General anesthesia is defined as a controlled state of unconsciousness accompanied by a loss of protective reflexes, including the ability to maintain an airway independently and respond purposefully to physical stimulation or verbal command.
Properties of an ideal anesthetic
For the patient
- Should not cause nausea or vomiting.
- Induction and recovery should be fast with no after effects.
For the surgeon
- It should provide adequate analgesia, immobility and muscle relaxation.
- It should be noninflammable and nonexplosive so that electric cautery may be used.
For the anesthetist
- Its administration should be easy, controllable and versatile.
- Margin of safety should be wide. Heart, liver and other organs should not be affected.
- It should be potent so that low concentrations are needed and oxygenation of the patient does not suffer.
- Rapid adjustments in depth of anesthesia should be possible.
- It should be cheap, stable and easily stored.
- Patients who cannot cooperate due to a lack of psychological or emotional maturity and/or mental, physical, or medical disability.
- Patients for whom local anesthesia is ineffective because of acute infection, anatomic variations, or allergy
- The extremely uncooperative, fearful, anxious, or uncommunicative child or adolescent
- Patients requiring significant surgical procedures
- Patients for whom the use of general anesthesia may protect the developing psyche and/or reduce medical risk
- Patients requiring immediate, comprehensive oral/dental care.
- A healthy, cooperative patient with minimal dental needs
- Predisposing medical conditions which would make general anesthesia inadvisable.
cardinal features of general anesthesia
- Loss of all sensations, especially pain
- Sleep (unconsciousness) and amnesia
- Immobility and muscle relaxation
- Abolition of somatic and autonomic reflexes
STAGES OF ANESTHESIA
General anesthetics cause an irregularly descendingvdepression of CNS, i.e. the higher functions are lost first and progressively lower areas of the brain are involved. Guedel (1920) described four stages with ether anesthesia but the precise sequence of events differs among each anesthetic.
Stage of analgesia
Starts from beginning of anesthetic inhalation and lasts up to the loss of consciousness. Pain is progressively abolished during this stage. Patient remains conscious, can hear and see, and feels a dream like state. Reflexes and respiration remain normal. Though some minor and even major operations can be carried out during this stage, it is rather difficult to maintain so its use is limited to short procedures only.
Stage of delirium
From loss of consciousness to beginning of regular respiration. Apparent excitement is seen in many patients and he may shout, struggle and hold his breath; muscle tone increase, jaws are tightly closed, breathing is jerky; vomiting, involuntary micturition or defecation may occur. Heart rate and blood pressure may rise and pupil dilates due to sympathetic stimulation. No stimulus should be applied or operative procedure carried out during this stage. This stage can be cut short by rapid induction and appropriate premedication. It is inconspicuous in modern anesthesia.
Extends from onset of regular respiration to cessation of spontaneous breathing. This has been divided into four planes, which may be distinguished as:
- Plane 1: Roving eyeballs. This plane ends when eyes become fixed.
- Plane 2: Loss of corneal and laryngeal reflexes.
- Plane 3: Pupils start dilating and light reflex is lost.
- Plane 4: Intercostal paralysis, shallow abdominal respiration, dilated pupil.
Includes the stage from cessation of breathing to failure of circulation and death. Pupil is widely dilated, muscles are totally flabby, pulse is imperceptible and blood pressure is very low.
DRUGS USED FOR GENERAL ANESTHESIA
- It is a colorless, odorless, heavier than air, non-inflammable gas supplied under pressure in steel cylinders.
- It is nonirritating, but low potency anesthetic unconsciousness cannot be produced in all individuals without concomitant hypoxia: MAC is 105 percent implying that even pure N2O cannot produce adequate anesthesia at 1 atmosphere pressure. Patients maintained on 70% N2O + 30% O2 along with muscle relaxants
often recall the events during anesthesia, but some lose awareness completely.
- Onset of N2O action is quick and smooth (but thiopentone is often used for induction), recovery is rapid: both because of its low blood solubility.
- Nitrous oxide is generally used as a carrier and adjuvant to other anesthetics. A mixture of 70% N2O + 25–30% O2 + 0.2–2% another potent anesthetic is employed for most surgical procedures. In this way concentration of the other anesthetic can be reduced to 1/3 for the same level of anesthesia.
- Ether is a potent anesthetic, produces good analgesia and marked muscle relaxation by reducing acetylcholine output from motor nerve endings.
- It is highly soluble in blood and induction is prolonged and unpleasant with struggling, breath holding, salivation and marked respiratory secretions (atropine must be given as premedication to prevent the patient from drowning in his own secretions).
- Recovery is slow; postanesthetic nausea, vomiting and retching are marked.
- Ether is not used now in developed countries because of its unpleasant and inflammable properties. However, it is still used in developing countries, particularly in peripheral areas because it is cheap, can be given by open drop method without the need for any equipment, and is relatively safe even in inexperienced hands.
- It is a volatile liquid with sweet odor, nonirritant andnoninflammable. Solubility in blood is intermediate induction is reasonably quick and pleasant.
- It is a potent anesthetic but precise control of administered concentration is essential.
- For induction 2-4% and for maintenance 0.5-1% is delivered by the use of a special vaporizer.
- Halothane causes direct depression of myocardial contractility by reducing intracellular Ca2+ concentration.
- Halothane causes relatively greater depression of respiration and ventilatory support with added oxygen is frequently required.
- It inhibits intestinal and uterine contractions. This property is utilized for assisting external or internal version during late pregnancy. However, its use during labor can prolong delivery and increase postpartal blood loss.
- It is a newer all fluorinated congener of isoflurane which has gained popularity as an anesthetic for outpatient surgery in Western countries.
