Day 3: General Anaesthetic Agents: Inhalation Flashcards
synonyms of inhalational agents
anaesthetic agents
anaesthetic gases
volatiles
What were some of the first volatile agents used in anaesthesia?
Ether and chloroform were among the first volatile agents used in anaesthesia.
When was halothane introduced, and what was significant about it?
Halothane was introduced in 1956 as the first modern non-flammable hydrocarbon, marking a significant advancement.
What characterizes modern volatile agents besides nitrous oxide?
Modern volatile agents, besides nitrous oxide, are characterized by being halogenated hydrocarbons, with halogen atoms (F, Cl, Br) attached to a carbon skeleton.
How are volatile agents delivered to patients?
Volatile agents are liquid at room temperature and require a special vaporiser for delivery into the patient’s inspired gas mixture.
How did anaesthetic vaporisers evolve by the 1980s?
By the 1980s, anaesthetic vaporisers had evolved considerably with several modifications and safety features, enhancing their efficiency and safety in anaesthesia practice.
agent specific vaporizers
colour coded and specific for each particular volatile
anaesthetic back bar
interlocks so only one vaporiser can be switched on at a time
What factor primarily determines the speed of induction (loss of consciousness) with anaesthetic agents?
The concentration of the anaesthetic agent in the brain primarily determines the speed of induction.
What are the three main factors affecting the concentration of volatile agents in the brain?
The factors affecting the concentration of volatile agents in the brain are:
1. Delivery of the agent to the lungs
2. Uptake of the agent from the lungs to the bloodstream
3. Uptake of the agent into other tissues besides the brain.
What factor governs the partial pressures of anaesthetic in all body tissues?
The alveolar partial pressure.
How is the alveolar concentration of an anaesthetic agent measured?
The alveolar concentration of an anaesthetic agent is proportional to the alveolar partial pressure, measured in percentage, similar to the percentage of oxygen in air.
How do factors affecting alveolar concentration influence the induction of anesthesia?
Factors that decrease alveolar concentration slow down the induction of anesthesia, while factors that increase alveolar concentration expedite the induction process.
How does inspired concentration of the agent influence the induction of anesthesia?
Inspired concentration of the agent: Higher concentration leads to faster induction.
What effect does alveolar ventilation have on the speed of induction?
Alveolar ventilation: Increased ventilation speeds up induction.
How does reduced ventilation impact the induction of anesthesia?
Reduced ventilation slows down induction, such as in cases of respiratory depression or airway obstruction
delivery to the lungs
- inspired concentration of agent
-alveolar ventilation
uptake from lungs
solubility in blood
cardiac output
How is the uptake of volatile agents into the bloodstream calculated?
Uptake = Solubility in blood x Cardiac Output x (Alveolar Partial Pressure - Venous Partial Pressure).
Do all volatile agents dissolve easily in blood?
No, not all volatile agents dissolve easily in blood.
How does the solubility of an agent in blood affect the speed of induction?
Agents that dissolve easily in blood result in slower induction, while those that are insoluble induce anesthesia faster.
Why is the rate of induction slower if the agent is more soluble in blood?
The rate of induction is slower if the agent is more soluble in blood because the circulation constantly carries the agent away, preventing the alveolar concentration from building up quickly. As a result, the concentration in the brain rises slowly too.
What happens to the agent that has dissolved in blood?
The agent that has dissolved in blood is “hidden” from the brain.
Why must an anaesthetic agent be fat soluble to be effective?
An anaesthetic agent must be fat soluble to be effective because it needs to bind to the fatty tissue in the brain.
Why does a faster cardiac output result in a slower rate of induction?
A faster cardiac output results in a slower rate of induction because blood needs enough time to travel through the capillary/alveolar interface to pick up enough anaesthetic agent. If the cardiac output is higher, the blood will not be as saturated with the agent.
How does shunting affect induction?
Shunting, which refers to blood passing through alveoli that are not ventilated, adversely affects induction by reducing the amount of agent available to be absorbed into the bloodstream
uptake by the tissues
- tissue solubility
- tissue blood flow
- concentration gradient between blood and tissues
Which organs experience the greatest uptake initially, and why?
The greatest uptake occurs in vessel-rich organs such as the heart, brain, lungs, kidneys, and liver. This is because these organs receive the largest portion of cardiac output and reach equilibrium with the anaesthetic agent more quickly.
Second phase – uptake by the muscle group
equilibrium is achieved within 1-3 hours
Last phase – uptake by the poorly vascularised tissues (fat, bone)
equilibrium takes many hours
FASTER INDUCTION
High inspired concentration of agent
Increased ventilation
Less soluble agent (in blood)
Decreased cardiac output
Decreased shunting
slower induction
Lower inspired concentration of agent
Decreased ventilation
More soluble agent (in blood)
Increased cardiac output
Increased shunting
What factors influence the speed of recovery from anaesthesia?
The same factors that affect “going to sleep” influence “waking up” in reverse order.
