21 Pharmacology of Therapeutic Gases and Inhalational Aneshtetics Flashcards
Therapeutic gases:
Oxygen
- Oxygen is administered to prevent or treat…which results from…
- Administration of high amounts of oxygen for prolonged periods of time may lead to adverse effects
- Any oxygen-enriched atmosphere also constitutes…
- Oxygen toxicity
- In part a consequence of…
- Primarily results in…
- Oxygen is administered to prevent or treat…
- Tissue hypoxia, which results from…
- A failure of the lungs to normally oxygenate the blood
- Inadequate delivery of oxygen to the tissues
- Impaired oxygen utilization
- Tissue hypoxia, which results from…
- Administration of high amounts of oxygen for prolonged periods of time may lead to adverse effects
- Promote absorption atelectasis
- Dry and irritate mucosal surfaces of the airway and the eyes
- Decrease mucociliary transport and clearance of secretions
- Any oxygen-enriched atmosphere also constitutes…
- A fire hazard
- Oxygen toxicity
- In part a consequence of excess generation of free radicals
- Primarily results in alterations in pulmonary structure and function
Therapeutic gases:
Oxygen:
Administration
- Low-flow systems
- The FIO2 of these systems
- Examples of these systems
- High-flow systems
- For precise delivery of defined FIO2, closed systems require…
- Low-flow systems
- Oxygen flow is lower than the inspiratory flow rate
- Have a limited ability to raise the FIO2 because they depend on entrained room air to make up the balance of the inspired gas
- The FIO2 of these systems
- Extremely sensitive to small changes in the ventilatory pattern
- Examples of these systems
- Nasal cannulae
- Simple face masks, with or without reservoir bags
- High-flow systems
- Deliver oxygen at flows that exceed the patient’s minute ventilation by three to four times
- Provide a relatively constant FIO2 at high flow rates
- For precise delivery of defined FIO2, closed systems require…
- Employing an endotracheal tube (or sealed mask) with an oxygen blender
Therapeutic gases:
Oxygen:
Monitoring
- Monitoring and titration are required to…
- Cyanosis
- Invasive approaches for monitoring oxygenation
- Noninvasive monitoring of arterial oxygen saturation
- Now widely available with…
- Useful for…
- Monitoring and titration are required to…
- Meet the therapeutic goal of oxygen therapy
- Avoid complications and side effects
- Cyanosis
- A physical finding of substantial clinical importance
- Not an early, sensitive, or reliable index of oxygenation
- Invasive approaches for monitoring oxygenation
- Intermittent laboratory analysis of arterial or mixed venous blood gases
- Placement of intravascular cannulae capable of continuous measurement of oxygen tension
- Noninvasive monitoring of arterial oxygen saturation
- Now widely available with…
- Pulse oximetry, in which oxygen saturation is measured from the differential absorption of light by oxyhemoglobin and deoxyhemoglobin and the arterial saturation determined from the pulsatile component of this signal
- Useful for…
- The rapid evaluation and monitoring of potentially compromised patients
- Titrating oxygen therapy in situations where toxicity from oxygen or side effects of excess oxygen are of concern
- Now widely available with…
Therapeutic gases:
Nitric oxide
- Nitric oxide (NO)
- Inhaled NO
- This improvement in ventilation-perfusion matching leads to…
- Nitric oxide (NO)
- Important cell signaling molecule
- Acts primarily through the activation of soluble guanylate cyclase (sGC) and generation of cGMP
- Inhaled NO
- Preferentially dilates the pulmonary vasculature
- Limits effects on the systemic vasculature
- Delivered to areas of the lung that are well-ventilated, leading to…
- Increased pulmonary vasodilation in those areas
- Increased perfusion to well-ventilated areas of the lung
- This improvement in ventilation-perfusion matching leads to…
- A significant improvement in oxygenation
Therapeutic gases:
Nitric oxide
- Therapeutically, NO is administered to newborns to treat…
- Administration of high levels of NO for extended periods of time can lead to…
- Therapeutically, NO is administered to newborns to treat…
- Persistent pulmonary hypertension
- Diseases associated with increased pulmonary vascular resistance
- Pulmonary hypertension
- Pulmonary embolism
- Acute chest syndrome in sickle-cell patients
- Administration of high levels of NO for extended periods of time can lead to…
- Toxicity
- This may be related to oxidation of NO to nitrogen dioxide (NO2) in the presence of high concentrations of oxygen
- The development of methemoglobinemia can be a significant complication of inhaled NO at higher concentrations, particularly in infants
Therapeutic gases:
Helium
- Helium
- A primary use of helium
- The low density of helium makes it useful to…
- Helium has high thermal conductivity, which makes it useful during…
- Helium has also been used as…
