blood pressure.

9. Mild drops in blood pressure.

10. Hypothyroidism

11. Chronic alcoholism probably doesn't affect MAC.

12. Anemia (unless sufficiently severe to cause tissue hypoxia)

F. Factors affecting MAC:

1. Significant hypoxia (below 40 mm Hg) decreases MAC.

2. Hypothermia decreases MAC.

3. Hyperthermia increases MAC.

4. Hyperthyroidism slightly raises MAC (about 15%)

5. Increasing age decreases MAC:

Neonatal Halothane MAC: 1.1

70 y.o. MAC: 0.63

6. Circadian rhythms alter MAC + 10%

7. Opioids reduce MAC, but there is a ceiling effect beyond which additional opioid does not lower MAC.

8. Diazepam reduces MAC.

9. Scopolamine slightly lowers MAC.

10. Other volatile agents reduce MAC:

MACs are additive:

50% N2O + .5MAC Halothane = 1 MAC of anesthetic

11. Drugs which reduce central catecholamines lower MAC:

Reserpine

Alpha methyl dopa (Aldomet)

chronic amphetamine use

clonidine

12. Drugs which raise central catecholamines raise MAC:

acute amphetamine intoxication

ephedrine

MAO inhibitors

13. Relaxants lower MAC slightly (but don't push your luck!)

14. Pregnancy decreases MAC

G. MACawake = minimum alveolar concentration at which patients still respond to commands. Originally thought to be about 0.6 MAC. Subsequent work suggests it is closer to 0.2 MAC, at least for isoflurane.

H. MACei = minimum alveolar concentration to block movement to endotracheal intubation. About 1.3 MAC.

I. MACbar = minimum alveolar concentration to block adrenergic responsiveness. About 1.5 MAC.

III Mechanisms of General Anesthesia

A. Wide variety of agents cause anesthesia:

Drug MAC

Halothane 0.77%

Isoflurane 1.11%

Enflurane 1.66%

Nitrous Oxide 110%

Nitrogen 30 ATM

Hydrogen 180 ATM

B. Interaction of anesthetics is through Van der Waals forces, not covalent bonding.

C. No structure-activity relationship for inhaled anesthetics.

D. Anesthetic potency correlates amazingly well with the anesthetic oil/gas partition coefficient: MAC * oil/gas partition coefficient = 2.08 + 20%.

- Strongly suggests a lipid site of anesthetic effect.

E. Inhaled anesthetics appear to act at many sites in the CNS.

F. Very high pressure (e.g. 5-50 atmospheres) can reverse the effects of anesthetics

IV Uptake of Inhaled Anesthetics.

A. Factors raising the alveolar concentration, assuming a constant inspired anesthetic concentration, and no uptake by blood:

1. The inspired concentration.

a. The rate of rise is directly proportional to the inspired concentration.

2. The alveolar ventilation

a. The larger the minute alveolar ventilation (Valveolar), the more rapid the rise in alveolar concentration.

b. Inspired gas is diluted by the FRC, so the larger the FRC, as a fraction of Valveolar, the slower the alveolar rise in anesthetic concentration.

c. For ventilatory rates over 4 half breaths, the ventilatory rate does not make any difference at the same Valveolar.

3. The time constant

The time required for flow through a container to equal the capacity of the container (e.g. volume/flow)

Time Constant % washin/washout

1 63%

2 86%

3 95%

4 98%

The time constant for the lungs is FRC/Valveolar.

The time constant for the anesthesia circuit is circuit capacity/FGF.

4. The larger the FRC, the slower the washin of a new gas.

5. The rate of rise of the alveolar concentration is greatly slowed by anesthetic uptake by the blood.

B. Factors determining uptake by blood.

1. Solubility in blood:

a. The blood/gas partition coefficient.

b. The relative capacity per unit volume of two solvents (e.g. gas and blood) to hold the anesthetic gas.

c. The relative molar amount in equal volumes of blood and gas when the partial pressures are equal.

