This study confirmed that LPS-induced ALI in mice is associated with the formation of ventilation defects that can be detected by HP 3
He MRI. Because the gaseous contrast agent in HP 3
He MRI directly visualizes airspaces, hypointense regions represent areas of the lungs that are not as well ventilated as surrounding regions. Thus persistent defects, which were found primarily in LPS-dosed mice, imply that prolonged periods of hypoventilation occurred in specific regions of the lung corresponding to the ventilation defects. Ventilation defects were recently described in a murine model of LPS-induced acute lung injury (17
What are the underlying mechanisms responsible for the development of persistent ventilation defects? Because none of the persistent defects involved an entire lobe of the lung, each lobe contained both normally ventilated and less well ventilated areas, confirming that regional ventilation is heterogeneous after the administration of LPS. None of the persistent defects appeared as large, wedge-shaped regions of hypoventilation, which would be characteristic of the obstruction or bronchospasm of a proximal, major bronchus. Likewise, these defects are not likely a result of persistent large-caliber airway closure, because no such decreased airway diameter was evident in our image analysis or histology. Furthermore, because persistent defects remained after animals received total lung capacity breaths, these defects were also unlikely to be the result of persistent atelectatic collapse of the airspaces. The resistance of persistent defects to TLC breaths raises the possibility that the decreased airway diameter of smaller bronchioles, alveolar interstitial edema, or pulmonary edema may be responsible for regionally reduced ventilation in these animals.
Unlike persistent defects, atelectatic defects were evident in saline-treated mice under low tidal volume ventilation. However, in LPS-dosed mice at low tidal volume ventilation, the atelectatic defects were accentuated in both size and number. Some degree of atelectasis is likely attributable to low-volume ventilation, which makes a smaller volume of gas available to fill alveoli. The accentuation of atelectasis in LPS-dosed animals supports the hypothesis that the decreased ventilation of respiratory units may be attributable to localized inflammation, induced by exposure to LPS. Indeed, atelectasis was shown to occur histologically, as was the case in other studies of LPS-dosed animals (18
). However, the results of our cytokine analyses suggest that increased inflammation was not present in ventilation defect areas of the lung. Consequently, regional differences in the inflammatory process may not fully explain the worsening of ventilation in LPS-dosed animals. Further studies will be necessary to determine if a localized reduction in surfactant concentrations or other mechanisms contribute to the increased size and number of atelectatic defects in mice exposed to a high dose of LPS. These studies will use the 30-μg LPS dose, given the nonsignificant differences we observed between the 30-μg and 50-μg LPS groups.
TLC breaths and larger tidal volumes may cause barotrauma in both LPS-dosed and saline-treated mice. Although especially prominent in LPS-dosed mice, saline-treated mice also demonstrated smaller diameters of right and left mainstem bronchi before TLC breaths than afterward ( and ). Because the increase in airway diameter after the TLC maneuver was especially dramatic in LPS-dosed mice, LPS-induced, airway-centric inflammation may further injure airways and predispose animals to worsened barotrauma. In this regard, a previous study using animals showed that LPS-induced acute lung injury can be exacerbated by barotrauma (19
The analysis of cytokines typically released after exposure to LPS did not indicate that increased inflammation occurred in ventilation defect areas of the lung relative to normally ventilated areas. Because the levels of proinflammatory cytokines were comparably increased in areas of ventilation defects and in surrounding normally ventilated lung parenchyma, we can surmised that LPS and the subsequent inflammatory response were distributed homogenously throughout the lung. However, because a margin of several millimeters was obtained around each defect to ensure it was properly sampled, any localized increase in cytokine concentrations within ventilation defects may have been diluted by the lung tissue in the margin.
Future research may apply several human imaging modalities to murine models of ALI. For example, computed tomography (CT) has contributed considerable insights into clinical ALI, and small-animal CT is becoming more widely available (20
). CT studies may enable the addition of lung tissue density measurements to the ventilation information provided by MRI. The 3
He used in our studies is in short supply (21
). In this regard, obtaining ventilation information from Xe-enhanced CT may be possible (22
). Indeed, the quality of hyperpolarized 129
Xe MRI is improving dramatically, and can image the transfer of gas into the pulmonary blood (23
), which could provide additional insights in models of ALI.
In conclusion, this study describes the identification and characterization of three types of hypointense regions observed via 3He MRI in mice after the administration of LPS. Histologic examination showed no unique tissue characteristics of the defects, except in the single case of an atelectatic defect visible using both MRI and histology. Similarly, cytokine analysis showed comparable inflammation in ventilation defect regions and adjacent unaffected regions of lung tissue. Our study demonstrates that HP 3He MRI may be a valuable tool in the further characterization of regional ventilation patterns in murine models of ALI, and may help elucidate the etiology of ventilation defects that are pervasive in pulmonary diseases in humans.