Regional function-structure relationships in lungs of an elastase murine model of emphysema

Atorvastatin dose-dependently promotes mouse lung repair after emphysema induced by elastase

Emphysema results in a proteinase - antiproteinase imbalance, inflammation and oxidative stress. Our objective was to investigate whether atorvastatin could repair mouse lungs after elastase-induced emphysema. Vehicle (50 μL) or porcine pancreatic elastase (PPE) was administered on day 1, 3, 5 and 7 at 0.6 U intranasally. Male mice were divided into a control group (sham), PPE 32d (sacrificed 24 h after 32 days), PPE 64d (sacrificed 24 h after 64 days), and atorvastatin 1, 5 and 20 mg treated from day 33 until day 64 and sacrificed 24 h later (A1 mg, A5 mg and A20 mg, respectively). Treatment with atorvastatin was performed via inhalation for 10 min once a day. We observed that emphysema at day 32 was similar to emphysema at day 64. The mean airspace chord length (Lm) indicated a recovery of pulmonary morphology in groups A5 mg and A20 mg, as well as recovery of collagen and elastic fibers in comparison to the PPE group. Bronchoalveolar lavage fluid (BALF) leukocytes were reduced in all atorvastatin-treated groups. However, tissue macrophages were reduced only in the A20 mg group compared with the PPE group, while tissue neutrophils were reduced in the A5 mg and A20 mg groups. The redox balance was restored mainly in the A20 mg group compared with the PPE group. Finally, atorvastatin at doses of 5 and 20 mg reduced nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and matrix metalloproteinase-12 (MMP-12) compared with the PPE group. In conclusion, atorvastatin was able to induce lung tissue repair in emphysematous mice.

A hybrid multibreath wash-in wash-out lung function quantification scheme in human subjects using hyperpolarized 3 He MRI for simultaneous assessment of specific ventilation, alveolar oxygen tension, oxygen uptake, and air trapping

PURPOSE

To present a method for simultaneous acquisition of alveolar oxygen tension (PA O2 ), specific ventilation (SV), and apparent diffusion coefficient (ADC) of hyperpolarized (HP) gas in the human lung, allowing reinterpretation of the PA O2 and SV maps to produce a map of oxygen uptake (R).

METHOD

An imaging scheme was designed with a series of identical normoxic HP gas wash-in breaths to measure ADC, SV, PA O2 , and R in less than 2 min. Signal dynamics were fit to an iterative recursive model that regionally solved for these parameters. This measurement was successfully performed in 12 subjects classified in three healthy, smoker, and chronic obstructive pulmonary disease (COPD) cohorts.

RESULTS

The overall whole lung ADC, SV, PA O2 , and R in healthy, smoker, and COPD subjects was 0.20 ± 0.03 cm2 /s, 0.39 ± 0.06,113 ± 2 Torr, and 1.55 ± 0.35 Torr/s, respectively, in healthy subjects; 0.21 ± 0.03 cm2 /s, 0.33 ± 0.06, 115.9 ± 4 Torr, and 0.97 ± 0.2 Torr/s, respectively, in smokers; and 0.25 ± 0.06 cm2 /s, 0.23 ± 0.08, 114.8 ± 6.0Torr, and 0.94 ± 0.12 Torr/s, respectively, in subjects with COPD. Hetrogeneity of SV, PA O2 , and R were indicators of both smoking-related changes and disease, and the severity of the disease correlated with the degree of this heterogeneity. Subjects with symptoms showed reduced oxygen uptake and specific ventilation.

CONCLUSION

High-resolution, nearly coregistered and quantitative measures of lung function and structure were obtained with less than 1 L of HP gas. This hybrid multibreath technique produced measures of lung function that revealed clear differences among the cohorts and subjects and were confirmed by correlations with global lung measurements. Magn Reson Med 78:611-624, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Hyperpolarized gas diffusion MRI of biphasic lung inflation in short- and long-term emphysema models

During lung inflation, airspace dimensions are affected nonlinearly by both alveolar expansion and recruitment, potentially confounding the identification of emphysematous lung by hyperpolarized helium-3 diffusion magnetic resonance imaging (HP MRI). This study aimed to characterize lung inflation over a broad range of inflation volume and pressure values in two different models of emphysema, as well as in normal lungs. Elastase-treated rats (n = 7) and healthy controls (n = 7) were imaged with HP MRI. Gradual inflation was achieved by incremental changes to both inflation volume and airway pressure. The apparent diffusion coefficient (ADC) was measured at each level of inflation and fitted to the corresponding airway pressures as the second-order response equation, with minimizing residue (χ² < 0.001). A biphasic ADC response was detected, with an initial ADC increase followed by a decrease at airway pressures >18 cmH₂O. Discrimination between treated and control rats was optimal when airway pressure was intermediate (between 10 and 11 cmH₂O). Similar findings were confirmed in mice following long-term exposure to cigarette smoke, where optimal discrimination between treated and healthy mice occurred at a similar airway pressure as in the rats. We subsequently explored the evolution of ADC measured at the intermediate inflation level in mice after prolonged smoke exposure and found a significant increase (P < 0.01) in ADC over time. Our results demonstrate that measuring ADC at intermediate inflation enhances the distinction between healthy and diseased lungs, thereby establishing a model that may improve the diagnostic accuracy of future HP gas diffusion studies.

