Advanced | Inhaled Anesthetics and clinical application Flashcards
What determines the rate of elimination of inhaled anesthetics?
A. solubility
B. oil:gas partition coeff.
C. vapor pressure
D. cardiac output
A. solubility
SOLUBILITY
Dictum: Poorly soluble agents have faster recovery from inhaled anesthetics.
Anesthesia is induced with halothane in a 3-year-old girl. Sixty seconds after administration of succinylcholine 1 mg/kg intravenously, heart rate decreases rapidly from 120 to 60 bpm. The most likely cause is:
(A) acute hyperkalemia
(B) failure to pretreat with a nondepolarizing relaxant
(C) halothane overdose
(D) muscarinic activity
(E) sympathetic ganglionic blockade
(D) muscarinic activity
Also, the NO depresses the myocardial activity thus accentuating the bradycardia seen in this patient.
The following are true pertaining to the Macroscopic sites of ACTION for Inhaled Anesthetics EXCEPT:
A. Amnesia: Hippocampus
B. Sedation: tubero-mamillary nucleus of hypothalamus
C. Ablation of movement: Spinal cord
D. Analgesia: Substansia gelatinosa
D. Analgesia: Substansia gelatinosa - FALSE
THE FOLLOWING are the action site where Inhaled Anesthetic takes place:
Ablation of movement = Spinal Cord
Amnesia = Hippocampus
Sedation = Tubero-mamillary nucleus of HYPOTHALAMUS
True or False
MAC is equivalent to the principle of ED50 of the intravenous drugs
TRUE
Which of the following is clinically considered TRUE gase?
A. Xenon
B. N2O
C. enflurane
D. Methyflurane
E. Both A and B
E. Both A and B
N2O
Xenon
TRUE or FALSE
The inspired concentration and the blood:gas solubility of an inhaled anesthetic is the major determinants of the speed of induction.
True
- Solubility alone determines the rate of elimination, provided there is normal cardiopulmonary function.
In order to minimize the accumulation of COMPOUN A, which of the following should be used in the circle system?
A. Na hydroxide
B. Barium hydroxide
C. Calcium hydroxide
D. K hydroxide
C. Calcium hydroxide
Calcium hydroxide is LEAST likely associated with Compound A.
This pertains to all the inhalation agents sharing a COMMON mechanism of action at the molecular level.
A. Unitary hypothesis
B. Meyer-overton
Unitary Hypothesis
Which of the following is considered to be the MOST POTENT clinical used Inhaled Anesthetic?
A. Isoflurane
B. Sevoflurane
C. Desflurane
D. Halothane
ISOFLURANE
DICTUM:
Potency is inversely related to MAC
“The higher the MAC, the less potent”
- Potency is oil;gas partition coeff.
- Onset is blood:gas partition coeff.
- MAC MAC is the alveolar concentration of an anesthetic
at one atmosphere (in volume%) that prevents movement in response to a surgical stimulus in 50% of patients.
This is the LEAST SOLUBLE clinically used inhaled anesthetics:
A. Isoflurane
B. Sevoflurane
C. Desflurane
D. Halothane
DESFLURANE
B:G partition coeff. = 0.42
Fluorination decreases blood and tissue solubility (the blood:gas
solubility of desflurane equals that of N2O), which results in a loss of
potency. It also results in a high vapor pressure owing to
decreased intermolecular attraction, requiring an electrically driven, heated, pressurized vaporizer to deliver a regulated concentration of desflurane as a gas.
Nitrous oxide (N2O) can expand a pneumothorax to double or triple its size in:
A. 10 to 30 minutes
B. 5 minutes
C. 1 hour
D. 120 seconds
A. 10 to 30 minutes
Nitrous oxide (N2O) can expand a pneumothorax to double or triple
its size in 10 to 30 minutes. Abrupt discontinuation of N2O inhalation can transiently lower alveolar concentrations of
oxygen and carbon dioxide, a phenomenon called diffusion hypoxia.
- It is relatively insoluble in blood. It is most commonly administered as an anesthetic adjuvant in combination with opioids or
volatile anesthetics during the conduct of general anesthesia.
Which of the following is CONTRAINIDACTED in the use of N2O?
A. Pneumothorax
B. Air embolism
C. Small bowel obstruction ,
D. pneumocephalus
E. Tympanic membrane grafting
F. All of the above
F. All of the above
Which of the following is the immune effect of N2O?
