![]() The Bohr equation can be used to calculate the amount of dead space in a lung. One can see an increase in the value of physiologic dead space in lung disease states where the diffusion membrane of alveoli does not function properly or when there are ventilation/perfusion mismatch defects. Therefore, physiologic dead space is equivalent to anatomical. In a healthy adult, alveolar dead space can be considered negligible. The respiratory zone is comprised of respiratory bronchioles, alveolar duct, alveolar sac, and alveoli. Physiologic or total dead space is equal to anatomic plus alveolar dead space which is the volume of air in the respiratory zone that does not take part in gas exchange. This volume is considered to be 30% of normal tidal volume (500 mL) therefore, the value of anatomic dead space is 150 mL. Anatomical dead space is represented by the volume of air that fills the conducting zone of respiration made up by the nose, trachea, and bronchi. The two types of dead space are anatomical dead space and physiologic dead space. Anaesthetic circuits and equipment will increase anatomical dead space.Dead space represents the volume of ventilated air that does not participate in gas exchange. In general, alveolar dead space increases in disease and anatomical dead space increases with increasing age. ![]() PaCO2 = arterial PCO2 PECO2 = expired gas PCO2 * The product of the alveolar carbon dioxide concentration and the amount of inspired air reaching the alveoli equals the expired carbon dioxide concentration (multiplied by the tidal volume. * Inspired air contains virtually no carbon dioxide.įil e Time in seconds Figure 7.1 Expired CO2 concentration during a normal breath. Alveolar pCO2 is thus similar to arterial pCO2. * Air ventilating perfused lungs will equilibrate with carbon dioxide in the blood. It can be measured by Bohr's method, which is based on the following principles: ![]() It comprises 25%-35% of the normal tidal volume (the VD/VT ratio). It consists of the anatomical dead space and the alveolar dead space (the volume of gas in alveoli that are ventilated but not perfused, and thereby not participating in gas exchange, i.e. This refers to the proportion of the tidal volume that does not participate directly in gas exchange. ![]()
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