Asthma is a serious worldwide health issue, but its clinical manifestations can be controlled with appropriate treatment [1]. Currently, a positive bronchodilator response is established based on an increase ≥ 12% and 200 ml in forced vital capacity (FVC) and/or forced expiratory volume in one second (FEV₁) compared with baseline values following administration of bronchodilators [2]. However, in clinical practice, patients with moderate or severe asthma may refer clinical improvement after bronchodilator use despite a negative bronchodilator test. In patients with chronic obstructive pulmonary disease (COPD), in whom the bronchodilator test is frequently negative, vital capacity (VC) and inspiratory capacity (IC) variation are used as complementary tools in order to evaluate bronchodilator response [3–6]. COPD patients have a persistent airway obstruction that may also be observed in moderate or severe asthma; however, only a few studies so far have evaluated the usefulness of VC and IC to assess bronchodilator response among asthmatic patients [7–13].

The aim of this study was to analyze the usefulness of VC and IC as additional parameters to assess bronchodilator response in asthma patients with moderate or severe airflow obstruction.

This study prospectively enrolled patients aged 15 years and older diagnosed with asthma and persistent airway obstruction according to Global Initiative for Asthma (GINA) criteria, and who were clinically stable at the time of enrollment [14]. Subjects were not eligible if they were current or former smokers (≥ 20 packs) or had clinical and/or radiographic evidence of heart failure, uncontrolled systemic arterial hypertension, pregnancy, focal fibrous scarring on chest X-ray with total area ≥ 1 pulmonary lobe, chest wall deformity or articular or neuromuscular disease, morbid obesity or previous lung resection. Also excluded were patients who used short-acting β2-agonist spray, long-acting β2-agonist spray or oral theophylline 8 h, 12 h or 48 h respectively before the pulmonary function tests, and patients experiencing an asthma crisis or exacerbation during the week before the pulmonary function tests. All patients were referred from the Outpatient Asthma Clinic at the Federal University of Rio de Janeiro Institute of Thoracic Diseases (ITD). They were evaluated at the ITD Pulmonary Function between June 19, 2006 and July 30, 2008. Written informed consent was obtained from all participants (or a legally responsible representative when applicable) and the study was approved by the Federal University of Rio de Janeiro Ethics Committee.

All patients answered a standardized interview and underwent physical examination as well as lateral decubitus and posteroanterior chest x-ray before the bronchodilator test [14]. Pulmonar function tests (PFTs) and a 6-minute walk test (6MWT) were performed before and 15 minutes after the bronchodilator test. Only one 6MWT was done before and after the bronchodilator. A bipolar visual analogue scale (VAS) was used to assess the perceived effect of the bronchodilator test on shortness of breath. The bronchodilator test was performed with salbutamol/400 μg spray under physician supervision (KRSA). PFTs included flow-volume and volume-time curves, inspiratory slow vital capacity maneuver and determination of static lung volumes using a Jaeger spirometer (model MasterScreen-PTF, Hoechberg, Germany), and were conducted according to American Thoracic Society (ATS)/European Respiratory Society (ERS) guidelines [15, 16]. Static lung volumes were calculated using the closed-circuit helium dilution method. For He-derived TLC, the end-of-test criterion (equilibration) was defined as helium concentration change of 0.02% or less during 30s rebreathing. The predicted normal values for spirometry and lung volumes were those of Knudson et al. [17]. Polgar/Promadhat and Goldman/Becklake [18, 19] respectively. The 6MWT was performed following ATS guidelines, with dyspnea score based on a Borg scale [20].

For the VAS, patients were asked to indicate on a 100 mm horizontal line, labeled “very much worse” on the left end (−100), “very much better” on the right end (+100) and “no change” (zero) in the middle, after the bronchodilator test [21]. Bronchodilator response was defined as an increase in FEV₁ or FCV ≥ 12% of the baseline value plus 200 ml [2], or a decrease in residual volume (RV) ≥ 20% of the predicted value and 300 ml compared with baseline [22]. Clinical bronchodilator response was defined as 6MWD ≥ 50 meters or at least 30 meters associated with a reduction greater than 2 points in the Borg scale score [5, 20], or any positive value in VAS. For VC and IC, an increase ≥ 12% and 200 ml compared with baseline indicated a positive bronchodilator test.

