VENTILATION-PERFUSION RATIO IN NORM AND PATHOLOGY

Frustration of ventilation-perfusion ratio (VPR) plays a key role in changes of gas composition of the arterial blood in lung diseases. In the article the concept of VPR is presented; mechanisms of formation of ventilation, perfusion and VPR gradients in healthy lungs are described. Factors that provoke a mismatch between ventilation and perfusion in diseases of respiratory organs: pathology of the airways and of lung parenchyma, derangement of pulmonary circulation, are considered. On an example of a three-component model of lung, the influence of disorders in VPR on composition of the alveolar gas and arterial blood is described. Causes and mechanisms of increase in the alveolar dead space and in its functional equivalent – regions of lungs with high VPR, are presented. It is shown that the existence of these regions reduces the effectiveness of ventilation and leads to increase in the minute pulmonary ventilation to maintain the alveolar ventilation on the level adequate to metabolic demands of an organism, which eventually increases the energetic cost of respiration. Causes and mechanisms of intrapulmonary shunting of blood and a functional equivalent of such shunting – gas exchange units with low VPR – are described. The consequences of reduction of VPR – a drop in the oxygenation of the arterial blood and increase in the alveolar-arterial oxygen difference – are considered. Potentials of volumetric capnography in the evaluation of VPR are shown. The interrelation between VPR, slope of the 3rd phase of capnographic curve and α angle is shown: the slope of the 3rd phase and the value of α angle are proportional to the extent of non-uniformity of VPR. A method for determination of the volume of physiological dead space and of its components (anatomic and alveolar dead spaces) using volumetric capnography is presented. The difference between the approaches of Bohr and Enghoff to calculation of physiological dead space is discussed.