Commonly used anaesthetic agents Flashcards
what are inhalational agents and examples
Inhalational agents
These are volatile liquids which readily vaporize, permitting administration by inhalation in O2-enriched air or an O2/N2O mix. They help maintain anaesthesia and decrease awareness (by an unclear mechanism).
Halothane
Isofluorane
Sevofluorane
Desflurane
What is volatility and how is it related to the gas flow and concentration
The ease with which the agent vaporizes in the chamber (=volatility) is a function of itssaturation vapour pressure(svp) and the temperature of the liquid. Agents with a highersvp(eg isoflurane) need less gas to flow over them to reach a given concentration.
How is potency of inhalational agents described and what does it mean
PotencyThe potency of an inhaled agent can be given by itsminimum alveolar concentration(mac), which is the concentration that will produce no movement response to incision of the skin in 50% of unpremedicated patients. It is not a useful clinical measurement, but it can be used to compare potency of the different inhalational agents. It is affected by a number of factors, eg age, thyroid status and anxiety state of the patient.
Propofol chemical structure clinical characteristics and uses potential problems contraindications doses
t½=30–600min
Lipophilic phenol derivative.
Produced commercially as an emulsion in soybean oil and has become the most commonly usedivanaesthetic agent in the developed world.
Its good recovery characteristics and anti-emetic effect make it popular, especially in day-case surgery.
It is fast-acting and its offset of action is due to rapid redistribution, and not metabolism.
•Dose examples:Induction = 2mg/kgiviat 2–4mg/sec. Maintenance = 4–12mg/kg/hivi.
nb:
Rapid injection can cause cardiovascular depression (↓bp), and respiratory depression can occur when combined withivnarcotics.
For procedural sedation: 0.5–1mg/kgiviover 1–5min. Dose needs to be reduced in the elderly, debilitated and shocked.
Dose also depends partly on premed use: with midazolam induction dose requirements for propofol are reduced by 20–50%. This effect seems to be more pronounced in the elderly.
•Uses:It is used in induction and maintenance of GA, and for sedation during regional anaesthesia, short procedures, and as a sedative inITU(contraindicated in children for this last use).
Once opened, use ampoules or discard, because of the risk of bacterial growth.
•Problems:Pain on injectionoccurs in up to 40% of patients. This can be minimized by either adding a small amount of lidocaine (eg 2mL of 1%) to the propofol, or by pre-injection of local anaesthetic.
Contraindications:
•Extremes of age
•
Thiopental sodium
chemical structure clinical characteristics and uses potential problems contraindications
(t½=11h)
a barbiturate that is typically mixed with water to give a 2.5% solution (ie 25mg/mL).
It has a rapid onset of action (arm–brain circulation time about 30sec).
Effects last 3–8min, and awakening is due largely to redistribution, not metabolism.
Some 30% of the injected dose is still present in the body after 24h, giving rise to a hangover effect.
It has been replaced by propofol as the most popular induction agent.
•Uses:Induction ofga; it is also a potent anticonvulsant—used in status epilepticus management when other measures have failed .
•Problems:Anaphylaxis is rare (1 in 20,000).
Is a negative inotrope so can drop cardiac output by 20%.
May also lead to bronchoconstriction.
Intra-arterial injectionproduces pain and blanching of the hand/limb below the level of injection due to arterial spasm, followed by ischaemic damage and gangrene—following inadvertent brachial artery puncture in the antecubital fossa. This is less of a problem now that an indwelling cannula is obligatory (compared with the historical use of a needle).
•Treatment:
1Leave the needle in the artery and inject a vasodilator, eg papaverine.
2Ask an experienced colleague to perform, brachial plexus or stellate ganglion block. (This should dilate vessels and ↓ischaemia.)
3Heparinivto stop thrombus forming.
4Give pain relief—postpone surgery unless desperate.
Extravascular injection causes severe pain and local necrosis.
Get expert help. Infiltrate with hyaluronidase 1500iudissolved in 1mL water through the cannula.
Contraindications: •Airway obstruction •Barbiturate allergy •Fixed cardiac output states •Hypovolaemia/↓bp •Porphyria •Compromised airway3
Etomidate
(t½=3.5h)
This is a carboxylated imidazole and is used as an induction agent.
Dose:
0.15–0.3mg/kg.
Uses:
Histamine release is not a feature, but rapidity of recovery and cardiovascular stability are; therefore it is often used in emergent patients with trauma/head injuries for whom avoidance of even a brief episode of hypotension is important.
Ketamine
(a phencyclidine derivative, produced as a racemic mixture;n-methyl-d-aspartate receptor antagonist; t½=2.2h)
Dose example:
2mg/kg usually gives 5–10min of surgical anaesthesia (for long procedures—seebnf15.1.1).
Uses:
Mainly for paediatric anaesthesia and procedural sedation. Cardiac output is unchanged or increased, and so it is a good ‘on site’ or ‘in the field’ agent, as it can be givenim, producing profound analgesia without compounding shock.
