COMMONLY USED GENERAL ANAESTHETIC AGENTS Flashcards
THIOPENTAL SODIUM (IV)
=A highly lipid soluble barbiturate
Induction is smooth and rapid (10 – 30 seconds)
=Has no analgesic effects
=Short-acting: Awakening from a moderate dose is rapid due to redistribution of the drug into other tissues
**Major adverse effects include laryngeal spasm, respiratory depression, hypotension
=Should be used with caution and reduce dose in severe congestive cardiac failure, shock, acute intestinal obstruction and renal impairment
**Contraindicated in acute intermittent porphyria or variegate porphyria
ETOMIDATE (IV)
=An induction agent that produces rapid recovery without hangover effects
Causes minimal cardio-respiratory depression during induction therefore may be used in CVS disease
=Has no analgesic effect
=Should not be used for maintenance anaesthesia as it suppresses adrenocortical function on continuous administration
**Adverse effects include pain at injection site, postoperative nausea and vomiting
PROPOFOL (IV)
=Has rapid onset of action and rapid recovery
=Has no hang-over effects, and does not cause post-operative nausea and vomiting
=Used for both induction and maintenance of anaesthesia
=Has no analgesic effect
=Preferred maintenance anaesthetic agent for day-care surgery
**Causes cardiovascular and respiratory depression, and should therefore used with caution in CVS disease and hypovolaemia. Monitor blood pressure closely.
**Causes bradycardia (antidote used is atropine)
KETAMINE (IV/IM)
=Mechanism of action: Blocks the effects of glutamate at NMDA (N-mono-methyl-D-aspartate) receptors (blocks the glutamate-activated NMDA receptor calcium channel)
=Not as rapidly acting as other IV anaesthetics
=Recovery is relatively slow and is associated with hallucinations and psychotic manifestations. The hallucinations can be reduced by a benzodiazepine.
**Analgesic at sub-anaesthetic doses
Increases sympathetic activity (therefore contra-indicated in hypertension) and increases intracranial pressure (avoid in increased intracranial pressure)
=Does not depress the cardiovascular system and has minimal respiratory depressant effect
=Useful for rapid induction of anaesthesia in patients who require a high degree of sympathetic activity (e.g. hypotension, shock, cardiac tamponade)
=Can be used in poor risk surgical patients e.g. patients with poor CVS function or hypovolaemia
=Also useful for repeated anaesthesia e.g. in burns, and short surgical or diagnostic procedures
=Used mainly for paediatric anaesthesia especially when repeated anaesthesia is required
ANESTHETIC VAPORS (VOLATILE LIQUIDS)
intro
=These are chlorofluorocarbons, which are delivered with precision from vaporizers and directly into the patient’s inhaled gas stream using oxygen as the delivery vehicle
=They may be mixed with nitrous oxide, a much weaker anesthetic gas
ANESTHETIC VAPORS (VOLATILE LIQUIDS)
examples
=The prototype of modern anesthetic vapors is halothane (halothane produces a hepatotoxic metabolite)
Others are sevoflurane, isoflurane, desflurane, enflurane and
=methoxyflurane (methoxyflurane is now obsolete because it produces a nephrotoxic metabolite)
DEPTH OF ANAESTHESIA WITH INHALATIONAL AGENTS
=Depth of anaesthesia is determined by partial pressure of the inhalational agent in cerebral arterial blood which in turn determines its diffusion into and partial pressure within brain tissue
=Partial pressure of the anaesthetic agent in arterial blood is determined by:
Partial pressure of the agent in alveolar gas
=Solubility of the anaesthetic agent in the blood (expressed as blood/gas partition)
=Partial pressure of the agent in alveolar gas depends on:
*Its concentration in inspired air
*Alveolar ventilation
*Rate of diffusion of the agent from the alveolar gas into blood (the more lipid soluble, the faster it will diffuse)
PROPERTIES OF INHALATIONAL AGENTS: PARTITION COEFFICIENT
=The more soluble in blood, the higher the partition coefficient
=The lower the partition coefficient (low solubility in blood), the more rapid the anaesthesia and recovery
=Agents which are not very soluble produce rapid effects and recovery is fast because it is the free form which easily diffuses