Gas Blood/Gas Brain/Blood

Nitrous Oxide 0.47 1.06

Isoflurane 1.41 2.60

Enflurane 1.78 1.45

Halothane 2.30 2.30

Diethyl Ether 12.1 1.03

Methoxyflurane 12.0 1.70

d. Other things equal, the more soluble the anesthetic, the more drug will be taken up by the blood, and the slower the rise in alveolar concentration.

2. Cardiac Output:

a. The flow of blood through the lungs determines the amount of blood available to remove anesthetic gas.

b. The greater the cardiac output, the slower the rise in alveolar concentration.

c. Mathematically, changes in cardiac output have exactly the same influence on anesthetic uptake from the lungs as changes in solubility, since both influence exactly the same process: the size of the storage capacity of the blood for anesthetic agent over a given time interval.

3. The mixed venous anesthetic concentration:

a. The higher the mixed venous concentration, the slower the anesthetic uptake.

b. Initially 0.

c. At equilibrium, the venous partial pressure = arterial partial pressure = alveolar partial pressure (e.g. uptake = 0).

d. The uptake from the lung, in liters of gas/minute:

[pic]

1

e. The rate of rise of the mixed venous concentration depends on the tissue uptake of the anesthetic.

4. Tissue uptake of anesthetic:

a. The tissue uptake (including blood) equals the uptake from the lungs.

b. The same factors which govern uptake by the blood from the lungs govern uptake by the tissues from the blood:

1. The tissue/blood partition coefficient (tissue solubility)

2. The tissue blood flow.

3. The tissue anesthetic concentration (analogous to the mixed venous tissue concentration)

c. Tissue uptake, in liters of gas/minute =

[pic]

2

d. The rate of rise in tissue anesthetic concentration is proportional to tissue blood flow.

e. The rate of rise in tissue anesthetic concentration is inversely proportional to the tissue capacity.

1. The tissue capacity is:[pic]

f. Just as discussed for the lungs, the tissues have a time constant:

1. [pic]

2. Each ml of grey matter gets about 1 ml of blood/minute, so the time constants for the brain equal the solubilities of the anesthetics (in minutes):

Gas 1 TC 2 TC 3 TC

Nitrous Oxide 1.06 2.12 3.18

Isoflurane 2.60 5.20 7.80

Enflurane 1.45 2.90 4.35

Halothane 2.30 4.60 6.90

3. By contrast, resting muscle has a blood flow of 3 ml/min / 100 ml tissue (in minutes):

Gas bl/m 1 TC 2 TC 3 TC

Nitrous Oxide 1.15 38 77 115

Isoflurane 4.0 133 267 400

Enflurane 1.7 57 113 170

Halothane 3.5 117 233 350

4. The blood flow to fat is similar to the blood flow to resting muscle. However, anesthetics are very soluble in fat, which results in very long time constants (in hours):

Gas bl/f 1 TC 2 TC 3 TC

Nitrous Oxide 2.3 1.3 2.6 3.9

Isoflurane 45.0 25 50 75

Enflurane 36.2 20 40 60

Halothane 60 33 67 100

Thus, eventually fat governs the uptake of all anesthetics, until equilibrium is reached (at several days)

g. The contribution of each tissue to the mixed venous partial pressure concentration is the tissue anesthetic partial pressure * the flow to that tissue.

h. The body can be roughly divided into 4 tissue groups, vessel rich group (brain, heart, lungs, kidney, splanchnic bed, glands), the muscle group, the fat group, and the vessel poor group (bones, cartilage, ligaments):

Group % MASS %CO TC Nitrous TC Halothane

VRG 10 75 1.3 3.3

MG 50 18 30 106

FG 20 5.5 100 2720

VPG 20 1.5 160 390

C. Unifying the above concepts:

1. Wash in of gas to the FRC occurs very fast (within 1 minute), so we will ignore it.

2. The alveolar/inspired partial pressure ratio approaches 1 with time.

3. [pic]

Where ULung = uptake from lung (seen on page 6). Note that [pic] is the rate of drug delivery to the lungs, while ULung is the rate of drug leaving the lung.

