[Physiology of pulmonary capillary recruitment]

At the apices of the lungs in man there is a poorly perfused zone (zone 2) which in certain circumstances can be recruited. The pulmonary capillary recruitment in this zone has been studied in the anaesthetised and ventilated dog. Following a thoracotomy a metal window with a small transparent viewing area was applied to the surface of the right lung, and the dog was placed in the left lateral decubitus position. The sub-pleural pulmonary microcirculation which is in this case in zone 2 was examined using a microscope. Alveolar hypoxaemia led to an increase in the number of pulmonary capillaries that were perfused. This recruitment of the pulmonary capillaries was independent of the cardiac output, the left auricular pressure and the pulmonary venous pressure. The recruitment disappeared if the rise in hypoxic pulmonary artery pressure was prevented using prostaglandin E1. There is a very good correlation between pulmonary arterial pressure and capillary recruitment. The hypoxic capillary recruitment is associated with a rise in capillary volume measured by DLCO. This capillary recruitment observed in the upper part of the lung is linked to a redistribution of the blood flow from the bases towards the apices of the lung as is shown in studies using radioactive microspheres. In conclusion, the response of the pulmonary circulation to generalised alveolar hypoxaemia may be outlined as follows: diffuse pulmonary arterial vasoconstriction leads to a rise in pulmonary artery pressure, which redistributes the blood flow towards the upper part of the lung recruiting capillaries and also increasing the pulmonary surface available for gas exchange.