Effects of methacholine on small airway function measured by forced oscillation technique and multiple breath nitrogen washout in normal subjects

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Abstract

The multiple breath nitrogen washout (MBNW) can be analysed to produce the parameters Scond and Sacin as measures of ventilation heterogeneity in conductive and acinar airways, respectively. The derivation of these parameters is based on a model of pulmonary ventilation and results of similar modelling suggest that respiratory system conductance (Grs) measured by forced oscillation technique (FOT) is also sensitive to heterogeneity and to airway closure. Therefore, Scond, the volume of gas trapping at FRC (VtrappedFRC) and Grs may be inter-related parameters. These relationships were examined in 12 normals under baseline and bronchoconstricted states. Specific Grs was measured at 5 Hz (sGrs5 = Grs5/FRC) and Scond, Sacin and VtrappedFRC by MBNW, before and after methacholine challenge. Scond was independently predicted by VtrappedFRC and FRC in a multivariate model (R2 = 0.68, p = 0.002). Post methacholine challenge, Scond related only to VtrappedFRC (R2 = 0.79, p < 0.0001). The absolute change in Scond induced by methacholine challenge were predicted by the changes in VtrappedFRC and sGrs5 in a multivariate model (R2 = 0.82, p = 0.0002). Sacin was unrelated to VtrappedFRC and sGrs5 before and after methacholine challenge. In conclusion, Scond and sGrs5 are measurements that are sensitive to changes occurring to the function of peripheral conducting airways, in particular heterogeneity and airway closure, while Sacin and presumably heterogeneity in terminal airways, are independent of these. Scond is also related to lung size. We review the current state of knowledge of FOT and MBNW in obstructive lung diseases and discuss future research directions.

Introduction

Disease of the small airways has long been considered an important aspect of the pathophysiology of obstructive lung diseases, especially in chronic obstructive pulmonary disease (COPD). This was recently affirmed by Hogg et al. (2004) who described persisting small airway inflammation in COPD, the severity of which was proportional to the severity of the COPD as defined by GOLD criteria (GOLD, 2004). The changes in the small airways was also associated with airway remodelling which increased the thickness of the airway walls and occurred despite patients having ceased cigarette smoking many years previously. Similarly in asthma, airway inflammation and remodelling are well recognised from findings of autopsy (Carroll et al., 1993) and biopsy studies (Ward et al., 2001). Functional abnormalities of the small airways have also been described from measurements made from small catheters wedged into small airways in both asthma (Kaminsky et al., 2000) and COPD (Hogg et al., 1968) and other complex tests which include the frequency dependence of compliance, response of total resistance to helium/oxygen breathing, single breath nitrogen washouts and increased airway closure (King et al., 1998, Macklem, 1998).

The challenge to clinical researchers and physiologist in studying small airways disease has been to find an accurate and simple measurement technique. As all the peripheral airways collectively only account for 10% of the total resistance, it is unlikely that patients will experience symptoms which would arguably be breathlessness during exertion, until the disease is well advanced and therefore present late to medical attention. Most measurements of small airway function are invasive, complex and technically difficult. The forced oscillation technique (FOT) and multiple breath nitrogen washout tests (MBNW) are both being increasingly used to measure information on the state of the respiratory system that are clearly different but complementary to traditional measurements of forced flow and static lung volumes. FOT and MBNW tests involve either tidal or near-tidal breathing from functional residual capacity (FRC), thus potentially reflecting airway function under more or less physiological conditions. The use of both tests owes a lot to modern technology, particularly to powerful but relatively cheap computing power which is required for the modelling, hardware control and data collection and processing. Used in conjunction with more traditional measurements of lung function, MBNW and FOT measurements will provide more complete descriptions of the airway abnormalities in obstructive lung disease. This has implications not only for clinical evaluation but also for genetic studies in which it is critical that clear phenotypic differences are defined. In this manuscript, we review the current state of knowledge of MBNW and FOT in relation to obstructive lung disease, present new data showing their inter-relationships and discuss areas that require future research.

