1. ANATOMY & PHYSIOLOGY for NITROUS OXIDE
dr shabeel pn

2. Respiratory System Two functional parts Conducting zone
Respiratory zone

3. Conducting Zone Transports gases from outside to respiratory zone
Includes anatomic deadspace (150 ml)

4. Conducting Zone Nasal passages Pharynx Nasopharynx Oropharynx
Hypopharynx

5. Conducting Zone (cont.)
Larynx
Trachea
Bronchi (left and right mainstem)
Bronchioles

8. Respiratory Zone
Portion of lung where exchange of gases occurs between blood and air

9. Respiratory Zone Respiratory bronchioles Alveolar ducts Alveolar sacs
Alveoli

11. Alveolus Unit in which actual gas exchange occurs
Around 300 million total
A pocket of air surrounded by thin membrane (1-2 micrometers in thickness) containing capillaries

12. Alveolus Wall consists of 4 thin layers Mucinous covering
Alveolar epithelium (incomplete)
Interstitial layer
Endothelial cells (pulmonary capillaries)

14. Physiology of Respiration
Inhaled gases travel through conducting zone to respiratory zone
Gases diffuse across alveolar membranes according to pressure gradients
Pulmonary capillaries are a pool of blood vessels (70 sq meters)

16. Respiratory Mechanics
Pre inspiration - resting - -5cm H2O pleural
Peak inspiration - more negative pressures
End inspiration - negative pleural, 0 alveolar
Peak expiration -positive alveolar
End expiration - -5cm pleural, 0 alveolar

17. Respiratory Mechanics
Muscles Involved
Primary - Diaphragm
Intercostals
Accessory - Abdominals, Scalenes,
Sternocleidomastoid
Some back muscles

20. Respiratory Mechanics
Resting pleural cavity pressure = - 5 cm H20
Thorax expands, increasing negative pressure
Air flows into lungs/alveoli until positive pressure develops and gas exchange occurs
Air flows out of lungs to return to resting pleural pressure of negative 5 cm H20

21. Nitrous Oxide Inhalational Sedation May Represent the Most Ideal Sedation Technique

22. PHARMACOLOGY OF NITROUS OXIDE

23. Preparation of Nitrous Oxide
Ammonium nitrate crystals heated to 240 degrees celsius
Decomposition to nitrous oxide and water

24. Preparation of Nitrous Oxide
heat
NH4NO3 =======> N2O + 2 H2O
240 deg.C

25. Preparation of Nitrous Oxide
N20 is chemically scrubbed 99.5% pure
Stored in compressed form in metal cylinders
30% in liquid form in full cylinder
Nitric oxide (NO) is the most dangerous impurity (use only medical grade N2O)

26. Physical Properties
N2O is a nonirritating, sweet-smelling, colorless gas
Only inorganic substance other than CO2 to have CNS depressant properties
Only inorganic gas used to produce anesthesia in humans

27. Physical Properties
N2O liquid requires heat for vaporization into gaseous state
Relatively insoluble in the blood; Blood-gas solubility coefficient is 0.47 at 37 deg. C

28. Potency of Nitrous Oxide
Least potent of anesthetic gases
35 more times soluble than N2 in plasma
100 times more soluble than O2 in plasma
N2O + O2 can produce CNS depression

29. Potency of Nitrous Oxide
N2O in subanesthetic doses can produce analgesia
N2O +O2 at 20%/80% is equal-analgesic to 10 to 15 mg of morphine
Optimal concentration is 35%

34. Pharmacology of Nitrous Oxide
N2O is rapidly absorbed into the CV system, due to large concentration gradient of N2O between alveolar sacs and blood
N2O rapidly fills air-filled body cavities

35. Pharmacology of Nitrous Oxide
Due to rapid uptake, two phenomena are seen
Concentration effect - higher concentrations cause more rapid uptake of N2O
Second gas effect - a second anesthetic gas will also be taken up more rapidly than usual when added to N2O

36. Concentration Effect Seen only when using high concentrations of a gas
The higher the concentration inhaled, the more rapidly the arterial tension of the gas increases
The diffusion gradient from the lungs into blood results in a greater uptake of gas into lungs

37. Second Gas Effect
Occurs when another inhalation anesthetic is used with N2O
Rapid uptake of N2O produces a vacuum in alveoli
Second gas also undergoes rapid uptake along with N2O

38. Absorption
CNS saturation occurs by displacement of N2 by N2O, usually in 3-5 minutes
Tissues with greater blood flow (brain, heart, liver, kidney) receive greater amounts of N2O

39. Absorption
Tissues with poor blood supply (fat, muscle, connective tissue) absorb small amounts
Slow absorption occurs once primary saturation is completed
Therefore no body reservoir present to slow recovery once N2O terminated

40. Biotransformation N2O undergoes no biotransformation in the body
Majority of N2O is exhaled unchanged 3 to 5 mins. following termination of delivery
1% eliminated via skin and lungs in 24 hours

41. Diffusion Hypoxia
Can occur following termination of N2O if patient is allowed to breathe only room air
“Hangover” effect (headache,nausea,lethargy) is produced
Prevented by having the patient breathe 100% O2 for minimum of 3 to 5 minutes

42. Diffusion Hypoxia
Rapid diffusion of N2O from blood to lungs results in decreased CO2 arterial tension with decreased stimulus for respiration
Rapid diffusion of N2O back into lungs dilutes alveolar O2 with resultant hypoxia

43. Pharmacology of Nitrous Oxide
N2O is non-allergenic
Least toxic of inhalational agents

44. Effects of Nitrous Oxide on Organ Systems

45. Central Nervous System
Actual mechanism unknown
Mild depression of CNS (cerebral cortex) in conjunction with physiological levels of O2 (greater than 20%)
Sensations depressed (sight, hearing, touch, pain)

46. Cardiovascular System
No changes in heart rate or cardiac output
Blood pressure remains stable with only slight decrease
Cutaneous vasodilation

47. Respiratory System N2O is non-irritating to pulmonary epithelium
Changes (drop) in rate and depth more likely due to anxiolytic effects
Slight elevation of resting respiratory minute volume at 50%/50%

48. GI System
No clinically significant effects, unless there is a closed space (obstruction)
N/V rarely seen unless hypoxia present
Can be used in hepatic dysfunction

49. Hematopoietic System
Long-term exposure (greater than 24 hours) can produce transient bone marrow depression

50. Musculoskeletal System
No direct relaxation of skeletal muscle
Anxiolytic effects help relaxation

51. Reproductive System Uterine contractions not inhibited
Pregnancy is a relative contraindication (avoid in first trimester)