Developing a New Marker of Dynamic Hyperinflation in Patients with Obstructive Airway Disease - an observational study

Tidal volume at peak exercise and vital capacity ratio (VTpeak/VC) and VTpeak/inspiratory capacity (IC) were used to differentiate lung expansion in subjects with normal health and chronic obstructive pulmonary disease (COPD) from that in subjects with restrictive ventilation. However, VC and IC variably change due to pseudorestriction of lung volumes. Thus, these variables are currently not recommended. In contrast, total lung capacity (TLC) does little change during exercise. The aims of the study investigated whether VTpeak/TLC is more significantly correlated with static air trapping and lung hyperinflation in patients with COPD than VTpeak/IC, VTpeak/FVC, and VTpeak/SVC (study 1), and developed a marker to replace dynamic IC maneuvers by evaluation of the relationship between end-expiratory lung volume (EELV) and VTpeak/TLC and identification of a cutoff value for VTpeak/TLC (study 2). One hundred adults with COPD (study 1) and 23 with COPD and 19 controls (study 2) were analyzed. Spirometry, lung volume, diffusing capacity, incremental cardiopulmonary exercise tests with dynamic IC maneuvers were compared between groups. An ROC curve was generated to identify a cut off value for VTpeak/TLC. In study 1, VTpeak/TLC was more significantly associated with airflow obstruction, static air trapping and hyperinflation. In study 2, VTpeak/TLC was highly correlated with EELV in the patients (r = −0.83), and VTpeak/TLC ≥ 0.27 predicted that 18% of the patients with static air trapping and hyperinflation can expand their VT equivalent to the controls. In conclusions, VTpeak/TLC was superior to other VTpeak/capacities. VTpeak/TLC may be a marker of dynamic hyperinflation in subjects with COPD, thereby avoiding the need for dynamic IC maneuvers. VTpeak/TLC < 0.27 identified approximately 82% of subjects with COPD who could not adequately expand their tidal volume. As most of our participants were male, further studies are required to elucidate whether the results of this study can be applied to female patients with COPD.

www.nature.com/scientificreports www.nature.com/scientificreports/ (or SVC or IC) lead to inconsistent values of V T /FVC, V T /SVC, and V T /IC. Therefore, these variables are not recommended to differentiate obstructive from restrictive ventilatory limitations 8 .
Dynamic IC measurements have been reported to be a good way to identify dynamic hyperinflation [2][3][4][5]7 . However, IC maneuvers have to be standardized and the study subjects have to become familiarized with the maneuvers, and IC measurements and analysis also have to be standardized by researchers 9 . Nevertheless, dynamic IC measurements are not recommended for ramp-pattern protocols in which V T cannot steadily proceed to perform IC maneuvers 9 . However, the ramp-pattern protocol is a widely used protocol to test incremental exercise.
TLC does not change or only changes a little during exercise in normal subjects and subjects with lung diseases [10][11][12] . We hypothesized that V T /TLC at peak exercise (V Tpeak /TLC) would be lower in subjects with COPD compared to normal subjects, and that it would be less variable than V Tpeak /FVC, /SVC and /IC. As TLC and V Tpeak are routinely measured during lung function tests and cardiopulmonary exercise tests (CPET), respectively, V Tpeak /TLC may be a convenient new marker of dynamic hyperinflation, thereby avoiding the need for dynamic IC measurements.
The aims of this study were (1) to investigate whether V Tpeak /TLC is less variable and more significant in correlation with static air trapping or lung hyperinflation in subjects with COPD than V Tpeak /IC, /FVC, and /SVC, and (2) to develop a new marker of dynamic hyperinflation to replace dynamic IC maneuvers by evaluation of the relationship between EELV and V Tpeak /TLC and identification of a cutoff value of V Tpeak /TLC.

