GENERAL ANESTHESIA

GENERAL ANESTHESIA

Dr.Arkan Jaafar , M.D. Anesthesiologist Medical college of Mosul

Definition of General anesthesia GA

Delivery of anesthetic drugs (inhaled or infused) to produce a level of CNS depression with the following goals : Anesthesia :Hypnosis , loss of consciousness (reversible) Analgesia : pain control Amnesia : loss of recall Areflexia : muscle relaxation ( this is not always required) Autonomic Areflexia : decrease sympathetic nervous system (SNS) function Anxiolysis : Intra-operative & Post-operative

GA
INDUCTION
MAINTENANCE
EMERGENCE

INTRAVENOUS ANESTHETICS IV

Thiopental (sodium thiopentone) Ultrashort acting thiobarbiturate Most commonly used as an induction agent Short duration of action 6-8 min After IV bolus , rapidly distributes to vessel rich organs (brain,liver,heart,kidney) thus achieves unconsciousness in approximately 30 sec Metabolism & elimination occur at a slower rate resulting in residual effects (usually sedation) during post-anesthesia recovery which may last hours

Propofol (Diprivan) Used for induction and/or maintenance of anesthesia Thick white soybean-based solution Metabolism & elimination much more rapid due to increase rate of liver metabolism compared to thiopental Less residual sedative effect, patients recover sooner, thus popular for outpatient surgery More suited for continuous infusion than thiopental due to rapid elimination More expensive

Ketamine: - Cause dissociative anesthesia; - that the patient appears conscious (e.g. eye opening, swallowing, muscle contracture) but unable to process or respond to sensory input. Use as i.v. & i.m. anesthetic agent. Can cause hallucination, but it is less common in children & in patient premedicated with benzodiazepines. Onset of anesthesia is slower than that with thiopental. Duration of action longer than thiopental, its (10-15 min). Cardiovascular stimulant (BP well maintained in poor risk patient e.g. shock state). Bronchodilator



Etomidate: - Onset is rapid. There is pain at injection site. It is least likely of commonly used agent to cause depression of the CVS or release of histamine. It may depress the cortisol production especially if given by infusion


Benzodiazepines ( eg. Diazepam.midazolam,lorazepam)Used as a premedication prior to induction or as an induction agent in combination with other drugsOral & injectable formulations are availableAct on a specific brain GABA receptors to produce selective anti-anxiety & sedative effects Duration of action long but variable/somewhat unpredictableBenzodiazepine antagonist→→ Flumazenil

Opioids Opioids used in anesthesia : morphine , codeine , meperidine , fentanyl, sufentanyl, alfentanyl, remifentanil Indications Opioids used for pre, intra, post-operative analgesia Also used as an induction agent alone or as adjuvant Can be administered IV , IM , PO


General effects of morphine (prototype opioid) CNS (depression) :analgesia , mood changes ,sedation , respiratory depression , decreased cough reflex CNS (excitation): miosis , nausea & vomiting ,hyperreflexia CVS : vasodilatation , orthostatic hypotension Respiratory :central depression , bronchial constriction GI : constipation , biliary colic GU : urinary retention Other : histamine release ,smooth muscle contraction (e.g. biliary & bladder sphincters) Opioid antagonists (e.g. naloxone , naltrexone)

Volatile inhalational agents : Halothane , Isoflurane , Enflurane , sevoflurane Characteristics : liquid , colorless, non-flammable ,non-explosive vaporizer delivers controlled concentration of anesthetic agent to respiratory system of the patient via anesthetic machine

Nitrous oxide N2O Characteristics : gas , colorless ,mild sweet odor at room temperature Uses : Analgesia Adverse effects : During emergence , N2O can diffuse rapidly from the blood to the alveoli , resulting in a dilution of O2 in the alveoli (diffusion hypoxia) it is therefore necessary to provide 100% O2 for several minutes until N2O is eliminated Bone marrow depression Chronic neuropathy Tends to diffuse into closed air spaces causing increased pressure & volume (important if there is trapped air e.g. air embolus , pneumothorax)

Muscle Relaxants

Muscle relaxants often required during surgical procedures for various reasons : Prevent muscle stretch reflex & suppresses muscle resting tone Facilitate intubation Facilitate controlled ventilation Allow access to the surgical field Muscle relaxants classified on the basis of the neuromuscular blockade they provide : Depolarizing Non-depolarizing And according to their duration of action Short Intermediate long

Depolarizing Succinylcholine (suxamethonium) Onset :rapid (30-60 sec) Duration : short (5 min) Metabolism of sch by plasma cholinesterase Side effects : Sch also binds to autonomic cholinergic receptors Muscarinic receptors in the heart can cause sinus bradycardia Muscarinic receptors in the salivary glands result in increase secetions Hyperkalemia Other side effects Increased intracranial ICP , intraocular IOP ,intragastric pressure Triggers malignant hyperthermia Sustained contraction in myotonia fasiculations



Non-depolarizing muscle relaxants Mivacurium , atracurium ,cis-atracurium , rocuronium ,pancuronium , doxacurium Reversing agents for non-depolarizing blockade (neostigmine , pyridostigmine) Reversible anticholinesterase Inhibit enzymatic degradation of Ach , increases the amount of Ach at nicotinic receptors ,displacing the non-depolarizing muscle relaxants With reversal ,Ach concentration will increase at muscarinic ( before nicotinic) sites causing bradycardia , salivation etc. Therefore simultaneous administration of atropine or glycopurrolate is necessary to decrease cholinergic side effects