Assessment of an Infant Whole-Body Plethysmograph Using an Infant Lung Function Model
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Copyright #ERS Journals Ltd 2001 Eur Respir J 2001; 17: 765±772 European Respiratory Journal Printed in UK ± all rights reserved ISSN 0903-1936 TECHNICAL NOTE Assessment of an infant whole-body plethysmograph usinganinfantlungfunctionmodel B. Reinmann*, J. Stocks#, U. Frey* Assessment of an infant whole-body plethysmograph using an infant lung function *University Hospital of Berne, Insel- model. B. Reinmann, J. Stocks, U. Frey. #ERS Journals Ltd 2001. spital, Switzerland. #Portex Anaesthe- ABSTRACT: In order to facilitate international multicentre studies and improve the sia, Intensive Care and Respiratory quality control of infant pulmonary function measurements, the European Respiratory Medicine Unit, Institute of Child Health, London, UK. Society-American Thoracic Society Task Force for infant lung function testing has recently developed speci®cations for standardized infant lung function equipment and Correspondence: U. Frey software. A mechanical infant lung model analogue has been developed to assess whether Dept of Paediatrics infant lung function equipment is able to meet these requirements. However, the University Hospital of Berne practical testing of infant lung function equipment using such models is highly complex Inselspital because of the need to use very small pressure and ¯ow changes, and the numerous 3010 Bern potentially confounding factors associated with both the design of the device and the Switzerland testing procedure. Fax: 41 316329484 The aim of this study was to determine whether the infant lung model is capable of Keywords: Infant assessing the overall function of an whole-body infant- plethysmograph, using the only respiratory function test infant plethysmograph that was commercially available at the time as an example. respiratory mechanics The mechanical characteristics of the model such as vibrations or noise did not disturb plethysmography the delicate plethysmographic measurements and thereby allowed a reliable assessment whole body of the system. A series of tests revealed that the plethysmograph was able to measure standardization airway resistance 1±3.5 kPa.L-1.s with an accuracy of 2.5% and lung volumes 75±300 mL with an accuracy of 2.5% under in vitro conditions. Received: September 8 1999 To conclude, the infant lung model is a useful means of assessing the overall in vitro Accepted after revision December 29 2000 performance of infant whole-body plethysmographs, but thermal, mechanical and frequency response characteristics of such a device must be taken into account when interpreting the results of such assessments. Eur Respir J 2001; 17: 765±772. To facilitate international multicentre studies and While the testing of isolated components of plethys- improve the quality control of infant pulmonary func- mographs has been described in detail in the literature tion measurements, the European Respiratory Society [10±15], there has been no guidance as to how the 7ERS)-American Thoracic Society 7ATS) working overall performance of infant-plethysmographs should group for infant lung function testing has recently be assessed. The latter is very important since even if the developed speci®cations for standardized infant lung physical properties of each component of the plethys- function equipment and software [1±7]. The assessment mograph adhere to speci®ed standards, their integrated of infant lung mechanics requires higher technical function might fail or display unexpected problems. demands of the equipment than when studying adults, To test the integrated overall performance of infant because of smaller signal to noise ratio and higher lung function devices, and particularly that of ple- respiratory rate. Whole-body infant-plethysmography thysmographs, a "mechanical model baby" which is 7IWBP) [4, 8, 9] is a particularly challenging technique partially able to simulate the breathing behaviour of with respect to these signal to noise and calibration infants of 2±12 kg body weight has recently been built problems, but remains an important method in that it [16]. However, the practical testing of infant lung allows simultaneous assessments of airway resistance function equipment using such a model is complex and 7Raw) and functional residual capacity 7FRCpleth)in reveals unexpected pitfalls. This is mainly due to the these young subjects. The criteria which infant-plet- fact that the plethysmographic measurements in infants hysmographs should meet have been described recently are potentially and critically in¯uenced by factors such [4, 8], as have general aspects of equipment and as signal to noise problems and thermal effects due processing [1, 2]. Based on these guidelines, it should to motor vibrations