L5 - General Anaesthetics Flashcards
How do local anaesthetics differ from general anaesthetics?
Local anaesthetics are injected into a local area and block pain sensation, whereas general anaesthetics cause a state of controlled unconsciousness.
What is the effect of local anaesthetics?
Local anaesthetics block the sensation of pain in a specific area, allowing the person to stay conscious but feel numb.
What are some common uses of local anaesthetics?
Local anaesthetics are commonly used for procedures such as:
Dental surgery
Epidurals
Caesarean sections
What is the purpose of regional anaesthesia?
Regional anaesthesia uses general anaesthetics to induce a state of controlled unconsciousness, with the person unable to be woken up until the drug effect wears off.
What is general anaesthesia?
General anaesthesia induces a controlled state of unconsciousness, impairing awareness and memory of the surrounding environment and surgical events. It can be administered via IV or anaesthetic gas.
What is sedation in the context of anaesthesia?
Sedation uses lower doses of general anaesthetic to produce a relaxed, sleepy, and forgetful state during an operation, but the person does not lose consciousness.
How is sedation different from general anaesthesia?
Sedation uses a lower dose of general anaesthetic, keeping the person relaxed and sleepy, while general anaesthesia induces a deeper, controlled unconsciousness.
What is the purpose of induction in anaesthesia?
Induction is the process of getting the patient to sleep, typically involving administration of general anaesthetic, neuromuscular blockers to relax muscles, and extra oxygen for pre-oxygenation.
Why are neuromuscular blockers used during anaesthesia induction?
Neuromuscular blockers relax muscles and aid intubation by ensuring muscle paralysis, making the procedure easier and more controlled.
What is the role of analgesics in anaesthesia?
Analgesics, such as opioids, are used during anaesthesia to reduce signals from stimuli, providing pain relief during surgery.
What happens during the maintenance phase of anaesthesia?
During maintenance, the general anaesthetic is continued throughout the operation, and neuromuscular blockers may be topped up as needed. Analgesics are also administered as required.
What is the purpose of neuromuscular monitoring during maintenance?
Neuromuscular monitoring ensures the correct dose of neuromuscular blockers is maintained, guiding top-up doses as necessary.
What occurs during the emergence phase of anaesthesia?
During emergence, the neuromuscular blockade is reversed, and the patient is given 100% oxygen. The endotracheal tube (ETT) or laryngeal mask airway (LMA) is removed once the patient starts spontaneously breathing and waking up.
What is balanced anaesthesia?
Balanced anaesthesia involves the simultaneous administration of multiple drugs to achieve an optimal anaesthetic state, ensuring the patient is asleep, relaxed, and pain-free during surgery.
What are the key phases of basic anaesthetic technique?
The key phases are:
Induction: Getting the patient to sleep.
Maintenance: Keeping the patient asleep during the procedure.
Emergence: Waking the patient up after the procedure.
How many main stages are there in anaesthesia?
There are four main stages of anaesthesia.
What is the goal of the anaesthetist regarding the stages of anaesthesia?
The anaesthetist aims to move the patient from Stage I to Stage III as quickly as possible.
What is the issue with Stage II of anaesthesia?
Stage II (excitement stage) can cause undesirable features such as involuntary movement and vomiting.
Why is Stage IV of anaesthesia avoided?
Stage IV is avoided because it can result in patient death.
How does recovery from anaesthesia occur?
Recovery from anaesthesia occurs in reverse order, meaning the patient moves backward through the stages.
What are the two main types of anaesthesia?
The two main types of anaesthesia are Intravenous (IV) and Inhalation anaesthesia.
What is the main use of intravenous (IV) anaesthesia?
IV anaesthesia is mainly used for induction, starting the process of making the patient unconscious.
What is the main advantage of IV anaesthesia in terms of stages?
IV anaesthesia allows for a quicker onset of action and helps move the patient through Stage II (excitement) more unpredictably and faster.
What are some examples of IV anaesthetics?
Some examples of IV anaesthetics include Propofol, Ketamine, and Etomidate.
