Efeitos Fisiológicos da Cânula Nasal de Alto Fluxo Medidos pela Tomografia de Impedância Elétrica na Doença Pulmonar Intersticial Crónica Grave

Abstract

Introduction Patients with Interstitial Lung Disease (ILD) are characterized by the presence of alveolar damage, inflammation, and fibrotic scarring of the pulmonary interstitium. Consequently, changes in lung function occur, with reduced lung expansion, decreased total lung capacity, and impaired gas exchange, leading to worsening respiratory symptoms, which may progress to progressive respiratory dysfunction and, eventually, respiratory failure. Oxygen therapy using a high-flow nasal cannula (HFNC) has well-recognized benefits in improving oxygen deficiency, while normalizing respiratory rate and reducing inspiratory effort, with additional benefits in humidifying the airways when compared to conventional long-term oxygen therapy alone. Electrical Impedance Tomography (EIT) is a noninvasive, radiation-free monitoring tool that allows real-time measurements of ventilatory parameters, regional lung ventilation and lung volume variations. This physiological study aims to evaluate, through EIT monitoring, the physiological differences in respiratory parameters and ventilatory variables during HFNC oxygen therapy with different flow values (from 10 to 60 L/min). Methods Adult patients diagnosed with ILD were monitored with EIT during conventional O2 via nasal cannula and during the application of O2 via HFNC with flow rates ranging from 10 to 60 L/min. Demographic data, respiratory rate (RR), peripheral oxygen saturation (SpO2), transcutaneous pressure of carbon dioxide (PtcCO2), variation in ventilation measured by impedance (TVz), TVz at two different times (Delta TVz), and end-expiratory lung volume variation (EELz) were analyzed during conventional O2 and HFNC with different flow rates (10 to 60 L/min). Results In this physiological study, four patients (two women) with severe chronic ILD were evaluated, with a median age of 74.5 (67.50-77.75) years, Forced Vital Capacity (FVC) of 1,695 (1,6575-1,7925) L, predicted %FVC of 60 (43-73) %, carbon monoxide diffusion capacity (DLCO) of 2,5000 (2,1725-2,9250) mL/min/mmHg, predicted %DLCO of 34 (30-47) %, and partial pressure of arterial oxygen (PaO₂) of 65.9500 (65.2000-74.3500) mmHg. All patients used conventional O2 at home with a flow rate of 3.00 (2.00-5.50) L/min at rest and 7.00 (6.00-8.75) L/min during the effort, as well as HFNC with a flow rate of 35.00 (35.00-38.75) L/min and an inspired oxygen fraction of 31.00 (29-34) %. Through EIT, it was possible to evaluate TVz values in conventional O2: 17,550 (7,825-26,675) ua, HFNC under a flow of 10: 16,800 (6,050-25,900) ua, HFNC under a flow of 20: 15,300 (6,225-23,175) ua, HFNC under a flow of 30: 17,450 (8,325-24,175) ua, HFNC under a flow of 40: 16,050 (7,875-24, 375) ua, HFNC under a flow of 50: 17,350 (9,350-23,475) ua, and HFNC under a flow of 60: 19,550 (8,825-25,400) ua. Significant differences were observed in TVz values when comparing flows of 20 L/min with 60 L/min (p<0.005). No significant differences in TVz were found when comparing the remaining HFNC flow values. The highest positive variation values for Delta TVz were observed when comparing flows of 20 L/min vs. 60 L/min: 21.00 (9.2-53) %, 10 L/min vs. 60 L/min: 19.5 (-1.2-53) %, and 20 L/min vs. 50 L/min: 18.00 (1.25-51.25) %. The highest positive EELz variation values were observed when comparing conventional O2 flows vs. 60 L/min: 15.0300 (2.7200-21.9625) ua, conventional O2 vs. 50L/min: 13.2550 (5.8950-27.7025) ua, and conventional O2 vs. 40L/min: 8.0850 (2.1350-15.7000) ua. Conclusion This physiological study with EIT monitoring allowed us to observe different values of TVz and EELz according to different HFNC flow settings in patients with ILD, in which the comparison of 20 L/min and 50 L/min produced greater variation in TVz, implying greater minute ventilation. The comparison of conventional O2 with high HFNC flows (60 L/min, 50 L/min, and 40 L/min) produced greater positive variation in EELz, implying greater dynamic hyperinflation and a probable increase in residual volume.