- The "Dead Space" of the lungs refer to those areas of the lung which do not participate in gas exchange. Even lungs of healthy individuals display some dead space as this represents the volume of air that would fill the conducting airways with each breath. Because the conducting airways do not possess alveoli, any air inspired into those areas cannot be used for gas exchange and thus is considered to exist in the dead space. However, in certain pathological scenarios, non-perfused alveoli may be also be considered part of the dead space as any air entering non-perfused alveoli cannot efficiently be used for gas exchange.
- Two basic methods have been devised to measure the dead space volume of the lung. Fowler's method essentially measures the volume of air that is inhaled into the conducting airways and cannot appreciate any dead space potentially contributed by non-perfused alveoli. Consequently, the dead space measured by Fowler's method is termed the "Anatomic Dead Space". Bohr's method takes advantage of the observation that only the areas of lung that participate in gas exchange are the source of exhaled carbon dioxide whereas the dead space areas cannot possibly contribute to carbon dioxide exhalation.
- Qualitatively, Bohr's method infers the volume of gas exchanging areas from the partial pressure of the exhaled carbon dioxide and subtracts this "Living Space" from the total inhaled gas volume to derive the dead space. Bohr's equation for calculating dead space is more complex than described above; however, the precise details are less important than understanding the concept behind this methodology. Because Bohr's method focuses on those areas which are actually involved in gas exchange, the dead space volume derived includes the conducting airways as well as any non-perfused alveoli and is thus referred to as the "Physiologic Dead Space"
- In a healthy individual the anatomic and physiologic dead space will be equal in volume as nearly all alveoli are well-perfused in a normal lung. However, in a variety of disease states, poor perfusion of alveoli will result in an expansion of the physiologic dead space although the anatomic dead space measured by Fowler's method will remain the same.