What is the primary use of inhalation anaesthesia?
Inhalation anaesthesia is usually used for maintenance during the operation.
What is the advantage of inhalation anaesthesia over IV anaesthesia?
Inhalation anaesthesia allows for easier control of the depth of anaesthesia over time and ensures a steady-state concentration.
What are some examples of inhalation anaesthetics?
Some examples of inhalation anaesthetics include Isoflurane, Desflurane, Sevoflurane, and Nitrous Oxide.
What did the Meyer-Overton observation suggest about the mechanism of action of general anaesthetics?
The Meyer-Overton observation suggested that general anaesthetics act non-specifically on the lipid component of the neuronal cell wall, altering the lipid environment of proteins and changing their function. However, this theory is not supported by research.
What is the unitary hypothesis in relation to general anaesthetics?
The unitary hypothesis proposed that although general anaesthetics are chemically diverse, they all work in the same way to reduce consciousness, implying a single mechanism of action, though the exact mechanism was unknown.
What was the finding of Franks and Lieb in 1984 regarding the mechanism of action of anaesthetics?
In 1984, Franks and Lieb demonstrated that most general anaesthetics act directly with proteins, specifically interacting with receptors or ion channels, rather than with lipid membranes.
What are some current hypotheses about how anaesthetics work?
The current hypotheses suggest that anaesthetics likely target specific receptors or ion channels and may work through mechanisms such as:
Decreasing excitation
Activating inhibition
Reducing overall brain activity
Altering neuronal networks and connections
What is the role of brain activity in the mechanism of anaesthesia?
The current hypothesis suggests that general anaesthetics work by reducing brain activity, specifically by decreasing excitation and activating inhibition within the neuronal network.
What are the most likely targets of general anaesthetics according to the current hypothesis?
The most likely targets of general anaesthetics include:
Voltage-gated potassium & sodium channels
NMDA receptors
Glycine receptors
GABAA receptors
What does the integrated neuronal network model suggest about the mechanism of anaesthetics?
The integrated neuronal network model suggests that new research focuses on neural networks involved in arousal and sleep, rather than individual receptors. Key brain regions involved include:
Ventrolateral preoptic area of the hypothalamus
Tuberomammillary nucleus
Ascending reticular activating system
(Note: These brain networks don’t need to be memorised in detail.)
How do general anaesthetics work to achieve their effects?
General anaesthetics work by:
Increasing inhibition (e.g., by activating the GABAA receptor) with drugs like propofol and isoflurane
Decreasing excitation (e.g., by blocking the NMDA receptor) with drugs like ketamine and nitrous oxide
What is the goal of administering general anaesthesia during surgery?
The goal is to administer enough anaesthetic to reach Stage 3 of anaesthesia, where the patient is unconscious, but not to reach Stage 4, which could result in death.
What does potency mean in the context of anaesthetics?
Potency refers to the concentration of a drug required to achieve a given pharmacological effect. For anaesthetics, potency is related to the minimal alveolar concentration (MAC).
What is the relationship between dose, response, and potency in general anaesthesia?
Increased dose leads to an increased response.
To achieve a 50% response (EC50), drug A may require a lower concentration than drug B.
The MAC (minimal alveolar concentration) is used to define potency: a lower MAC corresponds to a higher potency
What is the MAC in the context of general anaesthetics?
MAC is the minimal alveolar concentration required to abolish a response to a painful stimulus (e.g., surgical incision) in 50% of subjects. MAC is inversely related to the potency of the anaesthetic.
How do you determine how much general anaesthetic (GA) is required for surgical anaesthesia?
To determine how much GA is required for surgical anaesthesia (Stage 3), you need to know the potency of the anaesthetic, which is often expressed as the Minimum Alveolar Concentration (MAC).
What is potency in the context of general anaesthetics?
Potency refers to the concentration of a drug required to produce a pharmacological effect of a given intensity (e.g., EC50).
What is the Minimum Alveolar Concentration (MAC)?
MAC is the dose of anaesthetic required to abolish the response to a painful stimulus (e.g., surgical incision) in 50% of subjects.
