Alveolar-arterial oxygen gradient

Hypoxemia (alveolar-arterial oxygen gradient >30mmHg on room air saturation <94%) 6/10... 

SSc-ILD is characterized by a restrictive defect and a decreased DLco-PFTs, and especially DLco levels, correlate well with the extent of disease on HRCT (45,95,96). Rarely, scleroderma of the chest wall may cause extrathoracic restriction. Cardiorespiratory exercise testing may detect occult lung involvement by showing increased functional dead space ventilation and widened an alveolar-arterial oxygen gradient (96,97). The six-minute walk test, a simple, inexpensive and reproducible test that gives a good indication of patient s exercise capacity, has yet to be validated in SSc-ILD as an outcome measure (98,99). 

When performed in less severe cases, lung function tests usually show a mild restrictive ventilatory defect (without obstructive ventilatory defect), with reduced carbon monoxide transfer factor and increased alveolar-arterial oxygen gradient. At long-term follow-up, spirometry is generally normal (7,75,76). 

PMV and LTMV (18,36,37). For instance, the mean alveolar-arterial oxygen gradient in the 1419 patients enrolled in the Ventilation Outcomes Study Group was 127 mmHg (37). 

Figure 12 Changes in pulmonary hemodynamic and oxygenation parameters for an ARDS patient before and after aerosolized DETA/NO administration (a) pulmonary vascular resistance (PVR) index, (b) mean pulmonary arterial pressure (PAP), (c) intrapulmonary shunting, and (d) alveolar-arterial oxygen partial pressure gradient (A-aDC>2). The 20-min period of DETA/NO aerosol administration is indicated by ( ). [Adapted from Reference 27 with permission.]...

The PO2 in arterial blood, however, is further reduced by venous admixture (shunt), the addition of mixed venous blood from the pulmonary artery, which has a POj of -5.3 kPa (40 mm Hg). Together, the diffusional barrier, ventilation-perfusion mismatches, and the shunt fraction are the major causes of the alveolar-to-arterial oxygen gradient, which is normally 1.3-1.6 kPa (10-12 mm Hg) when air is breathed and 4.0-6.6 kPa (30-50 mm Hg) when 100% oxygen is breathed. 

Atmospheric air is breathed in during inspiration. This inspired air mixes with the gas already in the respiratory tree and some is drawn into the alveoli where gas comes into intimate relation with the blood circulating in the pulmonary capillaries. The gas here is called alveolar gas. Blood entering the lungs from the pulmonary artery has a partial pressure of oxygen which is lower than that in the alveolar gas. Oxygen diffuses from the alveolar gas into the pulmonary blood. For carbon dioxide, the partial pressure gradient is from blood to alveolar gas and so this is the direction in which carbon dioxide diffuses. 

The difference between alveolar and arterial P02 values is called the oxygen A-a gradient (also P(A-a)°2 (A-a)°2... 

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