ANATOMY & PHYSIOLOGY for NITROUS OXIDE

1
ANATOMY PHYSIOLOGYfor NITROUS OXIDE
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

6
(No Transcript)
7
(No Transcript)
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

10
(No Transcript)
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)

13
(No Transcript)
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)

15
(No Transcript)
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

18
(No Transcript)
19
(No Transcript)
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 OFNITROUS 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 gt 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

30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
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 Oxideon 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)