Inhaled Anesthetic Agents

Question 1

What are the factors that affect uptake of an anesthetic agent into the bloodstream?

Answer 1

1. Alveolar-venous partial pressure difference
2. Blood-gas coefficient
3. Cardiac output

Question 2

What MAC value prevents response to a surgical stimulus in 97% of patients? 99% of patients?

Answer 2

97%: 1.2 MAC

99%: 1.3 MAC

Question 3

Name the MAC values and blood-gas coefficients of Sevo, Des, Iso, and N2O

Answer 3

Sevo: 2.0 MAC, b-g: 0.63
Des: 6.0 MAC, b-g: 0.42
Iso: 1.14 MAC, b-g: 1.4
N2O: 104 MAC, b-g: 0.47

Question 4

How do right-to-left intracardiac shunts affect inhalation inductions?

Answer 4

Slows them down (dilution of pulmonary blood entering the left side of the heart with venous blood that has no inhaled anesthetics)

Question 5

How does cardiac output affect speed of induction?

Answer 5

Increased CO = slower onset of induction (blood spends less time with alveoli to pick up gas)

Question 6

What is the Meyer-Overton Hypothesis?

Answer 6

Lipid-solubility hypothesis: anesthesia is produced when sufficient numbers of molecules disrupt neuronal lipid membranes, causing anesthesia (outdated)

Question 7

What is the critical volume hypothesis?

Answer 7

Anesthesia occurs when anesthetic agents cause lipid-membrane expansion, disrupting membrane-protein function (at critically relevant concentrations)

Question 8

How do inhaled anesthetics affect the CNS?

Answer 8

Potentiate inhibitor neurotransmission (GABA or glycine) and inhibit excitatory transmissions (NMDA) both presynaptically and postsynaptically

Question 9

Describe the concentrating effect of anesthetic gases

Answer 9

As the inhalation agent is taken up by the blood, the total lung volume is decreased by the amount of gas taken up by the blood, concentrating the agent remaining within the lung

Question 10

Describe the second-gas effect

Answer 10

The uptake of a first gas reduces the total gas volume, increasing the concentration of the second gas via the concentrating effect.

Question 11

With what anesthetic gases does the concentrating effect play more of a role? The second-gas effect?

Answer 11

Concentrating: less soluble agents (i.e. Des or N2O)

Second-gas: more soluble second gas agents (i.e. Iso)

Question 12

At higher altitudes, how would an inhalation agent differ in its effect?

Answer 12

The same concentration would exert a lower partial pressure within the alveolus and have a reduced anesthetic effect.

Question 13

What is the standard stimulation used to define MAC in humans?

Answer 13

Skin incision

Question 14

How do opioids/benzos/barbs/propofol, ketamine, alpha-2 agonists, IV local anesthetics, amphetamines, ephedrine affect MAC?

Answer 14

Amphetamines and ephedrine increased MAC; the rest decrease MAC

Question 15

How does acute vs. chronic EtOH affect MAC?

Answer 15

Acute: decreases MAC
Chronic: increases MAC

Question 16

At what age is MAC the highest? How can you estimate MAC in the elderly?

Answer 16

3-6 months of age

MAC decreases 6% per decade of life after age 40

Question 17

How does hypothermia, hypotension, hypoxemia, anemia, and sepsis affect MAC?

Answer 17

Decreases MAC

Question 18

How does MAC change with temperature?

Answer 18

Decreases linearly with decreasing temperatures; 1C decrease reduces anesthetic requirement by 4-5%

Question 19

Does sex or patient size affect MAC?

Answer 19

No

Question 20

What is MAC-bar?

Answer 20

The MAC necessary to blunt adrenergic response in 50% of patients with skin incision (1.7-2.0 MAC)

Question 21

What is MAC-awake?

Answer 21

The MAC at which half of patients will open their eyes to command (hypnotic potency, 0.3-0.5 MAC)

Question 22

In general, what limits the uptake of anesthetic gases into the blood?

