Measurement of work of breathing in infancy.

Assessment of lung function in young children is extremely difficult unless sedation is given. If tidal exchange and intrathoracic pressure swings can be recorded, the severity of the respiratory problem can be measured by construction pressure-volume loops and calculating the work of breathing. The system we have used to measure tidal thoracic volumes is that of a soft, rubber, inflatable jacket with pressure-volume characteristics linear from inflation pressure of 2.5 to 7 cm H2O. Oesophageal (intrathoracic) pressures are measured using water-filled nasogastric tubes (FG6). Frequency responses showed that 63% rise times for the jacket and water-filled nasogastric tube assessed together to be 10.5 msec. Recordings were traced onto a computer digitizing tablet and print-outs were obtained with the following information: respiratory rate, tidal thoracic volume, minute thoracic volume, oesophageal pressure, resistive work per min, elastic work per min, and total work per min. Comparisons were made between volumes measured using the respiratory jacket (VJ) and by pneumotachograph and integrator (VInt) in six infants. Hysteresis arising from the jacket produces some change in resistive work of breathing, but the total work of breathing is only increased by a maximum of 10.5%. Studies on healthy newborn babies showed that the jacket system tends to underrecord the resistive work of breathing, but that total work of breathing values are very similar. Reproducibility of analysis showed the following coefficients of variation from the mean: tidal thoracic volume, 1.6%; minute thoracic volume, 1.7%; resistive work, 5%; elastic work, 2%; and total work, 2.5%. The extent of breath-to-breath variation is much larger. This system makes it possible to obtain serial measurements of work of breathing without the need for sedation in infants who may be acutely ill and in respiratory failure.