Titration of mechanical respiratory support for neonates with lung disease is hampered by the lack of a reliable and practical bedside tool to monitor end-expiratory lung volume (EELV), also denoted as functional residual capacity. Whole-body plethysmography (1) and gas dilution (2), both of which give absolute values for EELV, are not appropriate for repeated measurements in a clinical setting. Other methods, including respiratory inductance plethysmography (RIP) (3) and fiberoptic plethysmography (4), allow measurement of change in EELV (ΔEELV) relative to a baseline condition, and only after calibration can ΔEELV be expressed as a gas volume in milliliters. Despite these limitations, RIP in particular has been widely applied as a research tool in following EELV trends in neonates on respiratory support (3,5,6) but is not in regular clinical use for this purpose.

Apart from measurement of absolute or relative EELV, there is potentially much to be gained by measuring tidal volume (VT) in spontaneously ventilating subjects supported with continuous positive airway pressure (CPAP) applied using prongs or a face mask. At present, estimation of VT in such infants is achievable with RIP, but only if preceded by a calibration procedure involving removal of the CPAP device and delivery of tidal inflations of known volume through a close-fitting mask (7). VT monitoring in this setting may allow titration of CPAP pressure to optimize respiratory system compliance.

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