Both FOT and MBNW techniques have been comprehensively explored in modelling studies with the particular aim of deriving indices that were likely to reflect the contribution from the small airways to overall airway obstruction (Paiva and Engel, 1985, Verbanck and Paiva, 1990, Lutchen and Gillis, 1997, Thorpe and Bates, 1997). The MBNW has been adopted as a test of small airway function that distinguishes between ventilation inhomogeneities originating in the acinar versus the more proximal conducting airways and quantified by the indices Sacin and Scond, respectively. The greater the numerical values of Sacin and Scond, the greater the respective inhomogeneity of ventilation. Additional information about small airway abnormality can be obtained by including several inspiratory capacity inhalations of test gas at the end of the MBNW test (Christensson et al., 1981). This manoeuvre allows the volume of trapped gas at FRC (VtrappedFRC) to be quantified when relatively small airways that are closed or nearly closed during breathing around FRC, are able to open up during these deep inhalations and are then washed out.

The forced oscillation technique is even easier to perform than MBNW since it only requires passive breathing to which the oscillatory signal is superimposed at the airway opening, usually by a loudspeaker. This method allows measurement of lung impedance at frequencies of between approximately 4 and 20 Hz, depending on the characteristics of the loudspeaker and flow measurement systems used with the lower oscillation frequencies penetrating further into the lung. However, impedance measurements using frequencies less than 4 Hz are difficult due to problems in generating sufficient power and separating the oscillation signals from the breathing signals due to the closeness of their respective frequencies; this may be overcome by testing during voluntary apnoea using a modified form of high-frequency ventilation (Kaczka et al., 1997). However, the use of electrical piston pumps to generate wave-forms to both ventilate the subject and apply FOT during voluntary apnoea is very complex and at present, impractical in routine clinical practice. Furthermore, there are potential effects of laryngeal movement during voluntary apnoeas which may generate measurement distortions.

An additional advantage of the FOT derived indices is that these measurements have a high temporal resolution with several data points every second. This allows the intra-breath changes to be studied which may provide dynamic information on the mechanical state of the airways and possibly of the lung parenchyma. To do this the impedance data for individual frequencies are calculated and these can be then be related directly to changes in lung volume and time. The parameters derived from impedance data obtained by FOT include respiratory system resistance (the in-phase or flow-resistive part of impedance) and elastance—a component of reactance (the out-of-phase or reactive part of impedance). Parameters are commonly derived by fitting a model that contains an airway compartment that has a resistance, inertance and elastance, and a lung parenchymal compartment that has elastance and damping (Kaczka et al., 1997). Forced oscillatory resistance was found to have a hyperbolic relationship with frequency by Grimby et al. (1968). However, over narrow frequency ranges where the relationship is close to linear, the linear regression slope has been used as an approximation to describe this relationship. It has also been suggested that this slope represents the presence of small airways disease. Evidence to support this include correlations with the frequency dependence of lung compliance, another measure of small airways disease (van den Elshout et al., 1994), greater frequency dependence of resistance in smokers despite similar resting FEV1 (Brochard et al., 1987) and greater changes in frequency dependence in smokers after hyperventilation in the absence of any changes in FEV1 (Decramer et al., 1984). Results of modelling studies also suggest that frequency dependence of resistance and also of elastance particularly, are sensitive to heterogeneity of airway narrowing (Kaczka et al., 1999) (Lutchen and Gillis, 1997).

The FOT and MBNW tests may be considered to provide complementary and to some extent similar responses to induced changes in the mechanical properties of the airways such as those occurring during bronchoconstriction induced by inhalation of an airway smooth muscle agonist. The MBNW parameters Scond and Sacin were derived from models predicting their specific sensitivity to enhanced ventilation inhomogeneity in conducting and acinar airways, respectively. Strictly speaking it is impossible (intrinsic to MBNW theory) to attribute changes in Scond more specifically to heterogeneity in either the small or large conductive airways. However, results of several experimental studies in health and early disease strongly suggest that Scond abnormalities predominantly reflect changes in the small conductive airways. This was the case in mild asthma patients (Verbanck et al., 1999), in non COPD smokers with smoking histories of 10–20 pack/years (Verbanck et al., 2004) or in normal subjects following bronchoprovocation (Verbanck et al., 1997, Verbanck et al., 2001).