- Helium
- Inert gas with low density, low solubility, and high thermal conductivity
- A primary use of helium: pulmonary function testing
- Determinations of residual lung volume, functional residual capacity, and related lung volumes require a highly diffusible gas that is insoluble (and thus does not leave the lung via the bloodstream) so that, by dilution, the lung volume can be measured
- The low density of helium makes it useful to…
- Decrease work of breathing, particularly in patients with elevated airway resistance
- Helium has high thermal conductivity, which makes it useful during…
- Laser surgery on the airway, where it can prolong time to ignition of flammable materials (including oxygen)
- Helium has also been used as…
- An inhalational contrast agent for pulmonary magnetic resonance imaging
Therapeutic gases: Carbon monoxide (CO)
- CO production
- Both the toxic and potentially therapeutic effects of CO are a result of…
- CO and hemoglobin
- CO production
- As a result of the incomplete combustion of all carbon containing fuels or endogenously
- Humans generate ~ 10 ml of CO daily, as a by-product of conversion of heme to biliverdin by heme oxygenase (HO)
- Both the toxic and potentially therapeutic effects of CO are a result of…
- Its affinity for heme-containing compounds
- CO and hemoglobin
- CO reversibly binds to hemoglobin, to form carboxyhemoglobin (CO-Hb)
- The affinity of CO for the hemoglobin heme iron is 240 times that of O2
- The partial occupation of these binding sites by two CO molecules (half-saturation) inhibits the release of O2 from the remaining heme groups, leading to a leftward shift of the oxyhemoglobin dissociation curve
- This property of CO reduces the O2-carrying capacity of the blood to deliver O2, causing the asphyxiating properties of CO
Therapeutic gases: Carbon monoxide (CO)
- CO and other metalloproteins
- The physiological effects of CO have been related to…
- The exogenous application of non-toxic CO doses produces…
- The known physiological signaling effects of CO involve…
- Inhalation of CO has been demonstrated to be effective in animal models of…
- CO and other metalloproteins
- CO can also form complexes with reduced forms of other metalloproteins including myoglobin, sGC, inducible NO synthase, cytochrome P-450, NADPH oxidases and cytochrome C oxidase
- The physiological effects of CO have been related to…
- Its endogenous production from basal and inducible HO activity
- The exogenous application of non-toxic CO doses produces…
- Effects similar to those seen with up-regulation of HO-1.
- The known physiological signaling effects of CO involve…
- Relatively few defined mechanisms
- The modulation of sGC activity and subsequent stimulation of cGMP production is the most commonly observed
- Other mechanisms include the modulation of MAPK activation and the stimulation of Ca2+-dependent K+ channel activity
- Inhalation of CO has been demonstrated to be effective in animal models of…
- Inflammation, hypertension, organ transplantation, vascular injury, and ventilator-induced lung injury
Therapeutic gases:
Hydrogen sulfide
- Primarily known for its ability to…
- When delivered in a dose-controlled manner, H2S has been shown experimentally to…
- Its signaling actions are mediated through…
- Primarily known for its ability to…
- Limit mitochondrial respiration by inhibition of cytochrome C oxidase
- When delivered in a dose-controlled manner, H2S has been shown experimentally to…
- Limit some forms of cellular injury
- Its signaling actions are mediated through…
- Chelation of metallo-proteins
- Direct covalent modification of proteins
- Redox reactions such as reduction of disulfide bonds and reaction with free radical molecules
Pharmacology of general anesthetics:
Anesthesia
- Anesthesia
- An ideal inhaled anesthetic
- Anesthesia
- A generalized reversible depression of the central nervous system such that perception of all senses is ablated
- No single drug is available that satisfies all of the characteristics of an ideal anesthetic
- Typically, multiple drugs are used to achieve surgical anesthesia
- An ideal inhaled anesthetic
- Pleasant to inhale
- Potent
- Easy to administer and regulate
- Inexpensive
- Stable and safe
- Acts at specific sites in the CNS
- Devoid of side effects
- Provides postoperative pain relief
Pharmacology of general anesthetics:
History of anesthesia
- Ether
- Better inhaled anesthetic agents
- Newer agents
- Currently used agents
- Ether
- The ideal “first” anesthetic
- Easily synthesized and purified
- Relatively nontoxic to vital organs
- A liquid at room temperature
- Readily vaporizes
- Easy to administer
- Unlike less potent nitrous oxide (N2O), ether induces anesthesia without diluting the oxygen in room air to hypoxic levels
- Generally sustains respiration and circulation, important when assisted respiration and circulation was technically limiting
