ANATOMY & PHYSIOLOGYANATOMY & PHYSIOLOGYfor NITROUS OXIDEfor NITROUS OXIDE

Respiratory SystemRespiratory System

Two functional partsTwo functional parts

Conducting zoneConducting zone

Respiratory zoneRespiratory zone

Conducting ZoneConducting Zone

Transports gases from outside to respiratory Transports gases from outside to respiratory zonezone

Includes anatomic deadspace (150 ml)Includes anatomic deadspace (150 ml)

Conducting ZoneConducting Zone

Nasal passagesNasal passages

PharynxPharynx

NasopharynxNasopharynx

OropharynxOropharynx

HypopharynxHypopharynx

Conducting Zone (cont.) Conducting Zone (cont.)

LarynxLarynx

TracheaTrachea

Bronchi (left and right mainstem)Bronchi (left and right mainstem)

BronchiolesBronchioles

Respiratory ZoneRespiratory Zone

Portion of lung where exchange of gases Portion of lung where exchange of gases occurs between blood and airoccurs between blood and air

Respiratory ZoneRespiratory Zone

Respiratory bronchiolesRespiratory bronchioles

Alveolar ductsAlveolar ducts

Alveolar sacsAlveolar sacs

AlveoliAlveoli

AlveolusAlveolus

Unit in which actual gas exchange occursUnit in which actual gas exchange occurs

Around 300 million totalAround 300 million total

A pocket of air surrounded by thin membrane A pocket of air surrounded by thin membrane (1-2 micrometers in thickness) containing (1-2 micrometers in thickness) containing capillariescapillaries

Alveolus Alveolus

Wall consists of 4 thin layersWall consists of 4 thin layers

Mucinous coveringMucinous covering

Alveolar epithelium (incomplete)Alveolar epithelium (incomplete)

Interstitial layerInterstitial layer

Endothelial cells (pulmonary capillaries)Endothelial cells (pulmonary capillaries)

Physiology of RespirationPhysiology of Respiration

Inhaled gases travel through conducting zone to Inhaled gases travel through conducting zone to respiratory zonerespiratory zone

Gases diffuse across alveolar membranes Gases diffuse across alveolar membranes according to pressure gradientsaccording to pressure gradients

Pulmonary capillaries are a pool of blood Pulmonary capillaries are a pool of blood vessels (70 sq meters)vessels (70 sq meters)

Respiratory MechanicsRespiratory Mechanics

Pre inspiration - resting - -5cm H2O pleuralPre inspiration - resting - -5cm H2O pleural

Peak inspiration - more negative pressuresPeak inspiration - more negative pressures

End inspiration - negative pleural, 0 alveolar End inspiration - negative pleural, 0 alveolar

Peak expiration -positive alveolarPeak expiration -positive alveolar

End expiration - -5cm pleural, 0 alveolarEnd expiration - -5cm pleural, 0 alveolar

Respiratory MechanicsRespiratory Mechanics

Muscles InvolvedMuscles Involved

Primary - DiaphragmPrimary - Diaphragm

IntercostalsIntercostals

Accessory - Abdominals, Scalenes,Accessory - Abdominals, Scalenes,

SternocleidomastoidSternocleidomastoid

Some back musclesSome back muscles

Respiratory MechanicsRespiratory Mechanics

Resting pleural cavity pressure = - 5 cm H20Resting pleural cavity pressure = - 5 cm H20

Thorax expands, increasing negative pressureThorax expands, increasing negative pressure

Air flows into lungs/alveoli until positive Air flows into lungs/alveoli until positive pressure develops and gas exchange occurspressure develops and gas exchange occurs

Air flows out of lungs to return to resting pleural Air flows out of lungs to return to resting pleural pressure of negative 5 cm H20pressure of negative 5 cm H20

Nitrous Oxide Inhalational Sedation Nitrous Oxide Inhalational Sedation May Represent the Most Ideal May Represent the Most Ideal

Sedation TechniqueSedation Technique

PHARMACOLOGY OFPHARMACOLOGY OFNITROUS OXIDENITROUS OXIDE

Preparation of Nitrous OxidePreparation of Nitrous Oxide

Ammonium nitrate crystals heated to 240 Ammonium nitrate crystals heated to 240 degrees celsiusdegrees celsius

