The effects on gas exchange of a large mediastinal tumour

A 14-year-old boy with a mediastinal tumour large enough to cause dyspnoea at rest was anaesthetised for a biopsy. The tumour was believed to compress the tracheal bifurcation. After tracheal intubation, the expiratory breath sounds were heard to be intermittent. The expiratory flow pattern showed irregularities synchronous with the electrocardiogram. With constant flow inflation, the inspiratory pressure profile was suggestive of mechanical obstruction with a resistance which decreased during inspiration. The airway deadspace was greatly reduced, suggesting that the obstruction was near the tracheal tube opening and that there was pooling of carbon dioxide behind this obstruction. The alveolar deadspace fraction was increased and the shape of the carbon dioxide single breath test suggested severe airway obstruction. These findings can be explained on the basis of large airway obstruction, partly intermittent because of the heart's movement, and also reduced lung volume and uneven gas distribution caused by the tumour and pleural fluid.