Mild loss of lung aeration augments stretch in healthy lung regions

Abstract

Inspiratory stretch by mechanical ventilation worsens lung injury. However, it is not clear whether and how the ventilator damages lungs in the absence of preexisting injury. We hypothesized that subtle loss of lung aeration during general anesthesia regionally augments ventilation and distension of ventilated air spaces. In eight supine anesthetized and intubated rats, hyperpolarized gas MRI was performed after a recruitment maneuver following 1 h of volume-controlled ventilation with zero positive end-expiratory pressure (ZEEP), FiO2 0.5, and tidal volume 10 ml/kg, and after a second recruitment maneuver. Regional fractional ventilation (FV), apparent diffusion coefficient (ADC) of (3)He (a measurement of ventilated peripheral air space dimensions), and gas volume were measured in lung quadrants of ventral and dorsal regions of the lungs. In six additional rats, computed tomography (CT) images were obtained at each time point. Ventilation with ZEEP decreased total lung gas volume and increased both FV and ADC in all studied regions. Increases in FV were more evident in the dorsal slices. In each lung quadrant, higher ADC was predicted by lower gas volume and by increased mean values (and heterogeneity) of FV distribution. CT scans documented 10% loss of whole-lung aeration and increased density in the dorsal lung, but no macroscopic atelectasis. Loss of pulmonary gas at ZEEP increased fractional ventilation and inspiratory dimensions of ventilated peripheral air spaces. Such regional changes could help explain a propensity for mechanical ventilation to contribute to lung injury in previously uninjured lungs.

A hybrid multibreath wash-in wash-out lung function quantification scheme in human subjects using hyperpolarized3He MRI for simultaneous assessment of specific ventilation, alveolar oxygen tension, oxygen uptake, and air trapping

“…The acquisition scheme was closely related to three previously published imaging methods (9)(10)(11). Each of these methods has been shown to be useful in diagnosis or staging of COPD, detecting early changes in asymptomatic smokers, or to provide a meaningful imaging surrogate for specific clinical lung function tests (12)(13)(14)(15). The technique presented here also contains a computational technique for deriving oxygen uptake by the capillary blood in a manner that is much less sensitive to imaging noise than those previously explored (16)(17)(18)(19)(20).…”

A Physiologically Informed Strategy to Effectively Open, Stabilize, and Protect the Acutely Injured Lung

“…in the adjacent patent alveoli and that alveolar size actually decreases with increased airway pressure (i.e., PEEP) if lung tissue is recruited by a mechanism of gas redistribution (Cereda et al, 2011(Cereda et al, , 2013(Cereda et al, , 2016a. It has also been shown that gas redistribution in the microenvironment is not only pressure dependent but also time dependent (i.e., the longer the pressure is applied, the better the gas redistribution) (Figure 7) (Kollisch-Singule et al, 2014b).…”

“…Even with low Vt, tissue injury may occur regionally, since overdistension is not global but occurs in the microenvironment. Specifically, alveolar and alveolar duct overdistension occurs in open tissue surrounding a collapsed area of stress-focus or adjacent to areas of alveolar instability (Figures 4, 6, 7) (Mead et al, 1970;Cereda et al, 2011Cereda et al, , 2013Cereda et al, , 2016aKollisch-Singule et al, 2014b, 2018Makiyama et al, 2014;Nieman et al, 2017b;Ruhl et al, 2019). Indeed, it has been shown that atelectasis, not high Vt, causes overdistension FIGURE 7 | Rat lungs fixed at inspiration and expiration in normal lungs (Control) and in a Tween lavage ARDS model with four ventilation strategies:…”

Mechanical Ventilation Lessons Learned From Alveolar Micromechanics

“…In a murine model of surfactant depletion, atelectasis was found to occur primarily in dependent regions of the lung but demonstrated that ventilator induced injury occurred in the non-dependent/non-atelectatic regions (Tsuchida et al, 2006), likely because tidal volumes were distributed to the regions of increased compliance (Gattinoni and Pesenti, 2005). Another murine study of healthy lungs mechanically ventilated with Vt 10 mL•kg −1 and ZEEP plus recruitment maneuvers used CT and MRI imaging to determine regional lung variations in response to mechanical ventilation (Cereda et al, 2016a). Ventilation with ZEEP led to heterogeneity of and an increase in regional fractional ventilation as measured by MRI, yet did not demonstrate macroscopic atelectasis on CT scan (Cereda et al, 2016a).…”

“…Another murine study of healthy lungs mechanically ventilated with Vt 10 mL•kg −1 and ZEEP plus recruitment maneuvers used CT and MRI imaging to determine regional lung variations in response to mechanical ventilation (Cereda et al, 2016a). Ventilation with ZEEP led to heterogeneity of and an increase in regional fractional ventilation as measured by MRI, yet did not demonstrate macroscopic atelectasis on CT scan (Cereda et al, 2016a). This study demonstrated both the heterogeneous effect mechanical ventilation may have on the lung and also that subtle alterations in the micro-environment may not manifest on a macro-scale (Cereda et al, 2016a).…”