This is the first study to demonstrate the feasibility of cardiopulmonary exercise testing to qualitatively assess DLH. DLH is defined as a decrease in expiration relative to inspiration, with an increase in respiratory rate due to air trapping in respiratory physiology. Based on this idea, we analyzed data on a breath-by-breath basis using a gas analyzer and observed the characteristic findings in two representative cases: a long-term smoker, and a patient with COPD. Our results provide an initial indication that this method could be used to qualitatively evaluate DLH.
Measurement Of Tidal Volume
The exhaled gas analysis used in this study measured the amount of ventilation in each breath using a hot-wire flow meter.30 It is unclear whether the measurement of ventilation volume per breath, as in this study, can be applied to other gas analyzers, such as differential pressure flowmeters (pneumotachometers) and vortex flowmeters.
Aging deterioration of the hot wire, temperature difference between expiratory and inspiratory air due to poor environment, and poor calibration can all cause measurement errors; for example, aging of the hot wire results in an inability to measure ventilation, and poor calibration causes a difference between inspiratory and expiratory ventilation from rest. Therefore, calibration was performed according to standard protocols before conduction of the study. None of these problems were observed in the four cases in this study.
Relationship Between Cardiopulmonary Exercise Testing Indices And Dynamic Lung Hyperinflation
Ventilation efficiency is reduced by congestion owing to heart failure and obstructive ventilation. In cardiopulmonary exercise testing, VE/VO2 and VE/VCO2 are indicators of ventilatory efficiency. The VE/VCO2 slope increases in COPD patientན, but decreases with disease severity.31 The minimum VE/VCO2 ratio also decreased with severity. Furthermore, it has been reported that in COPD patients, the Y-intercept of the VE/VCO2 slope is both higher, and related to the FEV1.0%.32 In addition, the minimum VE/VCO2 and VE/VCO2 slopes were generally consistent in patients with heart failure. However, Murata et al. reported that as COPD progresses, the minimum VE/VCO2 and VE/VCO2 slope may diverge, or the VE/VCO2 slope may become pseudo-negative while the Y-intercept may be high.32, 33
In the two cases in this study, these indices were such that the VE/VCO2 slope was lower than the minimum VE/VCO2 and the Y-intercept was higher, as shown by Murata et al. Therefore, obstructive ventilatory impairment may have occurred. However, both indices are reports of observational studies on COPD, and do not examine the presence or degree of DLH. In addition, in case 4, the peak TV E/IC did not show a very high rate, yet the peak VE/MVV was high. These indices also indicate that DLH may have occurred.34 Existing cardiopulmonary exercise testing indices can indicate the possibility of obstructive ventilatory impairment, but cannot indicate the occurrence of DLH during exercise testing. It is necessary to interpret cardiopulmonary exercise testing up to the symptomatic limit in combination with spirometry testing to identify the possibility of dynamic lung overexpansion.
The Possibility Of Detection Of Dlh Using Tv E-i During Cpet
In the two healthy males in this study, the inspiratory and expiratory ventilation were equal during exercise testing, and the expiratory ventilation increased from the middle of the exercise testing. The increase in expiratory volume from the middle of exercise testing is thought to be due to carbon dioxide excretion by the buffering of lactate produced with increased exercise intensity. Conversely, in the COPD patient and long-term smoker, TV I increased more than TV E at the beginning of exercise, and the difference between TV I and E decreased in the middle of the exercise test. As the elasticity of the lungs reaches its limit, the difference between inspiration and expiration is thought to decrease, while expiration is thought to increase.
Based on our observation of this phenomenon, we believe that DLH may be a simple and easily detectable method to measure DLH. In the present study, Case 4, who underwent cardiopulmonary exercise testing and spirometry testing simultaneously, had a high peak VE/MVV and a low peak TV E/IC, which are suggestive of DLH.34 In addition, in Case 4, TV E-I decreased from the start of exercise, suggesting DLH. Based on these results, we believe that this study demonstrates the possibility of visually detecting DLH. Detecting DLH using differences in tidal volume (TV E-I) may be possible without performing cardiopulmonary exercise testing to the symptomatic limit. Conversely, a method that combines cardiopulmonary exercise testing and spirometry testing requires combining the results of cardiopulmonary exercise testing and spirometry testing performed on the symptomatic limit. The method using the differences in tidal volume is considered safe and visually detectable. However, this was only true for non-oxygenated participants.
Previous reports have described methods to observe a decrease in IC with effortful inspiration during exercise testing, or a decrease in IC with increased respiratory rate. In this study, we observed changes in TV-E and TV-I during exercise testing, but did not observe a decrease in IC during exercise. In future work, it will be necessary to verify whether the phenomenon shown in this study and the existing DLH evaluation methods are similarly detectable in the same individuals.
Meaning Of Tv E-i Accumulation
Since TV E-I accumulation is calculated as the accumulation per breath, we thought that it may be possible to roughly determine the total volume of air trapped by DLH. However, we calculated this parameter as 4.7 L in the smoking participant, and 5.2 L in the patient with COPD. The maximum value of TV during exercise was 1.5 to 2 L, the predicted VC was 3.0 to 4.0 L, and the TV E-I accumulation was well above these values. This TV E-I accumulation did not mean that the body (lungs) stored all the unexhaled respiratory volume. Whether TV E-I accumulation is a meaningful indicator of the severity of DLH, correction for body size, calculation methods, etc., the meaning of this accumulation value needs to be verified in a series of cases.
Limitations
This study has several limitations which should be noted. First, our study was the very low number of participants. We need to collect more cases to verify that this phenomenon is observed. Second, we did not compare the correct method to existing DLH assessment methods. Thus, it is difficult to prove the existence of DLH based on the results of the present study alone. Third, it is difficult to determine the severity of DLH because of only observes changes in exhaled and inspiratory ventilation. Therefore, it would be desirable to develop an appropriate analysis method for TV E-I.