Effect of Lung Volume Reduction Surgery on Pulmonary Diffusion Capacity in a Rabbit Model of Emphysema 1

BACKGROUND
While there is renewed interest in lung volume reduction surgery (LVRS) for treatment of emphysema, many aspects of the operation such as patient selection and surgical end points of excision are uncertain. We studied the effects of LVRS on measured lung volumes and diffusion capacity in an animal model to investigate optimal resection volumes.


METHODS
Emphysema was induced in 32 New Zealand white (NZW) rabbits using aerosolized elastase. Helium dilution lung volumes and single breath DLCO were measured concurrently at baseline, following induction of emphysema (preop), and 1 week postoperatively (postop) following LVRS. Bilateral upper and middle lobe stapled lung resections were performed through midline sternotomies with excision of variable amounts of lung tissue from 1.8 to 5.8 g.


RESULTS
FRC increased following induction of emphysema and decreased postoperatively. DLCO improved with increasing lung tissue resection up to 3 g of tissue and then decreased as even greater amounts were removed (r = 0.54).


CONCLUSIONS
Measured lung volumes increase with development of emphysema and appropriately decrease in response to LVRS in this rabbit model. DLCO improves with moderate resection but then decreases with excessive excision of lung quantities and may help define one physiologic operative end point. In this rabbit model, excision of approximately 30% of lung volume was optimal and prevented further decrease in diffusion capacity.


Background. While there is renewed interest in lung volume reduction surgery (LVRS) for treatment of em-
Lung volume reduction surgery (LVRS) has rephysema, many aspects of the operation such as patient cently been recognized as a potential palliative treatselection and surgical end points of excision are uncerment for refractory obstructive emphysema.Howtain.We studied the effects of LVRS on measured lung volumes and diffusion capacity in an animal model to ever, many aspects of LVRS procedures remain uninvestigate optimal resection volumes.Methods.Emphycertain and require further investigation.There is sema was induced in 32 New Zealand white (NZW) rabcurrently little clinical data available regarding exbits using aerosolized elastase.Helium dilution lung volchange of gases at the alveolar level following surgiumes and single breath DLCO were measured concurcal lung volume reduction.

rently at baseline, following induction of emphysema
Gas exchange is a function of accessibility to the alve-(preop), and 1 week postoperatively (postop) following olar-capillary bed as well as the membrane characteris-LVRS.Bilateral upper and middle lobe stapled lung retics of the alveolar lining.Obstructive emphysema desections were performed through midline sternotomies creases diffusion capacity by decreasing the ventilating with excision of variable amounts of lung tissue from 1.8 compartment of the lung, thereby decreasing the availto 5.8 g.Results.FRC increased following induction of able exchange surface area.Total diffusion capacity as emphysema and decreased postoperatively.DLCO immeasured by the diffusion of carbon monoxide (DLCO) proved with increasing lung tissue resection up to 3 g is decreased, as is alveolar volume adjusted DLCO of tissue and then decreased as even greater amounts (DLCO/VA) [1].
were removed (r Å 0.54).Conclusions.Measured lung LVRS has been suggested to improve airway support volumes increase with development of emphysema and and thereby increase expiratory flows [2].Volume reappropriately decrease in response to LVRS in this rabbit model.DLCO improves with moderate resection but duction is designed to remove nonventilating or poorly then decreases with excessive excision of lung quantities ventilating lung segments.How this intervention afand may help define one physiologic operative end fects the lung's overall ability to diffuse respiratory point.In this rabbit model, excision of approximately gasses is not well documented.

30% of lung volume was optimal and prevented further
End points for optimal resection volumes during sur- against potential reduction in gas exchange capabilities as larger volumes of lung tissue are resected.
For these reasons, we studied the effects of LVRS on measured lung volumes and single breath DLCO in a rabbit model of elastase-induced obstructive emphy-Lung volume reduction surgery.LVRS was performed 4 weeks

