Maintenance of Anaesthesia Flashcards
Methods of maintenance of anaesthesia
TIVA - Total Intravenous Anaesthesia
CRI - Constant Rate Infusions
Volatile agents
OR combination
What is inhalation anaesthesia?
Using volatile or inhalation agents (NOT GASES)
Vapour
Agents come from a bottle and poured into vapouriser used to deliver agent via breathing system
Usually includes oxygen but sometimes NO
How does a Vaporiser work?
Fresh gas flow on - Gas flows along bypass pathway to patient
If you turn control dial on - fresh gas flow is divided by splitting valve
Some continus along bypass, some goes through anaesthetic agent chamber and becomes saturated
Gas and vapour both flow out of outlet to patient
Delivery of anaesthetic agent depends on fresh gas flow
How do inhalation agents work?
Fresh gas and agent combine in vaporiser and enter alveoli of lungs
Agent is soluble in blood and therefore increases plasma concentrations - carried to tissues including brain
Once they reach CNS act on GABA, NMDA, ACh receptors (Don’t really know how they work)
Gases then diffuse out of tissues and return to lungs - Exhaled virtually unchanged
·This means that they need to be removed somehow - SCAVENGING
What affects rate of plasma increase?
Concentration of inhalation agent
Patient’s ventilation
Cardiac output -
If CO is higher there is less plasma conc rise
Solubility of agent in blood
If agent poorly soluble in blood will diffuse into tissues quicker
What is scavenging?
Collection and removal of anaesthetic gases
Why is scavenging important?
Anaesthetic agents are greenhouse gases - pollute environment
Anaesthetic agents would affect anyone in operating theatre - need to protect staff
Methods of Scavenging
Charcoal canister - absorbs cases
Passive scavenging - tube carrying waste gases out of the window
Active scavenging - negative pressure pump to withdraw gases
What is MAC?
Maximum Alveolar Concentration
Concentration of vapour in the alveoli of the lungs that is needed to prevent 50% of patients moving in response to surgical stimulus
Different values for different agents and species
What factors affect MAC?
Individual variation
Drugs given in pre-med or intraoperatively (usually decrease MAC)
Body temperature
Age
Examples of Inhalation Agents
Isoflurane - Dogs, cats, horses
Slightly lower MAC
Higher blood gas solubility
Sevoflurane - dogs, cats
Slightly higher MAC (end up using more)
Lower blood gas solubility (Diffuses into tissues quicker)
More expensive
Disadvantages of Inhalation Anaesthetic Agents
Profound depressive effect on cardiovascular pulmonary system
Negative inotropy
Peripheral vasodilation
Decrease in vascular reactivity
Overall reduction in autonomic tone
Causes a reduction in blood pressure and cardiac output
The respiratory depression is the result of a depressed ventilatory response to carbon dioxide
High concentrations of carbon dioxide normally stimulate respiration
Depressed hypoxic pulmonary vasoconstriction
Pulmonary arteries constrict in parts of the lung tissue in the face of hypoxia to direct blood to better ventilated areas
Almost complete suppression of the ventilatory response to hypoxia
Causes bronchodilation which increases dead space
When is Nitrous Oxide used?
Not as often - opioids used instead now
Has an extremely high MAC so not useful as sole anaesthetic agent
Reduces amount of volatile agent required
Provides good analgesia via NMDA receptor antagonism
Good cardiovascular and respiratory stability
Can be used to spare CV effects of iso and sevoflurane
Rapid Effect
Poorly soluble in blood
What are the environmental concerns of using inhalation agents?
Considered to be greenhouse gases
Deplete ozone layer
Relatively inert molecules
Persist for a long time
* The impact of nitrous oxide is significantly greater than for isoflurane or sevoflurane a
○ Both a GHG and ozone depleting
○ Persists in the environment for >100 years, compared with 2.6-5.9 for isoflurane and 1.1-5.2 years for sevoflurane
COSHH
Control of Substances Hazardous to Health
