Novel analysis of 4DCT imaging quantifies progressive increases in anatomic dead space during mechanical ventilation in mice

Increased dead space is an important prognostic marker in early acute respiratory distress syndrome (ARDS) that correlates with mortality. The cause of increased dead space in ARDS has largely been attributed to increased alveolar dead space due to ventilation/perfusion mismatching and shunt. We sought to determine whether anatomic dead space also increases in response to mechanical ventilation. Mice received intratracheal lipopolysaccharide (LPS) or saline and mechanical ventilation (MV). Four-dimensional computed tomography (4DCT) scans were performed at onset of MV and after 5 h of MV. Detailed measurements of airway volumes and lung tidal volumes were performed using image analysis software. The forced oscillation technique was used to obtain measures of airway resistance, tissue damping, and tissue elastance. The ratio of airway volumes to total tidal volume increased significantly in response to 5 h of mechanical ventilation, regardless of LPS exposure, and airways demonstrated significant variation in volumes over the respiratory cycle. These findings were associated with an increase in tissue elastance (decreased lung compliance) but without changes in tidal volumes. Airway volumes increased over time with exposure to mechanical ventilation without a concomitant increase in tidal volumes. These findings suggest that anatomic dead space fraction increases progressively with exposure to positive pressure ventilation and may represent a pathological process. NEW & NOTEWORTHY We demonstrate that anatomic dead space ventilation increases significantly over time in mice in response to mechanical ventilation. The novel functional lung-imaging techniques applied here yield sensitive measures of airway volumes that may have wide applications.

A role for decorin in a murine model of allergen-induced asthma

Decorin (Dcn) is an extracellular matrix proteoglycan, which affects airway mechanics, airway-parenchymal interdependence, airway smooth muscle proliferation and apoptosis, and transforming growth factor-β bioavailability. As Dcn deposition is differentially altered in asthma, we questioned whether Dcn deficiency would impact the development of allergen-induced asthma in a mouse model. Dcn−/− and Dcn+/+ mice (C57Bl/6) were sensitized with ovalbumin (OA) and challenged intranasally 3 days/wk × 3 wk. After OA challenge, mice were anesthetized, and respiratory mechanics measured under baseline conditions and after delivery of increasing concentrations of methacholine aerosol. Complex impedance was partitioned into airway resistance and tissue elastance and damping. Bronchoalveolar lavage was performed. Lungs were excised, and tissue sections evaluated for inflammatory cell influx, α-smooth muscle actin, collagen, biglycan, and Dcn deposition. Changes in TH-2 cytokine mRNA and protein were also measured. Airway resistance was increased in OA-challenged Dcn+/+ mice only ( P < 0.05), whereas tissue elastance and damping were increased in both OA-challenged Dcn+/+ and Dcn−/−, but more so in Dcn+/+ mice ( P < 0.001). Inflammation and collagen staining within the airway wall were increased with OA in Dcn+/+ only ( P < 0.001 and P < 0.01, respectively, vs. saline). IL-5 and IL-13 mRNA were increased in lung tissue of OA-challenged Dcn+/+ mice. Dcn deficiency resulted in more modest OA-induced hyperresponsiveness, evident at the level of the central airways and distal lung. Differences in physiology were accompanied by differences in inflammation and remodeling. These findings may be, in part, due to the well-described ability of Dcn to bind transforming growth factor-β and render it less bioavailable.

Adiponectin-deficient mice are protected against tobacco-induced inflammation and increased emphysema

Adiponectin is a cytokine with both proinflammatory and anti-inflammatory properties that is expressed in epithelial cells in the airway in chronic obstructive pulmonary disease-emphysema (COPD-E). To determine whether adiponectin modulates levels of lung inflammation in tobacco smoke-induced COPD-E, we used a mouse model of COPD-E in which either adiponectin-deficient or wild-type (WT) mice were exposed to tobacco smoke for 6 mo. Outcomes associated with tobacco smoke-induced COPD-E were quantitated including lung inflammation [bronchoalveolar lavage (BAL) and total and differential cell count], lung mediators of inflammation (cytokines and chemokines), air space enlargement (i.e., linear intercept), and lung function (tissue elastance) in the different groups of mice. Whereas exposure of WT mice to tobacco smoke for 6 mo induced significant lung inflammation (increased total BAL cells, neutrophils, and macrophages), adiponectin-deficient mice had minimal BAL inflammation when exposed to tobacco smoke for 6 mo. In addition, whereas chronic tobacco-exposed WT mice had significantly increased levels of lung mediators of inflammation [i.e., TNF-α, keratinocyte-derived chemokine (KC), and adiponectin] as well as significantly increased air space enlargement (increased linear intercept) and decreased tissue elastance, exposure of adiponectin-deficient mice to chronic tobacco smoke resulted in no further increase in lung mediators, air space enlargement, or tissue elastance. In vitro studies demonstrated that BAL macrophages derived from adiponectin-deficient mice incubated in media containing tobacco smoke expressed minimal TNF-α or KC compared with BAL macrophages from WT mice. These studies suggest that adiponectin plays an important proinflammatory role in tobacco smoke-induced COPD-E.

