Gravitational distribution of regional opening and closing pressures, hysteresis and atelectrauma in ARDS evaluated by electrical impedance tomography

Abstract

Background The physiological behavior of lungs affected by the acute respiratory distress syndrome (ARDS) differs between inspiration and expiration and presents heterogeneous gravity-dependent distribution. This phenomenon, highlighted by the different distribution of opening/closing pressure and by the hysteresis of the pressure–volume curve, can be studied by CT scan, but the technique expose the patient to radiations, cannot track changes during time and is not feasible at the bedside. Electrical impedance tomography (EIT) could help in assessing at the bedside regional inspiratory and expiratory mechanical properties. We evaluated regional opening/closing pressures, hysteresis and atelectrauma during inspiratory and expiratory low-flow pressure–volume curves in ARDS using electrical impedance tomography. Methods Pixel-level inspiratory and expiratory PV curves (PVpixel) between 5 and 40 cmH2O were constructed integrating EIT images and airway opening pressure signal from 8 ARDS patients. The lower inflection point in the inspiratory and expiratory PVpixel were used to find opening (OPpixel) and closing (CPpixel) pressures. A novel atelectrauma index (AtI) was calculated as the percentage of pixels opening during the inspiratory and closing during the expiratory PV curves. The maximal hysteresis (HysMax) was calculated as the maximal difference between normalized expiratory and inspiratory PV curves. Analyses were conducted in the global, dependent and non-dependent lung regions. Results Gaussian distribution was confirmed for both global OPpixel (r2 = 0.90) and global CPpixel (r2 = 0.94). The two distributions were significantly different with higher values for OPpixel (p < 0.0001). Regional OPpixel and CPpixel distributions were Gaussian, and in the dependent lung regions, both were significantly higher than in the non-dependent ones (p < 0.001). Both AtI and the HysMax were significantly higher in the dependent regions compared to the non-dependent ones (p < 0.05 for both). Conclusions Gravity impacts the regional distribution of opening and closing pressure, hysteresis and atelectrauma, with higher values in the dorsal lung. Regional differences between inspiratory and expiratory lung physiology are detectable at the bedside using EIT and could allow in-depth characterization of ARDS phenotypes and guide personalized ventilation settings. Graphic abstract

Personalized mechanical ventilation in acute respiratory distress syndrome

“…Notwithstanding, sensitivity of LUS to detect alveolar recruitment is variable across studies [ 68 ]. Conversely, EIT assesses regional differences between inspiratory and expiratory aeration and permits in-depth characterization of ARDS phenotypes at the bedside [ 72 ]. PEEP, when set according to EIT, may aid in optimizing lung recruitment and homogeneity of ventilation [ 73 ].…”

Atelectasis, Shunt, and Worsening Oxygenation Following Reduction of Respiratory Rate in Healthy Pigs Undergoing ECMO: An Experimental Lung Imaging Study

“…Atelectasis may trigger two key mechanisms of VILI, potentially outweighing the beneficial effects of decreased mechanical power, namely, reduced baby lung size (i.e., the normally aerated lung fraction) causing increased lung strain (Bellani et al, 2011) and larger fraction of lung units opening and closing during the respiratory cycle (atelectrauma) causing additional local stress by the sudden diffusion of gas flow between the epithelial cells (Caironi et al, 2010). A larger fraction of atelectasis occurred in the dependent lung, suggesting that this region may be particularly prone to these detrimental mechanisms, as shown by a previous publication (Scaramuzzo et al, 2020b).…”

Spatial Ventilation Inhomogeneity Determined by Electrical Impedance Tomography in Patients With Chronic Obstructive Lung Disease

“…This means that the studied lung section was located in the same plane with respect to the gravity vector. However, the gravity-dependent phenomena can be assessed by EIT when the studied subjects are in a horizontal posture, as demonstrated in healthy subjects ( Frerichs et al, 1996 ), experimental animals ( Kunst et al, 2000 ), or patients with respiratory failure ( Victorino et al, 2004 ; Pulletz et al, 2012 ; Scaramuzzo et al, 2020 ). Alternatively, the subjects can be examined with the electrodes placed sequentially at first in the cranial and then the caudal chest location (or vice versa) ( Reifferscheid et al, 2011 ).…”