ventilation perfusion inequality shunt vs dead space

Then, by changing the fraction of gases to their partial pressures, the equation takes the following form:

Where O 2 is O 2 consumption, F IO 2 is the fraction of inspired O 2, A represents alveolar ventilation, and F AO 2 is the fraction of alveolar O 2. According to the Fick equation, under steady state conditions, the quantity of O 2 taken up by the lungs equals the amount of O 2 removed from inhaled air: The alveolar gas equation helps us understand the pathophysiology of abnormal gas exchange by providing information on alveolar oxygen tension. Changes in P O 2, P CO 2, and end-capillary O 2 content in a lung unit as its ventilation/perfusion ratio is increased from shunt (V/Q = 0) to dead space (V/Q = ∞). Newer studies that achieved greater spatial resolution demonstrated that perfusion and ventilation are rather heterogeneous and that this heterogeneity serves to match ventilation and perfusion.ĭecreased minute ventilationDecreased cardiac outputĪlveolar-end capillary diffusion limitationįigure 41–1 Gas exchange in a single lung unit. Early studies supported the concept that vertical gradients of both ventilation and perfusion produced V A/Q matching. Lung units that are poorly ventilated in relation to blood flow (low V A/Q) produce blood with low oxygen content (desaturated blood), but units with high V A/Q ratios cannot compensate, because the oxygen content of blood leaving these areas has nearly the same oxygen content as that from normal V A/Q regions as a result of the sigmoid shape of the hemoglobin oxygen dissociation curve ( Figure 41-1). In children, V A/Q mismatch and intrapulmonary shunt cause most gas exchange abnormalities ( Table 41-1). However, as the largest organ in the body, the lung is influenced by external and internal factors that affect ventilation/perfusion relationships.

Efficient gas exchange requires the close matching of regional ventilation and perfusion (V A/Q). The primary function of the lung is to exchange oxygen and carbon dioxide between inspired air and blood.

Although positive end-expiratory pressure decreases the proportion of shunt, at high levels it also increases dead space.

Ventilation perfusion inequality shunt vs dead space