Protective mechanical ventilation does not exacerbate lung function impairment or lung inflammation following influenza A infection

The degree to which mechanical ventilation induces ventilator-associated lung injury is dependent on the initial acute lung injury (ALI). Viral-induced ALI is poorly studied, and this study aimed to determine whether ALI induced by a clinically relevant infection is exacerbated by protective mechanical ventilation. Adult female BALB/c mice were inoculated with 104.5 plaque-forming units of influenza A/Mem/1/71 in 50 μl of medium or medium alone. This study used a protective ventilation strategy, whereby mice were anesthetized, tracheostomized, and mechanically ventilated for 2 h. Lung mechanics were measured periodically throughout the ventilation period using a modification of the forced oscillation technique to obtain measures of airway resistance and coefficients of tissue damping and tissue elastance. Thoracic gas volume was measured and used to obtain specific airway resistance, tissue damping, and tissue elastance. At the end of the ventilation period, a bronchoalveolar lavage sample was collected to measure inflammatory cells, macrophage inflammatory protein-2, IL-6, TNF-α, and protein leak. Influenza infection caused significant increases in inflammatory cells, protein leak, and deterioration in lung mechanics that were not exacerbated by mechanical ventilation, in contrast to previous studies using bacterial and mouse-specific viral infection. This study highlighted the importance of type and severity of lung injury in determining outcome following mechanical ventilation.

The bimodal quasi-static and dynamic elastance of the murine lung

The double sigmoidal nature of the mouse pressure-volume (PV) curve is well recognized but largely ignored. This study systematically examined the effect of inflating the mouse lung to 40 cm H2O transrespiratory pressure (Prs) in vivo. Adult BALB/c mice were anesthetized, tracheostomized, and mechanically ventilated. Thoracic gas volume was calculated using plethysmography and electrical stimulation of the intercostal muscles. Lung mechanics were tracked during inflation-deflation maneuvers using a modification of the forced oscillation technique. Inflation beyond 20 cm H2O caused a shift in subsequent PV curves with an increase in slope of the inflation limb and an increase in lung volume at 20 cm H2O. There was an overall decrease in tissue elastance and a fundamental change in its volume dependence. This apparent “softening” of the lung could be recovered by partial degassing of the lung or applying a negative transrespiratory pressure such that lung volume decreased below functional residual capacity. Allowing the lung to spontaneously recover revealed that the lung required ∼1 h of mechanical ventilation to return to the original state. We propose a number of possible mechanisms for these observations and suggest that they are most likely explained by the unfolding of alveolar septa and the subsequent redistribution of the fluid lining the alveoli at high transrespiratory pressure.

Effects of heterogeneities on the partitioning of airway and tissue properties in normal mice

We measured the mechanical properties of the respiratory system of C57BL/6 mice using the optimal ventilation waveform method in closed- and open-chest conditions at different positive end-expiratory pressures. The tissue damping (G), tissue elastance (H), airway resistance (Raw), and hysteresivity were obtained by fitting the impedance data to three different models: a constant-phase model by Hantos et al. (Hantos Z, Daroczy B, Suki B, Nagy S, Fredberg JJ. J Appl Physiol 72: 168–178, 1992), a heterogeneous Raw model by Suki et al. (Suki B, Yuan H, Zhang Q, Lutchen KR. J Appl Physiol 82: 1349–1359, 1997), and a heterogeneous H model by Ito et al. (Ito S, Ingenito EP, Arold SP, Parameswaran H, Tgavalekos NT, Lutchen KR, Suki B. J Appl Physiol 97: 204–212, 2004). Both in the closed- and open-chest conditions, G and hysteresivity were the lowest and Raw the highest in the heterogeneous Raw model, and G and H were the largest in the heterogeneous H model. Values of G, Raw, and hysteresivity were significantly higher in the closed-chest than in the open-chest condition. However, H was not affected by the conditions. When the tidal volume of the optimal ventilation waveform was decreased from 8 to 4 ml/kg in the closed-chest condition, G and hysteresivity significantly increased, but there were smaller changes in H or Raw. In summary, values of the obtained mechanical properties varied among these models, primarily due to heterogeneity. Moreover, the mechanical parameters were significantly affected by the chest wall and tidal volume in mice. Contribution of the chest wall and heterogeneity to the mechanical properties should be carefully considered in physiological studies in which partitioning of airway and tissue properties are attempted.

