In the GOLD 2023 guidelines, a newly introduced term called "Preserved Ratio Impaired Spirometry (PRISm)" refers to patients who have a normal forced expiratory volume in one second (FEV1)/forced vital capacity (FVC) ratio (FEV1/FVC≥0.7, after bronchodilator use), but exhibit impaired pulmonary ventilation function with FEV1 and/or FVC values below 80% of the predicted value. In the past, these patients were often classified as GOLD U or restrictive ventilatory impairment. In recent years, several large cross-sectional and longitudinal studies on PRISm have revealed its high prevalence, ranging from 5-20% due to population heterogeneity. Notably, approximately 10% to 40% of patients with PRISm tend to develop chronic obstructive pulmonary disease (COPD), resulting in frequent healthcare utilization. Studies have found that such patients are associated with increased respiratory symptoms, reduced quality of life, and increased burden of cardiovascular disease. A recent meta-analysis showed a significant increased risk of all-cause, cardiovascular, and respiratory-related mortality risk in PRISm patients. Currently, there are no diagnostic and treatment guidelines for PRISm, but studies highlights the importance of screening and identifying PRISm, with follow-up and early intervention if necessary.

Electrical Impedance Tomography (EIT) is an emerging imaging technology developing rapidly in recent years, with many advantages such as non-invasive, non-radiation and simple operation. Its principle is to measure the voltage or current signals through electrodes on the body surface, and then reconstructing images of the distribution of impedance changes. This technology is widely used in pulmonary diseases, including monitoring lung ventilation distribution, blood perfusion, and titration of positive end-expiratory pressure. EIT images have high spatial and temporal resolution, can display physiological and pathological changes in real time based on breathing. In 2022, China issued the clinical application consensus of electrical impedance tomography in critical respiratory management, further promoting the application of this technology in respiratory management. However, the number of clinical studies on EIT in identifying airway obstruction in COPD patients is limited. It has been proven that in COPD patients, EIT combined with pulmonary function tests can evaluate the spatial and temporal distribution of lung capacity in different regions and identify pathologically induced ventilation heterogeneity. At present, the pathophysiological mechanism of PRISm is still under discussion. Studies have found that PRISm is related to small airway dysfunction (SAD) and decreased total lung capacity. Computed Tomography (CT) can be used to evaluate small airway function and observe the morphological and structural changes of lung tissue, but it has but it has some disadvantages such as long duration and radiation exposure. Therefore, EIT may be a good choice for rapidly assessing regional lung function in PRISm patients. Due to the scarcity of EIT in PRISm studies, investigators have initiated a clinical study to evaluate whether ventilation heterogeneity in PRISm patients can be obtained through the combination of EIT and pulmonary function tests.