A. It affects chemotaxis and motility of PMNs
B. It irreversibly oxidizes the cobalt atom in vitamin B12
C. Reduces methionine synthase activity leads to increased homocysteine levels
A. It affects chemotaxis and motility of PMNs
All other statement are true but the evidence-based immune effect is on the motility of PMNs and chemotaxis
TRUE or FALSE
Volatile anesthetics depress cerebral metabolic rate in a dose-dependent manner but at LOWER concentrations will
increase cerebral blood flow
FALSE
Volatile anesthetics depress cerebral metabolic rate in a dose-dependent manner and at HIGHER concentrations will
increase cerebral blood flow
Which Inhaled anesthetic with the lowest boiling point?
A. N2O
B. halothane
C. Isoflurane
D. Desflurane
A. N2O
The boiling point is -88
At room temperature, most of the potent agents have a vapor pressure that is below atmospheric pressure. If the temperature is raised, the vapor pressure increases. The boiling point of a liquid is the temperature at which its vapor pressure exceeds atmospheric pressure in an open container.
Desflurane is bottled in a special container because its boiling point of 23 °C makes it boil at typical room temperatures.
Boiling does not occur within the bottle because it is countered by buildup of vapor pressure within the bottle, but once opened to air, the desflurane would quickly boil away.
A 50 year old female is scheduled for elective laparoscopic surgery. Her BMI is 50. Which inhaled anesthetics is ideal for this patient?
A. Desflurane
B. Isoflurane
C. Sevoflurane
D. Enflurane
DESFLURANE
Desflurane has the lowest blood:gas solubility of the potent volatile anesthetics; moreover, its fat solubility is roughly half that of the other volatile anesthetics.
- Thus, desflurane requires less downward titration toward the end of long surgical procedures to achieve a rapid emergence by virtue of decreased tissue saturation. This may be particularly advantageous in the morbidly obese patient.
The pharmacologic property of XENON which is responsible for its analgesic property is:
A. NMDA inhibition
B. GABA-A inhibition
C. Analgesic effect at the spinal cord level
D. Blunting of pain receptors at the cerebral cortex
A. NMDA inhibition
Xenon provides some degree of analgesia. Its blood:gas partition coefficient is 0.115, and unlike the other potent volatile anesthetics (except methoxyflurane), xenon provides some degree of analgesia.
- This action is likely due to N-methyl-D-aspartate (NMDA) receptor inhibition.
TRUE OR FALSE
Poorly soluble inhaled anesthetics have faster RECOVERY?
True
Agent B:G partition coeff. MAC
A 0.42 6.6
B 0.65 1.8
C 1.4 1.17
D 2.4 0.75
- Given the following parameters, which of the following will achieve the HIGHEST Fa/Fi rise?
A. Agent A
B. Agent B
C. Agent C
D. Agent D
A. Agent A
This is the B/G partition coeff of DESFLURANE.
Dictum:
Low blood/gas partition > less soluble > LESS uptake > less time needed to reach Fa/Fi > FAST INDUCTION or ONSET!
Which if the following has a potential toxic effects on cell function via inactivation of vitamin B12?
A. Sevoflurane
B. N2O
C. Isoflurane
D. Desflurane
B. N2O
Despite a long track record of use, controversy has surrounded N2O in four areas: its role in postoperative nausea and vomiting (PONV), its potential toxic effects on cell function via inactivation of vitamin B12, its adverse effects related to absorption and expansion into air-filled structures and bubbles, and lastly, its
effect on embryonic development.
The CNS partial pressure of inhaled anesthetics equals what pressure, which in turn equals alveolar pressure if cardiopulmonary function is normal:
Arterial partial pressure
This determines the speed of induction of the inhaled anesthetics?
A. Concentration of blood:gas solubility
B. MAC
C. oil:gas coefficient
D. Vapor pressure
A. Concentration of blood:gas solubility
This is the concentrations of inhaled anesthetics that provide loss of awareness and recall are about?
