This works fairly well as long as you remember that this assumption will usually over estimate dead space, as arterial CO2 is slightly higher than alveolar, and will also be affected by:Īnatomical dead space is measured using Fowler’s method. The Enghoff modification simply extends this with another assumption that the alveolar partial pressure of CO2 can reasonably be approximated by the arterial partial pressure of CO2, which makes it all a lot easier to measure. ![]() Therefore we can generate the equations above, and rearrange as demonstrated to form the Bohr equation. Therefore the entire expired CO2 is only going to be coming from the alveolar ventilation. We can reasonably assume that a tidal volume is comprised of alveolar volume and dead space volume, and we can also reasonably assume that inspired CO2 is minimal, if there is no rebreathing occurring. It is usually around 200-350ml in normal tidal breathing. Total or physiological dead space is measured using Bohr’s equation. This can be physiological, such as in hypoxic pulmonary vasoconstriction, or pathological, as seen in pulmonary embolism. ![]() It is composed of anatomical and alveolar dead space.Īnatomical dead space refers to the volume occupied by the conducting airways that supply the alveoli, but don’t undertake gas exchange themselves, and this is generally the first 16 airway generations.Īlveolar dead space refers to the alveoli that are ventilated but do not receive enough blood to undertake gas exchange. The total dead space is called the physiological dead space. Dead space refers to the areas of the respiratory tract that are ventilated but not perfused and therefore do not undertake gas exchange with the blood.
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