Which types of anaesthetic agents lead to slow recovery?
Recovery is slow from soluble agents like halothane.
Which types of anaesthetic agents lead to fast recovery?
Recovery is fast from poorly soluble agents like desflurane and sevoflurane.
How are volatile anaesthetic agents removed from the body at the end of anesthesia?
The major route of removal of volatile anaesthetic agents at the end of anesthesia is via the lungs through alveolar ventilation.
What percentage of volatile agents is metabolized in the liver?
A varying percentage of volatile agents is metabolized in the liver.
Can you provide the “Rules of 2’s” for the metabolism of different volatile agents?
-“Rules of 2’s” for metabolism:
Halothane: 20%
Enflurane: 2%
Isoflurane: 0.2%
Sevoflurane: 3-4%
Desflurane: 0.02%
What harmful effects are associated with certain metabolites of volatile anaesthetic agents?
The CF3 group in halothane can cause “Halothane Hepatitis.”
Fluorides from older agents like methoxyflurane and enflurane can lead to renal impairment.
“Compound A” from sevoflurane has been associated with renal impairment in rats.
stage 1 of anaesthesia
Analgesia: From induction to LOC
stage 3 of anaesthesia
Surgical anaesthesia
light: until eyeballs become fixed
medium: increasing intercoastal paralysis
deep: diaphragmatic respiration
stage 2 of anaesthesia
Excitement: LOC to automatic breathing; characterised by excitement, breath-holding, vomiting, coughing, swallowing, hiccoughing
stage 4 of anaesthesia
Overdose: From diaphragmatic paralysis to apnoea and death. All reflex activity is lost and pupils are widely dilated. Medullary paralysis
What is potency, and how is it measured for volatile anaesthetic agents?
Potency refers to a drug’s efficacy at a given dose. A more potent agent requires a lower dose to achieve the same effect, while a less potent drug requires more.
What does MAC stand for, and what does it measure?
MAC stands for Minimum Alveolar Concentration. It measures the minimum alveolar concentration of an anaesthetic required to prevent 50% of patients from moving in response to a standard surgical stimulus (such as incision) at sea level
How does MAC relate to the potency of volatile anaesthetic agents?
A low MAC indicates higher potency (e.g., Halothane with a MAC of 0.75%), while a high MAC indicates lower potency (e.g., desflurane with a MAC of 6%).
MAC is decreased by
Sedatives
N2O
Analgesics
Increased age
Hypotension
Hypothermia
Myxoedema
Less effect:
Hypoxia, anaemia, pregnancy
MAC is increased by
Alcoholism
Children
Hyperthermia
Thyrotoxicosis
What does MAC stand for, and what is the MAC of nitrous oxide?
MAC stands for Minimum Alveolar Concentration. The MAC of nitrous oxide is 105%.
What is another name for nitrous oxide, and what is its common mixture called?
Nitrous oxide is also known as laughing gas or Entonox. Its common mixture is 50% oxygen and 50% nitrous oxide.
What are the main properties of nitrous oxide in terms of anaesthesia?
Nitrous oxide is a potent analgesic but has poor anaesthetic properties. It also reduces the MAC of other agents when used in combination.
How does nitrous oxide contribute to induction when used as a carrier gas, especially in pediatrics?
When used as a carrier gas, induction with nitrous oxide is pleasant and rapid, particularly in pediatric patients.
adverse effects of nitrous oxide
-Negatively inotropic
-Augments respiratory depression of other anaesthetic drugs
-Diffuses into air-filled cavities of body (more soluble than N2) – increase volume and pressure
**Bowel, middle ear, eye, lungs (tension pneumothorax)
-PONV
-Bone Marrow Suppression
-Hypoxic mixture possible
What is diffusion hypoxia, and when can it occur in anesthesia?
Diffusion hypoxia is a condition where the partial pressure of oxygen in the alveoli drops due to the rapid elimination of nitrous oxide (N2O) at the end of anesthesia. It can occur when finishing an anesthetic with high concentrations of N2O.
Why does diffusion hypoxia occur specifically with nitrous oxide?
Nitrous oxide enters the alveoli from the blood faster than nitrogen (N2) enters the blood from the alveoli. This rapid elimination of N2O reduces the partial pressure of oxygen in the alveoli, leading to diffusion hypoxia.
Is diffusion hypoxia easily detectable in all patients?
Diffusion hypoxia is not usually detectable in young, fit patients.
What is the recommended practice to prevent diffusion hypoxia when finishing an anesthetic with nitrous oxide?
To prevent diffusion hypoxia, it is recommended to administer a high concentration of oxygen when switching off an anesthetic with nitrous oxide.
What are volatile agents, and what do they refer to?
Volatile agents refer to halogenated hydrocarbons that are typically liquid at room temperature. These agents are commonly used in anesthesia.
Name some examples of volatile agents in common use today.
Common volatile agents used today include halothane, isoflurane, sevoflurane, and desflurane.
What is the minimum alveolar concentration (MAC) of halothane?