Age-dependent changes of airway and lung parenchyma in C57BL/6J mice

In the current study, we hypothesize that senescent-dependent changes between airway and lung parenchymal tissues of C57BL/6J (B6) mice are not synchronized with respect to altered lung mechanics. Furthermore, aging modifications in elastin fiber and collagen content of the airways and lung parenchyma are remodeling events that differ with time. To test these hypotheses, we performed quasi-static pressure-volume (PV) curves and impedance measurements of the respiratory system in 2-, 20-, and 26-mo-old B6 mice. From the PV curves, the lung volume at 30 cmH2O pressure (V30) and respiratory system compliance (Crs) were significantly ( P < 0.01) increased between 2 and 20 mo of age, representing about 80–84% of the total increase that occurred between 2 and 26 mo of age. Senescent-dependent changes in tissue damping and tissue elastance were analogous to changes in V30 and Crs; that is, a majority of the parenchymal alterations in the lung mechanics occurred between 2 and 20 mo of age. In contrast, significant decreases in airway resistance (R) occurred between 20 and 26 mo of age; that is, the decrease in R between 2 and 20 mo of age represented only 29% ( P > 0.05) of total decrease occurring through 26 mo. Morphometric analysis of the elastic fiber content in lung parenchyma was significantly ( P < 0.01) decreased between 2 and 20 mo of age. To the contrary, increased collagen content was significantly delayed until 26 mo of age ( P < 0.01, 2 vs. 26 mo). In conclusion, our data demonstrate that senescent-dependent changes in airway and lung tissue mechanics are not synchronized in B6 mice. Moreover, the reduction in elastic fiber content with age is an early lung remodeling event, and the increased collagen content in the lung parenchyma occurs later in senescence.

Choosing the frequency of deep inflation in mice: balancing recruitment against ventilator-induced lung injury

Low tidal volume (Vt) ventilation is protective against ventilator-induced lung injury but can promote development of atelectasis. Periodic deep inflation (DI) can open the lung, but if delivered too frequently may cause damage via repeated overdistention. We therefore examined the effects of varying DI frequency on lung mechanics, gas exchange, and biomarkers of injury in mice. C57BL/6 males were mechanically ventilated with positive end-expiratory pressure (PEEP) of 2 cmH2O for 2 h. One high Vt group received a DI with each breath (HV). Low Vt groups received 2 DIs after each hour of ventilation (LV) or 2 DIs every minute (LVDI). Control groups included a nonventilated surgical sham and a group receiving high Vt with zero PEEP (HVZP). Respiratory impedance was measured every 4 min, from which tissue elastance ( H) and damping ( G) were derived. G and H rose progressively during LV and HVZP, but returned to baseline after hourly DI during LV. During LVDI and HV, G and H remained low and gas exchange was superior to that of LV. Bronchoalveolar lavage fluid protein was elevated in HV and HVZP but was not different between LV and LVDI. Lung tissue IL-6 and IL-1β levels were elevated in HVZP and lower in LVDI compared with LV. We conclude that frequent DI can safely improve gas exchange and lung mechanics and may confer protection from biotrauma. Differences between LVDI and HV suggest that an optimal frequency range of DI exists, within which the benefits of maintaining an open lung outweigh injury incurred from overdistention.

Developmental changes in airway and tissue mechanics in mice

Most studies using mice to model human lung diseases are carried out in adults, although there is emerging interest in the effects of allergen, bacterial, and viral exposure early in life. This study aims to characterize lung function in BALB/c mice from infancy (2 wk) through to adulthood (8 wk). The low-frequency forced oscillation technique was used to obtain impedance data, partitioned into components representing airway resistance, tissue damping, tissue elastance, and hysteresivity (tissue damping/tissue elastance). Measurements were made at end-expiratory pause (transrespiratory system pressure = 2 cmH2O) and during relaxed slow expiration from 20 to 0 cmH2O. Airway resistance decreased with age from 0.63 cmH2O·ml−1·s at 2 wk to 0.24 cmH2O·ml−1·s at 8 wk ( P < 0.001). Both tissue damping and tissue elastance decreased with age ( P < 0.001) from 2 to 5 wk, then plateaued through to 8 wk ( P < 0.001). This pattern was seen both in measurements taken at end-expiratory pause and during expiration. There were no age-related changes seen in hysteresivity when measured at end-expiratory pause, but the pattern of volume dependence did differ with the age of the mice. These changes in respiratory mechanics parallel the reported structural changes of the murine lung from the postnatal period into adulthood.