Department of Medicine, University of California, San Diego, La Jolla, CA, USA.
Compr Physiol. 2011 Jul;1(3):1541-53. doi: 10.1002/cphy.c091007.
By far the commonest cause of impaired gas exchange in patients with lung disease is ventilation-perfusion inequality. This is a complicated topic and much can be learned from computer models. Ventilation-perfusion inequality always causes hypoxemia, that is, an abnormally low PO2 in arterial blood. However, it is also the commonest cause of an increased arterial PCO2, or hypercapnia, in patients with chronic obstructive pulmonary disease (COPD). There is often confusion in this area with some people attributing the CO2 retention to "hypoventilation" when in fact these patients are usually moving much more air into their lungs than normal subjects. A patient with COPD can often return the arterial PCO2 to normal by increasing the ventilation. However, this does not return the arterial PO2 to normal because of the different shapes of the oxygen and carbon dioxide dissociation curves. Increasing pulmonary blood flow in the presence of ventilation-perfusion inequality usually raises the arterial PO2 but much less than increasing ventilation. Raising the inspired oxygen concentration is typically very effective in increasing the arterial PO2. Ventilation-perfusion inequality interferes with the transfer of all gases by the lung including the anesthetic gases. The gas exchange behavior of a lung depends greatly on the pattern of ventilation-perfusion inequality. It is theoretically possible to find a distribution that improves the transfer of some gases but this requires bizarre conditions that can never occur in practice.
到目前为止，肺部疾病患者气体交换受损最常见的原因是通气-血流比例失调。这是一个复杂的话题，可以从计算机模型中学到很多知识。通气-血流比例失调总是导致低氧血症，即动脉血氧分压异常降低。然而，它也是慢性阻塞性肺疾病（COPD）患者动脉血 PCO2 升高（即高碳酸血症）的最常见原因。在这个领域经常存在混淆，有些人将 CO2 潴留归因于“通气不足”，而实际上这些患者通常比正常受试者向肺部输送更多的空气。COPD 患者通常可以通过增加通气来使动脉 PCO2 恢复正常。然而，由于氧和二氧化碳解离曲线的不同形状，这并不能使动脉 PO2 恢复正常。在存在通气-血流比例失调的情况下增加肺血流量通常会升高动脉 PO2，但升高幅度远小于增加通气。提高吸入氧浓度通常非常有效地增加动脉 PO2。通气-血流比例失调会干扰肺部所有气体的转运，包括麻醉气体。肺的气体交换行为在很大程度上取决于通气-血流比例失调的模式。理论上，可以找到一种改善某些气体转运的分布，但这需要在实践中永远不会出现的奇异条件。
