Lecture 10: Inhalation Anaesthetic and Intravenous Anaesthetics Flashcards
What is the “triad of anaesthesia”?
What we mean by “anaesthesia” 1) Cause Hypnosis/Amnnesia -e.g. IV of volatile agents 2) Muscle relaxant - immobility 3) Opioids -Autonomic areflexia Enough ether can achieve all 3, but now the modern approach is “balanced anaesthesia”.
Describe the equilirbium of uptake of inhaled anaesthetic agents
We do not metabolise the inhaled anaesthetic agents.
At the beginning of the anaesthetic, the equilibrium shifts to the right
At the end of the anaesthetic, the equilibrium shifts to the left.
What is a hypotheses of how inhaled anaesthetics work?
1) Meyer-Overton hypothesis
They suggested that general anaesthetics may act by dissolving in the fatty fraction of brain cells and removing fatty constituents from them, thus changing activity of brain cells and inducing anaesthesia.
This hypothesis had problems, as not all lipophilic volatile agents produce anaethesia.
What is MAC?
The means of describing potency and dose
MAC is the minimal alveolar concentration producing imobility on standard surgical stimulus in 50% of patients
More potent agents with a lower MAC
The use of another drug can move the MAC.
Name some things that increases or decreases the MAC
Increases: (need higher dose to have the same effect)
1) Young age
2) Hyperthermia
3) Hyperthyroid
4) Heavy alcohol
5) Drugs e.g. amphetamine
Decreases: (need lower dose to the have the same effect)
1) Old age
2) Hypothermia
3) Hypothyroid
4) Drugs (e.g. opioids, depressants)
5) Pregnancy
6) Low O2, high CO2.
MAC for desflurane is 6%. Does setting 6% on the vaporizer mean the patient has an alveolar concentration of 6%?
No. because what happens is the drug goes into the alveolar, but some also go into the blood.
What does pharmacodnyamics mean?
The branch of pharmacology concerned with the effects of drugs and the mechanism of their action.
Describe the phamacodynmaics effect anaesthetics has on the CNS
Pharmacokinetics is the study of what the body does to the drug. Pharmacodynamics is the study of what the drug does to the body.
- Cause: 1) Hyponosis, 2) Immobility, 3) Amnesia
- but also decrease CMRO2 (cerebral metabolic rate of oxygen)
- Dose dependent increase CBF (Cerebral blood flow) and ICP (Intracranial pressure) (You must take care with use in neurosurgery)
- Cerebral protection (but not demonstrated in humans)
Describe the phamacodynamics of anaesthesia on the CVS
1) Peripheral vasodilation, lower BP
2) HR unchanged (except desflurane- which causes an increase in HR)
3) Modern agents don’t really affect Stroke Volume greatly
4) Holthane pro-arrythmogenic
Describe the pharmacodynamics of INHALED anaesthesia on the Respiratory System
1) Respiratory Depressant (Sevoflurane is least)
2) Impair ventilatory response to hypoxia
3) Impair ventilatory response to CO2
4) Bronchodilation (but desflurane is an airway irritant- so is useful for COPD patients, but don’t give to acute patients)
How is desflurane unique?
Changes HR (unlike other anaesthetics)
Describe Nitrous Oxide- Anaesthesia
1) Odourless non-flammable gas
2) Low potency (MAC 101%- cannot just use this for anaesthesia)
3) Low blood-gas solubility, rapid onset
4) Analgesic
5) Many adverse effects (e.g. nausea and vommitting)
Describe Halothane- Anaesthesia
1) Sweet non-pungent halogenated alkane
2) Highly potent (MAC- 0.86%)
3) Intermediate blood-gas solubility, slow onset
4) Metabolised 2% in vivo
5) Rarely causes fatal hepatitis
Name 5 common inhaled anasthetics
1) Halothane
2) Isoflurane
3) Sevoflurane
4) Desflurane
5) Nitrous Oxide (not used as much)
Describe Isoflurane- Inhaled anaesthetics
1) Pungent
2) Potent (MAC- 1.1%)
3) Intermediate solubility, medium onset
4) Cardiovascular stability
Describe Sevoflurane- Inhaled Anaesthesia
1) Non-pungent and least respiratory depression (good for gaseous inductions)
2) Intermediate potency (MAC 1.7%)
3) Low sikyvukutym raoud onset
4) Little airway stimulation
5) Little airway stimulation
6) reacts with CO2 absorbent
(avoid in renal failure patients)
In adults- often administered intravenously.
What INHALED anaesthetics shoudl you avoid in renal patients?
Sevoflurane
What inhaled-anaesthetics are pungent and what are non-pungent?
Halothane- non-pungent (sweet)
Isoflurane - pungent
Sevoflurane -non-pungent
Desflurane -pungent
Describe Desflurane
1) Pungent
2) Intermediate potency (MAC 6.0%)
3) Low blood-gas solubility, rapid onset and offset (therefore is ideal for long cases where fast wake up is desired)
4) Sympathetic stimulation (where most causes depression)
Name 5 types of intravenous anaesthetic agents
1) Barbiturates (e.g. Thiopentone)
2) Phenols (e.g. Propofol)
3) Imidazols (e.g. Ethomidate)
4) Phencyclidine derivatives (e.g. Ketamine)
5) Benzodiazepines (e.g. Midazolam)
What are the mechanisms of action of intravenous anaesthetic agents?
1 (Thiopentone, profol, etomidate, midazolam)
Enhance GABA which prolongs Cl- current. THis causes hyperpolarisation
2 (Ketamine)
Bind to NMDA and antagonises glumatae and supresses exciation
Also has analgesic effects, whilst the other 4 do not.
Name an intravenous anaesthetic that has analgesic effects
Ketamine
Describe the Pharmacokinetics of Intravenous Anaesthetic agents
1) Highly lipid soluble and cross BBB easily
2) Drug from IV bolus taken up by VRG (vessel rich group e.g. brain, kidney, lung, liver, heart) organs, then leaves these organs as lean (less perfused) tissues take up drug and concentration in blood falls
3) Offset after single IV dose is therefore due to redistribution - patient may wake even though the total drug in body has not changed much
4) Metabolism is much slower for most agents
Describe the Intravenous anaesthetics: Thiopentone
1) Very rapid onset (10s)
2) Rapid offset by redistribution
3) Slow clearance (therefore will accumulate in multi-dosing or infusion)
4) Metabolized in liver, induces liver enzymes
5) Some decrease in Peripheral vascular resistance and BP (enahnced in shock)
6) Respiratory depression and loss of airway reflexes
Good for emergency cesarean sections. The mother falls asleep 10s after administration of the anaesthetics
