Fisiopato

The field of pulmonary gas exchange is mature, with the basic principles developed more than 60 years ago. Arterial blood gas measurements (tensions and concentrations of O₂ and CO₂) constitute a mainstay of clinical care to assess the degree of pulmonary gas exchange abnormality. However, the factors that dictate arterial blood gas values are often multifactorial and complex, with six different causes of hypoxaemia (inspiratory hypoxia, hypoventilation, ventilation/perfusion inequality, diffusion limitation, shunting and reduced mixed venous oxygenation) contributing variably to the arterial O₂ and CO₂ tension in any given patient...

The 100% oxygen shunt test for detecting right-to-left anatomical shunting was originally described 70 years ago. However, its clinical value is not yet established. We conducted an audit in 80 patients undergoing the test between 1996 and 2012 in a tertiary referral centre. A significant difference (p=0.02) existed between the median shunt percentages where anatomical shunting was identified (10.2%) and absent (5.0%). The area under the curve for a ROC plot was 0.70. A shunt percentage of 8.3 had a sensitivity of 80% and specificity of 75% for detection of an anatomic shunt...

This review provides an overview of the relationship between ventilation/perfusion ratios and gas exchange in the lung, emphasising basic concepts and relating them to clinical scenarios. For each gas exchanging unit, the alveolar and effluent blood partial pressures of oxygen and carbon dioxide (PO2 and PCO2) are determined by the ratio of alveolar ventilation to blood flow (V'A/Q') for each unit. Shunt and low V'A/Q' regions are two examples of V'A/Q' mismatch and are the most frequent causes of hypoxaemia...