A. 0.4 to 0.5 MAC
B. 1 MAC
C. 1.2 MAC
D. 0.5 to 1.5 MAC
0.4 to 0.5 MAC
This is the typical time to loss of consciousness when delivering 8% sevoflurane via the face mask:
A. 1 minute
B. 30 seconds
C. 2 minutes
D. 15 seconds
60 SECONDS | 1 Minute
Which volatile anesthetic is metabolized to trifluoroacetate, thereby causing hepatotoxicity through an immunologic mechanism involving trifluoroacetyl hapten formation and a resulting autoimmune response.
A. Sevoflurane
B. N2O
C. Isoflurane
D. Desflurane
E. Halothane
HALOTHANE
In an otherwise normal patient, which of following will increases the rate of FI/FA concentration:
A. Increased CO
B. Low minute ventilation
C. High blood/gas partition coeff.
D. High pulmonary to arterial venous pressure
D. High pulmonary to arterial venous pressure
The following will INCREASE the FI/FA concentration:
*Low Blood Solubility
*Low CARDIAC OUTPUT
*High Minute Ventilation
* High pulmonary to arterial venous pressure
Ether-based volatile anesthetics which MAINTAIN or INCREASE hepatic artery blood flow:
S I D
Sevoflurane
Isoflurane
Desflurane
Which of the following inhaled anesthetic is LEAST like safe in cases of severe hepatic disease?
A. Sevoflurane
B. N2O
C. Isoflurane
D. Desflurane
E. Halothane
A. Sevoflurane
At 1 MAC the volatile anesthetic that decreased hepatic blood flow the MOST is:
A. Desflurane
B. Halothane
C. Sevoflurane
D. Isoflurane
A. Desflurane
Which tissue group plays the greatest role in determining emergence time?
A. Fat
B. Skeletal muscle
C. Lung
D. Coronary system
FAT
What is the effect of a R to L shunt on speed on inhaled induction?
A. Slow induction
B. Fast induction
C. No effect
Slows the induction onset
MAC decreases approximately how many percent per decade:
A. 6%
B. 10%
C. 50%
6%
True or false
Volatile anesthetics decrease ventilatory response to hypercarbia and hypoxia and INCREASE respiratory rate.
TRUE
CO2 absorbent degrade, sevoflurane, isoflurane and desflurane to CO when the normal water content of the absorbent (13 to 15%) is markedly decreased to?
A. < 5%
B. < 10%
C. <20%
< 5%
TRUE or FALSE
Volatile anesthetics ionized.
FALSE
It is Non-ionized.
Potent volatile anesthetics are LIQUIDS at ambient temperature and pressure except for?
A. Desflurane
B. Isoflurane
C. N2O
D. Enflurane
A. Desflurane
Is the ratio of dissolved gas (by volume) in two-tissue compartment at equilibrium
Partition coefficient
Loss of inhaled anesthetic via the skin is greatest with what anesthetic?
A. N2O
B. Enflurane
C. Sevoflurane
D. Desflurane
A. N2O
Is the alveolar concentration of anesthetic that blunts adrenergic responses to noxious stimuli
A. 0.5 MAC
B. MAC-BAR
C. 1 MAC
MAC-BAR
50% higher than standard MAC
How much decrease in MAC at the age of 40 to 80?
A. 22%
B. 44%
C. 50%
22%
The alveolar concentrations of anesthetics increase more rapidly in children than in adults because of a greater
(A) blood volume (per kg body mass)
(B) cardiac index
(C) MAC
(D) ratio of alveolar ventilation to functional residual capacity
(E) tidal volume (per kg body mass)
(D) ratio of alveolar ventilation to functional residual capacity
Sevoflurane preserves autoregulation up to approximately how much MAC?
A. 1 MAC
B. 0.5 MAC
C. 1.5 MAC
1 MAC
The increase in CBF with increasing anesthetic dose occurs despite decreases in CMRO2. This phenomenon is referred to as?
A. Coronary steal
B. Uncoupling
C. Over-pressurization
Uncoupling
TRUE or FALSE
Nitrogen cannot be removed readily via the bloodstream in air-filled spaces. As a result, it diffuses from the blood into these closed gas spaces quite easily until the partial pressure equals that of the blood and alveoli.
TRUE
TRUE or FALSE
Seventy-five percent N2O can expand a pneumothorax to double or triple its size in 10 and 30 minutes
TRUE
Air-filled cuffs of pulmonary artery catheters and endotracheal tubes also expand with the use of N2O, possibly causing tissue damage via increased pressure in the pulmonary artery or trachea, respectively. Accumulation of N2O in the middle ear can diminish hearing postoperatively8 and is contraindicated for tympanoplasty because the increased pressure can dislodge a tympanic graft.
