Oxygen
Dr 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
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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
Oxygen
The 10 Most Abundant Elements in the Universe
Source: Exploring Chemical Elements and their Compounds; David L. Heiserman, 1992
Element
Abundance
measured 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 Atmosphere
Source: Definition of the U.S. Standard Atmosphere (1976)
CRC Handbook of Chemistry and Physics, 77th Edition
Gas
Formula Abundance
percent by
volume
Abundance
parts 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 atmosphere
1 Pa = 1 Newton per square meter
1 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
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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
EMERGENCY MOBILE MEDICAL UNIT Training
Weekend
• TREATMENT
– OXYGEN TO OBTAIN SaO2>90%
• IF THE PATIENT BECOMES TIRED THE PaCO2
MAY RISE
• IF RISING PaCO2 OR DROWSY PROVIDE
ASSISTED VENTILATION
EMERGENCY MOBILE MEDICAL UNIT Training
Weekend
HIGH CONCENTRATIONS OF INSPIRED
OXYGEN DO NOT DEPRESS VENTILATION IN
PATIENTS WITH ACUTE RESPIRATORY
FAILURE
EMERGENCY MOBILE MEDICAL UNIT Training
Weekend
• OXYGEN THERAPY ON GENERAL WARDS MUST BE
PRESCRIBED
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TYPE OF OXYGEN DELIVERY SYSTEM
FLOW RATE OF OXYGEN (OR %)
DURATION OF THERAPY
MONITORING TO BE UNDERTAKEN
Oxygen is a drug
EMERGENCY MOBILE MEDICAL UNIT Training
Weekend
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
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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
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Intrapulmonary ventilation/perfusion mis-match
Cardiac shunts
Arterial oxygen loss
Oxygen diffusion in water solution
EMERGENCY MOBILE MEDICAL UNIT Training
Weekend
SUMMARY
ALL CRITICALLY ILL PATIENTS REQUIRE OXYGEN
Oxygen cascade occurs regardless
Maximise inspired Oxygen
MONITOR