What is the relationship between MAC and potency in anaesthetics?
MAC is inversely related to potency. This means that a high MAC indicates a low potency, while a low MAC indicates a high potency of the anaesthetic.
What is the MAC of Halothane, and what does it indicate about its potency?
Halothane has a MAC of 0.8, which makes it the most potent among the examples listed.
What is the MAC of Isoflurane, and what does it indicate about its potency?
Isoflurane has a MAC of 1.2, indicating it is moderately potent.
What is the MAC of Sevoflurane, and what does it indicate about its potency?
Sevoflurane has a MAC of 2.1, indicating it is less potent than Halothane and Isoflurane.
What is the MAC of Nitrous oxide, and what does it indicate about its potency?
Nitrous oxide has a MAC > 100, indicating it is the least potent among the listed anaesthetics.
What MAC is typically used by anaesthetists during surgery, and why?
: Anaesthetists generally use approximately 1.3 MAC to ensure that more than 50% of patients do not respond to a surgical incision. This value is patient-dependent.
How does age affect the MAC of a general anaesthetic?
Age generally decreases MAC, meaning older patients typically require less anaesthetic to reach the same level of anaesthesia
How does an increase in body temperature affect the MAC of a general anaesthetic?
Increased body temperature increases MAC, meaning higher temperatures require more anaesthetic to achieve the same effect.
How do other general anaesthetics affect the MAC value?
When multiple anaesthetics are used, their MAC values can be added together (e.g., ½ MAC + ½ MAC = 1 MAC), meaning the overall dose is the sum of individual MACs.
How do CNS stimulants affect the MAC of a general anaesthetic?
CNS stimulants increase the MAC, meaning more anaesthetic is required to achieve the desired effect.
How do CNS depressants affect the MAC of a general anaesthetic?
: CNS depressants decrease the MAC, meaning less anaesthetic is needed to achieve the same effect.
How are gaseous general anaesthetics commonly defined?
Gaseous general anaesthetics are commonly defined in terms of percentage values or partial pressures.
What is partial pressure
Partial pressure is the pressure an individual gas exerts in a mixture of gases. It represents the concentration of that specific gas in the mixture.
How can the amount of each component in a gaseous anaesthetic be described?
The amount of each component in a gaseous anaesthetic can be described as either a concentration or a partial pressure.
In clinical practice, how are inhalation agents often quoted?
In clinical practice, inhalation agents are frequently quoted as a percentage of total gas flow since vaporizers are calibrated to deliver anaesthetics in this way.
When discussing gaseous anaesthetics in the body, what do we typically refer to?
When discussing gaseous anaesthetics in the body, we typically refer to their partial pressure.
How are gaseous anaesthetics absorbed?
Gaseous anaesthetics are inhaled into the lungs, then must diffuse into the bloodstream via the alveolus and are delivered directly to the venous bloodstream.
How are gaseous anaesthetics distributed in the body?
Gaseous anaesthetics are distributed to the brain via the bloodstream, as well as to other tissues/organs such as adipose tissue and the heart.
How are gaseous anaesthetics metabolized?
Less than 5% of gaseous agents are metabolized in the liver. For example, propofol is rapidly metabolized in the liver by conjugation to glucuronide and sulphate, producing water-soluble compounds.
How are gaseous anaesthetics eliminated from the body?
The majority of gaseous anaesthetics are eliminated via the lungs, while most drugs are excreted by the kidneys.
How are gaseous anaesthetics administered?
Inhalational anaesthetics include gases and volatile liquids, administered using vaporisers with a mix of oxygen or nitrous oxide-oxygen mixtures as the carrier gas. They always contain >25% oxygen to avoid hypoxia.
How are intravenous general anaesthetics (IV GAs) administered?
IV general anaesthetics (GAs) are injected directly into the venous bloodstream before being distributed to the brain.
What does the pathway of an inhalational anesthetic drug involve during induction and emergence?
