General Anesthetics

 

General Anesthetics are the drugs that enter the systemic circulation, and produce loss of all sensations throughout the body and loss of consciousness. These drugs are mostly used to carry out long surgical procedures.

The purposes of general anesthesia include –

-abolishing pain (analgesia) and other sensations

-unconsciousness

-adequate muscle relaxation

-amnesia (loss of memory to intra-operative, immediate pre-operative, and immediate post-operative events)… and –

-suppression of undesirable reflexes

-maintaining the cardiovascular and respiratory functions

-keeping the patient away from anxiety and apprehension

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SAQ/Viva

What is Induction of Anesthesia, Maintenance of Anesthesia, and Recovery from Anesthesia?

1. “Induction of anesthesia” – This means beginning of anesthesia.

2. “Maintenance of anesthesia”. This means continuing the anesthesia for longer time.

3. “Recovery from anesthesia”. This means the events taking place during the period when the anesthetic effect is getting over.

As quickly and smoothly the induction takes place, the patient is likely to feel minimum distress. Therefore the drugs which produce rapid induction are valuable. So also with such drugs the recovery from anesthesia is quicker and smoother. The newer general anesthetic agents such as desflurane, sevoflurane, isoflurane, enflurane, halothane – do produce fairly quick and smooth induction. The gas – Nitrous oxide – produces very rapid induction. (The older anesthetic ether – used to produce very slow and prolonged induction)

If an anesthetic is more potent, then it is likely to continue (maintain) the anesthesia for longer time. (Halothane, isoflurane, desflurane, sevoflurane, enflurane, ether – all are relatively potent anesthetics. The older anesthetic agent Ether – is also known to be a potent anesthetic).

If the anesthetic agent is less potent, its effect does not last longer; and hence is not useful for longer procedures. Nitrous oxide is an anesthetic which is less potent, hence it will be useful only for induction or as a sole (only) anesthetic if the procedure is very short.

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LAQ/SAQ/Viva

Classify General Anesthetics with examples.

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(A) Inhalational Anesthetics

1. Volatile liquids: Halothane, Enflurane, Isoflurane, Desflurane, Sevoflurane [All useful for induction as well as maintenance], and the older agent – Di-ethyl-ether

2. Gases: Nitrous Oxide (N2O) (Less potent, rapid acting, short duration anesthetic) (Useful for induction and for short procedures)

(B) Intravenous Anesthetics

1.  Fast Acting: (For Induction of anesthesia, and for short procedures)

            a. Barbiturates: Thiopental Sodium

             b. Non-barbiturates: Propofol, Etomidate

2. Slow Acting: Diazepam, Lorazepam, Midazolam

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[There are some more types of anesthesia, that are being classified here as special types of anesthesia. These are intravenous anesthetics, and should always be mentioned with intravenous anesthetics as well.]

(C) Special types of  Intravenous anesthesia

1. Dissociative Anesthesia: Example: Ketamine

2. Neurolept-Analgesia/Anesthesia: Example: Droperidol plus Fentanyl and/or Nitrous Oxide

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Now there is another way of asking a question on examination. Instead of Classify General Anesthetics with examples, there may be a question – Classify Intravenous Anesthetics. This means only and all intravenous anesthetics need to be mentioned.

The answer will be:

Intravenous Anesthetics

A. Intravenous anesthetics producing general anesthesia:

     a. Fast Acting:

             1. Barbiturates: Thiopental Sodium

             2. Non-barbiturates: Propofol, Etomidate

     b. Slow Acting: Diazepam, Lorazepam, Midazolam

B. Special types of intravenous anesthesia:

              1. Dissociative Anesthesia: Example: Ketamine

              2. Neurolept-Analgesia/Anesthesia: Example: Droperidol

                    plus Fentanyl and/or Nitrous Oxide

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SAQ/Viva

What do you mean by Pre-anesthetic Medication? Mention the drugs used as Pre-anesthetic medications, with the purpose of their use.

“Pre-anesthetic Medication ” is the term used to describe the medications used before anesthesia, to make the anesthesia and the pre-operative and post-operative course uncomplicated.

The purposes of pre-anesthetic medication include:

1. Preventing sudden cardiac stoppage or vagal bradycardia

2. To decrease the secretions

3. To relax the smooth muscles

4. To suppress the undesirable reflexes

5. To prevent vomiting, cough, laryngospasm

6.  To prevent anxiety and apprehension

Below are the classes of drugs used as Pre-anesthetic Medication, and their use/purpose.

1. Anticholinergic drugs: – Glycopyrrolate (0.1-0.3 mg), [rapid and long action, less tachycardia, no central effects], -Atropine (0.6 mg), Scopolamine: These drugs serve the following purposes:

• Prevent reflex vagal bradycardia and/or sudden cardiac arrest
• Decrease the exocrine secretions
• Relax the smooth muscles
• Prevent cough reflex, vomiting reflex, and laryngospasm
• Decrease gastric acidity

2. Opioids: Fentanyl, sufentanil, alfentanil, remifentanyl, morphine, pethidine, pentazocine: Analgesia, sedation, amnesia

3. Benzodiazepines: Diazepam, lorazepam, midazolamsedation:  Antianxiety action, prevent apprehension, produce amnesia

4. Phenothiazines: Promethazine, hydroxyzinesedation: Sedation and antiemetic effect 

5. Antiemetics/Prokinetics: ondansetron, domperidone, metoclopramide,cisapride, ranitidine, famotidine: Prevent vomiting and reflux, promote forward movement through the upper gastrointestinal tract, decrease gastric HCl secretion, prevent aspiration .

6. Skeletal muscle relaxants: may be used during surgery to provide adequate skeletal muscle relaxation.

• Succinylcholine : To help tracheal intubation
• Neuromuscular blockers:  D-tubocurarine, pancuronium, atracurium, mivacurium: To assist required skeletal muscle relaxation for surgery

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SAQ/Viva

Mention adverse effects of halothane.

Adverse effects of halothane can be remembered by 4 H –

-Hepatitis

-Hyperthermia (Malignant hyperthermia)

-Heart (Cardiac arrhythmias), and –

-Hypotension

1. Hepatitis:

• (a) Type 2: Immune related acute fatal liver failure: Halothane can cause delayed allergic hepatic necrosis due to its toxic metabolite – trifluoroacetyl chloride. This happens after repeated surgeries with halothane in 1:10,000 individuals
• (b) Type 1: Mild hepatitis. Elevated transaminase enzyme levels. Self limiting

2. Hyperthermia: (Malignant hyperthermia)

• Halothane can cause malignant hyperthermia in susceptible individuals (1 in 50,000), who have an abnormal ryanodine receptor (Ryr)  in the sarcoplasmic reticulum of skeletal muscles. This leads to excess calcium entry through the calcium channels and produces increased heat production, increased body temperature, persistent muscle contraction, and acidosis.
• Treatment of malignant hyperthermia involves a directly acting skeletal muscle relaxant Dantrolene by Intravenous route, intravenous sodium bicarbonate, external cooling, and 100% oxygen inhalation.

3. Heart : Cardiac arrhythmias, sensitization of myocardium to the action of catecholamines

4. Hypotension: By direct myocardial depression

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