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Johns Hopkins Medical Institutions, Baltimore, Maryland 21205
| ABSTRACT |
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functional residual capacity; lung volume distribution; respiratory mechanics; computed tomography
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| METHODS |
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Lung volume.
HRCT scans were obtained with a Somatom IV Scanner (Siemens, Iselin, NJ) using a spiral mode to acquire 24 contiguous images in 15 s at 137 kVp and 165 mA. The images were reconstructed as 1-mm slice thickness and a 512 x 512 matrix using a 12-cm field of view and a high spatial frequency (resolution) algorithm that enhanced edge detection, at a window level of -450 Hounsfield units (HU) and a window width of 1,350 HU. In larger animals, these settings have been shown to provide accurate measurement of airway lumen size in airways approaching 1 mm (6, 14), and since the mouse thorax is of comparable size to conducting airways in larger species, these settings provide good visualization of the lung and thoracic wall boundary. The pixel dimension in these images was 0.23 x 0.23 mm.
The CT images were then transferred to a Macintosh computer for analysis using NIH Image software. The following procedure was used. The Hounsfield unit scale was first converted to percent air by selecting areas with pure vascular tissue (the heart) as 0% and pure air (outside the thorax) as 100%. A fixed threshold was selected and used for all images. This threshold (29% air) was chosen to ensure that all of the air in the lung was incorporated, without adding additional pixels with low radiodensity. The total thresholded area of each section was then measured and stored along with the measure of the percent air in this area. The percent air per thresholded slice was multiplied by the thresholded area to get the total amount of lung air in that slice. The total lung air volume was then determined by adding the air volumes of all 1-mm thick slices. This procedure of measuring lung air volume was described by Olson and Hoffman (11), who used it to study volume changes in pneumonectomized rabbits. The voxel resolution of current scanners, however, is over an order of magnitude greater than that of the Imatron scanner used in that study. In addition to total lung volume, we also calculated the left and right individual lung volumes.
Calibration protocol.
Five mice were used to validate the accuracy of the CT measurement system for lung air volume. The anesthetized mice were tracheostomized with an 18-gauge needle and connected to a mouse ventilator as previously described (5). They were ventilated with 100% oxygen for 10 min, and then the lungs were sealed. This procedure allows all the gas in the lung to be absorbed to bring the lung air volume to zero (13). The dead animals were then placed in the CT scanner, and lung volumes were measured after sequentially injecting known air volumes in each animal.
Lung volume measurement protocol.
In each of the experimental mice, the lung volumes were measured at weekly intervals from 4 to 12 wk of age. C3H/HeJ and A/J strains were statistically compared using ANOVA with repeated measures.
| RESULTS |
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| DISCUSSION |
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We should also note that our use of the term FRC is not precisely analogous to what it normally indicates in humans, i.e., the lung volume determined solely by the passive recoil of the lung and thoracic wall, for convenience. The mice we studied are breathing, so the mean lung volume we measure is slightly higher than the end-expiratory lung volume. For a mouse breathing with a nominal inspiratory-to-expiratory (I:E) ratio of 1:2 and tidal volume of 200 µl, the lung volume we measure would be expected to be 67 µl (
2025%) greater than the actual end-expiratory lung volume.
Our results demonstrate substantial differences in FRC between the two strains studied. This was clearly apparent even with the limited sample used to evaluate the method. We found FRC values averaging 350 µl in the C3H/HeJ animals and 270 µl in the A/J. There was considerably more variability in the C3H/HeJ strain, and we do not know of any physiological reasons for this. Given the small number of animals, however, this may just reflect normal statistical variation. Using a gas dilution method, Lai and Chou (8) reported FRC measurements from C57BL/6 mice (weighing 22 g) to average 250 µl, a value close to that in the A/J strain. The roughly 35% larger FRC volumes in the C3H/HeJ animals, compared with the A/J animals persisted over the 8-wk study period. We know of no other studies where similar measurement or comparison of in vivo lung volumes was made in different mouse strains. Recent work by Tankersley et al. (13) compared the pressure-volume curves in these strains. Their results showed that total lung capacity (TLC; defined as the lung volume at 30 cmH2O) in the C3H/HeJ animals was 44% larger than that in the A/J animals, a magnitude consistent with the differences in FRC that we found here. They also defined an FRC in the dead mice as the lung volume at 0 transthoracic pressure, and found FRC in the C3H/HeJ and A/J mice to be 370 and 270 µl, respectively. These values are in the range of what we found over time in the same strains in vivo, even considering the slight overestimation that results from tidal breathing.
