This is an extract of our Integrated Physiology document, which we sell as part of our Pre Clinical Systems Based Teaching Notes collection written by the top tier of Bristol University students.
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Integrated Physiology Applied Physiology of Acid Base Disorders ABCs of Arterial Blood Gases A - Oxygenation Sa O2 <90%
Pa O2 <60mmHg B - Ventilation Pa CO2 <35mmHg HYPOcapnia Pa CO2 >45mmHg HYPERcapnia C - Perfusion
results in hypoxic hypoxia/hypoxaemia Increased ventilation Insufficient ventilation
Hypoxaemia Causes V:Q mismatch Hypoventilation Altitude Shunt Diffusion defect
A-a Gradient Difference between PAO2 (alveolar) and PaO2 (arterial) Helps to determine the difference between hypoventilation and VQ mismatches/shunts
Calculating alveolar partial pressure - Alveolar Gas Equation
Normal A-a gradient is up to 15mmHg in non-smokers and 30mmHg in smokers The A-a gradient will rise by 5-7mmHg for every 10% rise in FiO2 (fractional gas concentration) A person breathing 100% oxygen may have a normal A-a gradient of around 250-300mmHg The normal A-a gradient exists due to anatomical shunts by which deoxygenated blood from the right side of the heart bypasses the ventilated areas of the lungs to enter the systemic circulation without taking on O2
PACO2 and PaCO2 are determined by alveolar ventilation i.e. the rate of removal & the rate of production
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