RDS

Tidal volume and functional residual capacity are reduced in babies with RDS so they have a large physiological dead space. The infants may increase their respiratory rate to sustain alveolar ventilation but this is often not enough to compensate.

Atelectasis is collapse of the alveoli. This occurs when the distending pressure of the alveoli is not enough to hold it open – this affects the smallest alveoli first. This collapse occurs at end expiration when the alveoli are at their smallest and the distending pressure is lowest.

During inspiration some alveoli may reopen but this cycle of collapse and reopening is damaging to the epithelium. Infants may try to maintain functional residual capacity and therefore keep alveoli open by increasing the resistance to airflow at end expiration. They do this by breathing out through a partially closed epiglottis. This process is characterised by an ‘expiratory grunt’.

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