Copyright ©ERS Journals Ltd 2002 Effects of lung volume reduction surgery in hamsters with elastase-induced emphysema1 Respiratory Muscle Research Unit, Laboratory of Pneumology, 2 Dept of Thoracic Surgery, 3 Dept of Histopathology, University Hospitals, Katholieke Universiteit Leuven, Leuven and 4 Dept of Pneumology, Mont-Godinne Université Catholique de Louvain University Hospital, Yvoir, Belgium CORRESPONDENCE: M. Decramer, Respiratory Muscle Research Unit, Laboratory of Pneumology, University Hospitals, Katholieke Universiteit Leuven, B-3000, Leuven, Belgium. Fax: 32 16347126. E-mail: marc.decramer@uz.kuleuven.ac.be Keywords: airway obstruction, emphysema, maximal expiratory flow rate, pneumonectomy, respiratory mechanics
Received: September 6, 2001
This study was supported by FWO-Vlaanderen grant G.0175.99 and KULeuven Research Foundation grant OT98/27. G. Gayan-Ramirez is postdoctoral fellow of the FWO-Vlaanderen.
Lung volume reduction surgery (LVRS) has been shown to improve respiratory mechanics in selected patients with severe emphysema. This is thought to be due to an improvement in lung elastic recoil. This study was aimed at gaining further understanding about the effects of LVRS on respiratory mechanics and airway function. Control hamsters instilled with saline (Ctrl; n=8) were compared with emphysematous animals that underwent either a sham operation (Sham; n=7) or an LVRS (LVRS; n=7). As expected, there was a significant increase in the static lung volumes in the Sham as compared to the Ctrl group and a significant decrease of these volumes in LVRS as compared to the Sham group. Surprisingly, emphysema was associated with a significant increase and LVRS with a significant decrease in vital capacity. Despite a tendency toward an increase in lung compliance as compared to Sham, indices of maximal expiratory flows tended to decrease with LVRS. As opposed to humans, there was no change in the distribution of airway diameters in Sham compared to Ctrl. These findings appear to be largely explained by the high compliance of the hamster chest wall. This allows for better matching between the emphysematous lung and the chest-wall sizes than in humans. Lung volume reduction surgery (LVRS) has been shown to have a beneficial effect on lung function, submaximal exercise capacity and quality of life in selected patients with severe emphysema 13, although individual results classically demonstrate large scatter 1. Initially, it was proposed that the increase in maximal expiratory flow (MEF) observed after LVRS was mainly due to an improvement in lung elastic recoil after the surgical procedure 4. Although attractive, this hypothesis does not completely explain the increase in MEF in every patient 4, nor does it explain the increase in vital capacity (VC) generally observed after LVRS 5. Recently, Fessler and Permutt 5 proposed a mathematical model whereby they demonstrated that other factors were critical for the improvements in lung function after LVRS. According to their model, they predicted that the mismatch between the lung and the chest-wall dimensions were the most important determinants of airflow limitation in patients with severe emphysema. Hence, the effects of LVRS on airway function would be largely due to improvement of that match. Interestingly, pathological studies have provided evidence that a competition for space between emphysematous zones and airways could occur in emphysematous lungs. Using three-dimensional reconstructions of the small airways and septal system, Verbeken et al. 6 suggested that as well as intrinsic airway narrowing due to inflammation and loss of elastic lung recoil, the competition for space could also cause flow limitation. The aforementioned mismatch between lung and chest-wall volumes could also play an important role in these pathological findings. In an attempt to better understand the determinants of improvement in respiratory mechanics following LVRS, the effects of LVRS on respiratory mechanics, airway function and morphometry were assessed in a model of elastase-induced emphysema in hamsters. In contrast to humans, hamsters have a highly compliant chest wall. Accordingly, the mismatch between lung and chest-wall dimensions and the competition for space within the lung would be less marked than in emphysematous humans. Following the model of Fessler and Permutt 5 this would lead to a less marked or even no improvement in airway function after LVRS.