Asthma patients were stratified into two groups according to the value of FEV₁: moderate (60% < FEV1 < 80% of predicted values) and severe airflow obstruction (FEV1 ≤ 60% of the predicted values), according to GINA criteria [14].

If considering the ATS/ERS definition of obstructive abnormalities, FEV1/VC < 5th percentile of predicted value, only 2 patients did not fulfill this criterion, although they have FEV1 values below 80%, respectively 79.3 and 69.0%.

VC response was observed in 11 (48%) patients with severe obstruction and three (15%) with moderate obstruction (p = 0.02). No significant changes were observed between the two groups in the percentage of bronchodilator response for the other parameters.

Five patients with positive VC and/or IC response presented a negative FVC and/or FEV₁ response.

In the present study with asthma patients with moderate and severe obstruction, we observed that VC may be a useful complementary parameter to FVC and FEV₁ to assess bronchodilator response.

Asthma patients were assigned to a moderate or severe airflow obstruction group based on FEV₁ analysis. In addition, because clinical improvement is the main goal of asthma therapy, VAS (subjective criterion) and 6MWT (objective criterion) were also used to evaluate clinical bronchodilator response in this study [20, 21, 25].

Paré et al. have described two patterns of response to bronchodilator therapy: predominant increase in expiratory flow rate (flow responders) or FVC (volume responders). Volume responders presented lower expiratory flows and greater degree of air trapping [7]. Newton et al., also reported similar results [22]. More recently, Sorkness et al., studying 287 patients with stable but severe asthma, demonstrated that this group presented prominent air trapping in contrast to individuals with non-severe asthma [12]. Our patients with severe airflow obstruction had both greater VC response in the group and greater air trapping (Table 2).

The 6MWT is considered a good parameter to estimate exercise tolerance in patients with moderate or severe pulmonary impairment, even though it has not been extensively used in asthma patients [20]. In our patients a significant increase in 6WTD as well as a decrease in Borg scale were observed in both groups (Table 3). We made only one 6MWT before and after the BD because the reproducibility of the 6MWD has been considered excellent [20].

The post-BD response of VC was well correlated with the variation in RV/TLC ratio in the severe obstruction group. In this line, O’Donnell et al. studied 84 patients with obstructive disease without response in FEV₁ and found a good correlation between variation in VC and IC and FRC after use of bronchodilator [6]. Additionally, Newton et al. demonstrated that lung volume improvement was independent of changes in maximal expiratory flows in patients with moderate and severe hyperinflation [22].

Agreement between VAS scores and functional responses was statistically significant in only three parameters, VC and RV in the severe obstruction group and FVC in the moderate obstruction group, without agreement with IC response (Table 4). This is in accordance with previous studies reporting a better correlation between dyspnea and the volume of thoracic gas rather than FEV₁[9, 10, 13]. A poor correlation (p = 0.35) was observed between variations in VC versus variation in VAS in severe obstruction patients. Probably, the reduction in air trapping is not enough to improve exertional breathlessness without a simultaneous and marked decrease in end-expiratory lung volume. Moreover, the increase in VC and IC may represent unrelated effects of the bronchodilation. VAS has been shown to be sensitive to detect improvement in asthma patients [26].

FEV₁ is considered a good marker of improvement. However, in patients with severe airflow obstruction, FEV₁ presents a lack of sensitivity and is a poor predictor of improvement in exercise tolerance [22, 27–29]. Moreover, Teeter and colleagues demonstrated that asthma symptoms were poorly correlated with FEV₁ before and after therapy [30]. Therefore, in addition to FEV₁, other physiologic parameters, such as lung elastic recoil, should be considered to characterize asthma severity [31].

This study has some limitations that need to be addressed: 1) we were not able to correlate the reduction in air trapping with the increase in IC measured in rest, as previously described during exercises tests [3, 9, 29] or during methacholine challenge testing [8, 11] and 2) static lung volumes were measured using the helium dilution method, even though body plethysmography is considered the most sensitive method [25, 32]. However, we opted to use more routine tests because ergospirometry, bronchoprovocation test and plethysmography are not available in most pulmonary function laboratories.

The present findings suggest that VC may be useful in addition to FVC and FEV₁ to identify bronchodilator response in asthma patients with severe airflow obstruction.