oxygen and oxygen therapy

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CONTENTSOxygenThe Fick PrincipleOxygen ElectrodeOxygen TransportOxyhemoglobin Dissociation Curve (ODC)Transport of carbon DioxideCarbon Dioxide Dissociation CurveOxygen ContentOxygen DeliveryPostoperative HypoxemiaOxygen FluxOxygen CascadeCO Poisoning

CONTENTSHypoxiaOxygen Therapy and devicesPeak Inspiratory Flow RateLow Flow SystemsHigh flow systemsPerformance devicesVenturi effectMedium Dependency SystemNeonates and InfantsClean and SterilizationChoice of DevicesHazards of oxygen therapyOxygen Toxicity

OXYGENOxygen is a drug.It must be used meticulously & diligently.If abused it can cause complications. The major reason for O2 therapy is hypoxia.Hypoxia is deficiency of O2 at tissue levels.Hypoxemia refers to reduced O2 tension in arterial blood.A gas with chemical formula of O2Colourless, odorless, tastelessBoiling point -183 CMelting point 216.6CCritical temp. 118.4C , Critical pressure 736.9psiConstitutes about 20.95% of atmosphere

THE FICK PRINCIPLEUsed to measure widely differing physiological flow parameters.A substance is added or subtracted from the flow that it is desired to measure.Flow of liquid in a given period of time = the amount of substance entering or leaving the stream in the same period of time, divided by the concentration difference before and after the point of entry or exit.Two forms of indicator dilution technique can be used to measure blood flow :-An indicator is injected at a constant rate upstream and its concentration is measured from a downstream sample after mixing has taken place.Flow = rate of infusion of indicator/concentration of indicator in blood

THE FICK PRINCIPLENontoxic dyes such as Indocyanine green have been used for physiological measurements of blood flow. Radioactive tracers may also be used.The direct Fick principle is the basis of the calculation of cardiac output from measurements of the steady-state oxygen consumption of the body as follows :-CO = Oxygen consumption/Arteriovenous oxygen content differenceOxygen consumption is measured over a fixed period of time using a volumetric method or indirectly by measuring the inspired and expired volumes & oxygen concentrations.

Arteriovenous oxygen content difference = Arterial oxygen content - the mixed venous (Pulmonary artery) oxygen content

THE FICK PRINCIPLE2. A slug of indicator is injected and the concentration of the indicator is sampled continuously downstream.In the thermodilution measurement of cardiac output, a bolus of saline is injected into the right atrium and a thermistor continuously measures the temperature of the blood downstream in the pulmonary artery.The CO is inversely proportional to the area under the plot of temperature with time and is calculated using a modified Stewart-Hamilton equation, the denominator of which is the integral of the change in blood temperature.A modification of this technique uses a special catheter that pulse-heats blood in the right side of the heart and detects the rise in temperature in the pulmonary artery, which estimates the continuous estimation of CO.

OXYGEN ELECTRODEThe principle of the Clark-type, polarographic electrode for the measurement of the partial pressure of oxygen (Po2) in a blood sample involves the electrolytic reduction of oxygen at a cathode constructed of glass-coated platinum.The platinum electrode is charged negatively w.r.t. a suitable reference electrode, (Ag/AgCl2) in a solution containing oxygen.

At the cathode, the oxygen combines with water and the additional electrons to give hydroxyl ions in a reduction process :-O2 + 2H2O + 4 2H2O + 4 4OH

The hydroxyl ions are buffered by the electrolyte (e.g. potassium chloride) :-4OH + 4KCl 4KOH = 4Cl

OXYGEN ELECTRODEAt the anode of the PO2, oxidation or loss of electrons occurs. The electrons in the process are provided by the silver anode and chloride ions, which form silver chloride :-4Ag + 4Cl 4AgCl + 4e

The clark electrode has an oxygen-permeable, plastic membrane covering the tip of the cathode.

As the polarizing voltage increases, the current flow reaches a plateau at about 0.6 to 0.8V, minimizes interference due to electrochemical reactions with other gases and provides sufficient voltage to drive the reaction.

OXYGEN ELECTRODEThere is a linear relation between the current generated and the concentration of oxygen in the solution.

The dependence on temperature is controlled by using a precise operating temperature, such as with an electronic heat source.

The system is calibrated with two gases :-Containing no oxygen and Containing a known amount of oxygen

OXYGEN TRANSPORTAt breathing air, oxygen reserve in a 3-L FRC is about 500 ml.The blood stores about 850 ml of Oxygen.

Increasing the Inspired Oxygen Fraction (FiO2), increases oxygen stores in the FRC to nearly 3 L which increases dissolved oxygen but has less effect on the volume bound to hemoglobin, as it is nearly fully saturated at breathing air.

Oxygen is carried to the tissues by means of being dissolved in the plasma and bound to hemoglobin.

