Measurement of lung volume and ventilation distribution with an ultrasonic flow meter in healthy infants

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Abstract
Small airway disease in infants is characterised by abnormal lung volume and uneven ventilation distribution. An inert tracer gas washin/washout technique using a pulsed ultrasonic flow meter is presented to measure functional residual capacity (FRC) and ventilation distribution in spontaneously breathing and unsedated infants.With a pulsed ultrasound sent through the main stream of the flow meter, flow, volume and MM of the breathing gas can be calculated. Sulphur hexafluoride was used as a tracer gas. In a mechanical lung model (volume range 53–188 mL) and in 12 healthy infants (aged 38.3±9.2 days; mean±sd) accuracy and reproducibility of the technique was assessed. Indices of ventilation distribution such as alveolar-based mean dilution number (AMDN) and pulmonary clearance delay (PCD) were calculated.Mean error of volume measurement in the lung model was 0.58% (coefficient of variance (CV) 1.3%). FRC was in the low predicted range for normal infants (18.0±2.0 mL·kg−1) and highly reproducible (5.5±1.7% intra-subject CV). AMDN was 1.63±0.15 and PCD was 52.9±11.1%.Measurement of functional residual capacity and ventilation distribution using a sulphur hexafluoride washin/washout and an ultrasonic flow meter proved to be highly accurate and reproducible in a lung model and in healthy, spontaneously breathing and unsedated infants.

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