PHTY208 Lecture Notes - Osteoporosis, Gentamicin, Ketosis
Arterial Blood Gases:
What is a blood gas?!
-Dissolved gases include nitrogen, carbon dioxide, oxygen, water vapour and other gases!
-Each gas in a mixture gases exerts its own pressure as if no other gases were present!
-Pressure of a specific gas is partial pressure P!
Gas exchange:!
-Atmospheric pressure (760mmHg) !
-Each gas diffuses across a permeable membrane from the area where its partial pressure is
greater to the area where its partial pressure is less!
-The greater the difference, the faster the rate of diffusion!
Henry’s Law:!
-Quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas
and its solubility!
-Higher partial pressure of a gas over a liquid and higher solubility, more of the gas will stay in
solution!
-Much more CO2 is dissolved in blood than O2 because CO2 is 24 times more soluble!
-Even though the air we breathe is mostlyN2, very little dissolves in blood due to low solubility!
Partial pressure of Oxygen:!
-Alveolar Po2= 105mmHg!
-Pulmonary capillary Po2 = 40mmHg!
-Oxygen diffuses into capillary blood!
-Po2 of pulmonary veins = 100mmHg!
Partial pressure of Carbon dioxide:!
-deoxygenated blood PCo2 = 45mmHg!
-Alveolar air PCo2 = 40mmHg!
-Oxygenated blood of pulmonary veins has PCo2 of 40mmHg!
Rate of gas exchange:!
-Rate of gas exchange based on several forces!
-Partial pressure difference of the gases!
-Surface area available for gas exchange!
-Molecular weight and solubility of the gases!
-Diffusion distance!
Carbon dioxide transport:!
-Each 100mL of blood contains 53mL ofCO2!
-Transported in 3 ways:!
-Dissolved Co2 (7%)!
-Bound to amino acids (23%)!
-Most Co2 transported in blood plasma (70%)!
CO2 + H2O- H2CO3 - H + HCO3!
-HCO3 accumulates inside RBCs as they pick up carbon dioxide!
-Some diffuses out into plasma!
-To balance the loss of negative ions, chloride (Cl) moves into RBCs from plasma!
-Reverse happens in lungs- Cl moves out as moves back into RBCs!
Blood gas sampling:!
-An arterial blood gas sample provides key information about a patients respiratory and
metabolic systems!
-Collection of blood involves an arterial puncture usually to the radial artery, very invasive, rapid
generation of results, most critical care units have blood gas analyser!
Arterial blood gases:!
-pH- measure of acid/base balance and ventilation!
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-PaCO2- measure of ventilation!
-PaO2- measure of oxygenation or gas exchange!
-HCO3-measure of metabolic disturbance!
-Base excess- measure of metabolic disturbance!
Normal values:!
Compensation:!
-complete compensation- when one system is able to offset the change in the other and pH is
restored within normal limits!
-Partial compensation- when one system has begun to offset the change in the other but is not
sufficient to return pH to within normal limits!
Compensated examples:!
-If the primary problem persists for long enough the body will compensate!
-Need to determine which system is compensating!
-If the pH is within normal range then it is compensated, if the pH is on the limits of normal or
slightly out of range partially compensated !
Respiratory system compensation- Metabolic Acidosis:!
-H+ + HCO3- (buffer) - HCO3 (carbonic acid) - CO2 + H2O (carbon dioxide)!
-If there are free H+ ions (acidity) they can combine with bicarbonate to form carbonic acid
which is broken down into water and carbon dioxide!
-Therefore there is less bicarbonate available which stimulates the respiratory centre to increase
respiratory rate and excrete the excess carbon dioxide!
Respiratory system compensation- metabolic alkalosis:!
-H+ + HCO3- (buffer) - H2CO3 (carbonic acid) - CO2 + H20 (carbon dioxide)!
-If there is a decreased number of free H+ions (alkalinity) and therefore increased bicarbonate
ions the respiratory centre reacts by decreasing ventilation to retain carbon dioxide!
Metabolic compensation:!
-Excess H+ ions are excreted by the kidneys!
-If the PaCO2 is high the blood entering the nephrons is acidic and the excess H+ ions are
excreted by the kidneys while the HCO3 is reabsorbed therefore the HCO3- rises!
-If the blood entering the nephrons is alkaloid the reverse occurs!
-If the PaCO2 is low the blood entering the nephrons is alkalotic and H+ ions are retained by the
kidney while the HCO3 is excreted therefore the HCO3- decreases!
-Kidneys will always fully compensate, lungs will not- Survival mechanism!
-The lungs compensate quickly (Minutes) but this compensation can not be maintained!
-The kidneys compensate slowly (days) and this compensation can be prolonged (respiratory
acidosis)!
Pulse Oximetry:!
-Measures the % of O2 carried in the Hb!
-The principle of pulse oximetry is based on the red and infrared light absorption characteristics
of oxygenated and deoxygenated haemoglobin. Oxygenated haemoglobin absorbs more
infrared and allows more red lights to pass through. Deoxygenated (or reduced) haemoglobin
absorbs more red light and allows more infrared light to pass through!
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-Pulse oximetry uses a light emitter with red and infrared LEDs that shines through a reasonably
translucent site with good blood flow !
-Sites finger, toes or pinna or lobe of the ear!
-Infant sites are the foot or palm of the hand and the big toe or thumb!
-Opposite the emitter is a photodetector that receives the light that passes through the
measuring site- Signal varies with the pulsate!
Advantages:!
-Non-invasive!
-Continuos monitoring!
Inaccurate:!
-Low perfusion (cold extremities) or hypotension!
-Movement!
-Pigmentation!
-Nail polish!
-No treasure of PaCO2 so patient may be going into type 2 respiratory failure without hypoxemia !
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