Inhalation increases bronchial diameter and length, effectively increasing the anatomic dead space. Numerous physiologic factors can influence the anatomic dead space, owing to variations in its function from posture, sleep, and the anatomy of the upper airway itself, as well as the associated bony and soft tissue structures. In comparison, ambient air is comprised of 0.04% carbon dioxide and 21% oxygen. The composition of end-expiratory dead volume air is 5 to 6% carbon dioxide and 15 to 16% oxygen. At the end of expiration, the dead volume consists of a gas mixture high in CO2 and low in O2 compared to ambient air. Effectively, 1/3 of a TV of inhaled air is rebreathed due to dead space. Anatomic, and therefore physiological, dead space normally is estimated at 2 mL/kg of body weight and comprises 1/3 of the TV in a healthy adult patient it is even higher in pediatric patients. Īlveolar dead space is typically negligible in a healthy adult. Finally, inspired air is humidified in the upper airways, which is important to its temperature and gas exchange function. Particulate matter is trapped in the mucus that lines the conducting airways, allowing it to be removed by mucociliary transport and thus performing a first-line barrier function to foreign matter. Inspired air is raised or lowered to body temperature, increasing the affinity of hemoglobin for O2 and improving O2 uptake. Carbon dioxide is retained, resulting in bicarbonate-buffered blood and interstitium. Therefore, in situations (i.e., V/Q =infinity) in which the alveoli are ventilated but not perfused, gas exchange cannot occur, such as when pulmonary embolism increases alveolar dead space.Īlthough initially counter-intuitive, there are multiple functions performed by the non-gas exchanging upper airway, including the anatomical dead space, that are important to normal respiratory function. Alveoli with no perfusion have a V/Q of infinity (Q=0), whereas alveoli with no ventilation have a V/Q of 0 (V=0). It is then implied that V/VT represents the portion of a tidal volume that does not participate in gas exchange :ĭead space has particular significance in the concept of ventilation (V) and perfusion (Q) in the lung, represented by the V/Q ratio. Total ventilation (VE) is, therefore, the sum of alveolar ventilation (Valv) and VD:Įnghoff's equation compiles these variables with PaCO2, tidal volume (TV), and expired CO2 (PECO2). Thus:ĭead space ventilation (VD) is then calculated by multiplying VD-Phys by the respiratory rate (RR): Physiologic dead space (VD-Phys) is the sum of the anatomic (VD-Ana) and alveolar (VD-Alv) dead space. On the other hand, alveolar dead space refers to the volume of air in alveoli that are ventilated but not perfused, and thus gas exchange does not take place. These segments of the respiratory tract include the upper airways, trachea, bronchi, and terminal bronchioles. Īnatomic dead space specifically refers to the volume of air located in the respiratory tract segments that are responsible for conducting air to the alveoli and respiratory bronchioles but do not take part in the process of gas exchange itself. This is comprised of two segments: the anatomic dead space (parts of the airway that are not alveolar exchange membranes) and the alveolar dead space (alveoli that are ventilated but not perfused with pulmonary capillary blood flow). Structure and Functionĭead space of the respiratory system refers to the volume of inspired air in a given breath in which oxygen (O2) and carbon dioxide (CO2) gasses are not exchanged across the alveolar membrane in the respiratory tract. However, differences in the exact way of measuring this space result in clinically significant different results and, therefore, debate remains about the true value of this measured parameter. Indeed, it may serve as a prognostic factor in patients with acute repository distress syndrome (ARDS) who require ventilation. This phenomenon has clinical significance because, both in healthy and impaired lungs, properly calculating and accounting for this non-physiological space is important for the proper respiratory care of ventilated patients. This is therefore termed anatomical dead space as it serves no respiratory function. Anatomic dead space is an important phenomenon in respiratory physiology whereby, owing to the fact that upper airways do not function as locations for gas exchange, and because of the tidal nature of ventilation, there is always a fraction of the inspired air that does not perform a physiologic function of exchanging carbon dioxide for oxygen.
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