Decomposition to nitrous oxide and water Decomposition to nitrous oxide and water

Preparation of Nitrous OxidePreparation of Nitrous Oxide

heatheat

NHNH44NONO3 =======> 3 =======> NN22O + 2 HO + 2 H22OO

240 deg.240 deg.CC

Preparation of Nitrous OxidePreparation of Nitrous Oxide

N20 is chemically scrubbed 99.5% pureN20 is chemically scrubbed 99.5% pure

Stored in compressed form in metal cylindersStored in compressed form in metal cylinders

30% in liquid form in full cylinder30% in liquid form in full cylinder

NitricNitric oxide (NO) is the most dangerous oxide (NO) is the most dangerous impurity (use only medical grade N2O) impurity (use only medical grade N2O)

Physical PropertiesPhysical Properties

N2O is a nonirritating, sweet-smelling, N2O is a nonirritating, sweet-smelling, colorless gascolorless gas

Only inorganic substance other than CO2 to Only inorganic substance other than CO2 to have CNS depressant propertieshave CNS depressant properties

Only inorganic gas used to produce anesthesia Only inorganic gas used to produce anesthesia in humansin humans

Physical PropertiesPhysical Properties

N2O liquid requires heat for vaporization into N2O liquid requires heat for vaporization into gaseous stategaseous state

Relatively insoluble in the blood; Blood-gas Relatively insoluble in the blood; Blood-gas solubility coefficient is 0.47 at 37 deg. Csolubility coefficient is 0.47 at 37 deg. C

Potency of Nitrous OxidePotency of Nitrous Oxide

Least potent of anesthetic gasesLeast potent of anesthetic gases

35 more times soluble than N2 in plasma35 more times soluble than N2 in plasma

100 times more soluble than O2 in plasma100 times more soluble than O2 in plasma

N2O + O2 can produce CNS depressionN2O + O2 can produce CNS depression

Potency of Nitrous OxidePotency of Nitrous Oxide

N2O in subanesthetic doses can produce N2O in subanesthetic doses can produce analgesiaanalgesia

N2O +O2 at 20%/80% is equal-analgesic to N2O +O2 at 20%/80% is equal-analgesic to 10 to 15 mg of morphine10 to 15 mg of morphine

Optimal concentration is 35%Optimal concentration is 35%

Pharmacology of Nitrous OxidePharmacology of Nitrous Oxide

N2O is rapidly absorbed into the CV system, N2O is rapidly absorbed into the CV system, due to large concentration gradient of N2O due to large concentration gradient of N2O between alveolar sacs and bloodbetween alveolar sacs and blood

N2O rapidly fills air-filled body cavitiesN2O rapidly fills air-filled body cavities

Pharmacology of Nitrous OxidePharmacology of Nitrous Oxide

Due to rapid uptake, two phenomena are seenDue to rapid uptake, two phenomena are seen

Concentration effect Concentration effect - higher concentrations - higher concentrations cause more rapid uptake of N2Ocause more rapid uptake of N2O

Second gas effectSecond gas effect - a second anesthetic gas - a second anesthetic gas will also be taken up more rapidly than usual will also be taken up more rapidly than usual when added to N2Owhen added to N2O

Concentration EffectConcentration Effect

Seen only when using high concentrations of Seen only when using high concentrations of a gasa gas

The higher the concentration inhaled, the The higher the concentration inhaled, the more rapidly the arterial tension of the gas more rapidly the arterial tension of the gas increasesincreases

The diffusion gradient from the lungs into The diffusion gradient from the lungs into blood results in a greater uptake of gas into blood results in a greater uptake of gas into lungslungs

Second Gas EffectSecond Gas Effect

Occurs when another inhalation anesthetic is Occurs when another inhalation anesthetic is used with N2Oused with N2O

Rapid uptake of N2O produces a vacuum in Rapid uptake of N2O produces a vacuum in alveoli alveoli

Second gas also undergoes rapid uptake along Second gas also undergoes rapid uptake along with N2Owith N2O

AbsorptionAbsorption

CNS saturation occurs by displacement of N2 CNS saturation occurs by displacement of N2 by N2O, usually in 3-5 minutes by N2O, usually in 3-5 minutes