Similarly, results from modelling studies of lung resistance and elastance measurements derived from FOT are thought to be particularly sensitive to the heterogeneity of airway narrowing and closure. Although frequency dependence of resistance is most pronounced below 5 Hz, it is still present above 5 Hz. It is increased more in COPD than it is in asthma (van Noord et al., 1989) and also appears to be a sensitive indicator of airway injury (Skloot et al., 2004). However, given the hyperinflation known to occur in COPD and the volume dependence of resistance, specific resistance (or its reciprocal specific conductance sGrs = Grs/FRC) might in fact be more sensitive to heterogeneity than would frequency dependence of resistance at these frequencies. This requires further study.

Kaminsky et al. (2004) showed that peripheral airways had a large influence on changes in forced oscillation impedance after methacholine. Therefore, FOT indices of heterogeneity should represent alterations in the small conducting airways. Forced oscillation derived indices at around 5 Hz would be expected to represent responses resulting from alterations in the small conducting airways because of the physical nature of lower frequency oscillations which are able to penetrate further into the lung. Therefore, greater frequency dependence of resistance, i.e. a more negative slope of resistance versus frequency, represents more disease in small, peripheral airways than in more proximal airways. Evidence to support this has been cited above. Given the similarities between FOT and MBNW indices in terms of the size of the airways that they are likely to represent and in their particular sensitivity to heterogeneity, we would expect that Scond and sGrs would be correlated between individuals (given that there is sufficient inter-subject variability in both indices) and that they would be similarly affected by any increase in heterogeneity and airway closure during induced airway narrowing. In healthy subjects at least, the sites of airway narrowing and closure affected by bronchoconstriction are likely to be located more proximal to the acinar opening (Hughes et al., 1970). Thus, unless major heterogeneity is generated at the level of the transitional bronchioli beyond the acinar entrance, it seems unlikely that Sacin should show a correlation with Grs given that Sacin is predominantly generated by intra-acinar asymmetry (Verbanck et al., 1997).

We therefore hypothesised that in normal subjects, the degree of heterogeneity and gas trapping are reflected in both Grs and Scond measurements and by inference, that Grs and Scond would be correlated. We further tested their relationships under a simulated disease state of airway narrowing by repeating measurements following methacholine induced airway narrowing, when heterogeneity and airway closure may increase, and hypothesised that the changes in both FOT and MBNW parameters would remain related. Our aims were therefore to examine the inter-relationships between MBNW and FOT parameters in normal subjects before and after methacholine challenge tests.

Section snippets

Methods

Twelve normal subjects underwent standard measurements of spirometry, methacholine challenge tests, lung volumes by body plethysmography and skin prick tests for allergy to common airborne allergens. They underwent FOT and MBNW tests, at baseline and after completion of the methacholine challenge. The FOT measurements were made after spirometry and immediately before each MBNW test both at baseline and post methacholine challenge test. The challenge tests were conducted in the usual manner as

Results

The subject characteristics are shown in Table 1. All subjects had normal FEV1 in terms of percent of predicted and the mean FEV1/FVC ratio was 0.88 ± 0.03, all ratios were above 0.8 except that of subject 7 which was 0.76. No subjects had airway hyperresponsiveness as defined by a 20% or greater decrease in FEV1 in response to 6.1 μmol or less of methacholine. The mean decrease in sGrs was 32 ± 6% (p < 0.0001) and in FEV1 was 10 ± 6% (p = 0.009). Scond increased by 0.027 ± 0.017 L−1 (163 ± 97%, p = 0.012),

Discussion

In summary, the result the multivariate analysis suggests that Scond is independently related to the volume of gas trapping at FRC and to FRC itself. The relationship between Scond and VtrappedFRC (but not FRC) persisted after methacholine challenge which was associated with considerable changes in Scond and VtrappedFRC. Sacin however, was independent of gas trapping and of sGrs in both baseline and post-challenge states. The absolute changes in Scond induced by methacholine were strongly

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