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Dependence of FEV1 on preceding inspiration during methacholine challenge testing in asthmatic patients
ABSTRACT. Background: In normal subjects, and patients with obstructive and restrictive lung defects, flows during an FVC manoeuver are higher after a fast inspiration without an end-inspiratory pause as compared to a slow inspiration with an end-inspiratory pause of approximately 5 s (time dependence of FVC). Objective: In the present study, we investigated the influence of these two manoeuvres on PEF and FEV1 in asthmatic patients during methacholine challenge testing. Methods: Thirteen patients who were positive according to routine (non-standardized) methacholine challenge tests (PD20£1.57 mg) were studied. Their average(SD) lung function data were: FVC%pred=94(15), FEV1%pred=76(17) and FEV1/FVC%=68(12). The patients performed the two types of FVC manoeuvres in random order before and at the last methacholine dose. Results: Using the standardized FVC manoeuvres (fast before/fast after and slow before/slow after), only 10 patients (77%) had a positive test, while all 13 patients were positive with the routine test. Conclusion: These data indicate that different results can be obtained, if instead of the standardized, the non-standardized manoeuvres during methacholine challenge tests are used. The use of non-standardized FVC manoeuvres may lead to over-, or at least theoretically, to underestimation of patients with bronchial hyperresponsiveness. Therefore, standardization of FVC manoeuvres during airway challenge testing is required. Pneumon 2002, 15(2)168-175.

INTRODUCTION

Extensive guidelines have been provided for the measurement procedure of airway responsiveness1-5. In these guidelines, however, the inspiratory manoeuver preceding the expiratory effort has not been standardized. Thus, in practice, the FVC manoeuvres performed during challenge testing may be preceded by: 1) maximal inspirations made at different speeds, and 2) variable pauses at full inspiration. However, several studies have shown that in normal subjects6-9 and patients with chronic obstructive pulmonary disease9,10, asthma8,9 and lung restriction11 the peak expiratory flow (PEF), FEV1, and maximal expiratory flows at different lung volumes are significantly higher if the FVC manoeuver is performed after a rapid lung inflation without an end-inspiratory pause (manoeuver 1; fast) than after a slow inspiration with a 4-6 s end-inspiratory pause (manoeuver 2; slow). This time dependence is probably mainly caused by stress relaxation and time constant inequalities within the lung6-14. These results dictate standardization of the inspiratory time history prior to the FVC manoeuver.

In the present study we have performed a routine methacholine challenge in 13 patients with a history of asthma. As it is common practice, during this procedure the volume and time history prior to the FVC manoeuver were not standardized. Additionally, before the routine test and at the dose of methacholine at which FEV1 decreased by at least 20% (positive test), we measured FEV1 using the standardized FVC manoeuvres 1 and 2. We hypothesized that our results might indicate that different responses can be obtained with the routine (non-standardized) as compared to standardized methacholine testing.

METHODS

We studied 13 consecutive patients (7 females) with a history of asthma diagnosed in the past according to the American Thoracic Society criteria15. These were referred to our laboratory for routine methacholine challenge testing having no absolute contraindication for the test. Their anthropometric characteristics and baseline lung function data are given in table 1. All subjects were in a stable clinical and functional state for at least 4 weeks preceding the study and were receiving their usual medi-cation. Subjects on treatment refrained from any drug (i.e., inhaled bronchodilators and/or corticosteroids) at least from the day before testing. All patients had previous experience with respiratory manoeuvres and were non-smokers. Forced spirometry was measured with a screen pneumotachograph (Screenmate; Erich Jaeger GmbH & Co., Hφchberg, Germany). The total lung capacity (TLC) and its subdivisions were assessed by the helium dilution method, carbon monoxide transfer factor (TLCO) and transfer coefficient (KCO) by the single breath-hold method (P. K. Morgan, Kent, UK). The predicted spirometric values, static lung volumes, TLCO and KCO are from the European Coal and Steel Community1,2. The study had the approval of the hospital's ethical committee and informed consent was obtained from all patients. The patients were not aware of the specific aim of the study.

For the routine methacholine challenge testing the European Community Respiratory Health Survey's (ECRHS) short protocol16,17 was employed. During the methacholine test FEV1 and FVC were measured with a Biomedin spirometer (Biomedin srl, Padova, Italy). This device involves insertion of the mouthpiece into the mouth after inhalation to TLC, therefore does not allow recording of inspiratory manoeuvres preceding the FVC. Mefar MB3 Dosimeter (Mefar srl, Bovezzi, Italy) was used for the administration of methacholine solutions. Methacholine solutions were made up from lyophilized methacholine chloride (Provocholine, Hoffman La Ro Roche, Basel, Switzerland) by standard procedures16,17.