Answer 22

For highly soluble agents (i.e. ether), uptake is limited by ventilation. For poorly soluble agents (i.e. N2O), uptake is limited by blood flow.

Question 23

Is the impact of a right-to-left shunt greater with anesthetic agents with higher or lower solubility?

Answer 23

Lower solubility (since uptake is limited by blood flow and essentially you dilute out the very little anesthetic that is in the blood)

Question 24

How does a left-to-right shunt affect speed of induction?

Answer 24

Little change if systemic blood flow is normal

Question 25

Which would decrease volatile anesthetic uptake: increased B/G coefficient, decreased CO, decreased partial pressure of the anesthetic in blood, increased partial pressure of anesthetic in alveolus, decreased pulmonary shunting?

Answer 25

Decreased cardiac output; this is asking about uptake and not speed of induction

Question 26

How would mainstem intubating a patient affect speed of induction? What type of agents (soluble vs. insoluble) would be more affected?

Answer 26

1. Delay it because you have increased pulmonary shunting (blood that does not participate in gas exchange which dilutes the concentration of anesthetic in the arterial blood leaving the alveoli) 2. Soluble agents are less affected

Question 27

Comparing 6% to 2% sevoflurane, which is true: FA will approximate FI quicker at 6% than 2%, FA will approximate quicker at 2% than 6%, the FA/FI plots will be identical

Answer 27

FA will approximate quicker at 6% than 2% secondary to the concentration effect (in reality, likely no change)

Question 28

Which of the following increases MAC: anemia, hypoxia, hypercarbia, hypernatremia, hypercalcemia?

Answer 28

Hypernatremia

Question 29

At 1 MAC of a modern halogenated volatile anesthetic: dead space decreases, FRC increases, hypoxic pulmonary vasoconstriction is significantly blunted, minute ventilation increases, airway resistance decreases

Answer 29

Airway resistance decreases (bronchodilation) | HPV is only mildly blunted and MV decreases secondary to decreased TV (but increased RR)

Question 30

Which of the following effects of N2O is false: sympathetic stimulation, bradycardia, depressed myocardial contractility, increased PVR, preserved CO

Answer 30

Bradycardia

Question 31

How does N2O affect minute ventilation?

Answer 31

Preserved (unlike the halogenated gases) with no rise in apneic threshold; you will see increased RR and decreased TV but MV is preserved

Question 32

Does N2O potential muscle relaxation?

Answer 32

No (the halogenated gases do)

Question 33

What are some side effects seen with N2O?

Answer 33

1. Inhibition of methionine synthetase (used in myelin formation) 2. Inhibition of thymidylate synthetase (used in DNA synthesis) 3. Peripheral neuropathy 4. Pernicious anemia/megaloblastic anemia/depressed bone marrow production (decreased Vit B12)

Question 34

Which volatile anesthetic is metabolized to the greatest extent: sevoflurane, isoflurane, desflurane, N2O

Answer 34

Sevoflurane (5%) | Isoflurane is 0.2%, Desflurane and N2O are less than that

Question 35

Which produces the greatest extent of coronary vasodilation: sevoflurane, isoflurane, desflurane, N2O

Answer 35

Isoflurane

Question 36

What MAC level of isoflurane produces burst suppression?

Answer 36

1.5 MAC

Question 37

Transient increases in sympathetic tone occurs with rapid increases in concentration of which volatile anesthetic?

Answer 37

Desflurane and isoflurane

Question 38

Which volatile anesthetic can lead to increased carboxyhemoglobin levels?

Answer 38

Desflurane with a desiccated CO2 absorbent (produces carbon monoxide)

Question 39

Which CO2 absorbent does not produced Compound A with sevoflurane?

Answer 39

Calcium hydroxide; barium hydroxide lime or soda lime both can lead to production of Compound A

Question 40

What is the vapor pressure of desflurane? Sevoflurane? Isoflurane?

Answer 40

Des: 680 mmHg Sevo: 160 mmHg Iso: 240 mmHg