Achievement of Forced Random Noise Resistance Determination in Childhood Asthma

Measured values of R«, Rae, and H^ in the four to 17-year-old asthmatic children ranged from 2.8 to 11.4,3.3 to 10.4, and 3.2 to 10.2 cmH2OLi, respectively. Figure 2 shows a plot of these three parameters as a function of height along with separate regression lines for those with normal and abnormal spirometry. It is possible that a curvilinear relationship could have provided a better fit, but we did not feel that the limited amount of data (15 subjects in each group) justified this complexity. Subjects with an FEVi less than 80 percent of predicted or an FEF less than 70 percent of predicted generally had higher resistances than individuals with normal spirometry.

Individual coefficients of variation for Re, R*, and He, ranged from less than 1 percent to about 20 percent. The variability of these three resistance parameters was similar, and no age dependency in variability was apparent. Table 1 shows the mean values for the coefficients of variation for R«, R along with their ranges and standard deviations. All three mean values were less than 10 percent, suggesting that the expected variability in these three resistance parameters in asthmatics is less than 10 percent.


Table 2 shows the result of the correlation analysis relating random noise resistance parameters to maximum forced expiratory spirometric parameters. As examples of the observed agreement, Figure 3 presents individual data showing the relationship between the three random noise resistance parameters and FEVi. In Table 2, correlation coefficients ranged from 0.50 to 0.89; eight of 15 correlation coefficients were greater than 0.8, and all but three were greater than 0.7. All correlations were statistically significant at the 0.0001 level except that between R* and FEF^, which was statistically significant at the 0.005 level. All spirometric parameters correlated best with Re and worst with R*. All three random noise resistance parameters correlated better with spirometric parameters that depended on the early portion of the maximum forced expiration (eg, FEF1 and FEVi) than they did with those spirometric parameters that depended on the late portion of this maneuver (eg, FEFisJ).

After the bronchodilator, changes in Re (AR«) ranged from — 49 to 16 percent of the individuals baseline value; corresponding values for AR*, were — 49 to -I- 23 percent, and for they were — 51 to 5 percent. Mean values and standard deviation for these changes are given in Table 3. Changes in FEVj and FEEas.^ (AFEVi and AFEF^ts*) ranged from —23 to 48 percent and —37 to 190 percent, respectively; mean values and standard deviations of these two parameters also are given in Table 3 along with those for AFEF75%, AFEFjo*, and AFEF^. The largest average changes occurred in forced expiratory flow rates at mid and low lung volumes, that is, AFEFso*, AFEF^, and AFEF^tto, with mean values of 30.1, 52.1, and 54.2 percent, respectively. At the same time, these parameters had the largest variability among the individual responses as indicated by their large standard deviations, about 60 percent of the mean value. The change in FEVj was smaller, 7.4 percent, but it was more consistent among subjects with a standard deviation of 18.8 percent. Mean changes in the three resistance parameters were on the order of 10 percent to 20 percent with standard deviation approximately equal to the mean change; of the three resistance parameters, ABe had the largest mean value and the smallest standard deviation.


There was little correlation between bronchodilator-induced changes in the three resistance parameters and those in the maximum forced expiratory spirometric parameters. The largest correlation coefficient, 0.73, was found between in AR<> and AFEFtb*. Only three other correlations (te, AR* and AFEVb ARe and AFEFso«, and AFEE and AFEF-^ were statistically significant, but the correlation coefficients were less than 0.6.

The bronchodilator induced a significant decrease (ie, greater than two times the expected coefficient of variation) in R*, R», and H in 10, 8, and 11 of the 20 subjects, respectively; changes in the remainder of the subjects were less than two times the expected variation in repeated measurements. At the same time, the bronchodilator induced a significant increase in FEVj in nine subjects and in FEF^t^ in 11 subjects; however, the bronchodilator induced a significant paradoxic decrease in FEVX in five of the subjects and in FEFg5_75% in one of the subjects. Of the 11 subjects who showed a significant decrease in ^b-2b) six showed an increase in FEV1? two showed no change in this spirometric parameter, and three showed a decrease. Of the nine who showed no change in five showed a significant increase in FEVb two showed no change, and two showed a decrease in FEV1.

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Figure 4A shows the effective resistance as a function of frequency for a normal three-week-old infant measured with the modified system and the Bennet seal mouthpiece. The curve fluctuated somewhat, but this pattern is typical with forced random noise measurements. In all cases, the coherence values, which provide a measure of data reliability, were greater than 0.8, a minimum acceptable value. The average resistance for this infant was 10.2 cmH20*LTi. Figures 4B and 4C show before and after bronchodilator (nebulized isoetharine, 0.25 ml) data from two asthmatic patients ( In the first, an 11-month-old infant with no clinical signs of active asthma, the bronchodilator caused a small decrease in effective resistance with the average going from 11.0 to 10.4 cmH20*LTJ. Figure 4C shows data from a two-year-old who was symptomatic at the time of the study. With this patient, the bronchodilator caused a substantial change in the effective resistance at all frequencies, and the average resistance decreased 26 percent from 18.2 to 13.5. Figure 4D shows before and after bronchodilator data from a 23-month-old patient with bronchopulmonary dysplasia and asthma. Again, the bronchodilator caused a substantial decrease in resistance with the average value going from 9.7 to 7.8. Figure 5 shows the before and after bronchodilator values of H*.* for the 12 two- and three-year-old asthmatic patients studied with the modified system with a standard mouthpiece. Prebronchodilator resistances (read for more information) were generally higher than those seen in the older group measured in the standard way (Fig 2), which is consistent with the younger age of the subjects represented in Figure 5. All subjects showed a substantial decrease in after the bronchodilator with the percent change averaging 38 percent and ranging from 15 percent to 56 percent.

Figure-2Figure 2. Effective resistance at 6 and 26 Hz (R« and R») and average effective resistance over range 6 to 26 Hz (Re.*) a function of height along with the regression equations. Squares represent asthmatics with normal spirometry, while circles represent those with an abnormal FEV^ or PEF^.^*.

Figure-3Figure 3. Relationship between three resistance parameters and FEVj. The resistance parameters are the effective resistance at 6 and 26 Hz and the average effective resistance over the range 6-26 Hz flV*).

Figure-4Figure 4. Effective resistance as a function of frequency for (A) a normal 3-week-old infant; (B) an 11-month-old asthmatic with no clinical signs of active asthma; (C) a symptomatic 2-year-old asthmatic, and (D) a 23-month-old patient with bronchopulmonary dysplasia and asthma.

Figure-5Figure 5. Before and after bronchodilator values of average effective resistance over the range 6 to 26 Hz (R^m) for 2- and 3-year-old children as a function of height.

Table 1—Coefficients of Variations of Random Noise Resistance Parameters Expressed as Percentage

Parameter Mean Range SD
R. 9.6 1.220.6 5.6
R* 8.9 0.8-20.4 4.7
7.4 2.0-21.9 4.4

Table 2—Correlation Coefficients Between Random Noise Resistance Parameter and Forced Expiratory Spirometric Parameters

FEVj fef^ fef** fef** FEFm
Re 0.88 0.82 0.89 0.85 0.70
R* 0.77 0.64 0.71 0.65 0.51
Re-» 0.82 0.82 0.86 0.82 0.69

Table 3—BronchodUator-Induced Changes Expressed as Fercent

AK« AR* AFEVj AFEF&7B% afef*. afef*. afef**
Mean -17.3 -12.7 -15.6 7.4. 54.2 13.4 30.1 52.1
SD 8.2 19.3 13.1 18.8 62.5 58.3 60.7 66.0