The anatomic dead space is the sum of volume exhaled during the first phase and half of volume exhaled during the second phase. The first phase has no nitrogen, the second phase nitrogen concentration increases and third phase is plateau phase. The subject breathes out and deeply inhales from 0% nitrogen gas mixture and exhales into the equipment that measures nitrogen and gas volume. Physiological dead space is calculated as follows:Īlveolar dead space is calculated as follows: Slowly the expirate is switched to Douglas bag once the respiration became steady. The rate of respiration and minute volume are recorded. The expirate is passed through the dry gas meter and into the atmosphere via two-way tap. The latter is firmly supported in the mouth by the standard mouth piece. A nose clip is applied and the subject is asked to breathe in and out through Ruben non-return valve.
The examination is carried out by subject lying on the bed supported by pillow. The anatomic dead space is measured by Fowler’s method. The anatomic dead space is the volume of gas within the conducting zone and amounts to about 2 ml/kg in the upright position.
The alveolar dead space is the volume of gas within unperfused alveoli and is very negligible amount. Physiologic or total dead space is the sum total of alveolar dead space and anatomic dead space. The Dead space is the volume of breath that does not participate in the gas exchange and is the ventilation of the lungs without perfusion.
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