Forced Random Noise Resistance Determination in Childhood Asthma

Measurement of pulmonary function in infants and young children is particularly difficult because they are unable to perform the necessary maneuvers and do not tolerate the equipment required by standard spirometric and plethysmographic techniques. Also, complex reflexes from the maximum inspiration and forced expiration complicate the interpretation of forced expiratory spirometric data after administration of bronchoconstrictor or bronchodilator drugs.

Forced excitation measurements, which require no special maneuvers and only minimal equipment contact, have potential for providing interpretable pulmonary function data in this population. In this approach, either sinusoidal or complex fluctuations are applied to the respiratory system. Resulting pressure and flow signals are detected and processed to obtain measures of effective respiratory resistance and in a more general sense, respiratory resistance as a function of frequency.

Pulmonary function in  young children

In children, several groups have reported effective resistance data using forced excitation approaches. However, only two groups studied children under three years of age, and both used single frequency sinusoidal excitation. One of these, Wohl et al, used a technically complex variation of the forced oscillations approach in which acoustic fluctuations were applied at the body surface using a body box. The other, Rutter, Lenny, and associates, used a much simpler approach with a face mask to couple the acoustic fluctuations to the airway. We are exploring the use of random noise excitation with the latter approach. We believe that this approach can be used routinely to obtain data in these young subjects as well as in children over three years of age. Learn more information subject to curing of asthma at children: “Treatment of Childhood Asthma“.

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In this report, data are presented showing the variability of random noise resistance measurements and their correlation with more standard spirometric parameters in asthmatic children over three years of age. Bronchodilator-induced changes are also compared in these two sets of parameters in a similar group of subjects. Finally, prebronchodilator and postbron-chodilator random noise resistance data are presented from young children and infants less than three years of age.

Material and Methods

In this study, we used a commercial forced random excitation system based on the work of Landser, Nagels, and associates. This system, which is shown schematically in Figure 1, measured respiratory resistance at 2 Hz intervals in the 2 to 26 Hz range and the coherence of this measurement, a quantitative indication of the reliability of the resistance measurement. It contained a loud speaker for generating the forced random excitation, pressure and flow transducers for detecting these signals, a computer for processing these signals, a keyboard for entering commands and subject identification, and a printer for displaying the data. In this study, we report the resistances at 6 Hz (RJ and at 26 Hz (R^) and the average resistance computed using 6 to 26 Hz data (Re.). Data at 2 and 4 Hz were not included because coherence values frequently fell below 0.8, a minimum acceptable value.

We conducted two studies with older asthmatic children (four to 17 years of age with a mean age of 9.4 years, 20 male/ten female) to compare random noise resistance and standard spirometric measurements. All had asthma as defined by the American Thoracic Society and demonstrated a 15 percent increase in FEVt after the administration of an inhaled bronchodilator. In 30 subjects with asthma, we obtained three sets of resistance data within a few minutes of each other; from these, we computed mean values and coefficients of variation for Re, R, and R for each individual. Coefficients of variation from all individuals were averaged to provide an estimate of the variability of these resistance parameters. After obtaining the three sets of random noise measurements, we obtained maximum forced expiratory spirometric data on all 30 subjects using a computerized spirometric system. Each subject performed three acceptable maneuvers, and the system computed and printed standard spirometric parameters from the one with the largest sum of FVC and FEVlt Using the data from these 30 individuals, we correlated mean values of R* R^, and with FEVi, FEE», FEFtj*, FEF», and FEF using linear regression analysis.

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Inhaled bronchodilator

The second study involved 20 asthmatic children who had an FEV, less than 80 percent of predicted or an FEF less than 70 percent of predicted. After baseline measurements, each received 180 of albuterol from a metered dose inhaler. Both random noise and forced expiratory measurements were obtained ten minutes after bron-chodilator administration. Random noise resistance data was always obtained before the maximum forced expiration to preclude any effect of the latter maneuver on the resistance measurement. Changes in resistance parameters that exceeded two times the average coefficient of variation measured in the study described in the previous paragraph were defined as significant. Similarly, spirometric changes that exceeded two times the average coefficient of variation reported earlier for our laboratory were defined as significant.

In order to make measurements in supine infants and young children, we modified the forced random excitation system by placing the transducers, bias flow connections, and port for the mouthpiece at the end of a flexible tube, approximately 7.5 cm in length. The lengths and rigidity of all tubes were adjusted so that measurements made on a calibration device supplied with the forced random excitation system after the modification matched those made with the original system. When measuring infants and young children, the transducer assembly was connected to a Bennett seal mouthpiece in place of the standard adult mouthpiece. The rigid mouthpiece ensured a patent airway while the soft “mask” provided a good seal to the face and supported the cheeks. These modifications provided minimal effective dead space, thereby allowing measurements in infonts with small tidal volumes. With this modification, we made measurements in several children two years old or younger. In another 12 asthmatic children that were two and three years old, we also made before and after bronchodilator measurements with a standard mouthpiece attached to the extension tubing. The study was approved by the institutional committee on human research and informed consent was obtained from the subjects/ parents. More about it you can find on our site – https://onlineasthmainhalers.com.
Figure-1

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Figure 1. Schematic diagram of the forced random excitation system.