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 Atomic Number: 8 Atomic Weight: Melting Point: K ( °C or °F) Boiling Point: K ( °C or °F) Density: 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 ElementAbundance measured relative to silicon Hydrogen40,000 Helium3,100 Oxygen22 Neon8.6 Nitrogen6.6 Carbon3.5 Silicon1 Magnesium0.91 Iron0.6 Sulphur0.38

Composition of the Earth's Atmosphere Source: Definition of the U.S. Standard Atmosphere (1976) CRC Handbook of Chemistry and Physics, 77th Edition GasFormulaAbundance percent by volume Abundance parts per million by volume NitrogenN2N %780,840 OxygenO2O %209,476 ArgonAr0.934%9,340 Carbon Dioxide CO %314 NeonNe %18.18 HeliumHe %5.24 MethaneCH %2 KryptonKr %1.14 HydrogenH2H %0.5 XenonXe %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 = kPa = 1 atmosphere 1 Pa = 1 Newton per square meter 1 torr = 1 mmHg = 1/760 atmosphere Specific fluid density = g/cm 3 (Density of mercury at 0 o C) Specific gravity = m/s 2 (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: ( ) x = 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)] 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 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 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

EMERGENCY MOBILE MEDICAL UNIT Training Weekend SUMMARY ALL CRITICALLY ILL PATIENTS REQUIRE OXYGEN Oxygen cascade occurs regardless Maximise inspired Oxygen MONITOR