Study animals Male 9-week-old Syrian golden hamsters were purchased. One week after purchase, the animals were anaesthetized with sodium pentobarbital (Nembutal, Sanofi Santé Animale Benelux, Brussels, Belgium; 60 mg·kg1 body weight (bw) i.p.) and underwent transoral tracheal intubation with a 16GA catheter (Insyte-W catheter, Becton Dickinson, Madrid, Spain). Hamsters were instilled on a random basis with either porcine pancreatic elastase (Sigma 40 U·100 g bw1 and sodium chloride (0.9%) added to a total volume of 0.4 mL, n=20) or 0.4 mL sodium chloride (0.9%) (n=8, control (Ctrl)). One of the animals instilled with elastase died soon after extubation due to tracheal perforation. Eight animals were kept in each cage and provided with standard laboratory chow and water at will. They were the same as those studied in a previous paper on the effects of LVRS on the diaphragm 7. All experiments received the approval of the animal experiments committee of the Medical Faculty of the Katholieke Universiteit Leuven.
Study design
Surgical procedure Three hamsters of the LVRS group died soon after the end of the surgical procedure. Autopsy revealed a significant amount of blood in one of the pleural cavities in the three cases. A fourth animal of the LVRS group died on the fifth operative day in a comatose state. Air leakage was never a problem.
Lung volume measurements The forced expirations were performed using chest compression. For this purpose, the body plethysmograph was used as an airtight box in order to induce negative pressure (and inspiration) and positive pressure (expiration). The animals were hyperventilated in order to induce apnoea. Negative pressure was then applied to the body box with a large syringe airtight-connected to the body box. Once a pressure of 30 cmH2O was reached, air was abruptly pumped in the body box to induce a pressure >30 cmH2O. Volume changes applied to the syringe were standardized in order to achieve reproducibility in pressure changes in the body box. Flow was measured at the airway opening during the forced expiratory manoeuvre. The following indices were used to analyse forced expiration: the time needed to exhale 75% of the forced vital capacity (FVC), defined as the difference between maximal lung volume before the forced expiration and minimal volume at the end of expiration; forced expiratory volume in 100 ms (FEV0.1); and maximal mid-expiratory flow (MMEF), defined as the mean maximal expiratory flow between 2575% of FVC. All measurements were performed at least in triplicate in order to achieve three reproducible measurements. These were averaged for statistical analysis.
Airway morphometry
Statistical analysis The three groups were compared using a one-way analysis of variance (ANOVA) test and a Newman-Keuls multiple comparison post-test for data with a Gaussian distribution. A Kruskal-Wallis test with a Dunn's multiple comparison post-test was used for data without a Gaussian distribution. The distribution of airway diameters was analysed using a Kolmogorov-Smirnov test. The significance level ( ) was set at 0.05.
Lung volumes As previously reported 7, both FRC and TLC25 differed significantly between groups. As can be seen from table 1
Lung compliance As expected, CL was highly increased in Sham as compared to Ctrl. Although not reaching statistical significance (p=0.09), there was a tendency for a decrease in CL after LVRS. The same pattern of changes was observed for CRS. Because of a smaller standard error, however, differences were significant between Sham and LVRS (table 2
Expiratory flows As shown in figure 2
Airway morphometry Figure 3
The present study allowed several important observations to be made. First, LVRS had no beneficial effects on MEF in hamsters with elastase-induced emphysema. Secondly, elastase-induced emphysema in hamsters was associated with changes in static lung volumes that were strikingly different from those found in human emphysema. Thirdly, the changes in airway morphometry observed in the present study were also less marked than in human emphysema. All these observations point to important distinctions between respiratory system mechanics of emphysematous humans and hamsters. Its hypothesized that the difference in chest-wall compliance (CW) is the main determinant for the dissimilarities between humans and hamsters. An important difference between emphysema in hamsters and humans is the smaller effect of LVRS on MEF and the lack of positive effect of LVRS in hamsters. LVRS in emphysematous hamsters was associated with a reduction of expiratory flow limitation. The model of Fessler and Permutt 5 predicts that the per cent increase in FEV1 would be directly related to the RV/TLC ratio and inversely related to the slope of the pressure/volume curve of the chest wall under conditions of maximal inspiratory muscle activity (CWmax)/CL.
