Utility Inspiratory Capacity by Simple Spirometry as an indirect Measure of Air Trapping

Objective: To evaluate the utility of inspiratory capacity (IC) measured by simple spirometry as an indirect measure of air trapping. Method: Functional residual capacity (FRC) and IC were determined, respectively, by a body plethysmography and by spirometry. Air trapping was defi ned by a FRC higher than 120%. Spearman’s rho between IC and FRC is determined and the receiver operating characteristic (ROC curve) is calculated as predictive of air trapping for IC. The best cutoff of the overall test is calculated. Results: 148 patients were included with a mean FEV1 60% (SD 20%). There was signifi cant correlation between IC and FRC with a Spearman rho -0.673 (p <0.0001). The area under the ROC curve for IC and FRC was 0.803 (95% CI 0.709 to 0.897). The optimal cutoff for predicting air trapping was given for a forced IC less than 80% of its theoretical value, with a sensitivity of 82%, a specifi city of 73%, a positive predictive value of 73% and a negative predictive value 82%. Conclusions: Inspiratory capacity is useful in the assessment of air trapping and may determine a cut below 80%. Research Article Utility Inspiratory Capacity by Simple Spirometry as an indirect Measure of Air Trapping Alberto Herrejon*, Julio Palop, Susana Herrera and Alejando Betancurt Department of Pneumology, Peset University Hospital of Valencia, Spain Dates: Received: 13 February, 2017; Accepted: 11 March, 2017; Published: 13 March, 2017 *Corresponding author: Alberto Herrejon, MD, Department of Pneumology, Peset University Hospital of Valencia, Spain, Tel: 349612622437; Fax: 349612622437; E-mail:


Introduction
The Spanish guide COPD (GesEPOC) [1], shall hyperinfl ation as a characteristic of emphysema phenotype, whereas international standards GOLD [2], air trapping considered an early phenomenon in the development of COPD. Air trapping, defi ned as increased residual volume (RV) with normal total lung capacity (TLC), always a shift towards higher point of respiratory rest (FRC) volumes and has important clinical consequences such as limiting the exercise, the intensity of dyspnea and the possible development of alveolar hypoventilation [3].
The determination of static volumes is not a routine measure in clinical practice because of the diffi culty in implementation.
However inspiratory capacity (IC) is easily performed by simple spirometry, so it has been postulated that the reduction would be an indirect measure of air trapping since any change that happens in the FRC promotes another and of opposite sign in IC, always remain unchanged TLC.
The measurement of the IC can be useful not only in static situation but also in the assessment of air trapping that occurs in patients with COPD during exercise, in which there is a progressive increase in the FRC with the consequent decrease in IC as a result of increased airfl ow limitation in relation to an increase in ventilatory demand.
The importance of initial and evolutionary IC evaluation of COPD during exercise and in the evaluation of bronchodilator test is increasingly evident. Of course, the IC cannot replace the FEV 1 , but it is a very useful piece of information in the overall assessment of COPD. The measure FEV 1 and IC are not exclusive but complementary and especially in COPD with an FEV 1 less than 50% or if pulmonary hypertension is suspected [4].
The main objective of this study is to check whether the measure IC, in patients with COPD, can assess the state of the FRC and thus predict the existence of air trapping, detecting at what level of deterioration of the IC already derives a FRC increased. The secondary objective is to determine the correlation of the values of IC from a slow vital capacity maneuver and identifi ed with a forced vital capacity maneuver. All this would strengthen the clinical use of ICS in COPD, which would facilitate and generalize the approximate value of static volumes, without the need to perform a plethysmography or other more sophisticated methods.
IC slow values were higher than IC forced in 65 ml, representing an average theoretical percentage of 4%. These  to 0.897). The optimal cutoff for predicting air trapping was given for a forced IC less than 80% of its theoretical value with a sensitivity of 82%, a specifi city of 73%, a positive predictive value of 73% and a negative predictive value of 82 %. The value of the lower forced IC at 65% of its theoretical state has a specifi city of 100% air trapping, even with a sensitivity of only 51%.

