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.