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.Alveolar dead space refers to the alveoli that are ventilated but do not receive enough blood to undertake gas exchange.Anatomical 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.Physiological dead space = Anatomical dead space + Alveolar dead space Anatomical dead space Likewise if you have an adult patient on a ventilator and they're only achieving tidal volumes of 200ml, it's likely that most of that is dead space ventilation, and they're likely to become steadily more hypoxic and hypercapnoeic. Why does it matter?īecause if you take lots of rapid shallow breaths, you will be moving air in and out of your dead space, without sending any meaningful ventilation to the areas of your lung that can engage in gas exchange. It is composed of anatomical and alveolar dead space. The total dead space is called the physiological dead space. These areas therefore do not undertake gas exchange with the blood. If the air and the blood are being sent to different alveoli, then clearly the system isn't going to work very well, and this is called a ventilation/perfusion mismatch.ĭead space refers to the areas of the respiratory tract that are ventilated but not perfused. Not only does the air need to be drawn successfully into the alveoli, but blood needs to be pumped to those same alveoli in order to pick up the oxygen. The lungs represent an interface between ventilation and perfusion. This is why you start assessing your patient with an airway, breathing, circulation approach, because that's the most life-threatening order in which problems present. To survive, we need to get oxygen from the air into our blood, in order to supply the tissues. Dead space is important to think about, especially in paediatrics.
Ventilation/Perfusion matching is everything (sort of).
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