ANATOMY PHYSIOLOGY for NITROUS OXIDE Respiratory System Two
- Slides: 51
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 - -5 cm H 2 O pleural Peak inspiration - more negative pressures End inspiration - negative pleural, 0 alveolar Peak expiration -positive alveolar End expiration - -5 cm 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 H 20 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 H 20
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 NH 4 NO 3 =======> 240 deg. C N 2 O + 2 H 2 O
Preparation of Nitrous Oxide N 20 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 N 2 O)
Physical Properties N 2 O is a nonirritating, sweet-smelling, colorless gas Only inorganic substance other than CO 2 to have CNS depressant properties Only inorganic gas used to produce anesthesia in humans
Physical Properties N 2 O 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 N 2 in plasma 100 times more soluble than O 2 in plasma N 2 O + O 2 can produce CNS depression
Potency of Nitrous Oxide N 2 O in subanesthetic doses can produce analgesia N 2 O +O 2 at 20%/80% is equal-analgesic to 10 to 15 mg of morphine Optimal concentration is 35%
Pharmacology of Nitrous Oxide N 2 O is rapidly absorbed into the CV system, due to large concentration gradient of N 2 O between alveolar sacs and blood N 2 O 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 N 2 O Second gas effect - a second anesthetic gas will also be taken up more rapidly than usual when added to N 2 O
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 N 2 O Rapid uptake of N 2 O produces a vacuum in alveoli Second gas also undergoes rapid uptake along with N 2 O
Absorption CNS saturation occurs by displacement of N 2 by N 2 O, usually in 3 -5 minutes Tissues with greater blood flow (brain, heart, liver, kidney) receive greater amounts of N 2 O
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 N 2 O terminated
Biotransformation N 2 O undergoes no biotransformation in the body Majority of N 2 O 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 N 2 O if patient is allowed to breathe only room air “Hangover” effect (headache, nausea, lethargy) is produced Prevented by having the patient breathe 100% O 2 for minimum of 3 to 5 minutes
Diffusion Hypoxia Rapid diffusion of N 2 O from blood to lungs results in decreased CO 2 arterial tension with decreased stimulus for respiration Rapid diffusion of N 2 O back into lungs dilutes alveolar O 2 with resultant hypoxia
Pharmacology of Nitrous Oxide N 2 O 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 O 2 (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 N 2 O 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)
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