General Anesthesia

Dr. Israa

General anesthesia

• General anesthesia was not known until the mid-1800’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. Analgesia2. Sleep (loss of consciousness)3. Skeletal Muscle relaxation4. 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 & MethohexitalB. 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)

Pharmacokinetics– Rapid induction = shorter acting– Duration of effect proportional to redistribution

from brain to other tissue

Intravenous agents

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 conductanceAdverse 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 Agents1. 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