During phase A and B for the standardized mano-euvres (see below), flow (V^  ) was measured at the mouth, through the heated (30o C) Screenmate pneumotachograph. In contrast to Biomedin spirometer, the use of the latter device allows the recording of the inspiratory manoeuvres prior to FVC. The response of the pneumotachograph, which was calibrated at the above temperature along with the Biomedin spirometer with the same 2L volume syringe, was linear over the range of flows used. The calibration factor was corrected for the fall in temperature (6-7o C) of the expired air on passing through the cooler pneumotachograph18. The flow signal was sampled at a rate of 100 Hz, using a data acquisition system with a built-in 16-bit analog-to-digital converter (AT-Codas; DATAQ Instruments, Inc., Akron, OH, USA) controlled by a computer (Toshiba T5200/100; Toshiba Inc., Tokyo, Japan). Volume (V) was obtained by numerical integration of the flow signal after correction for drift. The resulting maximal expiratory volume-time (tFVC) and MEFV curves were used for subsequent analysis.

Procedure

On the study day, all subjects were studied seated with a nose-clip on. To minimize variations of the maximal expiratory volume-time curves19,20: 1) the first two manoeuvres were disregarded except the ones in phase B (see below), thus making the volume history of the subsequent curves about the same7-11; 2) care was taken to keep the neck at a fixed neutral position21; 3) all subjects were urged to maintain the maximal expiratory efforts for a similar duration7-11 (Table 2); and 4) all subjects were studied at nearly the same time of the day20. Before the methacholine challenge (phase A) each subject randomly performed two types of FVC manoeuvres (Table 2): a) fast: after a fast inspiration (mean flow of 2.59 l/s) without an end-inspiratory pause (manoeuver 1), and b) slow: after a slow inspiration (mean flow of 0.53 l/s) with an end-inspiratory pause lasting ~5s (manoeuver 2). It should be noted that closely similar results would have been obtained with a shorter (more realistic) pause as the stress relaxation phenomena (see discussion) are completed within 2s. However, the two manoeuvres were selected as such, in order to have comparable results with the previous reports6-11. Each FVC manoeuver was initiated from resting end-expiratory lung volume (FRC) after a period of tidal breathing through the screen pneumotachograph. All patients had been trained to perform these two manoeuvres prior to this study. For both manoeuvres, at least three FVC curves were obtained that fulfilled the ATS criteria19. The largest FVC and FEV1 were selected from all the acceptable curves, while PEF was chosen from the acceptable curve that exhibited the largest sum of FVC plus FEV1 ("best" test)19.

A short period after completing phase A, the patients underwent the standard routine methacholine challenge test17,18 with an independent member of the laboratory staff (chest physician). In this case the time history prior to the FVC manoeuvres was not controlled. Since during the routine methacholine challenge the FEV1 was measured with a device (Biomedin Spirometer) which involves insertion of the mouthpiece into the mouth after inhalation to TLC, the subjects necessarily exhibited an end-inspiratory pause of variable time, depending on their coordination. Similarly, since the speed of inspiration to TLC was not standardized, the duration of the maximal inspiration prior to the FVC manoeuvre was necessarily variable. The routine challenge was stopped at the cumulative methacholine dose at which FEV1 decreased by at least 20% (PD20) from baseline (positive test)17,18. Immediately thereafter, the patients proceeded to phase B of the study. In contrast to phase A, this consisted in randomly performing fewer acceptable FVC manoeuvres, i.e., 1 slow and 1 fast, in order to maintain the bronchoconstrictor effect of methacholine.

In phases A and B the following variables were assessed from the maximal inspiration preceding the FVC manoeuvre: 1) inspiratory capacity (IC), 2) duration of inspiration (tI), 3) mean inspiratory flow (VT/tI), 4) duration of end-inspiratory pause (tp), and 5) duration of forced expiration (tE). The variables characterizing the fast and slow FVC manoeuvres during phase A and B are presented in Table 2.

 

 

Statistical analysis

Values reported are means (SD). The values of the various variables were compared using the paired Student t-test. A value of p£0.05 was taken as statistically significant. Linear regression analysis was performed using the least squares method (Sigma Stat 2.03, Jandel Scientific, San Rafael, CA, USA).

RESULTS

The PD20 of the patients amounted to 0.28±0.44 mg (range: 0.01-1.57) and in all of them was consistent with airway hyper-responsiveness17,18.

The values of PEF and FEV1 of the 13 patients were significantly higher with manoeuvre 1 than 2 both before and after methacholine challenge (Tables 3 & 4), in line with previous observations7-11. Furthermore, as previously reported for asthmatic patients9, there were no significant differences in the percent changes of PEF and FEV1 obtained before and after methacholine administration (Table 4).

