- Ventilation-Perfusion Defects (V'/Q' Defects) refer to abnormalities in the total ventilation-perfusion ratio of the lung that lead defects in blood oxygenation.
- Although the normal upright lung does display some regional variation in the ventilation-perfusion ratio (V'/Q' Ratio), overall the V'/Q' Ratio of a healthy lung approximates 1. This means that on average, individual units of the lung display roughly equivalent rates of alveolar ventilation to pulmonary blood flow. This allows for efficient exchange of oxygen and carbon dioxide between blood entering the pulmonary capillaries and the alveolar space. However, in scenarios of V'/Q' Defects, individual units of lung do not receive properly coordinated rates of alveolar ventilation and perfusion. Some units may be perfused with little ventilation, in some cases approaching a right-left Shunt, whereas other units may be ventilated but receive little perfusion, resembling dead space. When such derangements in the V'/Q' ratio are widespread, gas exchange becomes highly inefficient and can lead to significant defects in blood oxygenation, yielding hypoxemia. Although elimination of carbon dioxide will also be affected by V'/Q' defects, the compensatory hyperventilation caused by hypoxemia usually allows for sufficient elimination of carbon dioxide that levels of arterial carbon dioxide remain largely normal.
- A-a Gradient
- Because partial pressures of arterial carbon dioxide are largely normal in scenarios of diffusion defects, the "Alveolar Gas Equation" described in alveolar oxygen calculates a largely normal theoretical value for the partial pressure of alveolar oxygen. However, as described above, diffusion defects lead to abnormal declines in the partial pressure of arterial oxygen. Consequently, diffusion defects are characterized by a widening of the A-a Gradient.
- Response to Oxygen Therapy
- Inhalation of pure oxygen should improve oxygenation of blood in poorly-ventilated areas of lung although it will have little effect in poorly-perfused areas of lung which display excessive high V'/Q' Ratio. Given the improved blood oxygenation in poorly-ventilated areas of lung, the arterial oxygen tension of individuals with V'/Q' Defects does improve and in some cases may be corrected with oxygen therapy.
- Ventilation-Perfusion Defects arise in a wide variety of lung diseases and are likely the most important source of hypoxemia in these pathologies. Importantly, when a V'/Q' Defect is suspected other basic causes of hypoxemia should be first be ruled out.