Methods
Study design. This observational cross-sectional study enrolled healthy normal subjects and subjects with COPD at two university teaching hospitals, and analyzed lung function and cardiopulmonary exercise data for aim 1 and aim 2. The Institutional Review Boards of Chung Shan Medical University Hospital (CS16174) and Chang Gung Memorial Hospital (201700899A3) approved this study, which was conducted in compliance with the Declaration of Helsinki.
Subjects. Subjects aged ≥40 years without any chronic diseases including uncontrolled diabetes mellitus, uncontrolled hypertension, anemia (hemoglobin <13 g·dL −1 in males and <12 g·dL −1 in females), and no acute illnesses in the recent 1 month were enrolled. Anthropometric measurements, leisure/sports activities, and cigarette smoking were recorded. Subjects with a body mass index ≤18 kg·m −2 or ≥32 kg·m −2 or with laboratory findings of cardiovascular, hematological, metabolic or neuromuscular diseases were excluded.
Study group. COPD was diagnosed according to the GOLD criteria 13 . Adult subjects who underwent lung function tests were enrolled only if their FEV 1 /FVC was <0.7 or the flow volume curve of spirometry revealed typical concavity 13 , and their forced expired volume in one second (FEV 1 )% predicted was <80% and had been stable for at least 1 month. A total of 131 subjects in the study group were screened, and 123 were retained for the study (Fig. 1). The reasons for exclusion included not meeting the inclusion criteria (n = 3), meeting the exclusion criteria (n = 2), and declining to participate (n = 2). The study group was divided into two cohorts, one for aim 1 and one for aim 2. To evaluate the bronchodilator effect on the relationship between V Tpeak /TLC and static air trapping or lung hyperinflation, the subjects with COPD in study 1 were not allowed to use medications before measurements, whereas the subjects with COPD in study 2 were used medications as normal. A total of 131 subjects with chronic obstructive pulmonary disease were screened along with 20 normal healthy controls. After excluding eight subjects with COPD, the remaining 123 were allocated to study 1 (n = 100) and study 2 (n = 23). After excluding one subject, the remaining 19 normal subjects were allocated to study 2.
Definitions. Static and dynamic air trapping and hyperinflation. Definitions of pulmonary hyperinflation and air trapping of the lung in the literature are inconsistent. In this study, static air trapping and hyperinflation www.nature.com/scientificreports www.nature.com/scientificreports/ were defined as RV/TLC > 0.45 14,15 , FRC%pred > 120%, or RV%pred > 120%; dynamic air trapping or hyperinflation was defined as V Tpeak /TLC < 0.4 2,3,5 .
CPET. Each subject completed an incremental exercise test to the limit of the symptom (MasterScreen CPX ™ , Carefusion, Wuerzburg, Germany). Work rate was selected at a rate of 5-20 W/min based on a derived protocol formula according to the oxygen-cost diagram scores 22 .  VO 2 (mL/min), CO 2 output (  VCO 2 ) (mL/min), and minute ventilation (  V E ) were continuously measured.  VO 2peak was symptom-limited and  VO 2peak predictions were performed as reported previously 22 . Cardiovascular stress level or exercise intensity was defined as heart rate at peak exercise/heart rate predicted maximum. The definition of ventilatory limitation was a breathing reserve of either <30% or <11-15 L/min 23 .
Dynamic inspiratory capacity measurement. The techniques used for performing and accepting IC measurements were as previously reported 9 . Dynamic IC was measured at the end of a steady-state resting baseline and unloaded cycling, and near the middle of loaded exercise and near end exercise. The middle of the loaded exercise was approximately 5-6 minutes after the start of loaded exercise, when dynamic IC near anaerobic threshold was measured. EELV was calculated as TLC minus dynamic IC. O'Donnell threshold was calculated as dynamic IC minus V T at peak exercise 7 . www.nature.com/scientificreports www.nature.com/scientificreports/ Statistical analysis. Data were summarized as mean ± standard deviation and percentage and 5 th and 95 th percentiles. The Student's t test was used for comparisons between two groups. Correlations were based on Pearson's correlation coefficients. A p value < 0.05 was considered to be significant. An ROC curve was generated to identify a cut off value for V Tpeak /TLC by comparisons with dynamic EELV at peak exercise. Statistical analyses were performed using SAS statistical software (SAS Institute Inc., Cary, NC, USA) and Origin v4.1 (Northampton, MA, USA). The sample size of study 2 was estimated to be 23 based on calculations with a 0.1 between-group difference and 0.1 of standard deviation for each group in V Tpeak /TLC with a power of 0.9 and significance level of 0.05.

Results
One hundred subjects (97 men) with COPD in study 1 and 23 (23 men) with COPD and 19 healthy subjects (19 men) in study 2 were analyzed ( Fig. 1 and Table 1). The majority of the subjects in study 1 had moderate airflow obstruction with hyperinflation and air trapping, and mild exercise hyperventilation and exercise ventilation limitation with mild exercise impairment (Tables 1 and 2). Compared to study 1, most of the subjects with COPD in study 2 had less severe airflow obstruction and hyperinflation and exercise impairment (Tables 1 and 2). V Tpeak /TLC and V Tpeak /SVC were significantly lower in the subjects in study 1 than in the COPD group in study 2, while V Tpeak /FVC and V Tpeak /IC were similar between the two COPD groups.

