Halothane. In this topic, simple English used will enable readers to easily understand and grasp the content and improve or better manager use the drug

Anaesthesia topic MBCHB

1. HALOTHANE (FLUTHAME) The drug is a flouronated carbon, colorless volatile liquid, kept in Amber colored bottle as 250mls. The boiling point is 50 c. It has a pleasant smell. It is relatively non-irritant to respiratory tract and non-inflammable as opposed to Ether. Decomposition occurs when exposed to light that’s why it is kept in and amber colored bottle with addition of Thymol (Thymol is a chemical substance in Halothane blocking decomposition). It’s stored in cupboard away from light. It is first of the modern Halogenated anesthetic agents, first used in 1956. It’s quite potent with quick induction and recovery. It was administered via calibrated vaporizer O.M.V i.e. Oxford miniature vaporizer. Its induction is achieved with 2-4%. Maintenance is 0.5-2%. It has no analgesic action, it’s best used with nitrous oxide which is a very potent analgesic gas. Has a M.A.C of 0.7 1

2. SYSTEM EFFECTS C.N.S -It induces sleep rapidly in 2-3 minutes and also quick recovery because it doesn’t have blood/gas partition coeffiency (less soluble). -It is able to achieve deep anesthesia with some relaxation but no analgesia. -It increases cerebral blood flow moderately increasing intercranial pressure. -Cerebral oxygen requirement is reduced. C.V.S -Cardiac output and pressure falls because of myocardial depression. -Vasodilation also occurs due to sympathetic ganglionic block. -Sinus (atrial) bradycardia may occur as a result of increased tone of vagus nerve further lowering the blood pressure. -Ventricular extrasystal (arrythmia [this is because halothane has releasing ability of adrenaline]) secondary to the heart sensitizing heart to catecholamal worsened by hypercapnia. 2

3. Respiratory tract -It abolishes ventilatory response to cause hypercapnia and hypoxia with resultant respiratory depression. Depression becomes worse as the anesthetic becomes deepened. -It has a sweet smell, good for gaseous induction -Broncho-muscles are relaxed causing Broncho-dilation. Useful in asthmatics -There is no Broncho-secretion. -Both bronchioles and laryngeal reflexed are depressed.   G.U.T -Uterine muscle relaxation causing bleeding especially in caesarian section when dose above 0.5% is used. Renal system -There is reduced renal blood flow with reduction of urinary output. 3

4. ADVANTAGES Cause no Bronchial secretion Cause Bronchial-dilation good for air entry to the lungs Rapid induction and quick recovery Non-irritant It potentiates the use of non-depolarizing muscle relaxant allowing reduced doses to be used. It’s used for inducing controlled hypotension for example, neurosurgery to avoid much bleeding It is used with soda lime in closed circuit because it doesn’t react with soda lime (soda lime is carbon dioxide absorbent) DISADVANTAGES It causes liver toxicity if repeatedly used causing halothane hepatitis Safety margin is quite small therefore in an overdose the respiratory and heart are both affected simultaneously It causes release of adrenaline leading to arrythmia It may trigger malignant hyperpyrexia 4

5. HALOTHANE HEPATITIS Causative factors; Metabolites of reductive pathways (Halothane should be used in oxygen exclusively, without oxygen i.e. reductive gives a toxic metabolite to the liver) Repeated administration of halothane to adults at an interval of less than 3 months worse with obese, aged, females. Note that liver disease is not a contraindication so that as long as the cause was not related to halothane. 5

6. NITROUS OXIDE (N 2 O) (LAUGHING GAS) This is a colorless sweet-smelling gas with boiling of -89. It is neither inflammable nor explosive but supports combustion. At 400 c it decomposes releasing oxygen. It’s is supplied in a blue cylinder, compressed to liquid at a pressure of 650Ibl/m 2 (4485 Kpa ) ANESTHESIA It is a good analgesic but poor anesthetic, so it needs to be combined with a potent anesthetic such as halothane to achieve deeper anesthetic. Maximum safe concentration that can be administered in 70% (N 2 O) and 30% halothane to avoid hyperxia . It’s used as a vehicle of potent volatile agent such as halothane. 6

