Due to the advancement of ultrasound in recent years, we have seen an increase in the number of diaphragm muscle studies using this tool, and in normal individuals, some authors have already determined the relationship between diaphragmatic excursion and pulmonary function.15,16,17 However, the relationship between its mobility and lung function and severity in patients with COPD is not clear. We chose ultrasound as a way of evaluating the diaphragm muscle, which proved to be a very practical method of conducting the proposed research. The advantages of ultrasound over other imaging modalities include its portability, relatively low cost, and lack of contraindications. Our study was performed by a single professional trained in the management of ultrasound, and measurements were obtained with the patient in the supine position, with the transducer positioned in the right subcostal region, at the midclavicular line. This method has been described by most published articles and, for that reason, performed here.14, 15, 16, 17 Diaphragmatic mobility measurements were acquired with the patient breathing calmly (tidal volume) and after a deep inspiration maneuver (total lung capacity). All patients were able to perform the requested maneuvers properly, and we found it feasible to perform this test in COPD patients, even in those with severe disease. Although we observed a good correlation between the DM measurements performed at rest and during maximal inspiration (r = 0.63; 95% CI 0.43 to 0.78; r2 = 0.4; p < 0.0001), the correlations between DM and functional parameters were negative when we used data collected from breathing at rest, while we observed that there was a correlation when the measurement was performed at the TLC level. This fact may be explained by the smaller craniocaudal displacement of the diaphragm muscle during breathing at rest. Thus, small changes in the measure may have impacted the statistical evaluations. Alternatively, we can infer that hyperinflation may be more apparent during deep inspiration. Lowering the diaphragm in these patients would allow ventilation at the tidal volume level to be less impacted, while the wider excursion, measured with maximal inspiration, would be more compromised.
Although some authors have determined a relationship between DM and pulmonary function, not all studies point to the same results. While Scott et al reported that dynamic measurements using diaphragm ultrasonography do not correlate well with pulmonary volumes obtained by whole-body plethysmography16, Schulz et al published a recent article suggesting a good relationship between DM and residual volume15. Another study, published by Rocha et al. evaluated the DM of 25 COPD patients through chest radiography and related these findings to airway obstruction (FEV1) and the perception of dyspnea measured by the mMRC scale.18 However, they were unable to relate it to the measurement of daily activities and did not perform measurements using whole-body plethysmography to assess lung hyperinflation. A recent meta-analysis that included 8 articles evaluating DM by ultrasound in patients with COPD reported that the DM was lower in the severe COPD group than in the mild-to-moderate COPD group (WMD = 0.50; 95% CI: -0.01, 1.01; P = .06), but the difference was not significant.19
Our study evaluated the relationship between DM and variables that measured airway obstruction, especially FEV1, and with lung hyperinflation, considering RV and RV/TLC ratio, and we observed a relationship with all of them when measurements were performed in deep inspiration but not with the patient breathing at rest. Our results suggest that patients with lower DM also have lower FEV1, that is, they are more obstructive and have more hyperinflation. This data is confirmed by the literature.15,19 The difference in the data obtained by Rocha can be explained, at least in part, by the methodology used. Ultrasonography is a more suitable method for studying DM than chest radiography performed in both phases of breathing.
A recent study evaluated 55 COPD patients in the supine position using ultrasound to measure the DM on the right side of the chest, i.e., a measurement methodology like ours, and found that during the exacerbation the patients had a reduction of diaphragmatic excursion (40.4 mm to 30.8 mm).20 These data suggest that diaphragmatic function may be influenced by factors other than those determined by the degree of bronchial obstruction. All our patients were stable at the time of evaluation; however, patients with the worst performance on the 6MWT and those who scored higher on the BODE had lower DM. Both the field test and the BODE index are measures that relate to the prognosis of patients and to mortality.13,21 Yamaguti et al. evaluated the DM in 42 COPD patients and divided them into low (≤ 33.9 mm) and high (≥ 34 mm) diaphragmatic mobility.22 These patients were followed for 2 years to assess mortality. The authors found that the 4 deaths that occurred in that period were detected in the group with the worst DM. Moreover, the BODE index and the distance achieved in the 6MWT performed at the beginning of the study were related to the DM. The authors suggested that low DM, as well as the 6MWT and the BODE index, may reflect a worse prognosis for patients with COPD.
In our study, BODE index and 6MWT correlated with DM, suggesting that DM measured sonographically correlates with different parameters related to COPD severity. Other studies performed in COPD patients with lung hyperinflation reported a correlation between DM and distance covered on the 6MWT and a negative correlation with dyspnea.23 Using the same methodology, our study, performed in stable COPD patients, not necessarily for hyperinflation, found similar results, confirming the relationship between exercise capacity and DM.
Interestingly, the study by Rocha et al., using radiography in forced inhalations and exhalation, correlated the findings of DM measurements with the dyspnea assessed by the MRCm scale.18 Likewise, our study, which evaluated DM through ultrasound, also reached the same results. We found that most symptomatic patients had less diaphragmatic mobility (p = 0.0059).
Our study has some limitations that should be noted. The study was conducted in a single tertiary center, and the patients had a higher than average illness severity. Whole-body plethysmography was only performed on 36 of the 49 patients analyzed, as we were restricted from doing so during the period of the COVID-19 pandemic. We must also consider that we did not follow the patients to assess prognosis. Thus, despite the positive results, we are aware of the need for further studies to confirm the data.