Clinical values of diaphragmatic movement in patients with chronic obstructive pulmonary disease

This single-center, prospective, cross-sectional observational study recruited participants from a tertiary hospital outpatient respiratory clinic between April 2020 and April 2021. The inclusion criteria were: 1) patients 18 years old or older diagnosed with COPD by a pulmonologist; COPD diagnostic criterion was a post-bronchodilator FEV1/forced vital capacity (FVC) ratio < 0.70 based on the Global Initiative for Chronic Obstructive Lung Disease (GOLD), 2) patients who could maintain the required posture for diaphragm function measurement by ultrasonography and stable breathing during the examination such as 6MWT. Patients unable to cooperate with the examination and unstable patients requiring immediate medical intervention were excluded. Patients with interstitial lung disease featured on chest computed tomography (CT) that could affect diaphragm movement were also excluded.

Sixty-nine patients were enrolled, six of whom with combined interstitial lung disease on CT were excluded. Two patients were lost to follow-up, and one died before all examinations were completed. Finally, 60 patients completed all examinations for the study protocol and were included in the analysis.

All patients provided informed consent before participating in the study. Each patient’s clinical information was collected from four domains: pulmonary function, exercise capacity, body composition, and diaphragm function. Pulmonary function was evaluated through spirometry, MIP, and maximal expiratory pressure (MEP). Exercise capacity and body composition were assessed using the 6MWT and bioelectrical impedance analysis (BIA). Diaphragm dysfunction is defined as loss of muscle contractility [16]. To evaluated diaphragm dysfunction, we was assessed using ultrasonography in both the M-mode and B-mode for excursion and thickness, respectively.

For patients who had performed a PFT within 1 month of participating in the study, the previous results were used and no retesting was performed. Patients who had no available PFT results within 1 month of participating in this study were reevaluated after enrollment. The Carefusion Vmax 20 (VIASYS Healthcare Inc. Sensormedics; Yorba Linda, CA, USA) was used for PFTs and FEV1, FVC, diffusing capacity of the lungs for CO, and total lung capacity were measured using the body plethysmography test. Regarding spirometry, the patients sat in a small booth and breathed into a mouthpiece. One technical expert from the Department of Respiratory Medicine conducted all the tests to maintain the consistency of the results.

MIP (PONY FX, COSMED Inc.; Rome, Italy) and MEP (PONY FX, COSMED Inc.; Rome, Italy) were measured in the sitting position using a portable mouth pressure meter. Three consecutive MIP and MEP measurements were taken, and the best result was recorded. The PFT was measured in a sitting position. A flanged mouthpiece was applied to the short and rigid tube of the measuring instrument and air leakage was checked around the mouthpiece before testing. The test was performed by an experienced examiner who has conducted the test for more than 8 years. MIP was measured by exhaling as deep as possible and inhaling as hard as possible for at least 1.5 s. MEP was measured by inhaling as deep as possible and exhaling as hard as possible for at least 1.5 s. Both measurements were made three times, and patients recovered to normal breathing patterns with at least a minute of break between measurements. The highest of the three measurements was recorded [17].

The 6MWT was performed according to the American Thoracic Society standards under the direction of a well-trained respiratory therapist at a 30 m indoor walking course [18]. Patients were encouraged by the instructor every minute and were allowed to rest or quit the test at any point. We measured the total distance and peripheral saturation with the portable oxygen meter. The patients’ body compositions were estimated indirectly using the BIA from a supine position (InBody S10, InBody, Co. Ltd., Seoul, Korea).

Diaphragm function was assessed using ultrasonography (LOGIQ E9, GE Healthcare; Chicago, IL, USA) obtained from both supine and sitting positions. It is generally accepted that there are positional differences in diaphragm contractility. The effects of gravitational loading on the diaphragm length-tension and body position-mediated changes in intra-abdominal pressure may explain the differences found. Not only that there is also a difference in the excursion between right and left. The excursion of the right diaphragm shows a lower value than that of the left diaphragm because the liver in the abdominal cavity restricts the movement of the right diaphragm. We also measured the diaphragm function in two positions based on this information. The supine position involved lying on the back or with the face upward while the sitting position was semi-seated (45–60 degrees). Both M-mode and B-mode imaging were used to evaluate diaphragmatic excursion and thickness, respectively. The mid-clavicular line and the liver were used as anatomical landmarks on the right side and the spleen on the left side to visualize the diaphragm in the M-mode. B-mode ultrasonography was used to measure the diaphragmatic thickness at the bilateral zone of apposition [19]. The diaphragm thickness was measured during quiet spontaneous breathing without peak inspiratory or expiratory maneuvers. The diaphragmatic thickness fraction was calculated as the difference between thickness at the end of inspiration and thickness at the end of expiration divided by thickness at the end of expiration x 100. The diaphragmatic excursion was measured as follows. The highest position of the diaphragm movement taken by the M-mode was considered to be the end-expiratory phase, whereas the lowest position was considered as the end-inspiratory phase.

The dyspnea scale used St. George's Respiratory Questionnaire (SGRQ) and the modified Medical Research Council scale (mMRC scale). The SGRQ is a self-administered questionnaire with 76 items [20]. This can identify the patient’s symptoms and the activities of daily life. mMRC scale is most commonly used in the assessment of dyspnea in chronic respiratory diseases and is a very useful and unrecognized dyspnea scale [21].

