Validity of forced expiratory flow volume loops in neonates.

It is claimed that suddenly squeezing a newborn baby's trunk with a pressure of 3-4 kPa produces a flow volume curve that can be used to measure the function of the small airways. If the squeeze is applied during expiration rather than at the end of inspiration, however, anomalous results may be obtained. One possible explanation is that the babies are limiting expiratory flow by making inspiratory efforts in response to the applied external pressure. The response of 10 healthy term neonates to forced expiration was studied by using an oesophageal balloon. The squeeze was provided by an inflatable jacket, and measurements of oesophageal pressure and jacket pressure were recorded, as well as flow and volume changes at the mouth. Two hundred and twenty one squeezes were performed at different points in the respiratory cycle. In 188 squeezes an inspiratory effort was evident before the oesophageal pressure reached a plateau (mean time to peak pressure = 155 ms). For the remaining squeezes a plateau pressure was associated with closure or narrowing of the upper airway in most of the babies. When the squeeze was applied at low lung volumes the inspiratory effort was significantly earlier and stronger than around end inspiration. Thus a baby makes a reflex inspiratory response to chest compression that may interfere with the measurement of airway function when this technique is used.