In line with previous reports8,9, both before and at the last methacholine dose the difference in FEV1 between the fast and slow FVC manoeuvres, expressed as a percentage of FEV1 slow, were similarly correlated to FEV1 slow, expressed as percent of predicted FEV1 (Figure 1).

 

Based on the standardized (see below) FVC manoeuvres only 10 patients (77%) had a positive test (Table 5). Therefore, three patients (#6, 11, 13) who had a positive response during the routine (non-standardized) test, were not positive during the standardized tests. Furthermore, the patients with a positive test were the same using either the fast or slow FVC manoeuver. Except in patient #10, the difference in % change in FEV1 between the standardized fast (fast before/fast after) and slow (slow before/slow after) combination of FVC manoeuvres was less 4.7% (Figure 2).

 

DISCUSSION

In the present study we found that by using the standardized FVC manoeuvres only 10 patients (77%) had a positive methacholine test1-5,16-18 i.e., a fall in FEV1 ³ 20% (Tables 3 & 5). Furthermore, the same 10 patients had a positive test using either the fast or slow standardized FVC manoeuvres (Figure 2). Similarly, three patients (#6, 11, 13) who were positive with the routine test, were not positive during both fast and slow standardized FVC manoeuvres at the same methacholine dose. These results indicate that using non-standardized as compared to standardized methacholine testing different results can be obtained.

An argument may arise on the small sample size of the study. However, the behavior of these 3 different patients must be explained even when a much bigger sample with no new different patients would have been obtained. Therefore, the authors considered unethical to continue the study.

Table 5 shows the percentage decrease in FEV1 after the last dose of methacholine using various combinations between fast and slow manoeuvres, and the number of positive tests that would have occurred with each combination. As previously noted, if the fast or slow FVC manoeuver were made both before challenge and at the last dose of methacholine, 10 patients would have been labelled as positive responders at that dose. However, with a slow baseline manoeuver and a fast at the last methacholine dose, only 6 patients would have exhibited a fall in FEV1 ³20%. In contrast, with the fast manoeuver before and the slow after, all 13 patients would have been reported as responders, as was the case during the routine challenge test. Although we have no direct proof, it is likely that patients (#6, 11 and 13) exhibited such a fast-slow combination of FVC manoeuvres during the routine challenge test. In fact, as it is common practice, in the latter condition the FEV1 was measured with a device which required insertion of the mouthpiece into the mouth after inhalation to TLC. This necessarily involves an obligatory end-inspiratory pause which lasts a variable time, depending on the coordination of the subject. Furthermore, during the routine challenge test the speed of inspiration to TLC was not controlled, and hence it was also axiomatically variable. Since the speed of inspiration to TLC and the duration of the end-inspiratory pause were not controlled false responses to methacholine challenge would be expected.

The three patients (#6, 11 and 13), who were not responders with the standardized manoeuvres at the last methacholine dose, could have been positive at a higher dose. However, accurate determination of PD20 is a pre requisite for serial measurements of methacholine responsiveness22-24. Indeed, changes in PD20 play an important role in the follow up of the effects of treatment and are used to predict when a reduction in treatment might be tried. Furthermore, serial measurements of PD20 are commonly used to determine whether occupational exposure is the cause of workers asthma and to assess the effects of immunotherapy. Therefore, standardization of FVC manoeuvres during airway responsiveness should be mandatory.

At the start and at the end of the challenge the standardized and non-standardized manoeuvres were measured by different observers. Examining the data reported in Table 5, they do not seem to be far from the 20% fall and figure 2 shows that only one patient had a really big difference in response with the standardized manoeuvres from that with the routine test. In other words, the different results seem only a slight variation in PD20 FEV1. However, the differences in FEV1 (Table 3 & 4) can not be explained on the variability of the measurement alone, as in all instances but one the values with the fast manoeuvres were higher. Although, there are several potential mechanisms6-14,25-30 which might explain the higher values of PEF and FEV1 with manoeuver 1 relative to 2 (differences in TLC, differences in expiratory effort, gas compression artifacts, gas density and gas composition, airway and parenchymal hysteresis, and time constant inequality within the lungs) the time dependence of the FVC manoeuver appears to be mainly caused by differences in lung recoil due to viscoelastic behaviour (stress relaxation) of the pulmonary tissue6-14.

The concept of PD20 FEV1 may be an overly simplistic one if we want to diagnose the disease exclusively on it. Many scientists would be more interested on whether the routine not standardized tests may affect the entire bronchial dose-response curve rather than a single parameter like PD20FEV1, which may sometimes be misleading. Further studies are required to quantify the effect of the aforementioned factors on the time dependence of FVC during the entire procedure of bronchial challenge testing.

In conclusion, the results of the present study indicate that assessment of airway hyper-responsiveness and accurate determination of PD20 require standardization of the FVC manoeuvres in order to minimize their variability.

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