V. Hydraulic Model (intuitive model)

A. The model consists of 5 upright cylinders.

1. The cylinders represent the inspired reservoir, the alveolar gas, vessel rich group, the muscle group, and the fat group.

2. The arrangement of the cylinders is as follows:

Inspired reservoir

|

|

\|/

FG MG

|

|

\|/

Vessel rich group

3. The cross sectional surface of each cylinder corresponds to its capacity (volume * tissue solubility)

4. The four peripheral cylinders are connected to the alveolar gas cylinder by pipes at their bases. The diameter of the pipes correlates to the blood/gas partition coefficient times the cardiac output to each group, (except for the connection between the inspired reservoir, where the diameter of the pipe represents alveolar ventilation.)

5. The height of the column of fluid in each cylinder corresponds to the partial pressure of the anesthetic in that cylinder.

VI. The concentration and second gas effects:

A. The concentration effect (applies to the effects of nitrous oxide on the uptake of nitrous oxide):

1. The higher the inspired concentration, the more rapid the rise in alveolar concentration. Explanations:

a. The concentrating effect: As gas is taken up by the blood, the remaining gas is present in a smaller volume, which diminishes the change in partial pressure that might otherwise be expected.

b. The ventilation effect: As gas is taken up, more gas is brought in to the lungs to replace the lost volume, which both increases Valveolar and lowers the influence of uptake on reducing Falveolar

2. Examples:

Example 1: We instantaneously fill a 4 liter box (the lungs) with 50% nitrous oxide. The concentrating effect tells us that if half of the nitrous oxide is taken up (1 liter), we will have 1 liter of nitrous in a 3 liter box, for a concentration of 33%, not 25% as might be expected. If we consider the ventilation effect: the 1 liter of nitrous taken up will be replaced with another liter of 50% nitrous (the inspired gas), which will add another .5 liters of nitrous to the box. This results in 1.5 liters of nitrous in a 4 liter box, for a final concentration of .38%.

Example 2: We start with a 4 liter box the lungs filled with 100% nitrous oxide (don't try this), connected to gas supply that is also 100% nitrous oxide. No matter how much nitrous is taken up, the concentration cannot fall below 100%.

3. The concentration effect is only significant for gases present in high concentrations (e.g. nitrous). It is negligible for the potent agents.

B. The second gas effect (applies to the effects of nitrous oxide on the uptake of a another gas):

1. Analogous to the concentration effect, but relates instead to a the use of a potent agent concurrently with a second gas present in large quantity, usually nitrous oxide.

2. The higher the inspired concentration of the second gas (nitrous), the more rapid the rise in alveolar concentration. Explanations:

a. The concentrating effect: As the second gas (nitrous) is taken up in significant volumes, all remaining gases (including the potent agent, which is the "first" gas) are concentrated in the remaining volume.

b. The ventilation effect: As the second gas is taken up in significant volumes, additional fresh gas in brought into the lungs. This increases Valveolar, which increases the rate of rise of anesthetic concentration.

3. Example:

We instantaneously fill a 4 liter box with 50% nitrous oxide and 400 cc (1%) isoflurane. The concentration effect tell us that if half of the nitrous oxide is taken up, the concentration of isoflurane will increase from 1% to 1.33% (400 cc / 3 liters). The ventilation effect tells us that another liter of fresh gas, containing 500 mls of nitrous, and 100 mls of isoflurane will be brought into the lungs, resulting in a concentration of 1.25%. Although the ventilation effect has lowered the concentration of isoflurane slightly, it has increased Valveolar, which will more than offset the change.

VII. Changes in Ventilation and Circulation

A. Changes in Ventilation

1. Increasing ventilation increases the rate of rise of alveolar concentration.

2. The change is greatest for more soluble anesthetics:

a. Nitrous rises quite fast, regardless of Valveolar.

b. Changing Valveolar will notably increase the rate of rise of halothane, but it will be less than a proportional change (i.e. a 50% increase will result in less than a 50% acceleration of induction).

c. With ether, which is highly soluble, the change is proportional (i.e. a 50% increase will accelerate induction by 50%).

