Stages of Anesthesia:
|Stages of anesthesia|
Pharmacokinetic principles of volatile anesthetics:
- Anesthetic first saturates the alveoli. After the saturation of alveoli, it enters the blood.
- This depends upon blood/gas partition coefficient.
- Higher the blood/gas coefficient more time will be required for the saturation of blood (Induction time high).
- After saturation of blood, anesthetics enter in the tissue (including brain) & anesthesia achieved.
- This also depends on tissue/blood partition coefficient.
- Increase to 1.3 MAC = Immobility in 99% patients.
- As at equilibrium, con. in alveoli is equal to the con. in brain (site of action), hence MAC is used represent the potency of anesthetic.
- Increased catecholamines level CNS.
- Hyper thermia
- Decreased catecholamine in CNS
Theories about the mechanism of anestehesia:
- Later, postulated, anesthetics interacts with hydrophobic portion of membrane ----> distortion of Na+ pumps ------> Anesthesia.
- In the presence of anesthetics, membrane bloat & squeeze over Na+ channels ----> Interferes with depolarization.
- Not all lipid soluble anesthetics are potents.
- Chloride Channels:
- Sodium channels:
- Potassium Channels:
Classification of General anesthetics:
|Classification of general anesthetics|
Pharmacology of individual class of general anesthetics:
- Alone not used as low potency.
- Induction & recovery quick (low blood solubility).
- Thiopentone is used along for induction.
- Low effect on respiration, BP % heart.
- Potent anesthetic.
- Increased blood solubility.
- Increased irritation & inflammable (Not used in developed countries).
- Potent anesthetic.
- 2-4% for induction & 0.5-1% for maintenance.
- Cough suppression & bronchodilation- Suitable for asthamtics.
- During labour: Increased delivery & blood loss.
- Malignant hyperthermia: Abnormal RyR (Ryanodine receptor) Ca channels. Increased release of Ca2+ -------> Increased muscle contraction & heat (Succinyl choline increase the symptoms). IV dantrolene + O2 inhalation is used for the treatment.
- Isomer of enflurane.
- Safer in patients with myocardial ischemia.
- Preferred for neurosurgery.
- Very low blood solubility.
- Other properties similar to isoflurane.
- Ultrashort acting thiobarbiturate. High solubility in water & must be prepared fresh.
- Increased lipid solubility reaches brain quickly.
- Poor analgesic.
- Thiopentone & succinylcholine reacts chemically- Should not be mixed in same syringe.
- Superceded Thiopentone sodium.
- Bradycardia common otherwise safe drug.
- Poor analegiscs.
- Diazepam- Most common.
- Lorazepam- 3 times more potent than diazepam,
- Midozalam- Slower & faster acting.
- Dissociative anesthesia.
- Feeling of dissociation from one's body.
- Acts at cortex & subcortical region.
- Increased heart rate, blood pressure & carbon monoxide (Dangerous in hypertensives).
- Related to hallucinogen phenytoin.
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