![]() The A-a gradient can be calculated by subtracting the alveolar partial pressure of oxygen, which is calculated using the alveolar gas equation, by the arterial partial pressure of oxygen, measured with arterial blood gas (ABG). All three of these conditions can contribute to an increase in A-a gradient and cause hypoxemia. Diffusion problems can occur when the alveolar walls are fibrosed and oxygen cannot diffuse across the alveolar wall into the blood stream. High V/Q is also called dead space and low V/Q is called shunting. V/Q mismatch means a mismatch of ventilation to perfusion (V/Q) ratio and can represent either too little perfusion due to blood flow obstruction (high V/Q) or too little ventilation due to airway obstruction (low V/Q). An increase in A-a gradient can occur in hypoxemia and the causes include V/Q mismatch such as dead space or shunting and diffusion problems. The normal A-a gradient is 10 to 15 mmHg. ![]() Interestingly, the second patient had some evidence of. In both patients, other causes of worsening hypoxemia, including pulmonary emboli or intracardiac shunting, were effectively excluded. The A-a gradient represents the difference in partial pressure of oxygen between the alveoli and the arteries. The Bohr equation gives the true dead space, whereas the Enghoff modification may overestimate the dead space as it reflects not only the dead space, but also the degree of intrapulmonary shunt. Two cases of primary pulmonary hypertension are reported in which intrapulmonary shunting and severe hypoxemia developed during treatment with epoprostenol. ![]()
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