The present study found that compared to single anti-reflux medication therapy, the combination of DEP can improve the effectiveness of GERC treatment. Compared to the control group, the intervention group showed more significant improvements in the overall evaluation of GerdQ, LCQ, PSQI, GAD-7 and PHQ-9.

The presence of GERC is an important extraesophageal manifestation of GERD. According to the pathogenesis of GERD, the weakening of the anti-reflux barrier function plays an important role in the occurrence and development of GERC. The LES and diaphragm are important components of the anti-reflux barrier. The LES is a circular muscle layer at the distal end of the esophagus. Its resting pressure is usually sufficient to prevent gastric contents from refluxing into the esophagus. However, when abdominal pressure increases, the diaphragm forms a second defense barrier to prevent reflux [31]. When LES is surgically removed, pressure can still be detected at the gastroesophageal junction [32], indicating that the diaphragm continues to maintain the anti-reflux barrier function, emphasizing the important role of the diaphragm in the anti-reflux barrier. Several studies have shown that respiratory training can increase diaphragm function [33, 34]. The DEP technique mainly completes deep, slow and regular breathing through diaphragm contraction and relaxation. Eherer et al. found that DEP reduced acid reflux exposure in GERD patients, improved reflux symptoms and speculated that DEP training can train the crural diaphragm and reinforce the anti-reflux barrier [14].

Studies have also shown that most reflux events in GERD occur during periods of transient lower esophageal sphincter relaxation (TLESR) [35]. In addition to LES relaxation, the inhibition of the diaphragm muscle is an essential part of TLESR occurrence [31]. Banovcin et al. found that acid stimulation of the esophageal nerves can enhance gastric distension and cause a TLESR reflex, possibly by acid-activating sensory nerves in the esophagus and increasing the frequency of TLESR [36]. The use of PPIs can alleviate acid exposure-induced TLESR to some extent but cannot reduce reflux caused by LES and diaphragm dysfunction or decrease the frequency of reflux. Coughing caused by reflux is related to the total amount of proximal reflux and prolonged esophageal reflux exposure, rather than the pH value of the reflux, so most patients cannot benefit from acid suppression therapy [37].

Halland et al. found that DEP training can significantly reduce the frequency of reflux and decrease postprandial acid exposure, further improving cough symptoms in GERD [38].

Previous studies have indicated that both TLESR and the diaphragm muscle are regulated by the vagus nerve [39]. The nerve regulator baclofen is a γ-aminobutyric acid (GABA) receptor agonist that can regulate the vagus nerve pathway, reduce the occurrence of TLESR and decrease the frequency of reflux, thereby relieving cough symptoms in GERD, which is applied clinically [40]. However, some patients cannot tolerate baclofen due to the central nervous system side effects such as dizziness, drowsiness and fatigue [41]. The use of DEP training can directly or indirectly regulate the balance between sympathetic and parasympathetic nerves and is used in GERD, anxiety and other diseases [12, 14, 42]. Perhaps through the above mechanism, it can indirectly reduce the occurrence of TLESR, improve diaphragm function, reduce the use of baclofen and increase patient compliance with treatment.

Currently, the treatment for GERC includes medication, surgery, as well as non-pharmacological and non-surgical intervention. As people’s quality of life demands continue to rise, physical exercise and lifestyle modifications interventions for GERC are increasingly important. The guideline also points out that for suspected GERC patients without symptoms of acid reflux or heartburn, PPIs should not be the first choice and lifestyle and behavioral interventions should be prioritized [43]. Although non-pharmacological or lifestyle modifications interventions have been widely recommended for GERD patients in recent years, they are rarely mentioned for GERC patients. To the best of our knowledge, this study is the first clinical randomized controlled study on deep diaphragmatic breathing training interventions for GERC and it was concluded that this type of intervention could significantly improve the clinical symptoms of GERC patients in conjunction with medication therapy.

Based on the above mechanisms and research results, it is hypothesized that DEP training can improve the clinical symptoms of GERC patients by improving diaphragm muscle function, strengthening the anti-reflux barrier, regulating the vagal reflex, reducing the occurrence of TLESR.

To further confirm the mechanism of DEP training on the diaphragm, this study objectively evaluated diaphragm function through multiple methods. Transdiaphragmatic pressure is the main indicator for evaluating diaphragm contraction function [44], but it is invasive and difficult to widely implement in clinical practice. In recent years, studies have shown that diaphragm ultrasound can indirectly evaluate diaphragm contraction force assessing DE and DTF [45]. DE and DTF had be used to evaluate diaphragmatic function and predicted weaning from mechanical ventilation in many researches. To our knowledge, the usefulness of this technique in evaluating the changes in diaphragm function before and after DEP and speculating the effect of respiratory training on GERC has not been reported. The results showed that during DEP, diaphragm mobility was significantly increased compared to calm breathing, indicating that the diaphragm function increased accordingly, consistent with the results of Yamaguti et al. [13], and the DTF was significantly increased at post-treatment contrast to control group, indicating that DEP effectively trains the diaphragm. Compared with Wu W, et al. research on diaphragm mobility before and after rehabilitation [30], the change value did not change much and the ultrasonic sampling will be subject to echo error, for which the possibility of error cannot be excluded. The clinical significance of DEP needs to be further confirmed by large sample and multi-center independent studies. Moreover, the cause-and-effect relationship between the changes in the diaphragm and cough has not been established. Therefore, further research is necessary.