Ketamine has potent bronchodilatory properties, so can be considered during intubation in status asthmaticus.31
Problems:
Hypertonus and salivation, but there is some maintenance of laryngeal reflexes (but do not rely on this).
Recovery is slow. Emergence phenomena are troublesome (delirium, hallucinations, nightmares; all made worse if the patient is disturbed during recovery).
Avoid in the hypertensive patient, those with a history of stroke or raised intracranial pressure (further ↑ produced), patients with a recent penetrating eye injury (risk of ↑ intra-ocular pressure), and psychiatric patients. Avoid adrenaline infiltrations.
In theuk, the Home Office classifies it as a class C drug as it is prone to misuse (‘Special-k’).
To avoid excessive dosage in obese patients it may be best to calculate the dose on the basis of ideal body weight (ibw).
ideal but imaginary iv anaesthetic agent
The idealivagent would be stable in solution and in the presence of light, be water-soluble and have a long shelf-life.
It would be painless when giveniv; non-irritant if injected extravascularly (with a low incidence of thrombosis) with some pain (as a warning) if given intra-arterially. Furthermore:
•It should act rapidly within one arm–brain circulation.
•Recovery should be quick and complete with no hangover effect.
•It should provoke no excitatory phenomena.
•Analgesic properties are advantageous.
•Respiratory and cardiovascular effects should be minimal.
•It should not interact with other anaesthetic agents.
•There should be no hypersensitivity reactions.
•There should be no post-op phenomena, eg nausea or hallucinations.
ideal but imaginary anaesthetic agent
Inhaled agents have advantages (eg noivaccess required, more precise control) and disadvantages (eg claustrophobic) overivagents. The ideal inhaled agent should:
•Have low solubility in blood and tissues (to allow rapid recovery).
•Be resistant to any degradation.
•Have no injurious effects on vital tissues.
•Be administrable in a reliable and known concentration.
halothane
This is a colourless, pleasant-smelling gas (which, unlike the other agents, isnotan ether) first used as an anaesthetic agent in the 1950s.
It has little analgesic effect, but decreases cardiac output (vagal tone ↑, leading to bradycardia, vasodilation, and hypotension).
It sensitizes the myocardium to catecholamines (beware in patients with arrhythmias; surgical infiltration with local anaesthetic and adrenaline).
Halothane is no longer used in theuk—it has been replaced by safer inhalational agents (below), due to the rare but serious complication of postoperative hepatitis (up to ~1 in 4,000 for multiple exposures; less frequently in children)
isoflurane
Isoflurane
(fig2) This is a halogenated ether. Theoretically induction should be quick, but isoflurane is irritant, so coughing, laryngospasm, or breath-holding may complicate the onset of anaesthesia.
sevoflurane
his is a halogenated ether which is well tolerated. It is the agent of choice for inhalation induction of general anaesthesia due to its combination of being low irritant, and having relatively fast onset—offset.
desflurane
another halogenated ether with a rapid onset of anaesthesia, and quick recovery.
Having perhaps the lowest absorption into fat, desflurane is often chosen for surgery in the morbidly obese as it provides for the quickest recovery post surgery.
However, it is more of a respiratory irritant than sevoflurane, so is sometimes only used for maintenance rather than induction of anaesthesia.
It may also have greater depressant effects of the cardiovascular system than sevoflurane.
Stopping inhalation reverses all the above effects—except for hepatitis resulting from drug metabolism.
Neuromuscular blockers
types and examples
Depolarising agents: suxamethonium(parial afonist for ACh receptors). ianctivated by cholinesterases
Non-Depolarising agents: competitive antagonists of ACh competing at NMJ. can be reversed by anti-cholinesterases (eg neostigmine). Eg Rocuronium, vecuronium, atracurium
suxamethonium
Suxamethonium (=succinylcholine) is the only one commonly used. It is a partial agonist for acetylcholine receptors and causes initial fasciculation through depolarization of the post-synaptic membrane, then paralysis by inhibiting the restoration of normal membrane polarity. Suxamethonium is rapidly inactivated by plasma cholinesterases.
Dose:
1–1.5mg/kg
ivuses:
It has been the most popular paralytic agent in rapid sequence inductions (rsi) for several decades due to its rapid onset (30–60sec), and short duration (3–5min). Both these aspects are important in rapid sequence induction. The rapid onset lessens the time between induction and intubation—decreasing the risk of aspiration and potential hypoxia.
The short duration means that if intubation is impossible, the patient regains muscle tone, and starts protecting their own airway again.
Its popularity may start to wane with the increasing use of rocuronium (below).
Side effects:
• ↑K+(enough to raise the plasma K+by ~0.5–1.0mmol/L—avoid in paraplegia and burns!). Beware that K+liberation is increased with multiple sclerosis, Guillain–Barré, stroke, and crush injury. Suxamethonium increases intra-ocular pressure (eg increases risk of vitreous extrusion).
• 30% of patients get post-operative muscle pains.
• Repeated doses of suxamethonium may lead to bradycardia—more common in children—treat with atropine.
Beware suxamethonium apnoea ([link]).
