Gas exchange in the partially atelectatic lung.

An 11-month-old girl was anaesthetised for resection of a coarctation of the aorta. The child took part in a research project in which functional residual capacity (FRC) and alveolar deadspace were being monitored on-line. Initial measurements during controlled ventilation showed an unexpectedly low PaO2 and an alveolar deadspace more than double the normal value. After manual hyperventilation and tracheal suction, FRC increased by 25% and PaO2 and the alveolar deadspace fraction reverted towards normal values for an infant. We therefore had strong grounds for suspecting atelectasis, although the shunt through the atelectatic lung was, in theory, too small to account for the increase in alveolar deadspace. We also noted that the shape of the expired CO2 trace was improved after hyperinflation, as was the time needed to wash out SF6 (the tracer gas used to measure FRC) from the lung. From these observations, we conclude that atelectasis may be associated with more widespread disturbances of gas exchange than is generally realised, perhaps because of distension of adjacent lung regions.