OxygenDr Mark B Smith

Oxygen

• History– Discovered by

• Carl Wilhelm Scheele, 1772 (German-Swedish)• Joseph Priestley, 1774 (British)

– Named by• Lavoisier, 1777 (French)

– Lost his head selling cheap tobacco

Oxygen• Atomic Number: 8• Atomic Weight: 15.9994• Melting Point: 54.36 K (-218.79°C or -361.82°F)• Boiling Point: 90.20 K (-182.95°C or -297.31°F)• Density: 0.001429 grams per cubic centimeter• Phase at Room Temperature: Gas• Element Classification: Non-metal• Period Number: 2 • Group Number: 16• Group Name: Chalcogen

OxygenThe 10 Most Abundant Elements in the UniverseSource: Exploring Chemical Elements and their Compounds; David L. Heiserman, 1992

Element Abundancemeasured relative to silicon

Hydrogen 40,000

Helium 3,100

Oxygen 22

Neon 8.6

Nitrogen 6.6

Carbon 3.5

Silicon 1

Magnesium 0.91

Iron 0.6

Sulphur 0.38

Composition of the Earth's AtmosphereSource: Definition of the U.S. Standard Atmosphere (1976)

CRC Handbook of Chemistry and Physics, 77th Edition

Gas Formula Abundancepercent by volume

Abundanceparts per million by volume

Nitrogen N2 78.084% 780,840

Oxygen O2 20.9476% 209,476

Argon Ar 0.934% 9,340

Carbon Dioxide

CO2 0.0314% 314

Neon Ne 0.001818% 18.18

Helium He 0.000524% 5.24

Methane CH4 0.0002% 2

Krypton Kr 0.000114% 1.14

Hydrogen H2 0.00005% 0.5

Xenon Xe 0.0000087% 0.087

Oxygen

• Production– Photosynthesis

• Where it has all come from

– Heating mercuric oxide and nitrates• How it was discovered

– Fractional distillation of air• Supply for medicine and industry

– Colourless gas, pale blue liquid

Oxygen

• Daltons Law (John)

– total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture

• Significance– At fixed total pressure “adding” another gas reduces

partial pressure of other gases• Water vapour• Carbon dioxide

Oxygen: Under pressure

760mmHg = 101.325 kPa = 1 atmosphere1 Pa = 1 Newton per square meter1 torr = 1 mmHg = 1/760 atmosphere

Specific fluid density = 13.591 g/cm3

(Density of mercury at 0oC)Specific gravity = 9.80665 m/s2

(Standard gravity)

Oxygen• The purpose of the cardio-respiratory system is to extract

oxygen from the atmosphere and deliver it to the mitochondria of cells.

• Oxygen– exerts a partial pressure– determined by the prevailing environmental pressure– sea level atmospheric pressure is 760mmHg– oxygen makes up 21% (20.094% to be exact) air– oxygen exerts a partial pressure of 760 x 0.21 = 159mmHg

• The oxygen cascade– as one moves down through the body to the cell, oxygen is diluted

down, extracted or otherwise lost, so that at cellular level the PO2 may only be 3 or 4 mmHg.

Oxygen

• Oxygen Cascade– Oxygen moves down a stepwise series of partial

pressure gradients from the inspired air to the body's cells and their mitochondria.

Oxygen

• Water vapour– humidifies inspired air– dilutes the amount of oxygen– reduces the partial pressure by the saturated

vapour pressure (47mmHg)– effect on the PIO2 (the partial pressure of inspired

oxygen)– recalculated as: (760 - 47) x 0.2094 = 149mmHg.

Oxygen• Carbon dioxide

– alveolar carbon dioxide, the PACO2– is usually the same as the PaCO2

• measured by a blood gas analyzer– alveolar partial pressure of oxygen PAO2 calculated from the following

equation: PAO2 = PIO2 – PaCO2/R– R is the respiratory quotient

• represents the amount of carbon dioxide excreted for the amount of oxygen utilized• depends on the carbon content of food (carbohydrates high, fat low)

– let us assume that the respiratory quotient is 0.8– PAO2 will then be 149 – (40/0.8) = 100mmHg

– More Carbon dioxide means less Oxygen

Oxygen

• The next step– alveolus to artery– significant gradient, usually 5 –10 mmHg

• small ventilation perfusion abnormalities• the diffusion gradient• physiologic shunt (from the bronchial arteries).

