![]() In phase 4 there is an inflection and a sudden increase in the nitrogen concentration, and this occurs at the closing capacity. In phase 3 there is a plateau, representing only alveolar ventilation, as all dead space gas has now been exhaled. Towards the end of this phase, most of the dead space oxygen will have already been exhaled, and more alveoli will be now be draining their nitrogen and therefore the nitrogen concentration detected increases. ![]() This means that some alveoli will empty their nitrogen before others, so there will be mixing of alveolar and dead space gas. In phase 2 you’re detecting a gradually increasing concentration of nitrogen, because different alveoli have different time constants. To begin with, pure oxygen is exhaled from the dead space, so no nitrogen is detected - this is phase 1. The concentration of nitrogen detected is plotted against volume to look like the graph below, where Area A = Area B. ![]() Patient exhales all the way to residual volume into a pneumotachograph, which measures flow over time and therefore provides a volume measurement. Patient takes a vital capacity breath of pure (100%) oxygen, thereby removing all nitrogen from the anatomical dead space ( Remember that the alveoli still receive nitrogen from the blood). 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. This assumption is 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. The Enghoff modification simply extends this further with another assumption. Therefore we can generate the equations below, 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.
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