Moderate Aerobic Training Improves Cardiorespiratory Parameters in Elastase-Induced Emphysema

Aim: We investigated the therapeutic effects of aerobic training on lung mechanics, inflammation, morphometry and biological markers associated with inflammation, and endothelial cell damage, as well as cardiac function in a model of elastase-induced emphysema.

Methods: Eighty-four BALB/c mice were randomly allocated to receive saline (control, C) or 0.1 IU porcine pancreatic elastase (emphysema, ELA) intratracheally once weekly for 4 weeks. After the end of administration period, once cardiorespiratory impairment associated with emphysema was confirmed, each group was further randomized into sedentary (S) and trained (T) subgroups. Trained mice ran on a motorized treadmill, at moderate intensity, 30 min/day, 3 times/week for 4 weeks.

Results: Four weeks after the first instillation, ELA animals, compared to C, showed: (1) reduced static lung elastance (Est,L) and levels of vascular endothelial growth factor (VEGF) in lung tissue, (2) increased elastic and collagen fiber content, dynamic elastance (E, in vitro), alveolar hyperinflation, and levels of interleukin-1β and tumor necrosis factor (TNF)-α, and (3) increased right ventricular diastolic area (RVA). Four weeks after aerobic training, ELA-T group, compared to ELA-S, was associated with reduced lung hyperinflation, elastic and collagen fiber content, TNF-α levels, and RVA, as well as increased Est,L, E, and levels of VEGF.

Conclusion: Four weeks of regular and moderate intensity aerobic training modulated lung inflammation and remodeling, thus improving pulmonary function, and reduced RVA and pulmonary arterial hypertension in this animal model of elastase-induced emphysema.

Mild loss of lung aeration augments stretch in healthy lung regions

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.

Regional fractional ventilation mapping in spontaneously breathing mice using hyperpolarized ¹²⁹Xe MRI

The feasibility of ventilation imaging with hyperpolarized (HP) (129) Xe MRI has been investigated for quantitative and regional assessment of ventilation in spontaneously breathing mice. The multiple breath ventilation imaging technique was modified to the protocol of spontaneous inhalation of HP (129) Xe delivered continuously from a (129) Xe polarizer. A series of (129) Xe ventilation images was obtained by varying the number of breaths before the (129) Xe lung imaging. The fractional ventilation, r, was successfully evaluated for spontaneously breathing mice. An attempt was made to detect ventilation dysfunction in the emphysematous mouse lung induced by intratracheal administration of porcine pancreatic elastase (PPE). As a result, the distribution of fractional ventilation could be visualized by the r map. Significant dysfunction of ventilation was quantitatively identified in the PPE-treated group. The whole-lung r value of 0.34 ± 0.01 for control mice (N = 4) was significantly reduced, to 0.25 ± 0.07, in PPE-treated mice (N = 4) (p = 0.038). This study is the first application of multiple breath ventilation imaging to spontaneously breathing mice, and shows that this methodology is sensitive to differences in the pulmonary ventilation. This methodology is expected to improve simplicity as well as noninvasiveness when assessing regional ventilation in small rodents.

Intravenous and intratracheal mesenchymal stromal cell injection in a mouse model of pulmonary emphysema

The aim of this study was to characterize the evolution of lung function and -structure in elastase-induced emphysema in adult mice and the effect of mesenchymal stromal cell (MSC) administration on these parameters. Adult mice were treated with intratracheal (4.8 units/100 g bodyweight) elastase to induce emphysema. MSCs were administered intratracheally or intravenously, before or after elastase injection. Lung function measurements, histological and morphometric analysis of lung tissue were performed at 3 weeks, 5 and 10 months after elastase and at 19, 20 and 21 days following MSC administration. Elastase-treated mice showed increased dynamic compliance and total lung capacity, and reduced tissue-specific elastance and forced expiratory flows at 3 weeks after elastase, which persisted during 10 months follow-up. Histology showed heterogeneous alveolar destruction which also persisted during long-term follow-up. Jugular vein injection of MSCs before elastase inhibited deterioration of lung function but had no effects on histology. Intratracheal MSC treatment did not modify lung function or histology. In conclusion, elastase-treated mice displayed persistent characteristics of pulmonary emphysema. Jugular vein injection of MSCs prior to elastase reduced deterioration of lung function. Intratracheal MSC treatment had no effect on lung function or histology.