Discussion
In patients with COPD, in the absence of hyperinfl ation, you may be suspected the existence of air trapping by the value of the inspiratory capacity measured in a simple spirometry and can avoid the need for more complex studies. The IC value below 80% of its theoretical indicates with high probability the increase FRC and consequently the presence of air trapping.
It has been proposed valuation by slow vital capacity and forced spirometry to deduce air trapping. So it has been found that in normal individuals, slow vital capacity (VC) is superior to the

Agreement between slow and torced inspiratory capacity
Slow IC -Forced IC forced vital capacity (FVC), although differences small claims. However, in patients with obstructive respiratory disease the difference can be much higher, even above 1000 ml. The difference between VC and FVC correlates with the volume of gas intrathoracic and may refl ect the existence of an expiratory collapse of the airway secondary to loss of elastic recoil caused by pulmonary emphysema.
The small existing difference between VC and FVC in healthy people broad proportion to the intensity of the obstruction in patients with obstructive pulmonary disease. Therefore, the FEV 1 / FVC ratio underestimates the degree of obstruction regarding the FEV 1 /VC. In fact, in patients with severe obstruction, this difference can be up to 22%. Within the slow expiratory maneuver own VC is less than the inspiratory VC and this reduction is particularly evident in patients with moderate to severe obstruction. In a model predictive multiple regression hyperinfl ation, measured as the ratio of residual volume and total lung capacity (RV / TLC), the actual value of FEV 1 and its interaction with the difference between VC and FVC were predictors of hyperinfl ation and explained the 52% of the variability of the same [7].
The interest in the study of IC in COPD has increased in recent years for various reasons. Has proven its close relationship with exertional dyspnea, having a signifi cant correlation between the fall of the IC after testing the 6 minute walking (6MWT) and increased dyspnea measured by the Borg scale, suggesting the development of a dynamic lung hyperinfl ation, with the increase in lung volume at the end of expiration the best predictor of dyspnea [8].
Despite the close relationship between the fall of the IC and increased FRC, resulting in air trapping, this circumstance has been little studied in COPD. Previously it has been shown on one hand that the IC measured by spirometry and determined by plethysmography are practically identical in both absolute terms and as a percentage of theoric values [9], similar to that demonstrated by our study that the differences were clinically irrelevant. In our study we have shown the level of this association, determining a cutoff point IC below 65% in the coexistence of increased upper FRC 120% of its theoretical and a point of suspicion is demonstrated by below 80%, which is very likely the existence of air trapping, with acceptable sensitivity and specifi city. We believe that the best cutoff point is 80%. All without a corresponding increase in the TLC above normal. When we suspect that there is hyperinfl ation measured as greater TLC to 120% of its theoretical value associated with air trapping, the ratio mirror between the IC and the FRC is lost so that spirometry not be enough and we would point to the performance of other examinations more complex to demonstrate the presence of hyperinfl ation.
A characteristic fi nding of the graph of the fl ow / volume curve in situations of air trapping is to check that the image of the expiratory part of the tidal volume remains above that shown by the expiratory limb of forced expiration, indicating us dynamic collapse of the airway. However, this situation is diffi cult to quantify and depends largely on the extent of the tidal volume so as not useful screening air trapping, although its existence makes us suspect.
IC / TLC ratio known as inspiratory fraction is a powerful independent predictor of mortality in COPD, with a value less than 25% the best cut. It has been found that patients with moderate to severe COPD and an IC / TLC ratio of less than 0.23 have a higher lung hyperinfl ation and reduced exercise capacity [10].
There are many therapeutic or diagnostic benefi ts of predicting the existence of air trapping in COPD because bronchodilator treatment can modify the air trapping and improved dyspnea or exercise tolerance. Air trapping predict exercise capacity decline in COPD [11], COPD hospitalisations  [12], and survival in an emphysematous phenotype of chronic obstructive pulmonary disease [13].
It is concluded that the percentage of the theoretical inspiratory capacity measured by spirometry is useful in the assessment of air trapping and may determine an appropriate point of cutting suspicion. To assess hyperinfl ation associated spirometry should be complemented with other more sophisticated tests.