True or False
The lower the MAC the more potent the agent.
TRUE
MAC is the alveolar concentration of an anesthetic at one atmosphere (in volume%) that prevents movement in response to a
surgical stimulus in 50% of patients. It is analogous to the ED50 expressed for intravenous drugs and can be used to compare anesthetic potency, that is, the lower the MAC the more potent the agent.
True of MAC except:
A. MAC-BAR is the alveolar concentration of anesthetic that blunts adrenergic responses to noxious stimuli.
B. MAC-BAR has been approximated at 50% higher than standard MAC
C. Administering 0.5 MAC of a potent agent and 0.5 MAC of N2O is equivalent to 1 MAC of potent agent in terms of preventing patient movement
D. Increased central neurotransmitter levels have NO effect on MAC
D. Increased central neurotransmitter levels has NO effects on MAC
It is a false statement because anything that increases the central neurotransmitter levels increase the MAC.
True or False
Factors that decrease CNS metabolic activity, neurotransmission, and CNS neurotransmitter levels, as well as downregulated CNS responses to
chronically elevated neurotransmitter levels, all seem to decrease MAC
True
Which of the following factors decreases MAC value?
A. Duration of administration
B. Gender
C. Type of surgical stimulation
D. Chronic dextroamphetamine administration
E. Red-haired female
D. Chronic dextroamphetamine administration
True or False
All the potent agents increase CBF in a dose-related manner
TRUE
The ideal agent for neurosurgical procedure:
A. Sevoflurane
B. Isoflurane
C. Desflurane
Isoflurane
What is the effect of Hypercapnia on CBF?
Significant hypercapnia is associated with dramatic increases in CBF whether or not volatile anesthetics are administered.
On the other hand, hypocapnia can blunt or abolish volatile anesthetic-induced increases in CBF depending on when the hypocapnia is produced. This vasoreactivity to CO2 may be somewhat altered by the volatile anesthetics as compared with
normal.
True or False
Isoflurane DOES NOT abolish hypocapnic vasoconstriction
TRUE
Effect of ISOFLURANE EXCEPT:
A. Isoflurane to decrease CMRO2
B. Isoflurane increases ICP minimally
C. Isoflurane does not abolish hypocapnic vasoconstriction
D. Increased CO
D. Increased CO
- Heart rate is increased but the CARDIAC OUTPUT is unchanged!
- Isoflurane to decrease CMRO2
- Isoflurane increases ICP minimally
- Isoflurane does not abolish hypocapnic vasoconstriction
- When isoflurane is used to lower BP and cerebral perfusion, tissue oxygen content is improved as compared to a similar BP effect created by other pharmacologic means
True or False
At greater than 2 MAC, all of the potent agents can produce burst suppression or electrical silence
TRUE
TRUE or False
The anesthetic concentration that produces cardiovascular collapse exceeds the concentration that results in apnea.
TRUE
True or False
Nitrous oxide has unique cardiovascular
actions. It increases sympathetic nervous system activity and vascular resistance when given in a 40% concentration
TRUE
When N2O is combined with volatile anesthetics, systemic vascular resistance and BP are greater than when equipotent concentrations of the volatile anesthetics are evaluated without N2O
True or False
Oxygen consumption is decreased approximately 10% to 15% during
general anesthesia
TRUE
TRUE of hemodynamic effects of volatile anesthetics except:
A. Blood flow to liver, kidneys, and gut is DECREASED, particularly at deep levels of anesthesia
B. Blood flow to the brain, muscle, and
skin is DECREASED or not changed during general anesthesia
C. Spontaneous ventilation in theory would improve the safety of volatile anesthetic administration because the anesthetic concentration that produces cardiovascular collapse exceeds the concentration that results in apnea
D. N2O INCREASES sympathetic nervous system activity and vascular resistance when given in a 40% concentration
E. The volatile anesthetics prolong QTc interval, predisposing the patient to arrhythmias
B. Blood flow to the brain, muscle, and
skin is DECREASED or not changed during general anesthesia
TRUE of the ventilatory effect of volatile anesthetics EXCEPT:
A. Isoflurane does not increase respiratory rate above 1 MAC
B. Volatile anesthetics increases RR and tidal volume
C. Volatile agents produce a dose dependent Increase RR and decrease TV
D. gradual decrease in minute venitlation increases the resting PaCO2
E. When N2O is added to inhaled volatile anesthetics PaCO2 increases in a degree similar to equi-MAC concentrations of the volatile anesthetic in O2.