The pathway of an inhalational anesthetic drug is indicated by red arrows during induction and blue arrows during emergence. The large arrows indicate the direction of net movement.
What is speed of induction (onset) in gaseous anaesthetics?
Speed of induction (or onset) is the time taken from the first inhalation of the general anaesthetic (GA) to the depression of the CNS and the onset of general anaesthesia.
What is the recovery rate in gaseous anaesthetics?
Recovery rate is the time it takes for a patient to regain consciousness after the cessation of the general anaesthetic.
How does a gaseous anaesthetic achieve CNS depression?
To achieve CNS depression and general anaesthesia, the correct partial pressure of the GA must exist in the brain. The higher the partial pressure, the deeper the anaesthesia.
What is the process by which gaseous anaesthetics reach the brain?
The GA molecules move down their concentration gradient from the alveolus to the blood, and from the blood to the brain. Diffusion continues at each junction until equilibrium is reached.
What is equilibrium in the context of gaseous anaesthetics?
Once equilibrium is reached, the partial pressure of the GA in the alveolar air will be the same as the partial pressure in the brain.
How is the end tidal volume related to gaseous anaesthesia?
Since the drug in the brain cannot be directly measured, the end tidal volume (the partial pressure of GA in the alveolar air) is used as a proxy to determine the anaesthetic’s concentration in the brain.
What determines the speed of induction in gaseous anaesthetics?
The speed of induction is determined by the speed at which equilibrium is reached between each body compartment: alveolus, blood, and brain. The quicker the equilibrium is reached, the faster the speed of induction.
How does drug uptake occur in gaseous anaesthetics?
Drug uptake occurs in two main stages:
From the alveolus to the blood (partial pressure in alveolus to partial pressure in blood).
From the blood to the brain (partial pressure in blood to partial pressure in brain).
What are the three key partial pressures involved in drug uptake for gaseous anaesthetics?
The three key partial pressures are:
Partial pressure in the alveolus (PA)
Partial pressure in the blood (Pa)
Partial pressure in the brain (Pb)
What does the speed of induction depend on in gaseous anaesthesia?
The speed of induction depends on the speed at which equilibrium is reached between the partial pressures in the alveolus, blood, and brain.
hat analogy can be used to explain how gases move through the body during anaesthesia?
The tower of wine glasses analogy is used:
Wine represents the anaesthetic gas.
Top glass represents the alveolus,
Middle glasses represent the blood,
Bottom glass represents the brain.
The wine fills each glass in order, and once equilibrium is reached, the amount of wine in each glass is equal.
What does the analogy of wine glasses represent in gaseous anaesthesia?
In the analogy, the wine represents the anaesthetic gas, the glasses represent different compartments in the body (alveolus, blood, brain), and the process of wine filling each glass represents the equilibrium of gas partial pressures across the compartments.
How does the wine glasses analogy explain the concept of equilibrium in gaseous anaesthesia?
Just like wine filling the glasses in stages, the anaesthetic gas fills the compartments (alveolus, blood, brain) in order. Once equilibrium is reached, the partial pressure of the gas is the same in all compartments, just like the amount of wine is the same in each glass.
What is the relationship between the alveolus, blood, and brain in the analogy?
The alveolus is the first compartment where the gas enters (top glass), followed by the blood (middle glasses), and finally the brain (bottom glass). The gas must diffuse from one compartment to the next until equilibrium is reached across all compartments.
How does the red wine analogy explain the process of anaesthesia induction?
In the analogy, red wine represents the gaseous anaesthetic. The process of induction occurs in stages:
Alveolus fills first (reaching equilibrium with the inhaled gas).
Blood reaches equilibrium with the alveolus.
Finally, the brain reaches equilibrium with the blood. Once equilibrium is reached, anaesthesia is achieved when the partial pressure of the GA in the brain equals that in the alveolus.
What does the “speed of induction” refer to in the context of gaseous anaesthesia?
Speed of induction refers to the time it takes for the gaseous anaesthetic to reach the brain. The quicker equilibrium is reached between the alveolus, blood, and brain, the faster anaesthesia is induced.
