with pneumonia have a physical barrier within the alveoli, which limits the diffusion of oxygen into the capillaries. However, these patients can ventilate (unlike the patient with hypoventilation), which will result in a well-oxygenated respiratory tract (A) with poor diffusion of oxygen across the alveolar-capillary unit and thus lower oxygen levels in the arterial blood (a). The obstruction, in this case, would occur at the waterfall in our example, limiting the flow of water only through the second part of the river. Thus patients with hypoxemia due to pneumonia will have an inappropriately elevated A-a gradient (due to normal "A" and low "a"). Applying this analogy to different causes of hypoxemia should help reason out whether to expect an elevated or normal A-a gradient. As a general rule of thumb, any pathology of the alveolar-capillary unit will result in a high A-a gradient. The table below has the different disease states that cause hypoxemia. Because A–a gradient is approximated as: (150 − 5/4(P)) – at sea level and on room air (0.21x(760-47) = 149.7 mmHg for the alveolar oxygen partial pressure, after accounting for the water vapor), the direct mathematical cause of a large value is that the blood has a low , a low Pa, or both. is very easily exchanged in the lungs and low Pa directly correlates with high minute ventilation; therefore a low arterial Pa indicates that extra respiratory effort is being used to oxygenate the blood. A low indicates that the patient's current minute ventilation (whether high or normal) is not enough to allow adequate oxygen diffusion into the blood. Therefore, the A–a gradient essentially demonstrates a high respiratory effort (low arterial Pa) relative to the achieved level of oxygenation (arterial ). A high A–a gradient could indicate a patient breathing hard to achieve normal oxygenation, a patient breathing normally and attaining low oxygenation, or a patient breathing hard and still failing to achieve normal oxygenation. If lack of oxygenation is proportional to low respiratory effort, then the A–a gradient is not increased; a healthy person who hypoventilates would have hypoxia, but a normal A–a gradient. At an extreme, high levels from hypoventilation can mask an existing high A–a gradient. This mathematical artifact makes A–a gradient more clinically useful in the setting of hyperventilation. See also Pulmonary gas pressures References External links A-a Oxygen Gradient online calculator Respiratory physiology