– PaO2 = 90mmHg

Oxygen only moves through tissues dissolved in water

Oxygen

• Oxygen transport– arterial tree

• minimal extraction

– progressively extracted through the capillary network

• interstitial space

– return via venous system• partial pressure of oxygen in mixed venous blood,

PVO2, is approximately 47mmHg.

Oxygen

• Oxygen cascade interference – At 19,000 feet (just above base camp at Mount

Everest, the barometric pressure is half that at sea level, and thus, even though the FiO2 is 21%, the PIO2 is only 70mmHg, half that at sea level

– Increased barometric pressure such as in hyperbaric chambers, the PIO2 will actually be higher

• More oxygen…but dissolved not on haemoglobin

Oxygen

• Four factors influence transmission of oxygen from the alveoli to the capillaries– Ventilation perfusion mismatch

• Pneumonia, pulmonary embolism

– Right to left shunt• ASD, VSD

– Diffusion defects• Pulmonary fibrosis, pulmonary oedema, COPD

– Cardiac output• Pump failure, Exercise

Oxygen

• The amount of oxygen in the bloodstream is determined by;– serum haemoglobin level– percentage of this haemoglobin saturated with

oxygen– cardiac output– the amount of oxygen dissolved (see below).

Oxygen

• How much oxygen is in the blood?

• The amount of oxygen in the blood is calculated using the formula: [1.34 x Hb x (SaO2/100)] + 0.003 x PO2 = 20.8ml per 100ml blood

EMERGENCY MOBILE MEDICAL UNIT Training Weekend

HYPOVENTILATION

• A RISE IN PaCO2 CAUSES A FALL IN PaO2

THIS IS THE BASIS OF THE ALVEOLAR GAS EQUATION

PaO2 = FiO2(Pb-PaH2O)-PaCO2/0.8

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• TREATMENT– OXYGEN TO OBTAIN SaO2>90%

• IF THE PATIENT BECOMES TIRED THE PaCO2 MAY RISE

• IF RISING PaCO2 OR DROWSY PROVIDE ASSISTED VENTILATION

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HIGH CONCENTRATIONS OF INSPIRED OXYGEN DO NOT DEPRESS VENTILATION IN PATIENTS WITH ACUTE RESPIRATORY FAILURE

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• OXYGEN THERAPY ON GENERAL WARDS MUST BE PRESCRIBED

• TYPE OF OXYGEN DELIVERY SYSTEM• FLOW RATE OF OXYGEN (OR %)• DURATION OF THERAPY• MONITORING TO BE UNDERTAKEN

Oxygen is a drug

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OXYGEN DELIVERY SYSTEMS

• VARIABLE PERFORMANCE

• FIXED PERFORMANCE

VARIABLE PERFORMANCE SYSTEMS

NASAL CANNULA

• 24-40% DEPENDING ON FLOW RATE• INSPIRED CONCENTRATION VARY BETWEEN

BREATHS• DEPENDS UPON RATE AND DEPTH• MAXIMUM FLOW RATE APPROX 4 l/min

VARIABLE PERFORMANCE SYSTEMS

SIMPLE FACE MASK

• MASK INCREASES RESERVOIR • MINIMAL FLOW RATE APPROX 4 l/min• NO INCREASED INSPIRED OXYGEN ABOVE 15 l/min• INSPIRED OXYGEN OF UP TO 60%

VARIABLE PERFORMANCE SYSTEMS

PARTIAL REBREATHING MASKS

• FACEMASK AND RESERVOIR BAG

• INSPIRED OXYGEN >60%

• FLOW RATES OF APPROX 15 l/min REQUIRED

FIXED PERFORMANCE SYSTEMS

VENTURI MASK• ENTRAINS AIR

• HIGH FLOW MAINTAINS HIGH INSPIRED OXYGEN CONCENTRAION THROUGHOUT RESPIRATORY CYCLE

• COLOUR CODED VENTURI HEADS

FIXED PERFORMANCE SYSTEMS

ANAESTHETIC MACHINE– Open system

• Magill Circuit• Bains system• Maplesons C

– Closed system• Circle

Inspired Oxygen v Tissue Oxygen

• Inspired oxygen greater than alveolar oxygen– Oxygen cascade

– Water vapour– Carbon dioxide

• Normal and pathological physiological effects– Oxygen cascade, continued

– Intrapulmonary ventilation/perfusion mis-match– Cardiac shunts– Arterial oxygen loss– Oxygen diffusion in water solution

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SUMMARY

ALL CRITICALLY ILL PATIENTS REQUIRE OXYGEN

Oxygen cascade occurs regardless

Maximise inspired Oxygen

MONITOR