How is the speed of induction related to the equilibrium process in gaseous anaesthesia?
The speed of induction is determined by how quickly equilibrium is reached between the different compartments: alveolus, blood, and brain. The faster each compartment reaches equilibrium, the quicker the anaesthetic effect is achieved.
What happens when equilibrium is reached in gaseous anaesthesia?
When equilibrium is reached between the alveolus, blood, and brain, the partial pressure of the anaesthetic gas is equal in all compartments, leading to CNS depression and the induction of anaesthesia.
What role does the alveolus play in the induction of anaesthesia?
The alveolus is where the gaseous anaesthetic is first introduced, and it must reach equilibrium with the inhaled air before the anaesthetic moves to the blood and brain. This process sets the pace for the induction of anaesthesia.
What factors determine the speed of induction and recovery in gaseous anaesthesia?
The main factors include:
Properties of the anaesthetic:
Solubility in blood (blood:gas partition coefficient)
Solubility in fat (oil:gas partition coefficient)
Physiological factors:
Alveolar ventilation rate
Cardiac output
The blood:gas partition coefficient is the most important factor influencing speed of induction.
What is the blood:gas partition coefficient and why is it important?
The blood:gas partition coefficient reflects the solubility of the anaesthetic in blood.
A low blood:gas partition coefficient indicates faster induction (anaesthetic moves from the lungs to the blood more easily).
A high blood:gas partition coefficient means slower induction (the anaesthetic is more soluble in blood, requiring more time to reach the brain)
How does the oil:gas partition coefficient affect the speed of anaesthesia
The oil:gas partition coefficient measures the solubility of the anaesthetic in fat, which affects its potency and duration of action. A higher value indicates a more potent anaesthetic, which may lead to a longer recovery time due to its accumulation in fat tissues.
What physiological factors influence the speed of induction and recovery in gaseous anaesthesia?
Key physiological factors include:
Alveolar ventilation rate (how fast air is exchanged in the lungs)
Cardiac output (how much blood is pumped through the body, affecting drug distribution).
Why is the blood:gas partition coefficient considered the most important factor in speed of induction?
The blood:gas partition coefficient is the most significant because it directly affects how quickly the anaesthetic can move from the alveolus into the bloodstream and then to the brain, thus determining the speed of induction.
What is the blood-gas partition coefficient?
The blood-gas partition coefficient describes the relative affinity of a drug for two phases: blood and gas (air in the alveolus) when the drug’s partial pressures are in equilibrium. It reflects the drug’s solubility in blood compared to gas.
How does the blood-gas partition coefficient affect the speed of induction in anaesthesia?
A lower blood-gas partition coefficient means the anaesthetic is less soluble in blood, allowing it to move more quickly from the lungs to the blood, resulting in faster induction. Conversely, a higher blood-gas partition coefficient indicates the anaesthetic is more soluble in blood, leading to slower induction.
How do the blood-gas partition coefficients of Nitrous Oxide and Isoflurane compare?
Nitrous Oxide has a blood-gas partition coefficient of 0.5, meaning it is less soluble in blood and leads to faster induction.
Isoflurane has a blood-gas partition coefficient of 1.4, meaning it is more soluble in blood, leading to slower induction compared to Nitrous Oxide.
What does a blood-gas partition coefficient of 0.5 and 1.4 indicate about Nitrous Oxide and Isoflurane?
Nitrous Oxide (0.5) is less soluble in blood and reaches equilibrium faster, resulting in quicker induction.
Isoflurane (1.4) is more soluble in blood and reaches equilibrium more slowly, leading to a slower speed of induction.
Why does a more soluble drug in blood result in a slower speed of induction?
A more soluble drug takes longer to reach equilibrium between the blood and the alveolus, as more of the drug dissolves in the blood. Anaesthetic transfer to the brain begins only after the blood has fully saturated. Therefore, the more soluble the drug, the longer it takes for the drug to reach the brain and induce anaesthesia.
How can the wine glass analogy help explain the speed of induction for a highly soluble drug?
