ANATOMY & PHYSIOLOGY for NITROUS OXIDE

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ANATOMY & PHYSIOLOGY for NITROUS OXIDE

  1. 1. ANATOMY & PHYSIOLOGY for NITROUS OXIDE
  2. 2. Respiratory System <ul><li>Two functional parts </li></ul><ul><li>Conducting zone </li></ul><ul><li>Respiratory zone </li></ul>
  3. 3. Conducting Zone <ul><li>Transports gases from outside to respiratory zone </li></ul><ul><li>Includes anatomic deadspace (150 ml) </li></ul>
  4. 4. Conducting Zone <ul><li>Nasal passages </li></ul><ul><li>Pharynx </li></ul><ul><li>Nasopharynx </li></ul><ul><li>Oropharynx </li></ul><ul><li>Hypopharynx </li></ul>
  5. 5. Conducting Zone (cont.) <ul><li>Larynx </li></ul><ul><li>Trachea </li></ul><ul><li>Bronchi (left and right mainstem) </li></ul><ul><li>Bronchioles </li></ul>
  6. 8. Respiratory Zone <ul><li>Portion of lung where exchange of gases occurs between blood and air </li></ul>
  7. 9. Respiratory Zone <ul><li>Respiratory bronchioles </li></ul><ul><li>Alveolar ducts </li></ul><ul><li>Alveolar sacs </li></ul><ul><li>Alveoli </li></ul>
  8. 11. Alveolus <ul><li>Unit in which actual gas exchange occurs </li></ul><ul><li>Around 300 million total </li></ul><ul><li>A pocket of air surrounded by thin membrane (1-2 micrometers in thickness) containing capillaries </li></ul>
  9. 12. Alveolus <ul><li>Wall consists of 4 thin layers </li></ul><ul><li>Mucinous covering </li></ul><ul><li>Alveolar epithelium (incomplete) </li></ul><ul><li>Interstitial layer </li></ul><ul><li>Endothelial cells (pulmonary capillaries) </li></ul>
  10. 14. Physiology of Respiration <ul><li>Inhaled gases travel through conducting zone to respiratory zone </li></ul><ul><li>Gases diffuse across alveolar membranes according to pressure gradients </li></ul><ul><li>Pulmonary capillaries are a pool of blood vessels (70 sq meters) </li></ul>
  11. 16. Respiratory Mechanics <ul><li>Pre inspiration - resting - -5cm H2O pleural </li></ul><ul><li>Peak inspiration - more negative pressures </li></ul><ul><li>End inspiration - negative pleural, 0 alveolar </li></ul><ul><li>Peak expiration -positive alveolar </li></ul><ul><li>End expiration - -5cm pleural, 0 alveolar </li></ul>
  12. 17. Respiratory Mechanics <ul><li>Muscles Involved </li></ul><ul><li>Primary - Diaphragm </li></ul><ul><li>Intercostals </li></ul><ul><li>Accessory - Abdominals, Scalenes, </li></ul><ul><li>Sternocleidomastoid </li></ul><ul><li>Some back muscles </li></ul>
  13. 20. Respiratory Mechanics <ul><li>Resting pleural cavity pressure = - 5 cm H20 </li></ul><ul><li>Thorax expands, increasing negative pressure </li></ul><ul><li>Air flows into lungs/alveoli until positive pressure develops and gas exchange occurs </li></ul><ul><li>Air flows out of lungs to return to resting pleural pressure of negative 5 cm H20 </li></ul>
  14. 21. Nitrous Oxide Inhalational Sedation May Represent the Most Ideal Sedation Technique
  15. 22. PHARMACOLOGY OF NITROUS OXIDE
  16. 23. Preparation of Nitrous Oxide <ul><li>Ammonium nitrate crystals heated to 240 degrees celsius </li></ul><ul><li>Decomposition to nitrous oxide and water </li></ul>
  17. 24. Preparation of Nitrous Oxide <ul><li>heat </li></ul><ul><li>NH 4 NO 3 =======> N 2 O + 2 H 2 O </li></ul><ul><li>240 deg. C </li></ul>
  18. 25. Preparation of Nitrous Oxide <ul><li>N20 is chemically scrubbed 99.5% pure </li></ul><ul><li>Stored in compressed form in metal cylinders </li></ul><ul><li>30% in liquid form in full cylinder </li></ul><ul><li>Nitric oxide (NO) is the most dangerous impurity (use only medical grade N2O) </li></ul>
  19. 26. Physical Properties <ul><li>N2O is a nonirritating, sweet-smelling, colorless gas </li></ul><ul><li>Only inorganic substance other than CO2 to have CNS depressant properties </li></ul><ul><li>Only inorganic gas used to produce anesthesia in humans </li></ul>
  20. 27. Physical Properties <ul><li>N2O liquid requires heat for vaporization into gaseous state </li></ul><ul><li>Relatively insoluble in the blood; Blood-gas solubility coefficient is 0.47 at 37 deg. C </li></ul>
  21. 28. Potency of Nitrous Oxide <ul><li>Least potent of anesthetic gases </li></ul><ul><li>35 more times soluble than N2 in plasma </li></ul><ul><li>100 times more soluble than O2 in plasma </li></ul><ul><li>N2O + O2 can produce CNS depression </li></ul>
