Mechanism: Major targets are GABAA receptor gatedCl-channel.(Many inhalationanesthetics, barbiturates, benzodiazepines andpropofol) NMDA receptors are type of glutamatereceptor.(N2O,Ketamine)
Inducing agents: Drugs which on i.v. injection produce loss ofconsciousness in approximately 11 seconds. Used because of rapidity of onset of action. Then maintain by inhalational agents. Supplemented with analgesics and muscle relaxants.
Thiopentone sodium: An ultra-short-acting barbiturate usedcommonly in the induction phase. Injected i.v. (3-5mg/kg) produces unconsciousness in15-20 seconds. Consciousness regained in 6-10 min. Disposal mainly by hepatic metabolism (eliminationt1/2 is 7-12hrs).
Poor analgesic effect. Weak muscle relaxant. Respiratory depression is transient and severe in largedoses. Does not sensitizes the heart to adrenaline, cardiacarrhythmia are rare. Indications:- Minor surgery that are less painful.
Adverse effects: Laryngospasm-relived by atropine premedication. Fall in BP. Shivering and delirium. Acute intermittent porphyria-C/I.Other uses: Rapid control of convulsions. Narco analysis.
Methohexitone sodium: short-acting and has a rapid onset ofaction. Three times more potent and similar inits effects to thiopentone sod. Has a quicker and briefer action(5-8 min). Indication:-For oral surgery and dentistry. A/E:-Excitement during induction and recoveryperiod.
Propofol-Milk of amnesia Short-acting for both induction as well asmaintenance. An oily liquid employed as a 1% emulsion. Unconsciousness occurs 15-45 sec and lasts 5-10 min. Propofol is highly protein-bound and is metabolizedby conjugation in the liver. Intermittent injection or continuous infusion used fortotal i.v. anesthesia when supplemented by fentanyl. Most suited for out patient surgery.
Adverse effect: Fall in BP and bradycardia. Pain on injection. Dystonia and myoclonic movements are common. Propofol has reportedly induced priapism in someindividuals.Other uses: In sub-anesthetic doses, it is the drug of choice forsedating intubated patients in ICU.
Etomidate: Short-acting induction anesthesia. Has a briefer duration of action (4-8 min) thanthiopentone. Etomidate is highly protein bound and metabolised byhepatic and plasma esterases to inactive products. Onset of action: 30–60 seconds. Peak effect: 1 minute.Indications: As sedative, for short procedures such as reduction ofdislocated joints and cardioversion.
Slower acting drugs:Benzodiazepines: Used for induction, maintenance and supplementinganesthesia as well as for conscious sedation. In larger doses injected i.v. produce sedation, amnesiaand unconsciousness in 5-10mins. If no anesthetic or opioid is given, the patient becomesresponsive in 1hr. But, amnesia for 2-3hrs. Poor analgesics. Opioid or N2O is added, if the procedure is painful.Indication-in endoscopies, cardiaccatheterization, angiographies, regional anesthesia,fracture setting.
Action is rapidly reversed by flumazenil 0.5-2mg i.v.Diazepam: 0.2-0.5mg/kg , slow undiluted injection in a running i.v. This technique reduces burning sensation in vein andincidence of thrombophlebitis.Lorazepam: Potent, slower-acting, less irritating than diazepam. Amnesia is more profound. (Dose : 0.04 mg/kg)Midazolam: Water-soluble, shorter acting drug used for sedation ofintubated patients. (Dose : 1-2.5mg i.v.) In other critical care anesthesia as 0.02-0.1mg/kg/hrcontinuous i.v. infusion.
Ketamine: Related to phencyclidine-dissociative anesthesia. DA-characterised by profoundanalgesia, immobility, amnesia, feeling of dissociationfrom ones own body. Site of action-Cortical and sub cortical areas. Act by inhibiting NMDA receptors. Dose of 1-3mg/kg i.v. produces the above effects withina minute & recovery after 10-15 mins, but remainsamnesic for 1-2hr. Metabolized in the liver and has anelimination t1/2 3-4hrs.
Indications: Head and neck surgeries, asthmatics, Shortsurgeries, Burn dressings. Combined with diazepam used inangiographies, catheterization, and trauma surgeries.Adverse effects:-Heart rate, Cardiac output, BP increased.-Delirium, hallucination, involuntary movementsduring recovery.Contraindication:-HT,IHD.-Increased ICP.
Fentanyl: Short-acting, opioid analgesic related to pethidine. Has rapid onset and short duration of action about30-50 min. Strong agonist at the μ-opioid receptors. Used to supplement anesthetics in balancedanesthesia. Also used in neurolept analgesia in combination withdroperidol.
Indications: Combined with BDZs, used forendoscopic, angiographic, cardiac catheterization, burndressing and other minor procedures. used most often in operating rooms and intensive careunits.Adverse effects: Nausea, vomiting and itching oftenoccurs during recovery. Marked respiratory depression. Slight fall in BP.
Dexmedetomidine: An α2 agonist with sedative and analgesiceffect. Has a half life of 2-3hrs. Metabolized in liver and excreted mainly as inactiveurine metabolites. Adverse effects:-Hypotension and bradycardia.
Balanced anesthesia: Modern anesthesia involves a combination of i.v.(forinduction) and inhalational anesthesia(formaintenance). Sometimes volatile(sevoflurane) isused for induction. Muscle relaxant-facilitate intubation. Local anesthetics-tissue infiltration, peripheral nerveblock. Potent opioid analgesic & CV drugs(β blocker, cachannel blocker)used to control autonomic responsesto noxious surgical stimuli.
Conscious sedation: This technique refers to drug induction alleviation ofanxiety & pain in combination with an altered level ofconsciousness associated with the use of smallestdoses of sedative medication. Pt.retains ability to maintain patent airway &responsive to verbal comments. Drugs-diazepam,midazolam,propofol.
Neuroleptanalgesia Method of i.v. anesthesia which combines the use ofneuroleptic drug with an opioid analgesic. Subject is conscious & able to co-operate duringsurgery. most favoured combination- droperidol + fentanyl After administering nitrous oxide with oxygenneuroleptanalgesia can be converted toneuroleptanesthesia. C/I - patients receiving MAO inhibitors, abuse drugsor alcohol, with Parkinson disease.
Complications During Anesthesia:-Respiratory depression,-Cardiac arrhythmias,-Fall in BP,-Acid pneumonitis,-Laryngospasm and asphyxia,-Awareness,-Delirium and convulsions.
After anesthesia:-Nausea and vomiting,-Persisting sedation,-Pneumonia,-Atelectasis,-Nerve palsy,-Emergence delirium,-Organ toxicity.