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    • Respiratory System
      • Two functional parts
      • Conducting zone
      • Respiratory zone
    • Conducting Zone
      • Transports gases from outside to respiratory zone
      • Includes anatomic deadspace (150 ml)
    • Conducting Zone
      • Nasal passages
      • Pharynx
      • Nasopharynx
      • Oropharynx
      • Hypopharynx
    • Conducting Zone (cont.)
      • Larynx
      • Trachea
      • Bronchi (left and right mainstem)
      • Bronchioles
    • Respiratory Zone
      • Portion of lung where exchange of gases occurs between blood and air
    • Respiratory Zone
      • Respiratory bronchioles
      • Alveolar ducts
      • Alveolar sacs
      • Alveoli
    • 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
    • Alveolus
      • Wall consists of 4 thin layers
      • Mucinous covering
      • Alveolar epithelium (incomplete)
      • Interstitial layer
      • Endothelial cells (pulmonary capillaries)
    • 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)
    • 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
    • Respiratory Mechanics
      • Muscles Involved
      • Primary - Diaphragm
      • Intercostals
      • Accessory - Abdominals, Scalenes,
      • Sternocleidomastoid
      • Some back muscles
    • 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
    • Nitrous Oxide Inhalational Sedation May Represent the Most Ideal Sedation Technique
    • Preparation of Nitrous Oxide
      • Ammonium nitrate crystals heated to 240 degrees celsius
      • Decomposition to nitrous oxide and water
    • Preparation of Nitrous Oxide
      • heat
      • NH 4 NO 3 =======> N 2 O + 2 H 2 O
      • 240 deg. C
    • 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)
    • 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
    • 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
    • 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
    • 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%
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • Pharmacology of Nitrous Oxide
      • N2O is non-allergenic
      • Least toxic of inhalational agents
    • Effects of Nitrous Oxide on Organ Systems
    • 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)
    • Cardiovascular System
      • No changes in heart rate or cardiac output
      • Blood pressure remains stable with only slight decrease
      • Cutaneous vasodilation
    • 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%
    • 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
    • Hematopoietic System
      • Long-term exposure (greater than 24 hours) can produce transient bone marrow depression
    • Musculoskeletal System
      • No direct relaxation of skeletal muscle
      • Anxiolytic effects help relaxation
    • Reproductive System
      • Uterine contractions not inhibited
      • Pregnancy is a relative contraindication (avoid in first trimester)