ANESTHETICS-Contd...
General Anesthetics-1.Inhalers:-
1.Halothane
2.Enflurane
3.Isoflurane
4.Desflurane
5.Sevoflurane
6.Nitrous Oxide
MINIMUM ALVEOLAR CONCENTRATION(MAC):-
The concept of MAC is used to define the potency of the inhaler anesthetics.
MAC is the minimum alveolar concentration of the anesthetics necessary to suppress movements among 50% of the individuals challenged by a Standardised Skin Surgical Stimulus at a steady-state(assuming at a minimum concentration for 15 minutes remained in the alveolar chamber) at 1 atmosphere (at sea level)
The potency is calculated as an inverse proportion to the MAC. That means the greater the MAC for an agent the higher the amount of the agent needed to eliminate the movements of the individual in constant time and the lower the potency. (e.g.-Nitrous Oxide)
The MAC of a safer agent can be reduced by concomitant administration of some adjuncts such as analgesics or opioids.
1.HALOTHANE
The first generation halogenated anesthetic available in the market. Chemically it is Bromo- chlorotrifluoromethane.
It is a colorless, pleasant smelling halogenated ethane, unstable towards the light, and is stored in dark brown colored bottle mixed with thymol as a stabilizer. It is the only brominated anesthetic and because of its bromine content which increases its toxicity it is mostly replaced in developed countries by many modern inhalers.
Clinical Use:-
Because of its pleasant smell and lack of liver toxicity it is still used in pediatrics.
Kinetics and Metabolism:-
80% of the drug is eliminated in the expired air unchanged. The remaining 20% is metabolized in the liver to trifluoroacetic acid which may rarely cause (1 in 10,000) cause liver injury. Trifluoroacetic acid is excreted by the kidneys.
MAC of Halothane
MAC = 0.75%
Side Effects:-
1.Arrhythmia (by sensitizing the heart muscles to catecholamines.)
2.Decrease heart rate
3. Decrease cardiac output.
4.Hypotension with reduced peripheral resistance
5.Malignant hyperthermia-a common reaction of inhaled anesthetics composed of higher temperature, metabolic acidosis, tachycardia, and accelerated muscle contraction. Malignant Hyperthermia can be controlled by the administration of Dantrolene.
6.Liver toxicity particularly in adults (Halothane Hepatitis)
2.ENFLURANE:-
Enflurane is used to induce rapid anesthesia.
Metabolism and Kinetics
Approximately 2% of the drug is metabolized to a fluoride ion which is then excreted by the kidney.
The 98% remained is eliminated by expired air unchanged.
MAC of Enflurane
MAC = 1.6%
Side Effects:-
1.Cardio-Vascular-Decreased heart rate, Peripheral vascular resistance, and B.P.
Unlike halothane enflurane does not cause arrhythmias as it does not sensitize the myocardium to catecholamines.
2.Kidney damage by the fluoride ion after prolonged uses
Enflurane is contraindicated to those who have already kidney failure.
3.ISOFLURANE
Almost 100% of this drug is eliminated by expired air and is very safe as it does not pass into the system unless a fraction.
MAC=1.4%
Cardiovascular Effects
1.Increased heart rate
2.Decrease B.P by decreasing peripheral resistance
3.Malignant hyperthermia
Isoflurane does not sensitize the myocardium to catecholamines and hence does not cause
arrhythmias.
4.DESFLURANE
Very similar to isoflurane this drug also 100% eliminated through expired air unless a fraction can enter into the system on prolonged use.
MAC = 6%
Side Effects:-
1. Cardiovascular effects are similar to isoflurane
2.Higher risk of Malignant Hyperthermia
5.SEVOFLURANE
Metabolism is very similar to enflurane as a small quantity can enter into the system and the remaining portion is eliminated by the expired air unchanged. But it differs from enflurane it does no cause nephrotoxicity as it is hypothesized that it is not excreted by the kidneys.
MAC=2%
Side Effects
1.Cardiovascular-Increase heart rate, lower B.P by reducing peripheral resistance,
It does not cause arrhythmias as with enflurane.
2.Malignant hyperthermia.
6.NITROUS OXIDE
As it is a weaker anesthetic it is mostly used as an adjunct to induce anesthesia.
It can be administered either by inhalation or by I.V.
Nitrous oxide is expelled out without change as it is not metabolized.
MAC=100%, very weak as even if 100% nitrous oxide is given to the patients it does not attain the stage of surgical anesthesia.
Toxicity
1.Minimal cardiovascular effects
2.Bone marrow suppression if used for a long time
3.Neuropathies.
Contraindication
Pneumothorax-a condition in which there are closed cavities in the lung and the gas cannot escape out and will diffuse within the cavities to increase internal cavity pressure.
Precaution
The patient must be administered with sufficient oxygen during the recovery phase as nitrous oxide from the blood will diffuse into the alveoli and entirely replace the oxygen.
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