Inhalational Agents Flashcards
What are some terms for inhalational agents?
Anaesthetic gases, anaesthetic agents, volatiles.
What affects the uptake and distribution of volatile agents?
(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 governs the partial pressures of anaesthetic in all body tissues?
The alveolar partial pressure
What is the relationship between alveolar concentration of an agent and alveolar partial pressure?
Direct proportionality.
Factors decreasing alveolar concentration slow down induction of anaesthesia,
and vice versa
Relationship between ability to dissolve in blood and induction? Provide a reason.
Agent that dissolves easily=slow induction
Agent that is insoluble in blood=faster induction
If insoluble in blood–> fat soluble, like the brain tissue. Thus, agent is ready to bind to the fatty tissue in the brain.
Relationship between the cardiac output and rate of induction? Provide a reason.
Faster cardiac output= slower rate of induction.
Blood needs enough time to travel through the capillary/alveolar interface to pick up enough agent… if the cardiac output is higher, blood will not be as saturated with agent
SHUNTING (blood passing through alveoli that are not ventilated) will adversely affect induction
List the factors that leads to a faster induction of an agent
High inspired concentration of agent Increased ventilation Less soluble agent (in blood) Decreased cardiac output Decreased shunting
List the factors that leads to a slower induction of an agent
Lower inspired concentration of agent Decreased ventilation More soluble agent (in blood) Increased cardiac output Increased shunting
How does solubility affect recovery from anaesthesia?
Recovery is slow from soluble agents e.g. halothane.
Recovery is fast from poorly soluble agents eg desflurane and sevoflurane
Harmful metabolites from volatile agents
CF3 group causes “Halothane Hepatitis”
Fluorides from older agents like methoxyflurane and enflurane renal impairment
“Compound A” from Sevoflurane (renal impairment in rats
Explain potency of volatiles
Potency is the measure of a drug’s efficacy at a given dose
A more potent agent requires a lower dose to achieve the same effect; a less potent drug requires more
The concept of MAC is a measure of volatile potency
What does MAC stand for and how does it relate to potency of a volatile agent?
MAC (%): Minimum Alveolar Concentration (in %) required to prevent 50% of patients from moving in response to a standard surgical stimulus (incision) at sea level.
Hence a low MAC = more potent (e.g. Halothane = 0.75%) and a high MAC – less potent (e.g desflurane =6 %)
Which factors leads to a decrease in the MAC?
Sedatives N2O Analgesics Increased age hypotension hypothermia myxoedema Less effect: hypoxia, anaemia, pregnancy
Which factors leads to an increase in the MAC
Alcoholism
Increased in children
Hyperthermia
Thyrotoxicosis
What colour is nitrous oxide?
Blue
MAC of nitrous oxide
105%
Nitrous oxide- CNS effects
Potent analgesic
Poor anaesthetic
> 80% = hypoxic gas mix, always give max 70%
Second gas effect (need less [AA]) and speeds up induction
Nitrous oxide- CVS effects
Negative inotrope
Nitrous oxide- Resp effects
Augment depression of other agents
Diffusional hypoxia – after switch off NO2 from blood diffuse into alveoli and displace O2
Nitrous oxide- Muscle effects
no MH trigger
Nitrous oxide- additional info
Only ‘gas’ – stored as liquid.
Flow meter control
Diffuses into air filled cavities = rapid expansion (think pneumothorax and middle ear)
Colour and MAC of sevoflurane
Yellow
2%
sevoflurane- CNS Effects
Low sol = fast induction
Slight incr. CBF and ICP (Monroe Kelly doctrine)
Decr O2 consump
sevoflurane- CVS Effects
Mild -ve inotrope Slight BP drop No HR compensate = CO not maint Prolong qT No dysrhythm