In the analogy, the “blood” layer represents the part of the body where the drug is dissolving. Making the “wine glasses” (blood compartments) larger means it takes longer for them to fill with the drug, just as a highly soluble drug takes longer to reach equilibrium between the blood and the alveolus. This results in slower induction as the drug takes longer to saturate the blood before reaching the brain.
How does the solubility of a drug in blood affect recovery time?
Since a more soluble drug takes longer to leave the blood and return to the lungs, it also leads to slower recovery. This is because the drug needs to be exhaled from the lungs after being released from the blood, and this process takes longer for more soluble drugs.
How does solubility in blood relate to the speed of induction and recovery?
Higher solubility in blood: Slower induction (longer time to reach equilibrium in blood and brain) and slower recovery (longer time to leave the blood and be exhaled).
Lower solubility in blood: Faster induction (faster equilibrium between blood and brain) and faster recovery (faster elimination from blood and exhalation).
How does redistribution contribute to the recovery of a general anaesthetic (GA)?
Recovery occurs through redistribution, where the GA moves from the brain and other organs to either the liver for metabolism and excretion by the kidneys or to the lungs to be exhaled.
How does the blood:gas partition coefficient affect the recovery rate of gaseous anaesthetics?
Gaseous anaesthetics with a low blood:gas partition coefficient (low solubility) result in faster recovery, as they reach equilibrium quicker and are exhaled faster.
Why do highly lipid-soluble GAs (both inhalation and IV) have a slower recovery rate?
Highly lipid-soluble GAs are stored in fat tissue, which has a slower equilibration with the blood and brain, thus delaying their elimination from the body and extending recovery time.
How can the distribution of intravenous general anaesthetics (IV GA) affect recovery?
The recovery rate of IV GAs is influenced by their distribution in the body:
Rapidly equilibrating tissues (e.g., brain, blood) reach equilibrium quickly, while
Slowly equilibrating tissues (e.g., muscle, fat) store the drug longer, leading to slower recovery.
What can patients experience post-anaesthesia, even after recovery?
Cognitive function and judgment can be impaired for up to 24 hours after anaesthesia, potentially affecting decision-making and motor skills.
How do different tissues (blood, brain, muscle, fat) play a role in the recovery process from anaesthesia?
Blood & Brain: Rapidly equilibrating tissues, so they achieve equilibrium quickly and lead to faster recovery.
Muscle & Fat: Slow to equilibrate, causing the drug to be stored longer and recovery to take more time.
What is the CNS side effect of general anaesthetics?
General anaesthetics cause dose-dependent depression of the CNS, leading to reduced neural activity. Additionally, vasodilation of cerebral vasculature increases cerebral blood flow, which can raise intracranial pressure.
What is the effect of general anaesthetics on the cardiovascular system?
General anaesthetics cause dose-dependent depression of the cardiovascular system, affecting both the heart and blood vessels. They also sensitise the myocardium to catecholamine-induced arrhythmias, increasing the risk of abnormal heart rhythms.
What is a common gastrointestinal side effect of general anaesthetics?
Postoperative Nausea and Vomiting (PONV) is a common side effect, often linked to the type of operation, the drugs used, or motion sickness. Women and children are at an increased risk of PONV.
What is the respiratory side effect of general anaesthetics?
General anaesthetics cause dose-dependent depression of ventilation, which leads to a decrease in tidal volume and respiratory rate, resulting in an increase in CO2 levels. If the dose is too high, it can lead to respiratory arrest.
What are the hepatic and renal side effects of general anaesthetics?
General anaesthetics can cause mild hepatic dysfunction, reducing renal flow and glomerular filtration rate (GFR), which may affect kidney function.
What skeletal muscle side effect can be triggered by halogenated volatile anaesthetics?
Malignant hyperthermia can be triggered by halogenated volatile anaesthetics, which is a genetically conferred mutation in skeletal muscle. It can be life-threatening and requires immediate intervention. This side effect is also linked to neuromuscular blockers (NMBs), which will be discussed in more detail in the NMB lecture.
