In our study, all 33 ILD patients were diagnosed with OSA. A lower RR predicted a higher AI, and a lower DLco predicted a higher HI. These parameters were also predictive factors for AHI and ODI4%. These results indicate that ILD patients who lack rapid breathing pattern and who reveal impaired diffusion capacity are at a high risk of severe OSA. For such patients, clinicians should actively consider examinations of sleep disorders. Moreover, two other significant findings were present.
First, the high occurrence of OSA was not significantly related to BMI, while the high prevalence of this complication is consistent with previous reports4–8. The mean BMI of the patients was 23.9 ± 4.6, and either underweight or normal weight patients were the major population of this study (23 of 33 patients). It is noteworthy that the group with such a background exhibited 100% prevalence of OSA. Moreover, BMI was not a significant predictive factor for the severity of OSA. These results suggest that factors other than obesity are implicated in the occurrence of OSA in ILD patients. Previous reports have presented some hypotheses for the relationship of these diseases, such as craniocaudal traction of the trachea resulting from decreased lung volume, which increases the collapsibility of the upper airway lesion13,14. Ventilatory control system instability also results from intermittent hypoxia, which possibly exacerbates this complication15,16. However, no results supporting these hypotheses were obtained from our study.
Second, lacking rapid breathing pattern and impaired diffusion capacity were significant predictive factors for the severity of OSA. Advanced ILD typically shows rapid and shallow breathing17, which means that RR rises in severe ILD. On the other hand, diffusing capacity is likely to be impaired along with the progression of ILD, resulting in a lower DLco. Namely, these parameters are likely to change in the opposite direction in a clinical course of ILD progression, as an increase in RR and a decrease in DLco. Therefore, the results of our study indicate that AI is likely to decrease with the progression of ILD, while HI is likely to increase. In other words, hypopnea may be the major constitutive factor of OSA in severe ILD (Figure 1). These results indicate the possibility that minimal pressure may be reasonable as an initial treatment with continuous positive airway pressure (CPAP) therapy for OSA complicated with ILD.
The underlying mechanism of the inverse relationship between RR and AI is unclear. However, progression of ILD leads to a decrease in FVC, which is known to exhibit a rapid and shallow breathing pattern responding to stretch receptor afferents from the periphery of the lung sensing mechanical load of increased lung elastance17. Under the condition that the afferent signal is strongly stimulated, transient cessation of respiration may be less likely to occur. Such a mechanism may explain the relationship between RR and AI, but this speculation lacks support from scientific evidence.
The causal relationship between lower DLco and higher HI suggests that impaired diffusing capacity is likely to exhibit a drop in PaO2 only by a weak airflow limitation. Namely, craniocaudal traction of the trachea caused by restrictive lung disease or a shallow breathing pattern of ILD may lead to subtle airflow limitation. This may not be enough for a complete cessation of breathing but may lead to a more prominent drop in PaO2, especially in patients with lower DLco. Moreover, if the diffusing capacity is severely impaired, the baseline PaO2 is also likely to be lower. Such a condition means they are sitting on the steep slope of the oxygen dissociation curve, which also increases the likelihood of SpO2 dropping.
Previously, several authors have investigated the relationship between OSA and ILD, while very few of them have clearly distinguished AI and HI. However, considering the preciseness of their definition, AI and HI are strictly not synonymous parameters. Namely, AI reflects complete airflow cessation, HI reflects a likelihood of desaturation with subtle airflow limitation, and it seems that they correspond to different etiologies. Therefore, we investigated the relationship between ILD and OSA by distinguishing AI and HI, which was a unique point in this study.
There are some limitations of our study. First, this was a single-center study with a small sample size. Second, the study did not examine whether the treatment of ILD affects the severity of OSA. A future randomized control study is needed.
In conclusion, there was a high incidence rate of OSA in patients with ILD. RR and DLco were predictive factors of OSA severity. ILD patients should be actively evaluated for the presence of sleep disorders, especially when they lack rapid breathing pattern and reveal lower DLco.