The mean RV/TLC ratio in Sham was 26%. This compares to values of
As with other studies 10, 16, it was found that CW was higher than CL in control hamsters. As expected, the CL/CW ratio decreased after induction of emphysema because of the increase in CL (median value in Sham group: 0.74). For obvious reasons, CWmax could not be measured. It is reasonable, however, to postulate that CWmax is directly related to CW. Accordingly, the CWmax/CL ratio should also be very low in emphysematous hamsters as compared to humans. Considering that emphysematous hamsters have a low RV/TLC ratio and a high CWmax/CL ratio, the model of Fessler and Permutt 5 predicts a lack of improvement in airflow obstruction after LVRS.
Another striking difference between emphysematous hamsters and humans relates to VC. An increase in VC in Sham as compared to Ctrl and a significant decrease after LVRS was found. According to the model of Fessler and Permutt 5 and postulating an absence of airway tone (and, accordingly, a transpulmonary pressure of airway closure (PTM)=0):
The analysis of bronchial diameters on lung sections also showed striking differences as to the effects of emphysema in humans and hamsters. In humans, it has been shown that the distribution of the diameters of the peripheral airways is displaced towards smaller values in emphysematous lungs as compared to normal or senile lungs 6, 17. Using three-dimensional reconstruction of the septal system of human lungs, Verbeken et al. 18 were able to demonstrate that in addition to intrinsic inflammation and loss of elastic recoil, a competition for space between emphysematous zones and peripheral airways played a role in these findings and in airflow limitation. In the present study, the displacement of the distribution of peripheral airways diameters was also observed, but appeared to be much less pronounced than in humans. This can be seen when comparing figure 3
The authors believe that the high compliance of the chest wall in hamsters could explain these differences. As others have previously found 10, 16, CW (data not shown) was higher than CL in hamsters. Hence, the competition for space is predicted to be absent when elastase-induced emphysema develops. The repartition of the airway diameters observed in figure 3
The model used in the present study has several potential limitations. First, respiratory mechanics demonstrate significant differences with humans. These differences probably explain the relatively limited reduction in indices of MEF in emphysematous hamsters. However, as discussed earlier, these differences allowed for the testing of the hypotheses, which would be difficult to do in humans since respiratory mechanics are difficult to manipulate in severe emphysema. Secondly, the elastase-induced emphysema is diffuse and very closely simulates human panlobular emphysema 19. LVRS in panlobular emphysema associated with To conclude, the application of lung volume reduction surgery in emphysematous animals with strikingly different respiratory mechanics as compared to humans, allowed for the demonstration that an increase in elastic lung recoil is not sufficient to improve airway function in emphysema. This indirectly supports the assumption that the mismatch between lung and chest-wall compliances is an important determinant of the expected improvement in airway function after lung volume reduction surgery in humans 5. In the present study, it was hypothesized that the low slope of the pressure/volume curve of the chest wall under conditions of maximal inspiratory muscle activity/lung compliance ratio (and high residual volume/total lung capacity ratio) observed in humans compared to hamsters, actually promotes a competition for space within the thoracic cage when emphysema develops, thereby hampering airway function. These ratios are respectively much higher and much lower in emphysematous hamsters as is the competition for space. This was supported by the results of the airway morphometry study. The concepts derived from this study may have practical implications for the selection of patients for lung-volume reduction surgery. A high residual volume/total lung capacity volume as well as a low slope of the pressure/volume curve of the chest wall under conditions of maximal inspiratory muscle activity/lung compliance ratio, should be considered as predictors of good response in terms of forced expiratory volume in one second. An alternative for the high residual volume/total lung capacity ratio could be a low, slow vital capacity. Prospective clinical studies testing these suggestions are warranted.
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