The data were analyzed using IBM SPSS (version 27.0; Chicago, IL, USA). The level of significance was set at p < 0.05. Descriptive statistics, including numbers, percentages, means, and standard deviations, were used to summarize each variable (demographics, PFTs, 6MWT, and diaphragmatic ultrasound results). The results were analyzed by independent t-test, cross-analysis, and frequency analysis. The correlation between the variables was analyzed by Pearson’s Correlation Coefficient, which confirmed the linear relationship between two variables using a scatterplot. The cut-off value was calculated using the receiver operating characteristic (ROC) curve analysis. The reference plane was 0.5 or more in the ROC curve, and the p-value < 0.05; hence, this result was adopted. Consequently, the cut-off value was confirmed when sensitivity and specificity were plotted in a line chart, which is the point where the two graphs meet.

We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed throughout this study. The study procedures were reviewed and approved by our Pusan National University Yangsan Hospital Institutional Review Board [IRB No. 05–2020-217].

We assessed whether diaphragm function was associated with FEV1 (Fig. 1). In the total group analysis, both diaphragmatic excursion and thickness were associated with FEV1. However, the diaphragmatic excursion was more associated with FEV1 than thickness. Diaphragmatic excursion during forced breathing and in the supine position had a greater association with FEV1 than breathing at rest and in the sitting position. Additionally, when comparing the right and left under the same conditions, the right was more significant during forced breathing and in the supine position (r = 0.370, p = 0.004,). Moreover, diaphragmatic thickness at right end-expiration was associated with FEV1. In summary, right (r = 0.370, p = 0.004) and left (r = 0.257, p = 0.048) diaphragmatic excursion during forced breathing in the supine position and diaphragmatic thickness at right end-expiration (r = 0.310, p = 0.016) were significantly associated with FEV1.

To evaluate the function of the diaphragm muscle [22], the diaphragmatic excursion was measured at rest and during forced expiration (Supplement Table 1). In 60 patients, diaphragmatic excursion at rest in the supine position was 3.5 cm ± 1.2 on the right side and 3.5 cm ± 1.2 on the left side. During forced breathing, diaphragmatic excursion in the supine position was 6.9 cm ± 2.0 on the right side and 7.6 cm ± 1.6 on the left side. The total percent body fat was 24.2% ± 6.9. Segmental lean mass analysis was performed by direct segmental multi-frequency BIA. The lean mass was 90.5% ± 9.7 on the right arm, 88.1% ± 9.2 on the left arm, 94.5% ± 5.8 on the trunk, 95.7% ± 131.3 on the right leg, and 9.51% ± 8.8 on the left leg.

The ROC curve analysis of the diaphragm function variables was performed to identify the cutoff value for differentiating between FEV1 ≥ 50% and < groups. The cutoff value was ≤ 6.7 cm on the right diaphragmatic excursion at forced breathing with an area under the curve of 0.5 or more and p-value was 0.043. Right diaphragmatic excursion during forced breathing was less than the cut-off value of 6.7 cm for 26 patients and ≥ 6.7 cm for 43 patients (Table 1). There were no differences in age, sex, or smoking history between the two groups. The dyspnea scales such as mMRC, SGRQ, and GOLD were not significantly different between both groups. There were no differences in body mass index, percent body fat, or lean mass of the right or left legs between the groups. However, among the pulmonary function indicators, there were significant differences between the two groups. Specifically, FEV1, FVC, and MIP were significantly different (< 6.7 cm group vs. ≥ 6.7 cm group, FEV1: 49.2% ± 16.2 vs. 59.5% ± 17.2, p = 0.021; FVC: 76.2% ± 19.1 vs. 86.0% ± 15.5, p = 0.032; MIP: 67.4 cm H₂O ± 25.1 vs. 86.5 cm H₂O ± 28.7, p = 0.010). Concerning the 6MWT, there was a significant difference in SpO2 before 6MWT and the number of interruptions (SpO2 before 6MWT: 94.1% ± 2.7 vs. 95.3% ± 1.6, p = 0.038; number of interruptions: 4 [15.4%] vs. 0 [0%], p = 0.018). The left diaphragmatic excursion during forced breathing was also different between the two groups (6.8 cm ± 1.5 vs. 7.6 cm ± 1.3, p = 0.022) as well as the diaphragmatic thickness during right end-inspiration (0.3 cm ± 0.1 vs. 0.4 cm ± 0.1, p = 0.006). In addition, the ROC ≥ 6.7 cm group left diaphragmatic excursion was also measured with a value greater than that of the ROC < 6.7 cm group.

To identify the clinical significance of diaphragm function with the relationship between lung function, and COPD severity, the two groups classified as a right diaphragmatic excursion at 6.7 cm of forced breathing were further divided into groups based on FEV1 (< 50% or ≥ 50%) (Table 2). There were significant differences in age (65.0 ± 7.8 years vs. 72.7 ± 6.2 years, p = 0.011), the GOLD score (p < 0.001), FEV1/FVC (40.1% ± 14.7 vs. 55.%4 ± 11.4, p = 0.007), peak expiratory flow rate (183.3 L/min ± 80.4 vs. 275.8 L/min ± 113.8, p = 0.027), SpO2 after the 6MWT (85.9% ± 6.5 vs. 91.5% ± 2.2, p = 0.011), and left diaphragmatic excursion during forced breathing (6.2 cm ± 1.6 vs. 7.4 cm ± 1.0, p = 0.038).

When the group with the right diaphragmatic excursion ≥ 6.7 cm was further divided into subgroups according to FEV1 (< 50% or ≥ 50%) and analyzed, mMRC, GOLD score, FEV1/FVC, MIP, peak expiratory flow rate, 6MWT, SpO2 before and after the 6MWT, and right diaphragmatic thickness at end-expiration subgroups were significantly different between the two groups.