general anesthesia

General AnesthesiaDr. Maha talal

G A
General Anesthesia is the loss of response & perception of all external stimuli with loss of consciousness.

Ideal of anesthesia

Induce a smooth and rapid loss of consciousness . Allowing for a prompt recovery after administration is discontinued The drug would also possess a wide margin of safety and devoid adverse effects .

Balance anesthesia 

Modern anesthesia typically involves a combination of I.V (eg for induction of anesthesia) and inhaled (eg maintenance of anesthesia)Muscle relaxant are commonly used to facilitate tracheal intubation . .Potent opioid analgesic , and cardiovascular drugs (eg β-blocker , α 2 agonist, calcium channel blockers) and used to control transient autonomic responses , to noxious (painful) surgical stimuli .

Mechanism of action of general anesthesia 

1- old theory the anesthetic agent with lipid matrix of the nerve membrane , this lead to secondary changes in ion flux and interaction with membrane of the ligand – gated ion channel . 2- effect by facilitation GABA receptor function. 3- Antagonism the action of the excitatory neurotransmitter glutamic acid on the N-methyl-aspartate (NMDA) receptor.

4- causes membrane hyperpolarization (i:e inhibitory action) by their activation of potassium channel. 5- anesthetic decease the duration of opening nicotinic receptor – activated cation channel – an action that decrease the excitatory effect of acetylcholine synapse

Phases of Anesthesia

Induction: putting the patient to sleep Maintenance: keeping the patient asleep Recovery: waking the patient up

Preanesthetic medication

It is the use of drugs prior to anesthesia to make it more safe and pleasant.To relieve anxiety – benzodiazepines.To prevent allergic reactions – antihistaminics.To prevent nausea and vomiting – antiemetics.To provide analgesia – opioids.To prevent Bradycardia and secretion – atropine.

Classic Stages of Anesthesia

Depth of anesthesia is dependent upon the concentration of anesthetic in the central nervous system. Stages of Anesthesia divided into 4 stage : 1- stage I- Analgesia loss of pain sensation result from interference with sensory transmission in the spinothalamic tract . - The patient is conscious , Amnesia and reduced awareness of pain occur .

Classic Stages of Anesthesia

2- stage II – Excitement (delirium ) (from loss of consciousness to beginning of regular respiration ) - the patient experience delirium and possibility violent , combative behavior. -irregularity in blood pressure . - The respiratory rate may increase .- To avoid this stage of anesthesia , short acting barbiturate , such as Thiopental is given I.V before inhalation anesthesia

3- stage III - surgical anesthesia unconscious, no pain reflexes, regular respiration relaxation of the skeletal muscles occur in this stage . Eye reflex decease progressively , until the eye movement cease and the pupil is fixed . surgical may proceed during this stage .

4- stage IV – Medullary paralysis Severe depression of the respiration and vasomotor center occur during this stage . Death can rapidly occur unless measures are taken to maintain circulation and respiration . . Not all stages are observed with modern GAs. Seen mainly with Ether

Inhaled AnestheticsPharmacokinetics

Depth of anesthesia is dependent upon the concentration of anesthetic in the central nervous system.

Minimum Alveolar Concentration(MAC)

The minimum alveolar anesthetic concentration required to eliminate the response to a painful stimulus in 50% of patients (is the median effective dose(ED50)of the anesthetic ) MAC is a measure of GA potency. When several GAs are mixed, their MAC values are additive (e.g. nitrous oxide is commonly mixed with other anesthetics).


malignant hyperthermia
It’s a genetic disorder of skeletal muscle that occurs in susceptible individuals undergoing general anesthesia with volatile agent and muscle relaxant drugphysiology--hypermetabolic state by inhibition of calcium reuptake in sarcoplasmic reticulum

The malignant hyperthermia syndrome consist of the rapid tachycardia and hyperthermia ,severe muscle rigidity ,hyperkalemia ,and acid –base imbalance and acidosis that follows exposure volatile agent and muscle relaxant .