3. Anesthetics depress Valveolar, which slows the rate of rise of the alveolar concentration. However, the concentration only stops rising towards the inspired concentration when the patient stops breathing entirely.

a. This depression is sets an upper limit on the alveolar concentration which can be obtained in a spontaneously breathing patient. For halothane, patients stop breathing entirely when the concentration in the brain reaches 2.5%.

b. There is a delay caused by the time required for transport of the anesthetic from the lungs to the brain, and so in the first few minutes, patients may breath spontaneously with alveolar concentrations higher than 2.5%.

4. Hyperventilation reduces cerebral blood flow.

The effect on induction time is function of solubility:

a. For nitrous oxide, the reduction in cerebral blood flow more then compensates for the increased rate of rise of the alveolar concentration with hyperventilation, so induction time is actually slower in the hyperventilating patient (remember hyperventilation didn't help that much in the first place).

b. For halothane, the reduction in cerebral blood flow almost exactly balances the effect of the increased rate of rise of alveolar concentration.

c. For ether, the increased rate of rise of alveolar concentration more than compensates for the reduction in cerebral blood flow, so induction is faster.

B. Changes in Cardiac Output

1. Increasing cardiac output lowers the alveolar anesthetic concentration, and slows the rate of rise of alveolar anesthetic concentration.

2. The change is greatest for more soluble anesthetics (as was the influence of changes in ventilation)

a. Nitrous rises quite fast, regardless of cardiac output.

b. The potent agents are highly affected by cardiac output, with halothane the most affected, isoflurane the least.

3. If the cardiac output is lowered, the effect depends on the distribution of cardiac output.

a. If the cerebral circulation is less, then induction will be slower, even though the alveolar concentration will rise more quickly.

b. If the cardiac output is reduced, but cerebral circulation is maintained (the usual situation), then the cardiac alveolar concentration will rise more rapidly, and this rapid rise will be quickly reflected in the brain anesthetic concentration.

c. Thus, patients in hypovolemic shock will have very rapid rises in cerebral anesthetic concentration.

d. Patients in septic shock may have fairly slow rises in cerebral anesthetic concentration.

4. Anesthetics may reduce cardiac output, which will lower the rate of rise of anesthetic concentration.

VIII Effect of V/Q abnormalities

A. Ventilating unperfused alveoli (dead space)

Increases arterial - end tidal anesthetic partial pressure difference (as it also does for the carbon dioxide), but, assuming that the perfused alveoli are normally ventilated (e.g. that the patient is eucapnic), then the rate of rise of the alveolar concentration, and the rate of anesthetic induction, is unchanged.

B. Perfusing unventilated alveoli (shunting)

Delays the increase in arterial partial pressure, particularly for the poorly soluble anesthetics (e.g. nitrous oxide). The reason is that for a poorly soluble agent, the increased ventilation of the ventilated alveoli (assuming the patient is eucapnic) only delivers slightly more anesthetic to the lungs, and not enough to compensate for the unventilated alveoli. However, for a very soluble gas (e.g. the potent agents, but also for carbon dioxide, in the opposite direction, since the patient is eucapnic!) the increased ventilation of the ventilated alveoli is adequate to compensate for the lack of gas transport in the shunted blood. As mentioned, that is how the patient remains eucapnic in the first place.

C. Left-right arteriovenous or intracardiac systemic shunts

No effect on anesthetic uptake or the rate of rise in alveolar pressure. The shunted blood has equilibrated with the current alveolar pressure, and is superfluous to both tissue perfusion (since it is shunting) and to the alveoli (since it has equilibrated and thus neither adds nor removes anesthetic from the lungs).

However: a left to right systemic shunt will antagonize the effects of a intrapulmonary (e.g. right to left) shunt that were discussed above. This is easily visualized by imagining that the blood which shunted through the lung without exchanging gas is rerouted back to the lung, by the systemic left to right shunt, for another try at gas exchange!

IX. Nitrous Oxide transfer to closed gas spaces

A. Key concept: every gas equilibrates to its partial pressure, regardless of the partial pressures of other gases present.

