Drugs used for general anesthesia are an important topic in the NEET PG exam because anesthesia is a critical component of many medical procedures. A thorough understanding of the topic is essential for any medical professional.
In the exam, questions related to the pharmacology, mechanism of action, adverse effects, and clinical applications of anesthetic drugs are often asked, making it all the more to study for the NEET PG exam.
Xenon: Carrier gas, but due to high cost, currently not in use
PHARMACOKINETICS AND PHARMACODYNAMICS
Properties of potent inhalation agent
Chemistry
Halothane
Isoflurane
Sevoflurane
Desflurane
Halogenated Ethane
Halogenated Ethers
Isoflurane and Desflurane are structural analogues
Enflurane is isomer of isoflurane
Boiling point
50° C ± 2° C
Room Temp. 22° C
TEC 2,3,4,7 or any other vaporizer can be used
Highest vapor pressure
TEC 6 vaporizer is used
Color coding
Red
Purple
Yellow
Blue
MAC50
0.75
1.12
2
6
Important Information
MAC: Minimal Alveolar concentration of the vapor / anesthetic agent to prevent movement (Motor Response) in 50% of subject in response to surgical stimulus.
↑↑ MAC : Potency ↓↓
↓↓ MAC : Potency ↑↑
Blood gas partition coefficient or blood gas solubility
Halothane
Isoflurane
Sevoflurane
Desflurane
Blood gas partition coefficient
2.25
1.3
0.63
0.42 Fastest speed of induction and Recovery
Blood gas partition coefficient tells about the
Speed of Induction
Speed of Recovery
Low blood gas partition coefficient: Faster speed of induction and recovery (Vice Versa).
Important Information
For induction we seldom use desflurane because of its irritant property: Causes Bronchospasm and Laryngospasm.
Sevoflurane is inhalational induction agent of choice, has sweet smell
Sevoflurane and Desflurane: Both good for day care surgery, since they give fast recovery.
Stability
Halothane
Isoflurane
Sevoflurane
Desflurane
Most unstable
Most metabolized
Can cause autoimmune hepatitis
Preservative Needed: Thymol (0.01%)
Stable
Stable
Most stable
Very good
Agent for the maintenance
Smell of Agents
Halothane
Isoflurane
Sevoflurane
Desflurane
Fruity smell
Used both for induction & maintenance
Little bit irritant
Only used for maintenance
Sweet smell
Induction & maintenance
Inhalational induction agent of choice
Irritant (max)
Maintenance
For maintenance isoflurane > sevoflurane
PHARMACODYNAMIC PROPERTIES
CNS Effects
Halothane
Isoflurane
Sevoflurane
Desflurane
↓ cerebral metabolic O2
↓ Cerebral metabolic O2
↓ Cerebral metabolic O2
↓ Cerebral metabolic O2
CBF increase (maximum)
↑↑↑ ICP maximum
CBF increase ↑ ICP
CBF increase ↑ ICP
CBF increase ↑ ICP
↓ EEG
↓ EEG
↓ EEG
↓ EEG
Important Information
There is ↓ cerebral metabolic O2 requirement in all.
Inhalational agent are uncouplers of brain, they dilate the cerebral blood vessel: ↑ Cerebral Blood Flow (CBF) because of intrinsic vasodilation of cerebral blood vessel
In IV Anesthetic Agent it is opposite, ↓ Cerebral Metabolic O2: ↓ CBF.
All inhalational Agent cause ↑ICP (intra cranial pressure).
Halothane not used for Neurosurgery, Rest of the inhalational agents can be used.
Best Agent of Neurosurgery: Desflurane
2nd best agent for Neurosurgery: Isoflurane
Effects on CVS
Halothane
Isoflurane
Sevoflurane
Desflurane
Direct Myocardial Depressant
↓ HR
↓ CO
↓ BP
Sensitize Heart to the Arrhythmogenic effect of catecholamines.
Contraindicated for CVS surgery
Cardio stable
↓ Systemic Vascular Resistance
For neurosurgery: Isoflurane > Sevoflurane
Cause the Coronary steel phenomenon
All 3 can be used for CVS surgery
Important Information
Least cardio depressant: Desflurane > sevoflurane > isoflurane.
Best agent for CVS surgery: Desflurane.
Halothane not used for neurosurgery
Desflurane is best agent for neurosurgery
Agent most commonly used for CVS surgery: Isoflurane because desflurane is not commonly available.
Sevoflurane not used for prolonged hours.
Effects on Respiratory system
All of the inhalational agents are respiratory depressants.
Blunts hypoxic/ hypercapnic drive.
All are bronchodilators.
Best bronchodilator: Halothane
Sevoflurane is also a very good bronchodilator, so mostly used in developed countries.
Effects on liver
Halothane
Isoflurane
Sevoflurane
Desflurane
Autoimmune hepatitis
Disrupts dual blood flow of liver
Bad for liver
All 3 can be used for Hepatic surgery.
Contraindicated for liver surgery
Good for liver surgery.
Best for liver transplant surgery.
Best for liver surgery.
Important Information
For liver surgery Desflurane > isoflurane.
Effects on kidney
Halothane
Isoflurane
Sevoflurane
Desflurane
No effect
No effect
Bioproduct of metabolism (Compound – A Nephrotoxic)
All Inhalational agents provide good muscle relaxation but none of them provide analgesia.
All of them can precipitate malignant hyperthermia
MALIGNANT HYPERTHERMIA
Inhalational agents predisposes to this condition, but IV agents are safe in it.
Pharmacogenetic Disease: AD form is expressed.
Drugs triggering
1. SuccinylCholine
All halogenated Inhalational Agent
2. Potent inhalational
Lignocaine
Mechanism
When a person having gene receives these drugs it gets activated and it stimulates SR which causes massive outflow of calcium inside the cell activating CGMP pathway and this increases the metabolism of our body by 2-3 times.
Clinical features
It presents with
Hypercarbia
Tachycardia
Hyperthermia
Increased BP
Increased HR
Arrhythmias
All occur due to hypermetabolism and result in cell lysis, and further cause myoglobinuria, acidosis, renal failure, hyperkalemia.
Management
Discontinue the triggering Agent.
Intravenous Dantrolene
Central Muscle Relaxant.
Stops release of Ca2+.
Side Effects- Respiratory insufficiency and muscle weakness.
Symptomatic Management
Anesthesia safe in Malignant Hyperthermia: TIVA (Total Intravenous Anesthesia)
Propofol: Agent of choice.
CARRIER GAS
N2O
Xenon
MAC: 104, least potent
MAC: 70, more potent than N2O
Blood gas solubility: 0.45
Blood gas solubility: 0.11-0.19 (fastest)
CVS unstable sympathetic stimulation
CVS stable
Not metabolically inert Peripheral neuropathy Megaloblastic anemia Bone marrow depression
Metabolically inert
Environmental pollutant
Environment friendly
-
Expensive, not in clinical use
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