The sEMG can also quantify the work of respiratory muscles and serve as a non-invasive method to indirectly reflect respiratory muscle function [46]. In this study, sEMG was used to measure the diaphragm electromyographic activity of patients during DEP and calm breathing to evaluate changes in diaphragm contraction force. After 8 weeks of treatment, the diaphragm sEMG activity in the training group was increased in quiet breathing and deep abdominal breathing compared with those before training, in line with DE and DTF, indicating that the diaphragm function was improved under DEP. In the control group, the diaphragm electromyography activity showed an increasing trend at quiet breathing and a decreasing trend at abdominal deep breathing. The DE and DTF were not significantly or slightly increased. It may reflect that the diaphragm is prone to fatigue and its function has not improved and may be gradually deteriorating. Cough symptoms may reappear after drug withdrawal, which needs further study. The contamination of the signal picked up by surface electrodes aimed at recording diaphragm activity has also been reported. But, Similowski, et al., and Verin E, et al. [47, 48] found that when two recording EMG electrodes are placed very close to one another, they are much more likely to record near-field potentials than far-field potentials. And the surface electrodes could be silent in response to cervical magnetic stimulation in patients with phrenic paralysis. Therefore, we believe that, surface electrodes may provide an uncontaminated diaphragm signal. And we will further to study the correlation of sEMG, di with EMGdi.

GERC is a special type of GERD manifested by a prominent cough symptom. Eherer et al. [14] research demonstrated that diaphragmatic breathing significantly reduced acid exposure and improved symptoms of GERD. Compared to the research, the patients in our study had a much wider age range, were fatter, and the standard of living was higher, leading to more difficulty in curing. Our study showed that the intervention group showed significant improvement in their gastroesophageal reflux symptoms and quality of life compared to the control group, in line with Eherer et al. research. However, the cough symptoms relief was faster than gastroesophageal reflux symptoms. Some research showed that GREC pathogenesis mainly includes two theories: reflux theory and reflex theory. DEP may not only improve diaphragmatic function, but also is significantly associated with increased thalamic GABA levels and reduced sensitivity of the cough center. The pathogenesis of GERD is complex and the prime is reflux exposure, so it is slower to relieve than cough symptoms.

In recent years, the incidence of GERC has been increasing due to changes in people’s lifestyles, improvements in corresponding diagnostic techniques and increased awareness of the disease, which is making an increasingly significant impact on people’s quality of life [5]. The LCQ, GAD-7 and PHQ-9 can measure the quality of patients’ lives. Comparing GAD-7 and PHQ-9, LCQ can comprehensively evaluate the impact of cough on patients’ lives from the physiological, psychological and social aspects. This study used the LCQ score to comprehensively evaluate changes in patients’ quality of life and found that patients who underwent DEP training were able to improve their quality of life more quickly, strengthening their treatment compliance. For chronic cough patients, especially during the pandemic, long- term uncontrollable coughing can lead to anxiety and depression, and frequent nighttime coughing can affect sleep quality, exacerbating emotional disorders. Psychological disorders can worsen patients’ sensitivity to symptoms and reduce their treatment compliance and GERD patients are more prone to comorbid anxiety and depression, leading to treatment difficulties [10, 49] and a detrimental cycle. The DEP training is a relaxation technique that may upregulate GABA [50], regulate the balance of the sympathetic and parasympathetic nervous systems, reduce cortisol secretion, lower respiratory rate and increase heart rate variability, relieving patients’ anxiety and other emotions [13, 51] and reducing symptom sensitivity caused by these disorders. Gu et al. found that DEP training improved patients’ psychiatric disorders and improved sleep quality by reducing negative emotions [42]. The changes in cough symptoms, anxiety and depression and sleep quality in the intervention group in this study were consistent with the above research results, further supporting the benefits of DEP training for GERC.

Gabapentin, a widely used neural regulator in clinical practice, is a GABA derivative that inhibits synaptic neurotransmitter release, thereby inhibiting the sensitivity of the cough center to reduce coughing [7]. Previous studies in this department have found that gabapentin is effective for refractory GERC, possibly because these patients have cough center hypersensitization [52] and Streeter C, et al. found that breathing was significantly associated with increased thalamic GABA levels using magnetic resonance spectroscopy [50]. , which may be another mechanism for alleviating coughing in GERC patients.

HARQ and capsaicin cough sensitivity test were related to cough hypersensitivity. In this study, the HARQ and capsaicin cough sensitivity test showed an improvement trend after 8 weeks of training while these values showed no statistically significant difference(Supplementary Table 4), which further confirms the DEP may inhibit the sensitivity of the cough center and relieve cough symptoms in patients with GERC.

This study had some limitations. (1) In view of the pain of the examination, patients did not want to repeat the examination, especially after the symptoms improved, so we did not require the acquisition of esophageal manometry and MII-PH data in the design of the study protocol. While the improvement in diaphragmatic muscle function was observed through B-mode ultrasound and sEMG, the changes in pressure at the gastroesophageal junction and acid exposure could not be obtained. The direct relationship between diaphragmatic muscle strength enhancement and reflux cannot therefore be confirmed. (2) The ultrasonic sampling will be subject to echo error, for which the possibility of error cannot be excluded. (3) The sample size of this study is also relatively small, mainly because the proportion of these GERC patients was very low, and it is difficult for some patients to persist in training DEP, and larger studies may be needed to support the conclusions.