B. Compliant walls (e.g. bowel)

1. Assumption: The nitrous is highly soluble, and diffuses readily, when compared to the other gases in the bowel (although nitrous is insoluble compared to the potent agents, it is 30 times more soluble than nitrogen).

2. Nitrous will diffuse across the bowel wall, while the other gases don't move, until the partial pressure of nitrous within the bowel = the arterial partial pressure = alveolar partial pressure.

3. In other words, if the inspired concentration of nitrous is 66%, then 2 liters of nitrous will be added to each liter of bowel gas, making an intrabowel concentration of 66%, and the bowel 3 times larger.

4. The formula for the theoretical limit of bowel distention is Vnew = Vold * 1/(1-FiN20).

5. A simple way of remembering is: set the O2 flow to 1 liter/minute, and the adjust the nitrous flow to the desired inspiratory percentage. The theoretical limit is the total fresh gas flow (e.g. if the O2 is 1 l/min, and the nitrous is 2 l/min, then the maximum bowel extension is three fold).

6. The actual expansion is substantially less, owing to simultaneous, albeit slower, diffusion of nitrogen and other gases, in the opposite direction, and metabolic uptake of any oxygen present.

7. Nitrous oxide takes 2.5 hours to double the size of the bowel.

8. Nitrous oxide can double the size of a pneumothorax in 10 minutes!

C. Non-Compliant walls (e.g. inner ear)

1. The volume, by definition, doesn't change.

2. Since nitrous will, at equilibrium, have the same pressure on both sides of any membrane, the theoretical limit on the pressure is the original pressure (atmospheric), plus the partial pressure of nitrogen. If the patient is breathing 70% nitrous oxide, then the maximum pressure is 1.7 atmospheres.

X. Recovery from an inhalational anesthetic

A. Overall, it is the reverse process of the anesthetic induction:

1. The rate of fall in alveolar concentration determines anesthetic recovery.

2. Increased solubility slows recovery

3. Increased cardiac output slows recovery

4. Increasing ventilation may help the recovery from potent agents, but with hyperventilation, the increased rate of fall in alveolar concentration is nearly balanced by the reduced cerebral blood flow.

5. Hyperventilation probably delays recovery from nitrous oxide, because there is little improvement in pulmonary wash-out, while wash-out from the brain is delayed from the reduced cerebral blood flow.

6. There is no concentration effect on emergence, as there was on induction, because the gas in high concentrations (nitrous oxide) is not being drawn from an infinite reservoir, as it was on induction.

B. Diffusion Hypoxia

1. Related to the large outpouring of nitrous oxide diluting the inspired oxygen at the conclusion of a case.

2. Only a risk for the first 3-5 minutes after terminating the nitrous oxide.

3. Easily managed (and now almost never seen) with supplemental oxygen for a few minutes following termination of the nitrous.

C. The low partial pressure of anesthetics in most peripheral tissues (especially fat) means that during the initial recovery from anesthesia, anesthetic is continuing to pass into these tissues, rather than being released from these tissues. Thus, recovery is enhanced by redistribution of the inhalational anesthetic into fat and muscle, just like it is for most intravenous anesthetics.

XI. How vaporizers work

A. Vaporizers transfer a fraction of the gas from the fresh gas flow through the vaporizer. This diverted gas passes over the anesthetic liquid, and becomes 100% saturated with the anesthetic vapor. It is then added back to the fresh gas flow.

B. In type specific vaporizers, which are nearly universal now, the fraction of the fresh gas flow diverted through the vaporizer is automatically adjusted for changes in temperature.

C. In copper kettles, you manually adjust the oxygen flow into the vaporizer. The entire flow passes through the vaporizer, and is fully saturated with anesthetic vapor on leaving the copper kettle. This flow is then added to the fresh gas flow set elsewhere on the machine. If your fresh gas flow is 0, then the patient will get very high concentrations of anesthetic! (32% halothane, 32% isoflurane, 23% enflurane).

