Transcript anatomy and physiology of nitrous oxide

ANATOMY & PHYSIOLOGY
for NITROUS OXIDE
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
PHARMACOLOGY OF
NITROUS OXIDE
Preparation of Nitrous Oxide
Ammonium nitrate crystals heated to 240
degrees celsius
Decomposition to nitrous oxide and water
Preparation of Nitrous Oxide
heat
NH4NO3 =======> N2O + 2 H2O
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)