  22. 29. Potency of Nitrous Oxide <ul><li>N2O in subanesthetic doses can produce analgesia </li></ul><ul><li>N2O +O2 at 20%/80% is equal-analgesic to 10 to 15 mg of morphine </li></ul><ul><li>Optimal concentration is 35% </li></ul>
  23. 34. Pharmacology of Nitrous Oxide <ul><li>N2O is rapidly absorbed into the CV system, due to large concentration gradient of N2O between alveolar sacs and blood </li></ul><ul><li>N2O rapidly fills air-filled body cavities </li></ul>
  24. 35. Pharmacology of Nitrous Oxide <ul><li>Due to rapid uptake, two phenomena are seen </li></ul><ul><li>Concentration effect - higher concentrations cause more rapid uptake of N2O </li></ul><ul><li>Second gas effect - a second anesthetic gas will also be taken up more rapidly than usual when added to N2O </li></ul>
  25. 36. Concentration Effect <ul><li>Seen only when using high concentrations of a gas </li></ul><ul><li>The higher the concentration inhaled, the more rapidly the arterial tension of the gas increases </li></ul><ul><li>The diffusion gradient from the lungs into blood results in a greater uptake of gas into lungs </li></ul>
  26. 37. Second Gas Effect <ul><li>Occurs when another inhalation anesthetic is used with N2O </li></ul><ul><li>Rapid uptake of N2O produces a vacuum in alveoli </li></ul><ul><li>Second gas also undergoes rapid uptake along with N2O </li></ul>
  27. 38. Absorption <ul><li>CNS saturation occurs by displacement of N2 by N2O, usually in 3-5 minutes </li></ul><ul><li>Tissues with greater blood flow (brain, heart, liver, kidney) receive greater amounts of N2O </li></ul>
  28. 39. Absorption <ul><li>Tissues with poor blood supply (fat, muscle, connective tissue) absorb small amounts </li></ul><ul><li>Slow absorption occurs once primary saturation is completed </li></ul><ul><li>Therefore no body reservoir present to slow recovery once N2O terminated </li></ul>
  29. 40. Biotransformation <ul><li>N2O undergoes no biotransformation in the body </li></ul><ul><li>Majority of N2O is exhaled unchanged 3 to 5 mins. following termination of delivery </li></ul><ul><li>1% eliminated via skin and lungs in 24 hours </li></ul>
  30. 41. Diffusion Hypoxia <ul><li>Can occur following termination of N2O if patient is allowed to breathe only room air </li></ul><ul><li>“ Hangover” effect (headache,nausea,lethargy) is produced </li></ul><ul><li>Prevented by having the patient breathe 100% O2 for minimum of 3 to 5 minutes </li></ul>
  31. 42. Diffusion Hypoxia <ul><li>Rapid diffusion of N2O from blood to lungs results in decreased CO2 arterial tension with decreased stimulus for respiration </li></ul><ul><li>Rapid diffusion of N2O back into lungs dilutes alveolar O2 with resultant hypoxia </li></ul>
  32. 43. Pharmacology of Nitrous Oxide <ul><li>N2O is non-allergenic </li></ul><ul><li>Least toxic of inhalational agents </li></ul>
  33. 44. Effects of Nitrous Oxide on Organ Systems
  34. 45. Central Nervous System <ul><li>Actual mechanism unknown </li></ul><ul><li>Mild depression of CNS (cerebral cortex) in conjunction with physiological levels of O2 (greater than 20%) </li></ul><ul><li>Sensations depressed (sight, hearing, touch, pain) </li></ul>
  35. 46. Cardiovascular System <ul><li>No changes in heart rate or cardiac output </li></ul><ul><li>Blood pressure remains stable with only slight decrease </li></ul><ul><li>Cutaneous vasodilation </li></ul>
  36. 47. Respiratory System <ul><li>N2O is non-irritating to pulmonary epithelium </li></ul><ul><li>Changes (drop) in rate and depth more likely due to anxiolytic effects </li></ul><ul><li>Slight elevation of resting respiratory minute volume at 50%/50% </li></ul>
  37. 48. GI System <ul><li>No clinically significant effects, unless there is a closed space (obstruction) </li></ul><ul><li>N/V rarely seen unless hypoxia present </li></ul><ul><li>Can be used in hepatic dysfunction </li></ul>
  38. 49. Hematopoietic System <ul><li>Long-term exposure (greater than 24 hours) can produce transient bone marrow depression </li></ul>
  39. 50. Musculoskeletal System <ul><li>No direct relaxation of skeletal muscle </li></ul><ul><li>Anxiolytic effects help relaxation </li></ul>
  40. 51. Reproductive System <ul><li>Uterine contractions not inhibited </li></ul><ul><li>Pregnancy is a relative contraindication (avoid in first trimester) </li></ul>

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