Hyperpolarized (3)He diffusion MRI and histology of secreted frizzled related protein-1 (SFRP1) deficient lungs in a Murine model

Secreted frizzled related protein-1 (SFRP1) plays a key role in many diverse processes, including embryogenesis, tissue repair, bone formation, and tumor genesis. Previous studies have shown the effects of the SFRP1 gene on lung development using the SFRP1 knockout mouse model via histological and physiological studies. In this study, the feasibility of ADC (acquired via HP (3)He) to detect altered lung structure in the SFRP1 knockout (SFRP1(-/-)) mice was investigated, and compared to analysis by histology. This study consisted of two groups, the wild-type (WT) mice and the knockout (KO) mice with n=6 mice for each group. (3)He ADC MRI and histology were performed on all of the animals. The global Lm values of WT and KO mice were 35.0±0.8μm and 38.4±3.8μm, respectively, which translated to an increase of 9.58% in the Lm of KO mice. The mean global ADCs for the WT and KO mice were 0.12±0.01cm(2)/s and 0.13±0.01cm(2)/s, respectively, which equated to a relative increase of 8.0% in the KO mice compared to the WT mice. In the sub-analysis of the anterior, medial and posterior lung regions, Lm increased by 10.50%, 6.66% and 11.84% in the KO mice, respectively, whereas the differences in ADC between the two groups in the anterior, medial, and posterior regions were 7.3%, 8.3%, and 4.6%, respectively. These results suggest that HP MRI measurements can be used as a suitable substitute for histology to obtain valuable information about lung geometry non-invasively. This technique is also advantageous as regional measurements can be performed, which can identify lung destruction more precisely. Most importantly, this approach extends far beyond the specific pathology analyzed in this study, as it can be applied to many other pathological conditions in the lung tissue, as well to many other embryonic studies.

Positive end-expiratory pressure increments during anesthesia in normal lung result in hysteresis and greater numbers of smaller aerated airspaces

BACKGROUND

Although it is recognized that pulmonary hysteresis can influence the effects of positive end-expiratory pressure (PEEP), the extent to which expansion of previously opened (vs. newly opening) peripheral airspaces contribute to increased lung volume is unknown.

METHODS

Following a recruitment maneuver, rats were ventilated with constant tidal volumes and imaged during ascending and descending ramps of PEEP.

RESULTS

The authors estimated peripheral airspace dimensions by measuring the apparent diffusion coefficient of He in 10 rats. In a separate group (n = 5) undergoing a similar protocol, the authors used computerized tomography to quantify lung volume. Hysteresis was confirmed by larger end-inspiratory lung volume (mean ± SD; all PEEP levels included): 8.4 ± 2.8 versus 6.8 ± 2.0 ml (P < 0.001) and dynamic compliance: 0.52 ± 0.12 versus 0.42 ± 0.09 ml/cm H2O (P < 0.001) during descending versus ascending PEEP ramps. Apparent diffusion coefficient increased with PEEP, but it was smaller during the descending versus ascending ramps for corresponding levels of PEEP: 0.168 ± 0.019 versus 0.183 ± 0.019 cm/s (P < 0.001). Apparent diffusion coefficient was smaller in the posterior versus anterior lung regions, but the effect of PEEP and hysteresis on apparent diffusion coefficient was greater in the posterior regions.

CONCLUSIONS

The authors' study results suggest that in healthy lungs, larger lung volumes due to hysteresis are associated with smaller individual airspaces. This may be explained by opening of previously nonaerated peripheral airspaces rather than expansion of those already aerated. Setting PEEP on a descending ramp may minimize distension of individual airspaces.

Imaging the interaction of atelectasis and overdistension in surfactant-depleted lungs

OBJECTIVE

Atelectasis and surfactant depletion may contribute to greater distension-and thereby injury-of aerated lung regions; recruitment of atelectatic lung may protect these regions by attenuating such overdistension. However, the effects of atelectasis (and recruitment) on aerated airspaces remain elusive. We tested the hypothesis that during mechanical ventilation, surfactant depletion increases the dimensions of aerated airspaces and that lung recruitment reverses these changes.

DESIGN

Prospective imaging study in an animal model.

SETTING

Research imaging facility.

SUBJECTS

Twenty-seven healthy Sprague Dawley rats.

INTERVENTIONS

Surfactant depletion was obtained by saline lavage in anesthetized, ventilated rats. Alveolar recruitment was accomplished using positive end-expiratory pressure and exogenous surfactant administration.

MEASUREMENTS AND MAIN RESULTS

Airspace dimensions were estimated by measuring the apparent diffusion coefficient of He, using diffusion-weighted hyperpolarized gas magnetic resonance imaging. Atelectasis was demonstrated using computerized tomography and by measuring oxygenation. Saline lavage increased atelectasis (increase in nonaerated tissue from 1.2% to 13.8% of imaged area, p < 0.001), and produced a concomitant increase in mean apparent diffusion coefficient (~33%, p < 0.001) vs. baseline; the heterogeneity of the computerized tomography signal and the variance of apparent diffusion coefficient were also increased. Application of positive end-expiratory pressure and surfactant reduced the mean apparent diffusion coefficient (~23%, p < 0.001), and its variance, in parallel to alveolar recruitment (i.e., less computerized tomography densities and heterogeneity, increased oxygenation).

CONCLUSIONS

Overdistension of aerated lung occurs during atelectasis is detectable using clinically relevant magnetic resonance imaging technology, and could be a key factor in the generation of lung injury during mechanical ventilation. Lung recruitment by higher positive end-expiratory pressure and surfactant administration reduces airspace distension.