B. Volatile anesthetics increases RR and tidal volume - FALSE statement
All potent volatile anesthetics increases RR but DECREASES tidal volume.
All volatile anesthetics decrease tidal volume and increase respiratory rate such that the decrease in minute ventilation at low inspired concentrations is relatively minor. The ventilatory depressant effects of volatile anesthetics at higher concentrations are dose dependent, with higher concentrations of volatile anesthetics resulting in greater decreases in tidal volume and greater increases in respiratory rate. Isoflurane is an exception in that it does not increase respiratory rate above 1 MAC.
Which of the following variables is MOST directly responsible for the depth of anesthesia using volatile anesthetics?
A. Blood-gas partition coefficient
B. MAC of the volatile agent delivered
C. Volume percentage of the volatile agent delivered
D. Partial pressure of the volatile agent delivered
A. Blood-gas partition coefficient
Anesthetic depth is determined by the partial pressure of volatile anesthesia delivered to the brain. Minimal anesthetic concentration (MAC) values and percentages of volatile anesthesia are commonly used when discussing the depth of anesthesia, but it is the partial pressure of volatile anesthesia in the brain that determines the depth of anesthesia.
The volume percent of gas can be calculated by the partial pressure of the gas in mmHg divided by the atmospheric pressure in mmHg. For isoflurane, 1% isoflurane is equal to 7.6 mmHg (the partial pressure) divided by 760mmHg (atmospheric pressure at sea level.) MAC values were calculated as percentages of volatile anesthesia delivered at sea level (volume/volume percent) and are useful clinically since the vaporizers are marked by percentage and not partial pressure. MAC is measure of relative anesthetic potency
Reference: Open Anesthesia Question of the day
TRUE of volatile anesthetic effect on neuromuscular system EXCEPT:
A. Relaxation of skeletal muscle is most prominent for potent volatile anesthetics above 1.0 MAC
B. Nitrous oxide does not affect skeletal muscle relaxation
C. It directly relax skeletal muscle through a dose-dependent effect
D. Above 1.0 MAC the effect of volatile anesthetic is enhanced by 40% in patients with myasthenia gravis
E. The potentiation of NMB agents with volatile anesthetic appears to be largely because of a presynaptic effect at the muscarinic acetylcholine receptor located at the neuromuscular junction
E. The potentiation of NMB agents with volatile anesthetic appears to be largely because of a presynaptic effect at the muscarinic acetylcholine receptor located at the neuromuscular junction - FALSE statement
- The potentiation of NMB agents with volatile anesthetic appears to be largely because of a POSTSYNAPTIC effect at the NICOTINIC acetylcholine receptor located at the neuromuscular junction.
True or False
The hallmark findings of MH are a rapidly rising temperature, increasing up to 1 °C every 5 minutes along with increasing end-tidal CO2, arrhythmias, mixed respiratory/metabolic acidosis, and skeletal muscle rigidity
TRUE
True or False
Uterine relaxation/atony can become problematic at concentrations of volatile anesthesia greater than 1 MAC
TRUE
Uterine relaxation/atony can become problematic at concentrations of volatile anesthesia greater than 1 MAC and might delay the onset time of newborn respiration. Consequently, a common technique used to provide general anesthesia for urgent cesarean sections is to administer low concentrations of the volatile anesthetic, such as 0.5 to 0.75 MAC, combined with N2O
What is the NIOSH recommended exposure limit for N2O?
25 ppm
What is the NIOSH recommended exposure limits for volatile anesthetics?
2 ppm
The following inhaled agents with their corresponding MAC values. Taken into consideration that MAC is at 1 atm.
Agent A - 2.2(MAC)
Agent B - 1.8 (MAC)
Agent C - 1.2 (MAC)
Agent D - 3.5 (MAC)
Question: Which among the agents is least potent?
A. Agent A
B. Agent C
C. Agent D
D. Agent B
E. Both A and B
Agent D is the least potent
The dictum is the less or the lower the MAC the higher the LIPID solubility of an agent, hence MORE POTENT.