- Though it is highly volatile, a thermostatically heated special vaporizer is used to deliver a precise concentration of pure desflurane vapor in the carrier gas (N2O + O2) mixture.
- Its distinctive properties are lower oil:gas partition coefficient and very low solubility in blood as well as in tissues, because of which induction and recovery are very fast.
- Postanesthetic cognitive and motor impairment is short lived and patient can be discharged a few hours after surgery.
- Degree of respiratory depression, muscle relaxation, vasodilatation and fall in BP, as well as maintained cardiac contractility and coronary circulation are like isoflurane. Lack of seizure-provoking potential or arrhythmogenicity and absence of liver as well as kidney toxicity are also similar to isoflurane.
- As such, desflurane can serve as a good alternative to isoflurane for routine surgery as well, especially prolonged operations.
- It is an ultrashort acting thiobarbiturate, highly soluble in water yielding a very alkaline solution, which must be prepared freshly before injection.
- Injected IV (3–5 mg/kg) as a 2.5 percent solution it produces unconsciousness in 15 to 20 sec. Its undissociated form has high lipid solubility enters brain almost instantaneously. Initial distribution depends on organ blood flow brain gets large amounts. However, as other less vascular tissues (muscle, fat) gradually take up the drug, blood concentration falls and it back diffuses from the brain: consciousness is regained in 6–10 min (t½ of distribution phase is 3 min).
- Its ultimate disposal occurs mainly by hepatic metabolism (elimination t½ is 7–12 hr), but this is irrelevant for termination of action of a single dose.
- Residual CNS depression may persist for > 12 hour. The patient should not be allowed to leave the hospital without an attendant before this time.
- It is similar to thiopentone but is three times more potent, has a quicker and briefer (5–8 min) action and is more rapidly metabolized (t½ 4 hr) than thiopentone, thus the patient will recover more quickly.
- Excitement during induction and recovery is more common.
- Currently, propofol has superseded thiopentone as an IV anesthetic, both for induction as well as maintenance.
- It is an oily liquid employed as a 1 percent emulsion. Unconsciousness after propofol injection occurs in 15-45 sec and lasts 5-10 min as it distributes rapidly(distribution t½–4 min).
- Elimination t½ (1–2 hr) is much shorter than that of thiopentone due to rapid metabolism.
- Intermittent injection or continuous infusion of propofol is frequently used for total IV anesthesia when supplemented by fentanyl.
- It lacks airway irritancy and is particularly suited for outpatient surgery, because residual impairment is less marked and shorter lasting.
- Incidence of postoperative nausea and vomiting is low; patient acceptability is very good.
- Disadvantages include induction apnea lasting 1 minute; bradycardia and dose-dependent respiratory depression.
- Dose: 2 mg/kg bolus IV for induction; 9 mg/kg/hr for maintenance.
- Benzodiazepines (BZDs) are frequently used for inducing, maintaining and supplementing anesthesia as well as for conscious sedation. Relatively large doses (diazepam 0.2–0.3 mg/kg or equivalent) injected IV produce sedation, amnesia and then unconsciousness in 5 to 10 min. If no other anesthetic or opioid is given, the patient becomes responsive in 1 hour or so due to redistribution of the drug (distribution t½ of diazepam is 15 min), but amnesia persists for 2-3 hours and sedation for 6 hours or more.
- BZDs are poor analgesics so an opioid or N2O is usually added if the procedure is painful.
- Lorazepam in a dose 2 to 4 mg (0.04 mg/kg) is three times more potent, slower acting and less irritating than diazepam.
- Midazolam is water soluble, nonirritating to veins, faster and shorter acting and is being preferred over diazepam for anaesthetic use in a dose of 1 to 2.5 mg IV followed by 1/4th supplemental doses.
- It is pharmacologically related to the hallucinogen phencyclidine; induces a so-called dissociative anesthesia characterized by profound analgesia, immobility, amnesia with light sleep and feeling of dissociation from one’s own body and the surroundings.
- Respiration is not depressed; airway reflexes are maintained, muscle tone increases; limb movements occur and eyes may remain open.
- A dose of 1 to 3 (average 1.5) mg/kg IV or 5 mg/kg IM produces the above effects within a minute, and recovery starts after 10 to 15 minutes, but patient remain amnesic for 1 to 2 hours.
- Ketamine is metabolized in the liver and has an elimination t ½ of 3 to 4 hours.
- This short acting (30–50 min) potent opioid analgesic related to pethidine is generally given IV at the beginning of painful surgical procedures.
- It is frequently used to supplement anesthetics in balanced anesthesia which permits use of lower anesthetic concentrations with better hemodynamic stability.
- After IV fentanyl (2–4 pg/kg) the patient remains drowsy but conscious and his cooperation can be commanded.
- Respiratory depression is marked, but predictable; heart rate decreases; nausea, vomiting and itching often occurs during recovery.
- Fentanyl is also employed as adjunt to spinal and nerve block anesthesia, and to relieve postoperative pain.
- Activation of central α2 adrenergic receptors has been known to cause sedation and analgesia. Clonidine (a selective α2 agonist antihypertensive) given before surgery reduces anesthetic requirement.
- Dexmedetomidine is a centrally active selective α2 agonist that has been recently introduced for sedating critically ill/ventilated patients in intensive care units.
- Side effects are hypotension, bradycardia and dry mouth.
COMPLICATIONS OF GENERAL ANESTHESIA
- Respiratory depression and hypercarbia.
- Salivation, respiratory secretions
- Cardiac arrhythmias, asystole
- Fall in blood pressure
- Aspiration of gastric contents: Acid pneumonitis
- Laryngospasm and asphyxia
- Delirium, convulsions
- Fire and explosion: Rare now due to use of noninflammable gases
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