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. Alveolar ventilation (V) is the amount of air that reaches alveoli in the lungs, measured in liters/minute (L/min) and perfusion (Q) is the pulmonary blood. 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. Gas exchanging units with little or no blood flow (high V9A/Q9 regions) result in alveolar dead space and increased wasted ventilation, i.e. The A-a gradient represents the difference in partial pressure of oxygen between the alveoli and the arteries.
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