Inhalation anesthetics.pptx

Inhalation anesthetics Inhaled gases are used primarily for maintenance of anesthesia. Depth of anesthesia can be rapidly altered by changing the inhaled concentration.  Uptake and distribution of inhalation anesthetic:- The principal objective of inhalation anesthesia is a constant...

Inhalation anesthetics Inhaled gases are used primarily for maintenance of anesthesia. Depth of anesthesia can be rapidly altered by changing the inhaled concentration.  Uptake and distribution of inhalation anesthetic:- The principal objective of inhalation anesthesia is a constant and optimal brain partial pressure (Pbr) of inhaled anesthetic (partial pressure equilibrium between alveoli [Palv] and brain [Pbr]). Factors raising the alveolar anesthetic concentration  Inspired concentration (Fi) is the concentration of an inhaled anesthetic in the inspired gas mixture; ↑ inspired anesthetic concentration → ↑ alveolar anesthetic concentration → ↑ rate of transfer into the blood → ↑rate of induction of anesthesia.  Alveolar ventilation is the volume of fresh gases entering the alveoli per minute: ↑ minute alveolar ventilation (i.e. ↑rate and depth of ventilation) ↑ alveolar concentration (FA)  Time constant: is the time required for flow through a container to equal the capacity of the container. The time constant for lung is FRC/V alveolar.  Functional Residual Capacity (FRC) is the volume of air present in the lung at the end of passive expiration ↑ FRC → ↓ washin of a new gas.  FA /FI Ratio; ratio of alveolar agent to inhaled agent is an important determinant of the speed of induction. - ↓ blood anesthetic solubility → ↑ FA /FI. - ↓ Cardiac output → ↑ FA /FI. - ↑ Minute ventilation → ↑ FA /FI N.B an inhalation anesthetic agent with low solubility in blood shows fast induction and also recovery time (e.g. N2O), and an agent with relatively high solubility in blood shows slower induction and recovery time ( e.g. halothane).  Classification of inhalation anesthetic agents 1. Gas: Nitrous oxide 105% 2.Volatile liquids: Ether not used now Halothane 0.75% Isoflurane 1.2% Enflurane 1.7% Sevoflurane 2% Desflurane 6% Nitrous oxide (N20) -Is an oxide of nitrogen, commonly known as laughing gas - At room temperature; it is a colourless volatile gas - Non-flammable gas with a slightly sweet odour and test - It is poorly soluble in blood - Very rapidly in induction and recovery Concentration; in general anesthesia 30% to 70% in combination with oxygen Safety; - Nitrous oxide does not depress respiratory and does not produce muscle relaxation - Little or no effect on CVS Side effects; Hallucination, postoperative nausea, vomiting and toxicity - Prolonged exposure may cause megaloblastic anemia (due to decrease methionine synthase activity) ETHER Physical property: Colorless, highly volatile, pungent odor, flammable, explosive and stored in cool area MAC 2-3% Advantages: CNS depression Excellent muscle relaxant Disadvantages: flammable, irritate mucous membrane (increase in respiratory secretion) Breath holding, nausea and vomiting Contraindications: Respiratory system: increase in respiratory secretion Kidney: decrease in urine out put  Halothane (Fluothane) Is colourless volatile liquid, non-flammable and pleasant smelling at room temperature. But light sensitive (packaged in dark-coloured bottles). - It is produce; rapid induction, and quick recovery - Non flammable, non irritate and non explosive - Low solubility in plasma - Sensitizes myocardium, not sufficient muscle relaxation NMB may be required ( dose must be adjusted) - 70% exhaled as such, 30% metabolized in liver - MAC 0.75 uses in induction and maintenance of general anesthesia Effects on system; I. Cardiovascular system (cvs) :  Myocardial muscle relaxation which decrease in oxygen demand so its good for patient with coronary artery disease  Induces vasodilation  Decrease in cardiac out put, and heart rate  Sever hypotension and circulatory failure may occur with over dosage II. Respiratory system: a) Non-irritate to respiratory tract. b) Dilates the bronchioles c) Decrease salivation and bronchial secretions. d) Depress pharyngeal and laryngeal reflexes N.B. Halothane and sevoflurane agents of choice in patients with airway problems ( not have pungent odour to stimulate respiratory reflexes) III. Liver - Liver damage ( Halothane Hepatitis) - Jaundice, fever and hepatic necrosis By repeated administration of Halothane ( at intervals of less than 6 weeks)  Main advantage - Rapid smooth induction - Minimal stimulation of salivary & brachial secretion - Bronchiodilatation - Muscle relaxant - Relatively rapid recovery  main disadvantages - Poor analgesia - Post operatively shivering - Post operative nausea and vomiting - Possibility of liver toxicity  Modern Inhalation Anesthetics:-  Isoflurane (Forane) - Is a halogenated ether. - Colorless, nonflammable liquid at room temperature. - Has a high degree of pungency - Causes hypertension & tachycardia due to vasodilation - Induction & recovery are quicker, but it is irritant to respiratory tract so induction is unpleasant - MAC 1.2% Effects on system I. Respiratory : dose dependent depression of ventilation and bronchodilator II. CVS : - - myocardial depressant - Coronary vasodilation (coronary steal syndrome) - Increase pulse rate III. Hepatic system : decrease hepatic blood flow Advantage : - Rapid induction and recovery. - Little risk of hepatic or renal toxicity. - Cardiovascular stability. - muscle relaxation. Disadvantages - Pungent odor. - Coronary vasodilatation.  Sevoflurane ( sevoane) - Stable and pleasant smell - Use for induction and maintenance of general anesthesia in adult and pediatric. - It has a rapid onset and recovery due to low blood solubility - Non flammable - MAC 2%  effects on systems I. Respiratory: - Non irritant to respiratory tract , (so it is best for inhalation induction in children) II. CVS : same as isoflurane Advantages - Well tolerated ( non-irritant, sweet odor) , even at high concentration - Rapid induction and recovery - Does not sensitize the myocardium to catecholamines as much as halothane - Does not result in carbon monoxide production with dry soda lime  disadvantages - Less potent than similar halogenated agents - Interacts with CO2 absorbers. In the presence of soda lime compound A is produced which is toxic to brain, liver, and kidney  Desflurane : (Suprane) - It is very rapid induction and recovery due to low blood solubility - MAC 6% - Like Isoflurane, it has a pungent odour - Used for maintenance of general anesthesia, but due to high cost may precludes its use - Need special vaporizer - It is a pulmonary irritant and can cause bronchospasm   Adverse effects of inhaled anesthetics 1) Malignant hyperthermia:  Is a rare (1:20000) but life-threatening condition that may occur when anesthetics are used with NMBs like succinylcholine.  It is due to genetic abnormality in Ca2+ release channels (ryanodine receptor) in sarcoplasmic reticulum (SR) of muscle cells, leading to massive release of Ca+2 from SR following exposure to inhaled anesthetics or succinylcholine.  Symptoms includes muscle spasm, hyperthermia, hypertension, tachycardia and hyperkalemia  Treatment includes dantrolene, a muscle relaxant that blocks ca2+ release channels + supportive measure 2) Uterus: inhaled anesthetics cause uterine relaxation which can be helpful when uterine relaxation is required for intrauterine fetal manipulation or manual extraction of a retained placenta. However,it can also lead to increased uterine bleeding after delivery when uterine contraction is desired.