- Better inhaled anesthetic agents
- Chloroform
- Cyclopropane
- Other highly flammable and explosive agents
- Halothane, a nonflammable fluorinated hydrocarbon
- Newer agents
- Halogenated alkanes and ethers modeled after halothane
- Currently used agents
- Isoflurane
- Sevoflurane
- Desflurane

Pharmacology of general anesthetics:
Principles of anesthesia
- The components of the anesthetic state
- Unlike the practice of every other branch of medicine, induction of surgical anesthesia is usually…
- Administration of general anesthesia has been driven by three general objectives
- The components of the anesthetic state
- Amnesia
- Immobility in response to noxious stimulation
- Attenuation of autonomic responses to noxious stimulation
- Analgesia
- Unconsciousness
- Unlike the practice of every other branch of medicine, induction of surgical anesthesia is usually…
- Neither therapeutic nor diagnostic
- Administration of general anesthesia has been driven by three general objectives
- Minimizing the potentially deleterious direct and indirect effects of anesthetic agents and techniques
- Sustaining physiologic homeostasis during surgical procedures that may involve major blood loss, tissue ischemia, reperfusion of ischemic tissue, fluid shifts, exposure to a cold environment, and impaired coagulation
- Improving postoperative outcome
Pharmacology of general anesthetics:
Measures of anesthetic potency
- The potency of inhaled general anesthetics is usually defined as…
- MAC
- MAC values are useful because they…
- MAC values
- The potency of inhaled general anesthetics is usually defined as…
- The minimum alveolar concentration (MAC)
- MAC
- That prevents movement in 50% of patients in response to a surgical incision
- Represents the ED50 for inhaled anesthetics: the lower the MAC value, the more potent the anesthetic
- MAC values are useful because they…
- (a) allow easy comparison of different anesthetics
- (b) are easily measured by sampling end tidal gas concentration
- (c) represent important clinical endpoints
- MAC values
- Consistent and reproducible measures within a patient population
- Decrease with age and body temperature and during pregnancy
Pharmacology of general anesthetics:
Measures of anesthetic potency
- General anesthetics
- Therapeutic index
- Anesthetics…
- For general anesthetics, the difference in dose between no effect, surgical anesthesia, and severe cardiac/respiratory depression…
- General anesthetics
- Among the most dangerous drugs administered to patients
- Therapeutic index
- The LD50/MAC
- Anesthetics…
- have very steep dose-response relationships
- Have low therapeutic indices (2-4)
- No antagonists exist
- For general anesthetics, the difference in dose between no effect, surgical anesthesia, and severe cardiac/respiratory depression…
- Is small
Pharmacology of general anesthetics:
Mechanism of action of inhaled anesthetics
- Dominant theory: single site of action
- Lipid solubility hypothesis
- Dominant theory: single site of action
- i.e., all anesthetics exert all their effects via the same mechanism
- Based largely on the observations that…
- A wide variety of structurally diverse chemical compounds produce anesthesia
- Anesthetic potency of a gas correlated with its solubility in olive oil
- Stereoisomers of inhaled drugs generally have equal potencies
- No specific anesthetic antagonists have been identified
- All of these points argued for a single, nonspecific mechanism of action
- Lipid solubility hypothesis
- Meyer-Overton correlation
- Powerful correlation between anesthetic potency and lipid solubility
- Predicted that the site of action was the lipid bilayer of neurons
- Anesthesia resulted from the accumulation of anesthetic molecules in the lipid bilayer and thus perturbed neuronal function
- Meyer-Overton correlation

Pharmacology of general anesthetics:
Mechanism of action of inhaled anesthetics
- Enantiomers with identical physical properties
- An anesthetic agent produces…
- Different anesthetic agents produce…
- Enantiomers with identical physical properties
- Have unique molecular actions, indicating that properties other than bulk solubility are important in determining anesthetic action
- This has led to a search for identification of specific protein sites where anesthetics may bind, such as ligand-gated ion channels
- An anesthetic agent produces…
- Different components of the anesthetic state via actions at different anatomic loci in the nervous system and may produce these component effects via different cellular and molecular actions
- Different anesthetic agents produce…
- Specific components of anesthesia via actions at different molecular targets

Pharmacology of general anesthetics:
Mechanism of action of inhaled anesthetics
- General anesthetics act by…
- The Meyer-Overton correlation was initially interpreted as evidence that…
- General anesthetic potencies correlate equally well with…
