Physiologic Dead Space & Ventilation Rates

Gas exchange requires the close physical association of ventilated alveoli and perfused pulmonary capillaries. However, in the dead spaces of the respiratory tract, one (or both) of these requirements is absent, and gas exchange does not occur. Anatomic dead space: conduction portion of the respiratory tract (we show the tracheobronchial tree in this image). Functional dead space: aka, alveolar dead space comprises alveoli where gas exchange does not occur (i.e., non-perfused alveoli). Physiologic dead space: includes the anatomical space and functional dead space; this is the total volume of the respiratory tract that does not participate in gas exchange. It can be calculated using the Bohr equation. The total rate of air-flow into and out of the lungs. Includes the air-flow through the tracheobronchial tree and to both the functional alveoli and non-functional alveoli. Refers to the rate of air-flow into and out of the functioning alveoli, only (not though the physiologic dead spaces). Partial Pressure Minute and alveolar ventilation rate can be used to calculate the partial pressure of alveolar oxygen, which is a key facet of clinical respiratory evaluations. Calculate the minute ventilation rate: VE = TD x Breaths/Min Tidal volume can be measured clinically using a spirometer. VA = VE - VD (clinically determined via the Bohr equation). Alveolar ventilation equation states that: PACO2 = (VCO2 x K)/VA K is usually 863 mmHg; this constant controls for variation in physical measurement conditions. Alveolar gas equation states: PAO2 = PIO2 - (PACO2/RQ) RQ = respiratory quotient (aka, respiratory exchange ratio) is typically 0.8; it describes the ratio of the amount carbon dioxide produced in metabolism to the amount of oxygen consumed. When alveolar ventilation rate decreases, the alveolar partial pressure of carbon dioxide increases while the partial pressure of oxygen decreases. This makes intuitive sense, as you can imagine the effects of reducing alveolar ventilation by holding your breath: Carbon dioxide is held in your lungs, and the amount of oxygen will necessarily decrease because you aren't bringing fresh air into your lungs. More specifically, the alveolar gas equation predicts the degree of change in alveolar partial pressure of oxygen for a given change in the partial pressure of alveolar carbon dioxide, as long as the respiration quotient stays at 0.8: For example, if alveolar ventilation is reduced by one half, alveolar partial pressure of carbon dioxide increases two-fold, and partial pressure of oxygen decreases by slightly more than one-half. Notice that the alveolar gas equation also informs us that if the respiratory quotient changes, the relationship between alveolar partial pressure of carbon dioxide and oxygen will also change.