7. SYSTEMIC EFFECTS C.N.S -It gives potent analgesia with poor anesthetic -It increases cerebral blood flow with the increase in I.C.P -Causes degree of euphoria (happiness) that’s why it is called laughing gas. C.V.S -There are minimal effects on the overall functions of the heart. Respiratory tract -It causes slight increase in R/R but decrease in tidal volume. -It increases ventilatory response to carbon dioxide and hypoxia. This is by blocking the R/C in the medulla oblongata i.e. the center can receive from oxygen or carbon dioxide accumulated No muscle relaxation is experienced and elimination in respiratory system unchanged. 7

8. ADVANTAGES A good analgesic effective like morphine Relative induction and quick recovery It’s relatively safe provided hyperxia is avoided DISADVANTAGES It diffuses air filled cavities of the body for example, middle ear, sinuses, gut and may cause rise in pressure in them (cavities) It also causes diffusion hypoxia at the end of anesthesia because it rapidly diffuses into alveoli diluting the alveolar oxygen. It’s usually advisable to give pure oxygen after switching off N 2 O to avoid diffusion hypoxia. It causes bone marrow depression by interfering with D.N.A synthesis. It has been documented to be cytotoxic to experimental rats but NOT human beings and this is due to inhibition of synthesis of DNA. 8

9. ISOFLURANE PHYSICAL AND CHEMICAL PROPERTIES This anew inhalational anesthetic now widely used today. It is halogenated methyl ethyl ether. It is a colorless volatile liquid, with a saturated vapor pressure similar to halothane. Some suggested using it in a halothane vaporizer but this is not recommended. Blood gas solubility is lower (1.4) than that of halothane. It therefore provides a more rapid onset of recovery from, anesthesia. Further, fat solubility is also lower, which has the same effect. Minimum alveolar concentration (MAC) is 1.15 in oxygen. It has an irritant smell. Isoflurane is almost totally excreted by the lungs. 9

10. SYSTEMIC EFFECTS OF ISOFLURANE Central nervous system CNS depression and anesthesia but it has no convulsant effect. Cerebral blood flow and therefore intracranial pressure increases, though this is less than with halothane. Cardiovascular system (CVS) Little effect on myocardial contractility. A fall in blood pressure due to a decrease in peripheral vascular resistance, though this depends on dose. Heart rhythm is stable. Isoflurane does not increase the sensitivity of the myocardium to adrenaline. Respiratory system Decrease in tidal volume. Increase in respiratory rate. Muscle tone Relaxes muscles. Potentiates non-depolarizing muscle relaxants. Liver and kidney Negligible changes in liver function. Negligible changes in renal function. An association with malignant hyperpyrexia is under investigation. 10

11. ADVANTAGES OF ISOFLURANE Rapid onset and recovery from anesthesia No convulsant effect on central nervous system even with hypocapnia. Low risk of cardiac arrythmias and no increased sensitivity to adrenaline. Low toxicity to liver and kidneys Non-explosive and non-flammable DISADVANTAGES Expensive ADMINISTRATION Isoflurane can be administered using a Tec vaporizer on a conventional anesthetic machine or in a draw-over system using an OMV with the appropriate scale. 11

12. SEVOFLURANE PHYSICAL AND CHEMICAL PROPERTIES This is a newer inhalational agent. It is non-flammable with a pleasant smell. It has a very low blood solubility/gas solubility and provides the most rapid induction and recovery of all currently available inhalational agents. It has MAC of 1.7%. 5% is metabolized in the liver. 12

13. SYSTEMIC EFFECTS Central nervous system No significant effect in ICP or CBF at clinical levels Cardiovascular system Does not sensitize the myocardium to catecholamines Respiratory system Decrease in minute volume Increase in respiratory rate No airway irritation 13

14. USES It is becoming the agent of choice for inhalational induction especially in children but also in adults. ADVANTAGES OF SEVOFLURANE Rapid induction Rapid recovery Minimum cardiovascular effects DISADVANTGES High cost If being used as a sole inhalational agent in a draw-over system it would require two OMVs in series to give an adequate concentration. 14