Low MAC => High POTENCY
The higher the OIL:GAS partition coefficient, the higher the potency!
Which of the following is ACCURATE pertaining to the rate of inhalational induction with a left to right cardiac shunt?
A. No effect or change
B. Slow induction
C. Fast induction
A. No effect or change
Which of the following is ACCURATE pertaining to the rate of inhalational induction with a right to left cardiac shunt?
A. No effect or change
B. Slow induction
C. Fast induction
B. Slow induction
It slows because there is less pulmonary blood flow and the anesthetic rich blood is mixing with the anesthetic poor blood.
Which of the following justifies the faster rate of rise in nitrous oxide in contrast to desflurane?
A. concentration effect
B. blood/gas partition coefficient
C. Vapor pressure
D. MAC
A. concentration effect
Which class of NMBDs do inhalational anesthetics work more synergistically with?
A. Aminosteroid
B. Benzylquinolone
A. Amino steroids
- The amino steroids: rocuronium, vecuronium, and pancuronium
True Learn
Which vessels does nitric oxide primarily dilate?
A. arterioles
B. coronary arteries
C. veins
D. capillaries
A. arterioles
True Learn
True of nitric oxide EXCEPT:
A. Decreases pulmonary vascular resistance
B. Off loads the Right chamber
C. Inhibits platelet aggregation
D. Decrease cGMP thereby causing smooth muscle relaxation
D. Decrease cGMP thereby causing smooth muscle relaxation - FALSE
INCREASES cGMP! > Smooth muscle relaxation
Which volatile anesthetic MOST likely can cause direct coronary vasodilation?
A. Isoflurane
B. Sevoflurane
C. N2O
D. Desflurane
A. Isoflurane
Theoretically, Xenon protect against?
A. neuronal ischemia
B. coronary ischemia
C. hepatic artery ischemia
D. cortical nephron ischemia
A. neuronal ischemia
True Learn
The following are risk factors in the accumulation of compound A EXCEPT:
A. Low flow anesthesia
B. Dry barium hydroxide
C. High sevoflurane concentration
D. Long anesthetic duration
E. Calcium hydroxide
E. Calcium hydroxide - Least potential to accumulate compound A.
TRUE or FALSE
Sevoflurane INCREASES portal vein flow.
FALSE
It actually decreases portal vein flow BUT increases hepatic artery flow.
CNS effects of Sevoflurane EXCEPT:
A. Increase in CBF
B. Increase in ICP
C. Increase in CMRO2
D. Uncoupling
C. Increase in CMRO2 - It is the opposite!
DECREASE CMRO2!
8 year old male underwent cleft palate surgery. It was uneventful however at the PACU, the patient manifested symptoms of emergence delirium. Which volatile anesthetic was MOST likely used?
A. Desflurane
B. Sevoflurane
C. Isoflurane
D. Halothane
B. Sevoflurane
In order from MOST to LEAST:
Sevo > Des > Iso > Propofol
Which drugs CANNOT attenuate the cardiac effect of desflurane with rapid increases in dosage?
A. Fentanyl
B. Clonidine
C. Esmolol
D. N2O
D. N2O - Wrong
- Desflurane has been associated
with tachycardia and hypertension when used with minimal opioids and, in select cases, myocardial ischemia when used in high concentrations or when
rapidly increasing the inspired concentration. - All these agents can attenuate the myocardial effect of desflurane:
Fentanyl
Clonidine
Esmolol
Which volatile agent is resistant to metabolism to serum trifluoroacetate, thus making immune-mediated hepatitis a rare occurrence:
A. Desflurane
B. Isoflurane
C. Sevoflurane
D. Halothane
A. Desflurane
At high altitudes the partial pressure of desflurane will be lower at a given vaporizer setting (volume percent) than at sea level, leading to:
A. Underdosing of the anesthetic to the patient
B. Overdosing of the anesthetic to the patient
C. No change
A. Underdosing of the anesthetic to the patient
The desflurane vapor pressure at 20°C atmosphere is (664 mm Hg). This required the development of a special desflurane vaporizer (Tec 6, Datex-Ohmeda), which electrically heats desflurane to 23°C to 25°C and pressurizes the vaporizer with a backpressure regulator to 1500 mmHg to create an environment which the anesthetic has a lower, but predictable volatility.
