Download PDF Notes & PPT: General Anaesthetics
Delve into comprehensive study materials on General Anaesthetics. This resource, available as a downloadable PDF, is crucial for students and professionals in medicine, anesthesiology, pharmacology, and critical care. You will find detailed notes and potentially PPT summaries covering the pharmacology of drugs used to induce a reversible state of unconsciousness.
Download these notes for offline study or view the document online. Understand the classification (inhalational and intravenous), mechanisms of action, pharmacokinetic properties, and clinical applications of various general anaesthetic agents used in surgical and medical procedures.
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Download PDF, General Anaesthesia, Anesthetic Drugs Notes, Inhalational Anaesthetics, IV Anaesthetics, Propofol, Sevoflurane, Isoflurane, MAC, Anesthesiology Pharmacology, Free Medical PDF, Slides By DuloMix.
Understanding General Anaesthetics: Pharmacology and Clinical Application
General anaesthesia is a reversible, drug-induced state of unconsciousness characterized by amnesia (loss of memory), analgesia (loss of pain sensation), akinesia (immobility/muscle relaxation), and attenuation of autonomic responses to noxious stimuli. This state allows patients to undergo surgical and other painful or distressing medical procedures without awareness or recall. The drugs used to achieve this are known as general anaesthetics, and their pharmacology is a critical area of study for anaesthesiologists, pharmacologists, and other healthcare professionals involved in perioperative care.
Goals and Components of General Anaesthesia
The primary goals of general anaesthesia include:
- Unconsciousness (Hypnosis): Loss of awareness of oneself and surroundings.
- Amnesia: Lack of memory for events occurring during anaesthesia.
- Analgesia: Absence of pain perception.
- Akinesia/Muscle Relaxation: Immobility to prevent reflex movements and provide optimal surgical conditions.
- Autonomic Stability: Control of haemodynamic and other physiological responses to surgical stress.
Often, a "balanced anaesthesia" technique is employed, using a combination of different drugs (e.g., intravenous agents for induction, inhalational agents for maintenance, opioids for analgesia, neuromuscular blockers for muscle relaxation) to achieve these components effectively and minimize side effects of any single agent.
Classification of General Anaesthetics
General anaesthetics are broadly classified into two main groups based on their route of administration:
1. Inhalational Anaesthetics
These are administered via the lungs and are typically used for the maintenance of anaesthesia, although some (like sevoflurane) can be used for induction, especially in paediatrics.
- Types:
- Volatile Liquids: These are liquids at room temperature that vaporize to be inhaled (e.g., Sevoflurane, Desflurane, Isoflurane). Halothane is an older agent, now rarely used due to risks like hepatotoxicity.
- Gases: These are gases at room temperature (e.g., Nitrous Oxide - N2O). Xenon is another gaseous anaesthetic but its high cost limits widespread use.
- Mechanism of Action: The exact mechanisms are not fully elucidated but are believed to involve multiple molecular targets. They enhance inhibitory neurotransmission (e.g., by potentiating GABAA and glycine receptors, activating two-pore domain K+ channels) and inhibit excitatory neurotransmission (e.g., by inhibiting NMDA receptors and neuronal nicotinic acetylcholine receptors). The "Meyer-Overton correlation" (potency correlates with lipid solubility) suggests interaction with lipid membranes, but specific protein interactions are considered primary.
- Pharmacokinetics: Uptake from alveoli into blood and distribution to the brain are key. Factors influencing this include:
- Inspired concentration (FI): Higher FI leads to faster induction.
- Alveolar ventilation: Increased ventilation speeds uptake.
- Blood:Gas Partition Coefficient (Solubility): Lower solubility (e.g., desflurane, N2O) leads to faster induction and emergence because less drug needs to dissolve in the blood to raise partial pressure. Higher solubility (e.g., isoflurane) means slower induction/emergence.
- Cardiac Output: High cardiac output can slow induction by removing anaesthetic from alveoli faster.
- Alveolar-to-Venous Partial Pressure Gradient: Drives uptake into tissues.
- Potency: Measured by Minimum Alveolar Concentration (MAC) – the alveolar concentration at 1 atm that prevents movement in 50% of patients in response to a standard surgical stimulus. MAC is additive.
- Elimination: Primarily via exhalation. Metabolism is minimal for modern volatile agents like sevoflurane and desflurane, reducing toxicity.
2. Intravenous (IV) Anaesthetics
These are administered directly into the bloodstream, typically used for induction of anaesthesia due to their rapid onset. Some can also be used for maintenance (Total Intravenous Anaesthesia - TIVA) or for sedation.
- Examples and Mechanisms:
- Propofol: Most common IV induction agent. Potentiates GABAA receptors. Rapid onset, short duration, antiemetic properties. Can cause hypotension and respiratory depression.
- Barbiturates (e.g., Thiopental, Methohexital): Potentiate GABAA receptors. Ultrashort-acting. Significant respiratory and cardiovascular depressants. Thiopental use has declined.
- Benzodiazepines (e.g., Midazolam): Potentiate GABAA receptors. Used for premedication, sedation, and induction at higher doses. Reversible with flumazenil.
- Etomidate: Potentiates GABAA receptors. Minimal cardiovascular depression, making it useful in haemodynamically unstable patients. Can cause adrenocortical suppression.
- Ketamine: NMDA receptor antagonist. Produces "dissociative anaesthesia" (analgesia, amnesia, catalepsy). Sympathomimetic effects (increases heart rate and blood pressure). Can cause emergence phenomena (hallucinations).
- Pharmacokinetics: Rapid distribution to the brain (highly perfused, lipophilic) leads to quick onset. Termination of effect after a single bolus is usually due to redistribution from the brain to other tissues (muscle, fat), rather than immediate metabolism. Context-sensitive half-time is important for infusions, describing the time for plasma concentration to fall by 50% after stopping an infusion of a specific duration.
Stages of Anaesthesia (Historical Perspective)
Guedel's stages of anaesthesia (described for ether) are historically important but less distinct with modern anaesthetics. They include Stage I (Analgesia/Amnesia), Stage II (Excitement/Delirium), Stage III (Surgical Anaesthesia, with four planes), and Stage IV (Medullary Depression/Overdose). Modern anaesthesia aims for rapid transition to Stage III.
Physiological Effects and Monitoring
General anaesthetics affect nearly every organ system, notably the cardiovascular (e.g., decreased blood pressure, altered heart rate) and respiratory systems (e.g., respiratory depression, decreased tidal volume). Continuous monitoring of vital signs (ECG, blood pressure, SpO2, EtCO2), depth of anaesthesia (e.g., Bispectral Index - BIS), temperature, and neuromuscular function (if blockers are used) is standard practice to ensure patient safety.
Conclusion
General anaesthetics are powerful drugs that enable a vast range of medical interventions. Their safe administration requires a profound understanding of their pharmacokinetics, pharmacodynamics, physiological effects, and potential interactions. The field continues to evolve with research into new agents and techniques to further improve safety and patient outcomes in anaesthesia.
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