Correlation of V tpeak /TLC and EELV or other variables.
In study 2, the levels of EELV at rest, unloading exercise, and near anaerobic threshold and near peak exercise were significantly larger in the subjects with COPD than in the normal subjects (Table 2). V Tpeak /TLC was significantly correlated with EELV/TLC in the subjects with COPD but not in the normal subjects (Fig. 3

Discussion
The key findings of this study were that V Tpeak /TLC was the best marker for dynamic expandability of lungs compared to V Tpeak /SVC, V Tpeak /FVC, and V Tpeak /IC in the subjects with COPD (Table 3, |r| = 0.45-0.62 vs. 0.001-0.49). These findings were noted in the subjects with COPD with or without withdrawal of bronchodilators before the test (Table 3 in study 2, |r| = 0.65-0.79 vs. 0.001-0.52). V Tpeak /TLC was highly correlated with EELV in the subjects with COPD but not in the normal subjects (Fig. 3, r = −0.83 vs. −0.13). A V Tpeak /TLC cutoff value of <0.27 further identified approximately 82% of the subjects with airflow obstruction and static air trapping or hyperinflation who developed dynamic air trapping during incremental exercise (Fig. 2). As V Tpeak /TLC can easily be obtained without the need for any maneuvers during exercise, the need for dynamic IC maneuvers during exercise test may be avoided. V tpeak /TLC, /SVC, /FVC, and /IC. V Tpeak /SVC and /IC have been used to differentiate normal and obstructive ventilation from restrictive ventilation during CPET 24,25 . V Tpeak /FVC has also been used as an alternative due to the ease of obtaining FVC. However, these variables are not recommended by the American Thoracic Society for such purposes 8 . In this study, V Tpeak /SVC, /FVC, and /IC showed much higher variability than V Tpeak /TLC in the correlation with static air trapping (Table 3). This may be because TLC changes little during exercise in normal subjects and those with airflow obstruction and interstitial lung disease [10][11][12] . As dynamic air trapping develops, V T is restricted and air cannot be removed sufficiently, thereby limiting the increase in V Tpeak /TLC. In contrast, static SVC, FVC, and IC may be variable due to pseudorestriction of lung volume even in subjects with the same TLC, so that dynamic hyperinflation cannot be precisely predicted by V Tpeak /SVC, /FVC, and /IC. Pseudorestriction is not uncommon in subjects with airflow obstruction, small airway obstruction and emphysema.
O'Donnell et al. reported that changes in dynamic IC were larger (−14%) during exercise in subjects with airflow obstruction than in normal subjects (4%, p < 0.0005) 4 , meaning that V Tpeak /dynamic IC was larger than V Tpeak /static IC (74% vs. 65%) 4 . However, in the current study, V Tpeak /dynamic IC was smaller but not significantly smaller than V Tpeak /static IC (71% vs. 77%) in the subjects with COPD, suggesting that dynamic IC was larger than static IC. This was also noted in the normal subjects in study 2 (i.e., 0.72 vs. 0.65; Table 2). This may be due to different severities of COPD in these two studies. In O'Donnell's study 4 , the patients with COPD had 37%pred FEV 1 in contrast to study 2 where the subjects with COPD had 55-68%pred FEV 1 .  Table 3. Correlation coefficients of the ratios of tidal volume at peak exercise (V Tpeak ) and TLC, SVC, FVC, and IC with markers of air trapping or lung hyperinflation and spirometry at rest in subjects with chronic obstructive pulmonary disease. Forabbreviations, please refer to www.nature.com/scientificreports www.nature.com/scientificreports/ V tpeak /TLC ratio and EELV or other variables. EELV is calculated as TLC minus dynamic IC. As TLC changes little during exercise [10][11][12] , changes in EELV must be inversely associated with changes in IC 4 . As mentioned, V Tpeak /TLC changed in line with dynamic IC, and was thus inversely related to EELV (Fig. 4, Table 4. Summary from this study and the literature regarding the ratio of operating tidal volume at peak exercise and total lung capacity (VTpeak/TLC) and other relevant ratios in subjects with chronic obstructive pulmonary disease (COPD) and subjects with interstitial lung disease (ILD) and normal healthy subjects. IPO 26 : impaired peripheral oxygenation. ∧ P < 0.0001 vs the normal subjects of the study 2, *P = 0.02 vs study 2 norm, SE Standard error. www.nature.com/scientificreports www.nature.com/scientificreports/ p < 0.0001). The area under ROC curve was high (0.861) and the sensitivity (88%) and specificity (77%) of VT peak / TLC of 0.27 to predict elevated EELV in the current study were acceptable. The importance of V Tpeak /TLC has not previously been addressed or reported, and our findings seem to suggest that V Tpeak /TLC could be used as a substitute for dynamic IC maneuvers performed during exercise. One reason is that V Tpeak /TLC is easily obtainable, and another is that in the ramp pattern exercise protocol but not a steady state protocol, dynamic IC maneuvers are not recommended 9 . Moreover, dynamic V T represents the difference between dynamic end-inspiratory lung volume (EILV) and EELV, and while measuring V Tpeak /TLC is straightforward, measuring EILV and EELV is more complex 5 . Interestingly, in the current study V Tpeak /TLC was correlated with  VO 2peak in the COPD group rather than in the normal group. This suggests that dynamic lung expansion played a role in the exercise capacity in patients with COPD but not in health. However, both groups reached a similar level of Borg dyspnea score (COPD vs. normal, 6 ± 3 vs. 6 ± 2, p = 0.83) but different levels of O'Donnell threshold/TLC (COPD vs. normal, 0.11 ± 0.07 vs. 0.19 ± 0.12, p = 0.02) at peak exercise. However, V Tpeak /TLC was not correlated with either variable. We speculate that Borg dyspnea score and O'Donnell threshold are more related to the plateau portion of the pressure-volume curve for the lungs and chest wall whereas V Tpeak /TLC involves lung volumes not only expanding to the plateau portion of the curve but also encroaching downward to expiratory reserve volume.
V tpeak /TLC ratio in this study and previous reports. Table 4 reveals that V Tpeak /TLC was 0.21 ± 0.06-0.24 ± 0.07 in study 1 and 2, which is larger than in our previous report on patients with COPD (0.14-0.2) 26 and reports on COPD from other researchers (0.15-0.24, the values were not reported in their studies but were re-calculated by the current authors) 2,5,27,28 . This may be due to differences in the severity of COPD and preconditioning strategy before the measurements in these studies. We re-measured V Tpeak /TLC from the figures of the previous studies, and found values of 0.31-0.42 in patients with interstitial lung disease 3,10 and 0.3-0.42 in normal healthy subjects 2,3,5,28 . In the normal subjects of this study, the V Tpeak /TLC was 0.31 ± 0.06. However, it is difficult to compare this value between our study and studies in the literature as the level of V Tpeak /TLC has not been reported.

