Transcript 12. anaesthesia general I

General Anesthesia
Dr. Israa
General anesthesia
• General anesthesia was not known until the mid1800’s
• Diethylether was the first general anesthetic used
for surgery
• General Anesthetics are divided into two classes:
– Inhaled anesthetics (usually halogenated
compounds)
– Intravenous anesthetics or induction agents
Modern Anesthesia
•
•
It combines the following:
1. Analgesia
2. Sleep (loss of consciousness)
3. Skeletal Muscle relaxation
4. Amnesia
5. Abolition sensory & autonomic reflexes
No single drug can produce all these effects
Ideal anesthesia
• Ideal anesthesia is:
– Induce loss of consciousness smoothly and rapidly
– Allow for prompt recovery of cognitive function
after its administration is discontinued
– Possess wide margin of safety
– Have no side effects
(No single drug can produce all these effects)
Stages of anesthesia
Stage 1:analgesia
• Decreased pain awareness, sometimes with
amnesia, conscious may be impaired but not lost
Stage 2:disinhibition
• Delirium, excitation, amnesia, enhanced reflexes,
irregular respiration and incontinence
Stages of anesthesia
Stage 3: surgical anesthesia
• Unconsciousness, no pain reflex, regular
respiration and maintained blood pressure
Stage 4:medullary depression
• Severe CVS and respiratory depression and
the patient require pharmacological and
ventilatory support
Anesthesia protocols
• For minor procedure, conscious sedation
techniques that combine IV agent with local
anesthetics are often used; these can provide
profound analgesia, with retention of the patient
ability to maintain a patent airway and response
to verbal commands
• For extensive surgical procedure protocol
commonly includes IV drug for induction, inhaled
agent (with or without IV) for maintenance and
neuromuscular junction blockers to cause muscle
relaxation
General Anesthetics
• Absence of sensation associated with a reversible
loss of consciousness, skeletal muscle relaxation,
and loss of reflexes.
• Drugs used for anesthesia are CNS depressants
with action that can be induced and terminated
more rabidly than conventional sedative and
hypnotics
General Anesthetics
• Most sensitive site of action for general
anesthetics is the reticular activating system of
the brainstem (RAS)
• Anesthetic dose: does not cause depression of
cardiac, vasomotor or respiratory centers
• Has a small margin of safety
1. Inhaled Anesthetics
• Include:
– Nitrous oxide
– Halothane
– Enflurane
– Isoflurane
– Desflurane
2. Intravenous Anesthetics
• Include:
A. Barbiturates
• Thiopental & Methohexital
B. Opioids
• Alfentanil, Meperidine, Fentanyl, Sufentanil (agonists)
• Naloxone (antagonist)
C. Benzodiazepines
• Diazepam, Midazolam
• Flumazenil (antagonist)
Intravenous Anesthetics
D. Miscellaneous Agents
– Etomidate: non-barbiturate hypnotic agent
without analgesic properties
– Droperidol: Neuroleptic (similar to Haloperidol)
combined with Fentanyl and is used for
neuroleptanalgesia (state of analgesia and
amnesia)
– Ketamine: dissociative anesthetic
– Propofol
Intravenous agents
Mechanism of action:
• Act at cell surface receptors:
• Barbiturates and benzodiazepine act at
GABA-A receptors to increase Cl- influx
• Opioids act on μ and other subtypes
• Ketamine antagonizes PCP site on NMDA
receptors (prevent excitation)
Intravenous agents
Pharmacokinetics
– Rapid induction = shorter acting
– Duration of effect proportional to redistribution
from brain to other tissue
A. Barbiturates: Thiopentone
• Ultra-short acting hypnotic with no analgesic action
• High lipid solubility promotes rapid entry to the
brain
• Eliminated by the liver
• Has rapid onset of action and recovery
Mechanism of action:
• Potentiates GABA, decrease glutamate activity,
increase chloride ion conductance
Adverse reactions:
• Decreased myocardial and respiratory activity
B.OPIOIDS
• Morphine and fentanyl are used in anesthesia
regimens.
• Respiratory depression due to these drugs
may be reversed by naloxone.
C. Benzodiazepines
• Midazolam is widely used adjunctive with
inhaled anesthetics and IV opioids
• It has slow onset and longer duration of action
if compared to thiopental.
• Flumazenil is an antidote if respiratory
depression occur.
D. Miscellaneous Agents
1. Etomidate
• Imidazole derivative that provide induction with
minimal change in cardiac function and respiratory rate
and has short duration of action
• It is not analgesic , and its primary advantage is in
anesthesia for patient with limited respiratory and
cardiac reserve
• Activates GABA receptors
• Side effects
– Myoclonus
– Post-operative nausea and vomiting
– Adrenal suppression if prolonged use.
2. Ketamine
• This drug produce dissociative state in which the
patient is patient remains conscious but has marked
catatonia, analgesia, and amnesia
• It is a chemical congener of the psychotomimetic
agent, phencyclidine (PCP)
• It is a cardiovascular stimulant drug
• Can cause increase intracranial pressure
• Emergency reactions include disorientation,
excitation and hallucination which can be reduced
by preoperative administration of benzodiazepines
Uses
• Induction of anesthesia
– in children
– in severely hypovolemic patients
Contraindications
• Increased intracranial pressure
• Ischemic heart disease
• Psychological disorders
3.Propofol
• Effects
– Hypnosis , Antiemetic
– Fast acting, short duration.
– Fewer peripheral side effects compared to barbiturates
• Uses
– Induction and maintenance of anesthesia
– As anesthetic agent at outpatient surgery
– Also effective in producing prolong sedation in patient in
critical care setting
• Contraindications
– Cardiovascular instability due to marked reduction in the
peripheral resistance
Preanaesthetic
Medication
GOOD LUCK