and heating effects of the pump be possible to assess the mechanical and thermal when a model is placed within the plethysmograph. properties of the plethysmograph itself, as well as the The aim of this study was to investigate whether the linearity and frequency response of the signal transdu- infant lung model can be used to test the integrated cers. performance of an infant plethysmograph by testing the 766 B. REINMANN ET AL. only currently available commercial model 7Master- 20 min and hence no shift in baseline volume in a Screen BabyBody; Erich Jaeger GmbH, Hoechberg, plethysmographic box. Germany). The authors particularly wanted to inves- tigate whether an infant-plethysmograph can measure In summary. Based on recent standards [1, 2, 4], the lung volumes, ¯ow resistances and amplitude and phase minimal expected volume displacement in the box relationships between two corresponding signals 7pres- during Raw measurements is 1 mL corresponding to a sure or ¯ow, respectively) accurately, and to develop pressure change of 0.001 kPa 7DPbox)ina100L recommendations regarding optimal testing procedure chamber. The Sqirec linear motor pump did not when performing such in vitro assessments. cause signi®cant signal to noise problems or baseline drift in this order of magnitude and was, therefore, suitable for use in an infant plethysmograph. Methods and results Study design Accuracy of in vitro volume measurements For clarity, the protocol is presented in four stages. Rationale. FRC is an important physiological para- For each section, the rationale, methods, results and a meter in infants. During plethysmography, FRC is short summary are presented. Firstly, the in¯uence of measured from the changes in box volume 7DVbox)and the model itself 7e.g. vibrations, warming) on plethys- pressure at the airway opening 7Pao) during an air- mographic box pressure signals 7signal to noise ratio) way occlusion. Measuring small lung volumes in was assessed. Secondly, the accuracy with which an infant plethysmographs is dif®cult, since the ratio of infant plethysmograph could measure a range of model the measured lung volume to plethysmographic box lung volumes [16] at various breathing frequencies volume, which determines the amplitude of DVbox was determined. Thirdly, the accuracy with which an and thus the signal to noise ratio, is relatively small. infant plethysmograph could measure a range of model Although an infant lung model can potentially be air¯ow resistors [16] was assessed. Finally, the ampli- used to determine the smallest measurable volume, tude and phase relation of box volume 7Vbox) versus such measurements are complex. For example, whe- pressure at the airway opening 7Pao)andVbox versus reas gas compression during the FRC manoeuvre is ¯ow 7V 9) was tested, at 0.25±10 Hz [2, 17]. isothermal in an infant, thermal effects during ventila- tion of the model volume elements, together with any pressure losses due to mechanical leaks in the model, Effect of the infant lung model on the plethysmo- must be considered [16]. Nevertheless, as described graphic signals previously, the infant lung model can simulate effec- tive lung model volumes 7Vm)ofy50, 75, 100, 150 Rationale. Box pressure measurements in infant-pleth- and 300 mL and thermal and leak effects can be ysmographs are prone to environmental noise that will taken into account [16]. in¯uence the signal to noise ratio and the quality of the A second problem that occurs during FRCpleth data. Any vibration or heat generated from the motor measurements concerns the time constant of the of an infant lung model will crucially in¯uence mea- plethysmograph. Plethysmographs have an in-built, surements. The potential in¯uence of the infant lung well-de®ned mechanical leak, which acts like a mec- model on plethysmographic measurements were there- hanical high pass ®lter. This leak allows equilibration fore assessed. of slow pressure changes due to warming of air that occurs when an infant is enclosed inside the plethys- Methods. The infant model consisted of a computer mograph. However, if the time constant of the box is driven linear motor with two different exchangeable very short, not only slow pressure changes, but also pistons 7Scireq Inc., Montreal, CA, USA) [18] and those due to breathing movements, will be at least various volume and resistor elements. Its characteris- partly attenuated, with subsequent underestimation of tics and accuracy have been described in the accom- FRC. Thus, an ideal leak time constant has to be panying paper [16]. The composition of the infant established that allows equilibration of temperature lung model can vary depending on whether infant changes, while being long enough not to affect the lung volumes, ¯ows or airway resistances are to be lung volume measurements. An infant lung model can simulated. The