Anesthetics

Isoflurane

Isoflurane is a newer, safer anesthetic.  The down side is that it stinks so badly that patients will hold their breath so you need to induce with something like thiopental.

Methoxyflurane and Sevoflurane

Just more halogens

N₂O

N₂O is probably the safest inhaled analgesic but is a poor anesthetic so others are needed in mixture.  It can cause hypoxia so at least 20% of the mixture must be O₂.

Prolonged N₂O exposure can cause vitamin B₁₂ deficiency from messing up thiamine synthase.  This thiamine synthase deficiency causes megaloblastic anemia.

IV Anesthesia

IV anesthetics are used for rapid induction.  Then gasses are used to maintain it.

Barbs

Thiopental is a good anesthetic only.  It is metabolized slowly but it’s rapid redistribution out of the brain cause a short lived anesthetic effect.

Barbs can cause respiratory trouble so don’t use with asthmatics.

Benzos

Diazepam, lorazepam and midazolam facilitate sedation.

Opoids

Fentanyl, morphine and others are good for analgesia only and depress CNS function.  They work well with N₂O for cardiac surgery.

Neuroleptianesthesia

Inovar is a mixture of fentanyl (opoid) and droperidol (neuroleptic) that is called neuroleptianesthesia.

Ketamine

Ketamine is a short acting analgesic/anesthetic where the patient appears to be somewhat awake but is really out of it.  It causes cerebral vasculature dilation so it can cause hallucinations as you come out of it.

Propofol

Propofol is a new short induction anesthetic that must be used with another to keep the patient out for more than 5 min.

Local Anesthesia

Local anesthesia works by blocking nerves in the inactivated and active stages.  It causes increased threshold.

Local anesthesia has a strong pH effect because it must be in the non-ionized form to diffuse into a neuron and then must be converted to it’s cationic form to bind to the receptors on the inside of the cell.

Hypercalcemia causes decreased effects while hyperkalemia enhanced LA.

Amides are the drugs with two “I’s” in the name while esters have only one “I”.  The amides have two “I’s” and are metabolized in the liver while the esters are metabolized by plasma enzymes.  This demonstrates that amides are more likely to experience systemic toxicity because they must go through systemic circulation, effecting high perfusion organs, on their way to the liver.

Problems develop if the anesthetic goes systemic causing CNS depression and methemoglobemia.

Use caution with cardiac disease, hyperthyroidism, diabetes and myasthenia patients.  Diabetics have problems because of pH with diabetic acidosis, and myasthenia patients have senses that are already dulled enough and it’s dangerous to do more.

Local Uses

There are six ways to use LA:

1)      Surface anesthesia is when you apply the LA directly to a mucous membrane or to broken skin.  This allows it go get into the bloodstream so you must use it cautiously.

2)      Infiltration is when you put it on good skin and let it filter in.

3)      IV regional is when you cut off circulation to a limb with a pressure cuff and give a local.

4)      Nerve block involves injection near a nerve.

5)      Spinal is when you inject it into the CSF (sub-arachnoid) and it will dull nerves from the injection level down (and maybe up a little).

6)      Epidural is to block the spinal nerve roots at a specific level.

Uses

Amides include lidocaine, prilocaine and bupivacine and are used for all types of LA.  Prilocaine can cause methomyglobemia while bupivcaine can cause cardiotoxicity.

Esters:

• Benzocaine is for contact dermatitis.
• Cocaine is for topical mucus membranes.  It is a vasoconstrictor so you don’t need to specifically administer a vasoconstrictor to keep it local.
• Tetracaine has a greater potential for toxicity.

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