Tissues with greater blood flow (brain, heart, Tissues with greater blood flow (brain, heart, liver, kidney) receive greater amounts of liver, kidney) receive greater amounts of N2ON2O

AbsorptionAbsorption

Tissues with poor blood supply (fat, muscle, Tissues with poor blood supply (fat, muscle, connective tissue) absorb small amountsconnective tissue) absorb small amounts

Slow absorption occurs once primary Slow absorption occurs once primary saturation is completedsaturation is completed

Therefore no body reservoir present to slow Therefore no body reservoir present to slow recovery once N2O terminatedrecovery once N2O terminated

BiotransformationBiotransformation

N2O undergoes N2O undergoes nono biotransformation in the biotransformation in the bodybody

Majority of N2O is exhaled unchanged 3 to 5 Majority of N2O is exhaled unchanged 3 to 5 mins. following termination of deliverymins. following termination of delivery

1% eliminated via skin and lungs in 24 hours1% eliminated via skin and lungs in 24 hours

Diffusion HypoxiaDiffusion Hypoxia

Can occur following termination of N2O if Can occur following termination of N2O if patient is allowed to breathe only room airpatient is allowed to breathe only room air

““Hangover” effect (headache,nausea,lethargy) Hangover” effect (headache,nausea,lethargy) is producedis produced

Prevented by having the patient breathe 100% Prevented by having the patient breathe 100% O2 for minimum of 3 to 5 minutesO2 for minimum of 3 to 5 minutes

Diffusion HypoxiaDiffusion Hypoxia

Rapid diffusion of N2O from blood to lungs Rapid diffusion of N2O from blood to lungs results in decreased CO2 arterial tension results in decreased CO2 arterial tension with decreased stimulus for respirationwith decreased stimulus for respiration

Rapid diffusion of N2O back into lungs Rapid diffusion of N2O back into lungs dilutes alveolar O2 with resultant hypoxiadilutes alveolar O2 with resultant hypoxia

Pharmacology of Nitrous OxidePharmacology of Nitrous Oxide

N2O is non-allergenicN2O is non-allergenic

Least toxic of inhalational agentsLeast toxic of inhalational agents

Effects of Nitrous OxideEffects of Nitrous Oxideon Organ Systemson Organ Systems

Central Nervous SystemCentral Nervous System

Actual Actual mechanism unknownmechanism unknown

Mild depression of CNS (cerebral cortex) in Mild depression of CNS (cerebral cortex) in conjunction with physiological levels of O2 conjunction with physiological levels of O2 (greater than 20%)(greater than 20%)

Sensations depressed (sight, hearing, touch, Sensations depressed (sight, hearing, touch, pain) pain)

Cardiovascular SystemCardiovascular System

No changes in heart rate or cardiac outputNo changes in heart rate or cardiac output

Blood pressure remains stable with only slight Blood pressure remains stable with only slight decreasedecrease

Cutaneous vasodilationCutaneous vasodilation

Respiratory SystemRespiratory System

N2O is non-irritating to pulmonary epitheliumN2O is non-irritating to pulmonary epithelium

Changes (drop) in rate and depth more likely Changes (drop) in rate and depth more likely due to anxiolytic effectsdue to anxiolytic effects

Slight elevation of resting respiratory minute Slight elevation of resting respiratory minute volume at 50%/50%volume at 50%/50%

GI SystemGI System

No clinically significant effects, unless there No clinically significant effects, unless there is a closed space (obstruction)is a closed space (obstruction)

N/V rarely seen unless hypoxia presentN/V rarely seen unless hypoxia present

Can be used in hepatic dysfunctionCan be used in hepatic dysfunction

Hematopoietic SystemHematopoietic System

Long-term exposure (greater than 24 hours) Long-term exposure (greater than 24 hours) can produce transient bone marrow can produce transient bone marrow depressiondepression

Musculoskeletal SystemMusculoskeletal System

No direct relaxation of skeletal muscleNo direct relaxation of skeletal muscle

Anxiolytic effects help relaxationAnxiolytic effects help relaxation

Reproductive SystemReproductive System

Uterine contractions not inhibitedUterine contractions not inhibited

Pregnancy is a relative contraindication Pregnancy is a relative contraindication (avoid in first trimester)(avoid in first trimester)