Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 34

Here the situation is different. Blood returning from low‐V/Q areas has a low PO₂ and low O₂ content, but there is a limit to how far this deficit can be made good by mixture with blood returning from high‐V/Q areas. Blood returning from a high‐V/Q area will have a high PO₂ but is unable to carry more than the ‘normal’ quantity of oxygen, as the haemoglobin will already be saturated.

 Areas of low V/Q result in a rise in arterial CO2 and a fall in arterial O2 content.

 Increased ventilation in areas of high V/Q may balance the effect on CO2 content but will only partially correct the reduction in O2 content; a degree of hypoxaemia is inevitable.

 It follows that, where arterial oxygen levels are lower than would be expected from consideration of PaCO2 (overall ventilation) alone, there must be a disturbance to the normal V/Q matching system in the lung; that is, there is likely to be an intrinsic problem with the lung or its vasculature.

When interpreting arterial blood gas results, it is often important to know whether an observed low P_aO₂ can be explained by underventilation alone or whether a problem with the lung or pulmonary vasculature is present. The tool we use for this task is the alveolar gas equation.