The MAC of halothane is 0.75%.
What type of compound is halothane?
Halothane is a halogenated hydrocarbon.
it is a colorless liquid
How is halothane typically stored?
Halothane is typically stored in amber bottles to protect it from light.
Is halothane irritating to the airways?
Halothane is non-irritating to the airways.
What is a characteristic feature of halothane?
A characteristic feature of halothane is its pleasant smell.
Is halothane suitable for gas induction?
Yes, halothane is suitable for gas induction.
Among volatile agents, which one is the oldest in common usage?
Halothane is the oldest of the volatile agents commonly used.
Is halothane an ether compound?
Halothane is the only volatile agent among the common ones that is not an ether compound.
Halothane: CNS
-Potent hypnotic, no analgesia
-↑ed CBF and ICP (minimised by hyperventilation)
Halothane: CVS
-Myocardial depression and vasodilatation
-↓ed BP
-Arrhythmias
Halothane: Respiratory
Dose-dependent respiratory depression
bronchodilator
Halothane: Skeletal Muscle
-Muscle relaxation
-Trigger for MH (malignant hyperthermia)
Halothane: Uterus
↓es uterine tone
Halothane: Liver
↑ed liver enzymes post-op
Halothane Hepatitis
What is the MAC of isoflurane?
The MAC of isoflurane is 1.14%.
Is isoflurane a halogenated ether?
Yes, isoflurane is a halogenated ether.
What is the color of isoflurane?
Isoflurane is colorless.
Is isoflurane more or less expensive compared to other volatile agents?
Isoflurane is generally less expensive compared to other volatile agents.
isoflurane: CNS
-Least effect on ICP
**Least effect on CBF
**↓ed CMRO2 consumption
**↓ed CSF production
-Ideal for neurosurgery
isoflurane CVS
Vasodilator
Drops the BP
Tachycardia
More CVS stable than Halothane
isoflurane: respiratory
Irritant to airways
Not suitable for gas induction
Bronchodilator
isoflurane: skeletal muscle
Relaxant
Potentiates NDMR
Triggers MH
isoflurane: uterus
Relaxes uterus
isoflurane liver
Low potential for toxicity
What is the MAC of sevoflurane?
The MAC of sevoflurane is approximately ±2%.
How is the smell of sevoflurane described?
Sevoflurane is described as very pleasant and sweet-smelling.
Is sevoflurane suitable for gas induction?
Yes, sevoflurane is considered best for gas induction.
Is sevoflurane expensive compared to other volatile agents?
Sevoflurane is relatively expensive compared to other volatile agents.
sevoflurane CNS
-Slight ↑ed CBF and ICP
-↓ed CMRO2 consumption
-Also good for neurosurgery
sevoflurane CVS
-Mild depression of cardiac contractility
-Slight drop in BP, but less than with Iso or Des
-No increase in HR, so CO not as well maintained as with others
sevoflurane Respiratory
Dose-dependent depression
bronchodilator
sevoflurane: skeletal muscle
Adequate muscle relaxation for intubation in children
sevoflurane: liver
No increased liver enzymes
sevoflurane: renal
Nephrotoxic end products
-Fluoride
-Compound A
-Considered safe if used at a higher gas flow rate (>2L/min)
What is the MAC of desflurane?
The MAC of desflurane is approximately ±6%.
Why does desflurane require a special vaporiser?
Desflurane requires a special vaporiser because its boiling point is very close to room temperature.
Is desflurane expensive compared to other volatile agents?
Yes, desflurane is very expensive compared to other volatile agents.
Is desflurane suitable for gas induction?
Desflurane is not suitable for gas induction as it is very irritant and pungent.
What can trigger malignant hyperthermia in susceptible individuals?
Malignant hyperthermia can be triggered by all volatile agents and the muscle relaxant suxamethonium.
What is the incidence of malignant hyperthermia?
The incidence of malignant hyperthermia is approximately 1 in 15,000.
What is the underlying cause of malignant hyperthermia?
Malignant hyperthermia is a pharmacogenetic disorder caused by an abnormal calcium receptor in muscle.
What are the potential consequences of malignant hyperthermia if not recognized and treated early?
If not recognized and treated early, malignant hyperthermia can be fatal due to the acute hypermetabolic state it induces after general anaesthesia
What is the most reliable sign of malignant hyperthermia?
Despite the name, hyperthermia is a late sign of malignant hyperthermia.
The most reliable sign of malignant hyperthermia is a rapidly increasing end-tidal CO2 level.
treatment of malignant hyperthermia
DANTROLENE iv and supportive care in ICU
clinical signs of malignant hyperthermia
-Tachycardia (and other CVS signs)
-Hypercapnia (increased end-tidal CO2 x3 ↑ )
-Tachypnoea (if not paralysed)
-High temperature (late sign)
–Skeletal muscle rigidity
-ABG: mixed metabolic & respiratory acidosis
-↑ed K, Ca, creatine kinase
-myoglobinuria
-Acute Renal Failure