This correspondence between the FRC measurements in living and dead mice was not entirely expected. Although FRC in most mammals is thought to result from a balance between the inward recoil of the lungs and outward recoil of the thoracic cages, in mice it was suggested by Leith (9) that FRC might be determined by active tone in the inspiratory musculature, that is, because mice have evolved with extremely compliant chests that provide them the ability to be able to squeeze through tiny holes. This high thoracic wall compliance thus essentially eliminates any outward recoil of the relaxed chest wall. Indeed, recent measurements of chest wall compliance in mice have confirmed that there is negligible recoil in the physiological range of thoracic volumes (13). It thus seems likely that mice would need to regulate their FRC with some active inspiratory tone, but the relative consistency we observed between FRC values in anesthetized spontaneously breathing mice and that in mice postmortem suggests that only small amounts of inspiratory tone might be required. In mechanically ventilated mice with exogenous muscle paralysis, one might expect the FRC to be lower than in the intact situation in vivo. We have observed this anecdotally in other experiments using anesthetized ventilated mice (1, 5). In such animals, even with an intact chest wall, dynamic lung compliance increases as the level of end-expiratory pressure is increased up to about 3 cmH2O (unpublished observations). This observation not only supports the idea that some active inspiratory muscle tone sets FRC in vivo, but also suggests that experimentally ventilated intact mice with no added positive end-expiratory pressure (PEEP) or periodic deep inspirations may become progressively stiffer with increasing atelectasis or airway closure.
We also found that the A/J strain has an additional 3.5% of total lung volume in the left lung, compared with the C3H/HeJ strain. We know of no functional significance to this observation, and there is scant information in the published literature with which this finding can be compared. Our measured quantitative volume fractions at FRC in the mouse left lung of 36.9 and 40.4% in C3H/HeJ and A/J, respectively, can be compared with that at TLC in 14-wk-old rabbits, where Yee and Hyatt (15) found 43% of total lung volume in the left lung. In two male human subjects scanned with an Imatron electron beam CT scanner, Hoffman found 45 and 46% of the lung air volume in the left lung at FRC (Eric Hoffman, personal communication). Thus it appears that mice may have a smaller fraction of the lung volume on the left, perhaps reflecting a larger heart or more leftward location. Why different mouse strains would have lung volume partitioned differently between left and right lungs is not clear, but if the airway branching at the carina were different, there could conceivably be some functional effect of air pollutants in the left and right lungs. Cardiac size might also be expected to be larger in the strain with a smaller fraction of lung volume on the left side.
Although the animals of both strains were continually growing during the 8-wk study period, the lung growth (as evidenced by lung volume) appears to be nearly complete by 6 wk of age. Thus the lung volume-to-body weight ratio rises slightly from 4 to 6 wk in both strains, then is stable or falls slightly with age. Because the body weights of the two strains were not very different, the higher lung volumes in the C3H/HeJ strain persist even when normalized to body weight.
In conclusion, we have shown that accurate, reproducible measurements of FRC in breathing mice can be made using a standard clinical CT scanner. This method may be useful for noninvasively assessing both structural and functional changes in the lungs of experimental mice.
| FOOTNOTES |
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Address for reprint requests and other correspondence: W. Mitzner, The Johns Hopkins Medical Institutions, 615 N. Wolfe St., Baltimore, MD 21205 (E-mail: wmitzner{at}jhsph.edu).
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