D. The % flow from a copper kettle equals the partial pressure of the anesthetic / 760 x(times 100%).

E. The vapor flow from a copper kettle equals:

[pic]

7

where PP is the partial pressure. The % vapor inspired is approximately:

[pic]

8

The exactly correct solution, which nobody uses, requires adding the gas inflow to the copper kettle to the denominator. Since this is usually less than 200 cc, while the fresh gas flow is several liters, leaving it out simplifies the equation without sacrificing much accuracy.

F. Useful Constants for Vaporizer questions:

Drug PP PP% [pic] Mol Wt. Density [pic]

Halothane 241 32% 1/2 197.4 1.86 227

Isoflurane 240 32% 1/2 184.5 1.50 196

Enflurane 175 23% 1/3 184.5 1.52 198

Sevoflurane 157 21% 1/4 200.1 1.52 183

Desflurane 669 88% n/a 168.0 1.47 211

The mls vapor/mls liquid calculation is as follows:

[pic]

G. When using a copper kettle, the "magic" fresh gas flow rate is 5 liters for halothane and isoflurane and 3 liters for enflurane. At these flow rates, the copper kettle inflow rate is approximately 100 times the % inspired vapor. For example, 100 ml flow into a copper kettle yields 50 mls of vapor, which is 1% if the fresh gas flow is 5 liters.

XII. Closed-Circuit Anesthesia

A. Fun, but difficult to do on our current machines.

B. Very efficient use of gases, reduces environmental hazards.

C. Need to denitrogenate by administration of 100% O2 prior to starting.

D. Give baseline oxygen (200-300 l/min)

E. Add nitrous oxide and potent agent as necessary to achieve desired anesthetic state.

F. Gas necessary in first minute:

[pic]

11

Note that this is the same as equation 3d on page 6, adjusted for Pvenous = 0.

G. Uptake at subsequent times (t) is:

[pic]

12

H. Assuming a cardiac output of 5000 l/min, and a target concentration of 1 MAC, the following table shows the solutions for halothane, enflurane, isoflurane and nitrous oxide for concentrations of 1.11%, 0.77%, 1.66%, and 70%, respectively:

Closed Circuit Administration Table

Isoflurane Halothane Enflurane Nitrous

Time gas liquid gas liquid gas liquid gas

1 78.3 0.40 88.6 0.39 147.7 0.75 1650

2 55.3 0.28 62.6 0.28 104.5 0.53 1160

3 45.2 0.23 51.1 0.23 85.3 0.43 950

4 39.1 0.20 44.3 0.20 73.9 0.37 820

5 35.0 0.18 39.6 0.17 66.1 0.33 740

6 31.9 0.16 36.2 0.16 60.3 0.30 670

7 29.6 0.15 33.5 0.15 55.8 0.28 620

8 27.7 0.14 31.3 0.14 52.2 0.26 580

9 26.1 0.13 29.5 0.13 49.2 0.25 550

10 24.7 0.13 28.0 0.12 46.7 0.24 520

15 20.2 0.10 22.9 0.10 38.1 0.19 420

20 17.5 0.09 19.8 0.09 33.0 0.17 370

25 15.7 0.08 17.7 0.08 29.5 0.15 330

30 14.3 0.07 16.2 0.07 27.0 0.14 300

35 13.2 0.07 15.0 0.07 25.0 0.13 280

40 12.4 0.06 14.0 0.06 23.4 0.12 260

50 11.1 0.06 12.5 0.06 20.9 0.11 230

60 10.1 0.05 11.4 0.05 19.1 0.10 210

90 8.2 0.04 9.3 0.04 15.6 0.08 170

120 7.1 0.04 8.1 0.04 13.5 0.07 150

150 6.4 0.03 7.2 0.03 12.1 0.06 130

180 5.8 0.03 6.6 0.03 11.0 0.06 120

210 5.4 0.03 6.1 0.03 10.2 0.05 110

240 5.1 0.03 5.7 0.03 9.5 0.05 110

300 4.5 0.02 5.1 0.02 8.5 0.04 90

360 4.1 0.02 4.7 0.02 7.8 0.04 90

420 3.8 0.02 4.3 0.02 7.2 0.04 80

480 3.6 0.02 4.0 0.02 6.7 0.03 80

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