- General anesthetics act by…
- Binding directly to proteins
- The Meyer-Overton correlation was initially interpreted as evidence that…
- Lipids of nerve membranes were the principal anesthetic target sites based on the correlation between anesthetic potency and lipid-water partition coefficient
- General anesthetic potencies correlate equally well with…
- Their ability to inhibit activity of the soluble firefly enzyme luciferase
- The crystal structure of luciferase is shown in the inset with bound anesthetic in red

Pharmacology of general anesthetics:
Anatomic sites of action
- Anesthetics exert their effects on pain and immobility (i.e., MAC) via…
- Structure that plays a role in amnestic effects
- Structure that plays a role in sedative/anxiolytic effects
- A common attribute of general anesthetics
- Major locus for transmission of peripheral sensation to the cortex
- Inhibition of this function may result in…
- Anesthetics exert their effects on pain and immobility (i.e., MAC) via…
- The spinal cord, with possible modulatory input from supraspinal sites
- Structure that plays a role in amnestic effects
- The hippocampus
- Structure that plays a role in sedative/anxiolytic effects
- The tuberomammillary nucleus of the hypothalamus
- A common attribute of general anesthetics
- Suppression of thalamic excitability
- Major locus for transmission of peripheral sensation to the cortex
- Thalamus
- Inhibition of thalamic function may result in…
- The transition from the awake to the anesthetized state
Pharmacology of general anesthetics:
Molecular targets
- Direct effect of anesthetics on…
- Chloride channels gated by the inhibitory GABAA receptor
- By increasing the sensitivity of the GABAA receptor to GABA, anesthetics…
- Point mutations at various sites on the GABAA receptor that abrogate the effects of specific anesthetic agents suggest that…
- The GABA binding site itself…
- Direct effect of anesthetics on…
- GABAA and NMDA receptors
- Two-pore K+ channels
- Chloride channels gated by the inhibitory GABAA receptor
- Responsive to a wide variety of anesthetics, including the halogenated inhalational agents
- By increasing the sensitivity of the GABAA receptor to GABA, anesthetics…
- Enhance inhibitory neurotransmission and depress nervous system activity
- Point mutations at various sites on the GABAA receptor that abrogate the effects of specific anesthetic agents suggest that…
- There may be specific binding sites for several anesthetic classes
- The GABA binding site itself…
- Does not appear to bind anesthetics
Pharmacology of general anesthetics:
Molecular targets
- Structurally closely related to the GABAA receptors
- Inhalational anesthetics…
- While anesthetic gases can also inhibit neuronal nicotinic acetylcholine receptors,…
- The NMDA receptor
- Structurally closely related to the GABAA receptors
- Other ligand-gated ion channels including glycine receptors
- Neuronal nicotinic acetylcholine receptors
- Inhalational anesthetics…
- Enhance the capacity of glycine to activate glycine-gated chloride channels, modulating responses to noxious stimuli in the spinal cord and brainstem
- While anesthetic gases can also inhibit neuronal nicotinic acetylcholine receptors,…
- The consequences of these effects are unclear, as this inhibition does not mediate immobilization
- The NMDA receptor
- A glutamate-gated cation channel
- Inhibited by nitrous oxide and xenon, as well as the intravenous anesthetic agent, ketamine
Pharmacology of general anesthetics:
Molecular targets
- Halogenated gases activate…
- Pre- vs. post-synaptic channel stimulation
- Other potential targets of inhaled anesthetics
- Receptors for excitatory vs. inhibitory neurotransmitters
- Halogenated gases activate…
- Two-pore domain channels, K+ channels that are present both pre-and post-synaptically
- Pre- vs. post-synaptic channel stimulation
- Stimulation of pre-synaptic channels results in hyperpolarization of the pre-synaptic terminal and a reduction in neurotransmitter release
- Post-synaptic channels regulate the resting membrane potential, so that activation may also result in hyperpolarization of these neurons as well
- Other potential targets of inhaled anesthetics
- Channels that are gated by acetylcholine, serotonin, glutamate, and others
- Receptors for excitatory vs. inhibitory neurotransmitters
- Receptors for excitatory neurotransmitters are inhibited by anesthetics
- Receptors for inhibitory transmitters are potentiated
Pharmacology of general anesthetics:
Pharmacokinetics of inhaled anesthetics
- Anesthetics distribute…
- At equilibrium, the partial pressure of a gas that is dissolved in a liquid or tissue, such as blood or brain, is equal to…
- However, the amount of gas that is dissolved in the liquid or tissue (concentration) depends on both…
- Thus, at equilibrium,…
- The partial pressures of a gas will…
- The concentrations will…