**If there is need for rapid recovery, select an agent with a very low partition coefficient
PROPERTIES OF INHALATIONAL AGENTS: MINIMUM ALVEOLAR CONCENTRATION (MAC)
=Potency of anaesthetic gases is expressed as Mean Alveolar concentration (MAC)
=MAC is the minimum concentration of gas in the lungs that will produce a state of anaesthesia in 50% of subjects
=An anaesthetic agent with a high MAC is not very potent
=MAC is an additive function for inhaled anaesthetic agents
=MAC decreases with increasing age, pregnancy, hypothermia and hypotension
=MAC decreases in the presence of adjuvant drugs such as other general anaesthetics, opioids, sedative-hypnotics, or other CNS depressants
=MAC is independent of gender and weight
CHARACTERISTICS OF AN IDEAL INHALATIONAL ANAESTHETIC AGENT
=High potency (low MAC value)
=Low solubility in blood and tissues (low partition coefficient)
=Resistance to physical and metabolic degradation
=Should not injure vital tissues
=Should not cause seizures, respiratory irritation, and circulatory stimulation
=Should produce anesthesia while allowing the use of a high concentration of oxygen
NITROUS OXIDE
=Weak anaesthetic agent that cannot be used as the sole anaesthetic agent
*For anaesthetic purposes, nitrous oxide is used in a 70:30 mixture with oxygen and is used for short procedures (less than one hour)
=Very potent analgesic agent: can be used for analgesia in sub-anaesthetic doses in a 50:50 mixture with oxygen (50% N2O/50% O2).
*Entonox is a commercial preparation and is used in obstetric analgesia and dressing burns.
**Prolonged administration of nitrous oxide has a risk of bone marrow suppression (inhibits DNA synthesis)
**Can not be used as an anaesthetic agent alone without causing hypoxia
**Its use in anaesthesia is mainly for its analgesic properties
HALOTHANE
=Potent, non-irritant and causes smooth induction
=Produces moderate muscle relaxation
=Potent anaesthetic but poor analgesic agent
=Can be used for gaseous induction in children
**20% is metabolized in the liver and the metabolite can cause hepatic dysfunction
=Depresses myocardial contractility and can induce arrhythmias
=Adverse effects include hepatotoxicity (due to a hepatotoxic metabolite), bradycardia, cardio-respiratory depression, hypotension and predisposes to ventricular arrhythmias
=Avoid adrenaline infusions with halothane use; halothane sensitizes the myocardium to catecholamines
ISOFLURANE
=Potent anaesthetic but poor analgesic agent
=Less cardio-depressant than halothane but causes greater respiratory depression
=Does not sensitize the myocardium to catecholamines
=Causes less reduction in blood pressure and has less risk of hepatotoxicity compared to halothane
=Reduces peripheral resistance and can cause reduction in the blood directed to the coronary circulation (‘coronary steal’): therefore avoid in ischaemic heart disease
ENFLURANE, DESFLURANE AND SEVOFLURANE
=Enflurane, desflurane and sevoflurane:
*Have lower potency compared to halothane
*Have less risk of hepatoxicity compared to halothane
=Enflurane
*Causes cardio-respiratory depression
*Reduces cardiac output and blood pressure more than halothane
*Increases risk of seizures; avoid in epilepsy
=Desflurane
*Faster onset and recovery compared to halothane
*Has a pungent smell and causes choking (it is irritant to the respiratory tract; therefore avoid in bronchial asthma and chronic obstructive pulmonary disease).
=Sevoflurane
Rapidly acting, recovery is very rapid therefore there is need for early post-operative analgesia
*Suitable for use as an induction agent
*Produces a nephrotoxic metabolite
SPECIAL ANAESTHETIC TECHNIQUES
①Dissociative anaesthesia: State of analgesia with light hypnosis (done with ketamine). The patient feels dissociated from their surroundings, there is analgesia and amnesia, with or without loss of consciousness. Premedication with a benzodiazepine is done to prevent the psychosis that occurs during recovery.
❷Neuroleptanalgesia: State of analgesia with the patient able to cooperate. A combination of a neuroleptic with a high efficacy opioid analgesic is used. Example is droperidol + fentanyl (or alfentanil)
③Neuroleptanaesthesia: Use of a neuroleptic and high efficacy opioid analgesic to supplement GA with nitrous oxide