The amount dissolved depends on the partial pressure and Solubility, giving 0.003 ml per 100 ml blood for each mmHg of Po2.

OXYGEN TRANSPORT (CONT.)Blood substitues such as Perfluoronated hydrocarbons have a high oxygen solubility & can carry as much oxygen as blood, at high oxygen tensions.

In theory, 1 gm of Hb can combine with 1.39 ml of oxygen i.e. Hoffman coefficient but in reality, this amount is less i.e. 1.34-1.36 ml because in vivo the heme group of a small fraction of hemoglobin is oxidized to methemoglobin , which is unable to release its oxygen to the tissues.

Nitrites & Sulfonamides oxidize hemoglobin, as can a congenital deficiency in the enzyme methemoglobin reductase.

OXYGEN TRANSPORT (CONT.)If the oxygen capacity of hemoglobin is 1.36 ml/gm, 15 gm of hemoglobin can combine with 20.4 ml of oxygen the oxygen content.

The oxygen saturation of Hb (SaO2) is expressed as a percentage of the oxygen capacity :O2 content = (1.36HbSo2/100) + (0.003Po2)O2 content= O2 bound to Hb+O2 dissolved in plasma

Where the O2 content is given as ml O2 per 100 ml blood, Hb is given as g per 100 ml and Po2 is given as mmHg

ODCThe shape of the curve have important physiologic advantages :-The flat upper portion implies that small changes in alveolar oxygen (breath holding, moderate altitude) have harmful effect on full oxygen saturation of blood.

At alveolar oxygen tensions of less than 70 mmHg(9.3kPa), small reductions in capillary oxygen tension result in large reductions in hemoglobin oxygen content, which facilitates the unloading of oxygen from hemoglobin to supply the tissues.

The position of the ODC is described by the term P50, which is the oxygen tension required for 50% saturation of hemoglobin and is normally 27 mmHg(3.6kPa). P50 may shift to the right or left under certain physiologic conditions.

Oxygen Hemoglobin Dissociation Curve

Normal Oxyhemoglobin Dissociation Curve97% saturation = 97 PaO2 (normal)90% saturation = 60 PaO2 (danger)80% saturation = 45 PaO2 (severe hypoxia)

SHIFT TO LEFT Increase in pH Decrease in CO2 Decrease in 2.3-DPG Decrease in temperature SHIFT TO RIGHT Decrease in pH Increase in CO2 Increase in 2,3-DPG Increase in temperature

ODCA right shift means that more oxygen is unloaded at a given Po2.BOHR EFFECT :- The effect of altering Pco2 is mainly associated with change in the hydrogen ion concentration.Conditions associated with a right shift in the ODC are found in active tissues, so that hemoglobin gives up its oxygen more easily where it is needed most.The compound 2,3-DPG is an endproduct of red cell metabolism and is increased :-AnemiaThyrotoxicosisChronic Hypoxia Lung disease & High altitudeStored blood has depleted 2,3-DPG and is less effective at oxygen delivery.

Bohr Effect : pCO2 will facilitate offloading of O2 and formation of deoxy Hb Right shift

TRANSPORT OF CARBONDIOXIDETension of CO2 in venous blood is 46mmHg and in alveoli the tension is 40mmHg .CO2 is distributed in the following manner-

1.In solution in plasma (5%) : This quantity is responsible for determining the tension of gas in the blood and also acts as an intermediate between the air in alveoli and inside the red cell .2.As carbamino compound (25%) : CO2 can combine with amino group of Hb. - 3.5 times greater affinity to CO2 as compared to oxy hemoglobin 3.As carbonic acid: CO2+H20H2CO3


4.As Bicarbonate(90)% : Most of CO2 passes to red cells where enzyme carbonic anhydrase aids its rapid hydration to form carbonic acid . Carbonic Anhydrase is not found in plasma.

-Hamburger shift

Carbon di oxide dissociation curveThe CO2 dissociation curve relates to CO2 content of blood to PCO2 to which it is exposed .The position of this curve depends on the degree of O2 of the blood. HALDANE EFFECT: The more deoxygenated , the blood becomes the more CO2 it carries at given PCO2 .

Deoxygenation of blood in peripheral capillaries facilitates loading of CO2 while oxygenation in pulmonary capillaries facilitates unloading of CO2.

Hence, Venous blood carries more CO2 as compared to arterial blood.

Carbon dioxide dissociation curve

CO2 stores are large : approx120L and primarily dissolved as CO2 and bicarbonate.

Equilibrium requires 20 to 30 mins, as compared to O2 which requires 4 5 mins.

Due to large capacity of intermediate + slow compartments , change in CO2 tension is slower than its fall, following acute ventilation.

Carbon dioxide Diffusion rate of CO2 20 times greater than oxygen