July 1997
SAN FRANCISCO — Over the last decade, some innovative, noninvasive methods have been developed for assessing pulmonary function in infants. Two of these methods were reported here at the American Thoracic Society meeting.
A new method for obtaining full maximum expiratory flow-volume (MEFV) curves in sedated toddlers and infants was introduced by Robert Castile, MD, of Children's Hospital in Columbus, Ohio. Castile's Multiple Inflation Method permits the measurement of adult-type maximal respiratory flow-volume curves and fractional lung volumes in sedated infants and toddlers. Castile's proposed to use this method to define normal levels of lung function for these children.
"As infants and toddlers cannot voluntarily perform forced vital capacity [FVC] maneuvers, the measurement of standard adult-type pulmonary function tests in these very young children has not been possible," Castile said. "Tests of an infant's pulmonary function have been limited to measurements that could only be made during normal tidal breathing."
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The current method for obtaining tidal range partial forced expiratory flow-volume curves in infants is by mechanical compression of the thorax and abdomen with an inflatable vest. But Castile found that the clinical utility of forced expiratory flows at functional residual capacity produced by this method were inherently limited because they provide information only at low-lung volumes and are highly variable in normal infants. He also found that infants inspire reflexively in response to sudden expiratory forcing, substantially limiting the ability to produce increases in flow by thoracoabdominal compression, Castile said.
According to the study, mechanical over-ventilation during sleep produces a pause in respiratory effort after a fall in arterial CO2 of only a few torr. To produce a MEFV curve not affected by the infant's respiratory muscle activity, Castile determined a method for electively interrupting tidal breathing was needed.
"We found that augmenting ventilation only minimally by delivering several large, sigh-like breaths produced a 2 to 4 torr fall in end tidal CO2 and resulted in a short respiratory pause at end expiration, which was long enough to permit unencumbered performance of a FVC maneuver," Castile said.
The infants were sedated with 75 mg to 100 mg chloral hydrate. End tidal CO2, oxygen saturation and heart rate were monitored throughout the procedure. Flow was measured with a facemask attached to a pneumotachometer. Medical putty was used between the mask and the face to prevent air leaks during the procedure.
Each infant's chest and abdomen was wrapped in an inflatable vest. To produce a maximal forced expiratory maneuver, 30 cm of H2O was applied to the mouth and nose via the facemask during one to 10 consecutive spontaneous inspiratory breaths. When the lungs were inflated to 30 cm H2O, the expiratory port was opened and the child expired passively. The procedure was repeated until a pause in respiration was noted at the end of expiration. The lungs were then re-inflated to 30 cm H2O, the expiratory port was opened and the inflatable vest was rapidly pressurized to produce forced expiration.
"The ability to perform [adult type] pulmonary function tests in infants and toddlers using the Multiple Inflation Method should, by permitting earlier detection of respiratory dysfunction and more efficient assessment of the effects of therapeutic interventions, contribute substantially to the effort to prevent, treat and cure lung disorders of early childhood," Castile said.
In another study conducted by French researchers testing the abilities of new lung function tests at assessing metholine-induced airway obstruction in infants, 55 Caucasian infants were recruited from a pediatric pulmonology unit in Paris between November 1992 and April 1993.
The infants' median ages were 5.2 months. All the infants had previously suffered recurrent wheezing but had been free of respiratory symptoms for more than one month prior to the study. Those on bronchodilators stopped treatment more than eight hours before testing. Infants were studied after sedation with 100 mg of chloral hydrate. All the children were then challenged with methacholine. The results were that 42 completed the challenge while 13 were interrupted due to early awakening from sedation.
Technically acceptable measurements were obtained from 39 of the infants tested. Tidal breathing pattern indices were obtained in all 55 of the infants tested. In 39 of 45 infants, the partial expiratory flow-volume curves were convex and six were slightly concave, while 50 of the 55 expiratory flow tidal volume curves were convex and in five were straight lines.
Forty-one of the infants responded to the methacholine challenge (doses of approximately 1,600 mg) by a constant change of less than two means units in transcutaneous oxygen tension and at least one or two lung function tests. In each infant transcutaneous oxygen tension decreased with increasing doses of methacholine. At the final dose, researchers reported changes in more than two means units of transcutaneous oxygen tension in 97% of the infants, of maximum flow at functional respiratory capacity in 83%, of peak tidal expiration flow in 88%, and of expiration time in 54% of the infants.
All peak expiratory flow-volume curves obtained with the final dose, including those for infants showing a change in maximum flow at functional respiratory capacity of less than two means units, were concave. In only 20 infants were compliance and resistance of respiratory systems obtained in both pre- and post-challenge. The most sensitive tests in terms of changes induced were the peak tidal expiratory flow, respiratory rate, maximum flow at functional respiratory capacity and compliance in respiratory systems.
The authors, whose study was among those discussed by Steven M. Stick, MBBS, of the department of respiratory medicine at Princess Margaret Hospital in Perth, Australia, concluded that this non-invasive lung function test, along with maximum flow at functional respiratory capacity and peak tidal expiratory flow, can be used to detect minor and moderate airway obstruction, including that caused by a bronchial challenge. However, more tests are needed to assess its effect on bronchial disease.
For more information:
- Castile RG. Measurement of forced expiratory flows. Presented at the American Thoracic Society. May 16-21. San Francisco.
- Benoist MR, Brouard J. Ability of new lung function tests to assess methacholine-induced airway obstruction in infants. Journal of Pediatric Pulmonology1994;18:308-316.
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