Treatment

The first and most important step is to immediately stop giving the triggering medication and to stop the surgery. Doctors then give the drug dantrolene (Dantrium). Dantrolene relaxes the muscles. It stops the dangerous increase in muscle metabolism. Dantrolene is given intravenously until a patient has stabilized. Then, the medication typically is continued in pill form for three days. Additional treatment can include: Lowering body temperature with: Cool mist and fans Cooling blankets Cooled intravenous fluids

Dantrolene sodium is a postsynaptic muscle relaxant that lessens excitation-contraction coupling in muscle cells. It achieves this by inhibiting Ca2+ ions release from sarcoplasmic reticulum stores .It is the primary drug used for the treatment and prevention of malignant hyperthermia


Gas
Volatile liquids*
Barbiturates
Opioids
Benzodiazepines
(nitrous oxide)
(halothaneisoflurane,desflurane, sevoflurane) Enflurane
(thiopental)
(midazolam)
(fentanyl)
(etomidate, propofol)
General Anesthetics
Inhalational
Parenteral
ketamine

Anesthesiology

Inhalational anesthetics :Non-halogenated gas: Nitrous oxideHalogenated hydrocarbons:HalothaneEnfluraneIsofluraneDesfluraneSevoflurane Methoxyflurane – nephrotoxicity.

Properties of Inhaled anesthetics

Nitrous Oxide(N2O) laughing gas MAC > 100% : Incomplete anesthetic Good analgesia Weak anesthesia Low soluble in blood Sedative and analgesic (used for induction only) Used in 30% conc with oxygen in dentistry Less effect on respiratory and cardiovascular system. Rapid onset & recover Used along with other anesthetic; fast induction & recovery
* fewer side effects also seen in children

Properties of Inhaled anesthetics

Halothane The first halogenated inhalational anesthetic Potent anesthesia ,weak analgesia Low MAC.(0.77) Used as maintenance Anesthesia Medium rate of onset & recovery Although inexpensive, its use has declined Sensitizes the heart to epi-induced arrhythmias

Halothane Side Effects

“Halothane Hepatitis” -- 1/10,000 casesfever, jaundice, hepatic necrosis, deathmetabolic breakdown products are hapten-protein conjugatesimmunologically mediated assault.

Halothane Side Effects

Malignant Hyperthermia-- 1/60,000 with succinylcholine to 1/260,000 without. high association with muscle disorders

Properties of Inhaled anesthetics

Enflurane: Rapid induction and recoverygood muscle relaxationMetabolize into fluoride ionSeizures occurs at deeper levels –contraindicated in epileptics.Caution in renal failure due to fluoride.


Inhalational anesthetics
Sevoflurane: Induction and recovery is fast.. It do not cause air way irritancy. Concerns about nephrotoxicity.

Parenteral Anesthetics (IV)

Barbiturates

Weak analgesia It also has anti seizure activity . Reduce hepatic blood flow and glomerular filtration rate . Unwanted effect cause: cough, laryngospasm (contraindicated in asthmatic patient)
& surgical anesthesia in <1 min. Rx: induction of anesthesia & short procedures


Adverse effect , myoclonic activity , post operative nausea and vomiting , adrenocortical suppression via effects on steroidogenesis , prolong infusion may result in hypotension and electrolyte imbalance and oliguria because of its adrenal suppressive effects

Benzodiazepines

Produce sedation and amnesia Potentiate GABA receptors Slower onset

Benzodiazepines

Sedative doses achieved within 2 min. (short duration) within 30 min Used for short procedure. The Benzodiazepines antagonist (flumazenil) can be used to accelerate recovery when excessive doses of IV administration. Used toe, amnesia & sedation prior to induction of GA with another agent.


Ketamine
“dissociative anesthesia”Stimulates central sympathetic pathways

Intravenous anesthetics

Ketamine : Dissociative anesthesia Produce - profound analgesia, cataleptic state, immobility, amnesia with light sleep. amnesia, eyes open, involuntary limb movement, unresponsive to pain . Heart rate and BP are elevated due to sympathetic stimulation. Respiration is not depressed and reflexes are not abolished.

Ketamine

Is the only IV anesthetic that possesses both anesthetic and analgesic properties as well as the ability to produce dose- related cardiovascular stimulation Mechanism of action blockade of the membrane effect of the excitatory neurotransmitter glutamic acid at NMDA receptor subtype .

Rapid onset and last for 5-10 min.(short acting) . & yodults for short procedures. topical use in some types of arthritis It is useful for burn dressing and trauma surgery.

Ketamine: Emergence delirium, hallucinations and involuntary movements occurs in 50% cases during recovery. unpleasant dreams (15%) (adult) therefore give diazepam before administration ketamine to reduce the incidence of these adverse effect. .

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