General Anaesthesia Flashcards
What is general anesthesia?
Reversible, drug induced loss of consciousness. Used for the purpose of medical procedures.
Name the 3 common IV anesthetic agents and their mechanism of action
Propofol
Thiopentone
Etomidate
Activate GABA(A) receptors to inhibit neuronal excitability and decrease consciousness and movement.
Name the 4 common inhaled anesthetic agents, their use and mechanism of action
Used to induce anesthesia in children and maintain anesthesia in adults.
NO
Isoflurane
Sevoflurane
Desflurane
Work by activating background K+ channels and inhibiting NMDA receptors reducing excitatory neurotransmission and reducing consciousness and movement.
What is Entonox and its uses?
‘Laughing gas’ - Entonox is a gas breathed in as short-term pain relief and it also causes feelings of relaxation which can help with anxiety around medical procedures.
Made of 50% NO and 50% O2.
Used for analgesic purposes, labour and trauma.
Describe the Overton and Meyer theory of GA mechanism of function
In a homologous series of anesthetics, increasing the chain length of the molecule increases the lipid solubility, resulting in an increase in the anesthetic potency. This means that:
The potencies of general anesthetics are directly proportional to their hydrophobicities.
However, beyond a certain chain length, the anesthetic effect disappears.
Define MAC and discuss its relevance to general anesthetics
MAC means minimum alveolar concentration which is the minimum concentration of an inhaled anesthetic at 1atm of pressure that prevents skeletal muscle movement in response to a surgical incision in 50% of patients.
This means that the higher the clinical potency of the general anesthetic, the lower the MAC.
Describe the relation between blood solubility and speed of onset of general anesthetics
The blood-gas partition coefficient is used to describe the solubility of inhaled general anesthetics in blood. It is the ratio of the concentration of the gas dissolved in blood to the concentration of the gas in contact with the blood.
If an anesthetic has a high blood-gas partition coefficient, this means it requires a large amount to be taken up in the body’s blood before it is passed to the lipid tissues of the brain where it will exert its effect.
The blood-gas partition coefficient is inversely related to induction rate. This means the higher the solubility of inhaled general anesthetics then the slower the rate of onset.
Describe the molecular mechanism of function of general anesthetics
Typically, general anesthetics potentiate the activation and sensitivity of inhibitory post-synaptic channels (GABA and glycine channels (Cl-)) or inhibit the activation of excitatory synaptic channels (5HT, ACh, and glutamate channels (Na+)).
GABA receptors are the main inhibitory NT of the brain. General anesthetics bind allosterically to activate GABA receptors and enhance inhibitory currents.
What are the drawbacks of general anesthetics on the body?
General anesthetics have a negative ionotropic effect on the heart (weaken the force of muscular contraction). This results in decreased vascular resistance, decreased stroke volume and decreased contractility.
General anesthetics decreases the FRC (functional residual capacity) of the lungs (volume in the lungs after a normal expiration). In IV agents it causes a decrease in respiratory rate and in inhaled agents it causes a decrease in TV. This results in a degree of respiratory depression.
Describe the meaning behind balanced anesthesia and selection process of the agent used
Anesthesia by using smaller doses of multiple agents rather than a large dose of a single agent for safety reasons.
Selection based on:
Health requests of the patient
Properties of the drug
Requirements of the surgery
Aim is to minimise patient risk and maximise patient safety and comfort.
