obstructive airways disease, parenchymal lung disease, pulmonary oedema, pulmonary vascular disease, intra-cardiac shunting, and the hepatopulmonary syndrome) where the gradient is increased. Hypoxaemia due to a ventilation-perfusion mismatch, right to left veno-arterial shunting of blood, or a barrier to oxygen diffusion (e.g. Hypoxaemia with a normal A-a gradient requires augmentation of ventilation, and oxygen therapy may be contra-indicatedĢ.neuromuscular disease, drug overdose, chest wall disease), where the gradient is normal (both alveolar and arterial oxygen are reduced) Hypoxaemia caused by alveolar hypoventilation alone (e.g. The gradient is used to distinguish two primary causes of hypoxaemia:ġ. (where constant k = PAN2 + PAH2O) CLINICAL USE The gradient is therefore measured and calculated as: PAH2O is the pressure of water in alveolar air, and is constant. If oxygen is being added to inspired air, PAN2 can be calculated by difference. PAN2 is the pressure of nitrogen, and is constant in normal air. If there is marked ventilation/perfusion inequality or arterio-venous shunting, the calculated gradient will be lower than the true gradient. This is not strictly valid, but because carbon dioxide is very soluble in water and diffuses at twenty times the rate of oxygen, the error is small in most patients. PaCO2, the measured arterial carbon dioxide pressure is substituted for PACO2, the alveolar carbon dioxide pressure. PB is the barometric pressure, which is measured by the blood gas analyser. Where the barometric pressure PB equals the sum of the individual partial pressures all of the component gases in alveolar air: Rearranging the equation PAO2 is calculated according to the alveolar air equation, which follows: PaO2 is measured in the arterial blood sample by the blood gas analyser. Where PAO2 is the partial pressure of oxygen in alveolar air and PaO2 is the partial pressure of oxygen in arterial blood. The Clinical Use and Reference Intervals are below the Calculation section. The gradient is best determined if the arterial blood is drawn after the patient has been breathing ambient air for at least 5 minutes Oxygen normally diffuses rapidly across alveolar membranes, and so the gradient is normally close to zero. The alveolar-arterial (A-a) oxygen gradient is a measure of the efficiency of the transfer of oxygen from the lungs to the blood. Short Description : Alveolar - Arterial Oxygen Gradient
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