- Anesthetic partition coefficients
- Anesthetic gases tend to be more soluble in some tissues, such as…
- Anesthetics distribute…
- Between tissues (or between blood and gas) along partial pressure gradients
- At equilibrium, the partial pressure of a gas that is dissolved in a liquid or tissue, such as blood or brain, is equal to…
- The partial pressure of the free gas
- However, the amount of gas that is dissolved in the liquid or tissue (concentration) depends on both…
- The partial pressure and the solubility of the gas in that particular liquid or tissue
- Thus, at equilibrium,…
- The partial pressures of a gas will be the same in different compartments of the body
- The concentrations will not
- Anesthetic partition coefficients
- The ratio of anesthetic concentration in two tissues when the partial pressures of anesthetic are equal
- Anesthetic gases tend to be more soluble in some tissues, such as…
- Tat compared with blood
- However, there is a wide range of solubility between the anesthetics
Pharmacology of general anesthetics:
Pharmacokinetics of inhaled anesthetics
- Equilibrium is achieved when…
- At this point, there is…
- For inhalational agents that are poorly soluble in blood and tissues, such as nitrous oxide, equilibrium…
- If an anesthetic gas is more soluble, equilibrium…
- The rise in end-tidal alveolar (FA) anesthetic concentration toward the inspired (FI) concentration
- Most rapid with…
- Slower with…
- Equilibrium is achieved when…
- The partial pressure in end-tidal (alveolar) gas (FA) is equal to the partial pressure in inspired gas (FI) i.e., when FA/FI is 1
- At this point, there is…
- No net movement of anesthetic from the alveoli into the circulation
- For inhalational agents that are poorly soluble in blood and tissues, such as nitrous oxide, equilibrium…
- Is achieved quickly
- If an anesthetic gas is more soluble, equilibrium…
- May take much longer, due to the large reservoir of body fat
- The rise in end-tidal alveolar (FA) anesthetic concentration toward the inspired (FI) concentration
- Most rapid with the least soluble anesthetics, nitrous oxide and desflurane
- Slower with more soluble anesthetics, such as halothane

Pharmacology of general anesthetics:
Pharmacokinetics of inhaled anesthetics
- An important parameter of anesthetic pharmacokinetics
- The anesthetic state occurs when…
- Because the brain is well-perfused, the anesthetic partial pressure in brain…
- Thus, anesthesia is achieved soon after…
- Highly soluble anesthetics such as halothane
- The rate of increase of FA will be…
- The speed of induction can be increased by…
- What influences the FA/FI ratio
- The magnitude of this influence is usually governed by…
- An important parameter of anesthetic pharmacokinetics
- The speed of anesthetic induction
- The anesthetic state occurs when…
- The partial pressure of the anesthetic in brain is equal to or greater than MAC
- Because the brain is well-perfused, the anesthetic partial pressure in brain…
- Equalizes with the partial pressure in alveolar gas quickly
- Thus, anesthesia is achieved soon after…
- Alveolar partial pressure (FA) reaches MAC
- Highly soluble anesthetics such as halothane
- The rate of increase of FA will be slower
- The speed of induction can be increased by delivering higher inspired partial pressures; this is called the concentration effect
- What influences the FA/FI ratio
- Changes in physiologic variables (e.g., ventilation, circulation, distribution of circulation, ventilation-perfusion abnormalities)
- The magnitude of this influence is usually governed by…
- Solubility

Pharmacology of general anesthetics:
Effects on circulation
- All inhalational anesthetics…
- However, cardiac output is maintained with…
- This suggests that…
- All inhalational anesthetics…
- Reduce systemic blood pressure in a dose-related manner
- However, cardiac output is maintained with…
- Isoflurane and desflurane
- This suggests that…
- The mechanisms for diminished blood pressure vary with the agent

Pharmacology of general anesthetics:
Effects on respiration
- All halogenated inhalational anesthetics…
- In patients whose ventilation is not controlled, this results in…
- All halogenated inhalational anesthetics…
- Reduce spontaneous minute ventilation in a dosedependent manner
- In patients whose ventilation is not controlled, this results in…
- An increase in arterial CO2 tension

Pharmacology of general anesthetics:
Xenon
- Rare gas that can only be obtained by extracting it from air
- Thus, only very limited quantities of xenon are available and the cost is high
- If costs can be reduced in the future, it is probable that xenon will become a widely used inhaled anesthetic
- Xenon is very close to being an ideal anesthetic
- Has very low blood and tissue solubility (results in rapid induction and recovery)
- Is potent
- Is not metabolized in the body
- Is nonflammable
- Has minimal side effects