Study limitations.
The normal subjects had significant cigarette consumption despite having normal spirometry. In addition, subjects with restrictive ventilation were not included as interstitial lung disease is rare in our institutions. However, V Tpeak /TLC in patients with interstitial lung disease as re-measured from the figures of previous studies was 0.31-0.42 2,3,10 . These values are quite different from those reported in the subjects with COPD in this study and in the literature. The number of participants in study 2 was small and the findings may not be generalizable to all populations. However, the sample size of study 2 was estimated to be 23 based on standard calculations. The cohort was all men because the incidence of COPD in women is very low in Taiwan (37:1 in our previous report 29 ). Dynamic IC measurements are recommended for subjects who can achieve a steady state of exercise. The exercise protocol in this study was the ramp pattern, and the relationship between V Tpeak /TLC and EELV may be different between two-minute incremental and ramp-pattern exercise. However, it can be difficult to reach a steady state in each stage of exercise despite using the two-minute incremental exercise protocol [2][3][4][5] . Lastly, the subjects did not undergo pre-test exercise testing including dynamic IC maneuvers to allow them to become familiar with the whole protocol. However, this may more accurately reflect cardiopulmonary exercise testing in the real world.

Conclusion
V Tpeak /TLC may be a potential marker of dynamic hyperinflation in subjects with COPD, and its use may avoid the need for dynamic IC maneuvers during incremental exercise. This marker is simple to derive and more stable than other V Tpeak -capacity ratios regarding the relationship with static air trapping or hyperinflation, and it was significantly associated with EELV. A cut-off value of V Tpeak /TLC < 0.27 identified approximately 82% of the subjects with COPD who had static hyperinflation and air trapping but could not expand their tidal volume to the same extent as the normal subjects. Further large-scale studies are warranted to investigate whether V Tpeak /TLC can replace dynamic IC maneuver and whether it can be used to identify the V Tpeak /TLC cut-off value. As most of our participants were male, further studies are required to elucidate whether the results of this study can be applied to female patients with COPD.