Anaesthesia Flashcards
Why are there so many anaesthetic drugs?
No single perfect agent
Combinations of anaesthetics sometimes used for synerstic and additive therapeutic effects
Risk of > adverse effects
Drug prescription much about minimising adverse effects as achieving therapeutic goals
Why sedation?
< pt anxiety > acceptability of prolonged/ extensive procedures -still effective time limit < 2 hrs Looking for -anxiolysis -cooperation -street fitness Not sedation???
Why GA
Extensive/ prolonged procedures Brief painful procedures -extractions Total lack of pt cooperation -intellectual impairment -children
Drugs available for sedation
Alcohol Tetrahydrocannabinol Opiates Major tranquilisers Minor tranquilisers
Alcohol for sedation
Disinhibition, nausea, slow recovery
THC for sedation
Nausea, illegal
Opiates for sedation
Euphoria, nausea, respiratory depression
Major tranquilisers for sedation
Chloropromazine, haloperidol
Profound anxiety, hypotension
Minor tranquilisers for sedation
Benzodiazepines
Drowsiness, prolonged effect
Oral benzodiazepines
Long interval, variable, small effect -although often enough for many pts IV -competence in IV cannulation -limited duration of action (repeat doses) -need for monitoring (SpO2 mandatory) -written record (pulse, BP, SpO2)
Benzodiazepines mechanism of action
Barbiturates / benzodiazepines –> bind to GABAa receptor at different allosteric sites –> facilitates GABA action –> Barbiturates > duration and frequency, benzodiazepines > frequency of opening of Cl- channel –> membrane hyperpolarisation –> CNS depression
At higher dose barbiturates can act as GABA mimetic
Types of benzodiazepams
Diazepam -irritant -skin necrosis Diazemuls -rapid onset -brief duration of action -psychoactive metabolites with long half life Midazolam -slow(er) onset -less predictable effect -rapid metabolism
Sedation with propofol
Believed to work at least partly via GABA receptor
Short acting and fast recovery
Can be used in subanaesthetic doses - good amnesia
BUT
-no algesia
-continuous infusion (secure IV access)
-expensive delivery system
Difference between sleep and unconsciousness
Unrousability Loss of protective reflexes: potential for -aspiration -obstruction -nerve damage due to prolonged pressure
Process of GA
Induction: rapid pleasant production of unconsciousness -IV or gaseous/ volatile Secure airway -optimum head position, LMA, ETT Maintenace -gaseous/ volatile or IV -analgesia -+/- muscle relaxant Emergence -reversal of muscle relaxation
Induction
IV
-Propofol, thiopentone, katime, etomidate
Volatile
-sevoflurane
Characteristic of IV anaesthetics: propofol
Rapid pleasant onset
Non cumulative
Hypotension (Vasodilation)
Painful on injection
Characteristic of IV anaesthetics: Thiopentone
Rapid onset Offset due to redistribution Slow metabolism Cumulative Hypotension (myocardial depression) Odd complications -arterial spasm -porphyria -anaphylaxis rare but serious
Characteristic of IV anaesthetics: ketamine
Rapid onset Hypertension Powerful analgesic Non cumulative Muscle rigidity Unpleasant hallucinations
Characteristic of IV anaesthetics: etomidate
CV stability Noncumulative Odd (major) complication -inhibition of steroid synthesis -death due to induced Addison's
Airway maintenance
Optimum head position
-neck flexed, head extended
Laryngeal mask (does not prevent aspiration)
Tracheal intubation largely eliminates danger of aspiration but use muscle relaxant to overcome laryngeal reflexes
-suxamethonium, atracurium, cisatracurium, rocuronium, pancurocium
Characteristics of suxamethonium
\+: Rapid onset and usually rapid offset -: Depolarising Muscle pain Occasional prolonged paralysis Malignant hyperthermia
Characteristics of atracurium
+: reliable spontaneous hydrolysis
-:
Histamine release
Hypotension
Characteristics of cisatracurium
+:Not so much histamine release
-:slow onset
Characteristics of rocuronium
+:
Long duration
Specific reversal agent
-: histamine release
Characteristics of pancuronium
+: medium duration
-:
Tachycardia
Renally excreted unchanged
Maintenance of anaesthesia
Gaseous/ volatile
-nitrous oxide
-isoflurance, sevoflurane, desflurane
IV: propofol
MAC
minimum alveolar conc at which 50% of pts do not react to initial surgical incision
Characteristics of nitrous oxide
MAC 104% Strong analgesia 0 pugency Isses: -dysphoria -association with miscarriage
Characteristics of isoflurane
MAC 1.2% (strong potency)
+/- analgesia
++ pungency
Characteristics of sevoflurane
MAC 1.8% (+++ potency)
+/- analgesia
+ pungency
Issues: acceptable for volatile induction
Characteristics of desflurane
MAC 6.6% (++ potency)
+/- analgesia
++++ pungency
Issues: rapid onset/ offset
Characteristics of propofol
\++++ potency 0 analgesia NA pungency Issues: -IV agent - access required -microprocessor controlled infusion pump - expensive -can be used for sedation
Preoperative assessment - why?
Perfect Preparation Prevents Poor Performance
Maximise pt safety
-optimise pt’s condition
-postpone procedure if necessary
Toxic drugs with significant side effects
-all have life threatening effects at therapeutic doses
-CV depressant
-respiratory depressant
Complications encountered during an operation
Pt
Anaesthesia
Surgeon
Patient complications
Pre-existing disease
- cardiac (aortic stensosis; coronary artery disease)
- respiratory (COPD; asthma)
- diabetes (common; associated with renal, CV pathology)
An anaesthetic history
Proposed procedure
PMH
-drugs/ allergies
-surgical: previous GA, problems (pt/ family)
-medical
–cardiac: IHD, valvular, other
–respiratory: obstructive/ restrictive diesases
-GI: aspiration risk - hiatus hernia
–renal: impairment affects drug handling
–diabetes
-airway assessment
Anaesthetic complications and ways of reducing them
Awareness/Overdose (Agent monitoring; Brain function monitors; Neuromuscular blockade)
Anaphylaxis (Learn the drill)
Hypoxia (SpO2)
Respiratory Depression (CO2 monitoring)
Aspiration (Preop starvation; Intubation)
Peripheral Nerve Damage (Careful positioning)
Haemorrhage signs
Falling BP
Rising HR
Suction container full of blood
Loads of wet swabs being piled in a corner
Treatment for haemorrhage
Permissive loss of haemoglobin (to Hb ~80)
Restore circulation with crystalloid
Chloride is current unfashionable anion