METHODS
following elastase induction of emphysema.The anesthetized and intubated rabbits were shaved and placed in a supine position.This protocol was approved by the Institutional Animal Care and Twenty-three rabbits underwent resection of varying quantities of Use Committee at the University of California, Irvine.All rabbits lung tissue.Nine control rabbits underwent the operative procedure were cared for in accordance with the NIH Guidelines for the Care with no excision of lung tissue (sham surgery).and Use of the Laboratory Animal.
Hypothermia was prevented with a surgical warming pad, and lactated Ringers solution was infused through an iv catheter in a Animal preparation.Thirty-two rabbits (3.0-4.5 kg) were anesthetized with 2:1 mixture of ketamine HCl (100 mg/ml):xylazine (20 marginal ear vein at 5-15 cc/h.The rabbits were mechanically ventilated.Oxygen saturation (Ohmeda Biox 3700 Pulse Oximeter, BOC mg/ml) at a dose of 0.75 ml/kg im.The rabbits were intubated with a 3-mm endotracheal tube and mechanically ventilated (Harvard Health Care), tidal CO 2 (Ohmeda 5200 CO 2 Monitor, BOC Health Care), and EKG (Hewlett Packard 78353B Continuous EKG Monitor, Apparatus Dual Phase Control Respiratory Pump-Canine, Harvard Co., South Natic, MA) with a tidal volume of 50 ml and a BioMedical Services) were monitored continuously.The chest was shaved, prepped with Betadine, and draped ster-frequency of 30-40/min.A 20-gauge iv catheter was placed in a marginal ear vein for iv access.Anesthesia was maintained with 0.3 ilely.The thorax was entered through a median sternotomy.Bilateral upper and middle lobes were excised using a linear thoracoscopic ml of 1:1 mixture of ketamine HCl (100 mg/ml):xylazine (20 mg/ ml) given as iv bolus as needed to maintain apnea throughout all stapler (Endopath ELC, Ethicon Endo-Surgery) with 3.5-mm staples.
Target quantity of lung tissue removed was 2-6 g.The quantity of procedures.
excised lung weight was carefully escalated.The excised lung tissue Induction of emphysema.Emphysema was induced in 32 rabbits weights were obtained intraoperatively to assess adequate target under general anesthesia by aerosolizing 15,000 units (7.89 ml) of resection.In the sham operations, no lung tissue was excised.Hemoporcine elastase (Product E1250, Sigma Chemical Company, St. stasis was obtained and a 12 Fr neonatal chest tube was placed Louis, MO) through the endotracheal tube over approximately 1 h.
under direct visualization into each pleural space.The two chest The nebulizer (Respirgard, Marquest Medical Products, Inc., Engletubes were connected to a 10-cm water suction.The sternum was wood, CO) was placed in the inspiratory arm of the ventilator circuit closed with 0 Silk and the chest wound closed in layers with absorbawith the tidal volume provided by the ventilator set at zero with a ble monofilament sutures.The rabbits were awakened from anestherate of 30 breaths/min.The O 2 flow through the nebulizer was adsia and extubated.There was usually a small airleak in the chest justed to maintain the peak airway pressure at 20 cm H 2 O, monitored tubes but all leaks sealed spontaneously within 1 h.All chest tubes by a pressure gauge placed at the side port of the endotracheal tube were removed within 1 h.which provided the tidal volume during induction.
Histologic preparation.The animals were sacrificed at 1 week Pulmonary function testing.Lung function measurements were following LVRS.The lungs were removed en bloc and inflated with obtained at baseline prior to induction of emphysema, immediately formalin (20 cm pressure) for histologic preparation.The lung secpreoperatively at 4 weeks following induction of emphysema, and 1 tions were prepared at 0.2 to 0.4 cm thickness and embedded in week postoperatively.Static lung compliance and gas dilution lung paraffin.Slides were stained with hematoxylin-eosin and studied volumes were measured at each time interval.by light microscopy.Gas dilution lung volumes.The inhalation gasses consisted of Statistical analysis.All helium dilution lung volume data and 9.30% helium, 60.50% oxygen, 29.05% nitrogen, 0.87% C 2 H 2 , and DLCO data for each rabbit were tabulated corresponding to baseline, 0.28% C 18 O (Liquid Carbonics Corp., Los Angeles, CA).All gas conpreoperative, and postoperative measurements.Comparisons of centrations were measured continuously by an on-line mass specbaseline to preoperative values and preoperative to postoperative trometer (MGA 1100, Perkins-Elmer Corp., Pomona, CA).Analog values were made using paired Student's t test.The percentage data were converted to digital information by an AD converter change in DLCO from preoperative to postoperative data was ana-(Keithley System 570, Cleveland, OH) sampling at 20 Hz and stored lyzed and graphed in relation to the excised lung weights.A best-fit on an IBM personal computer.
curve was generated from a polynomial equation and correlation The anesthetized and intubated rabbits were taken off the ventilavalue calculated.tor and placed in a left decubital position.The sampling tube of the mass spectrometer was connected to the side port of the endotracheal tube through which the inspired and expired gas concentrations were

RESULTS
continuously measured.A syringe was filled to 60 cc with inhalation gasses and connected to the endotracheal tube.A multibreath helium dilution maneuver was performed by manually insufflating and re-Mortality from elastase induction of emphysema was moving 50 cc of tidal volume by the syringe for 10 breaths at an 10%.Rabbits died from acute pulmonary hemorrhage approximate rate of 20-30 breaths/min.The initial and final helium or pneumothorax following induction.Of the 23 rabbits concentrations were used to calculate the functional residual capacsurviving induction, all survived LVRS.Excised lung ity (FRC).Two measurements of FRC were obtained at each trial and averaged.The rabbits were returned to mechanical ventilation mass ranged from 1.85 to 5.81 g.Nine rabbits underfollowing each procedure.
went sternotomy with no lung resection.

Microscopic evaluation confirmed histologic evidence of
Five-second breath-hold DLCO maneuvers were performed following moderate to severe emphysema in all rabbits (Fig. 1).
the above FRC measurement on each rabbit.All gas concentrations Helium dilution FRC volumes showed an increase in were measured continuously through the mass spectrometer.Of the FRC in response to induction of emphysema.LVRS inhalation gas 60 cc was insufflated into the lung through the endotracheal tube and held for 5 s.Of the inspired volume 30 cc was returned these volumes toward preemphysema values then withdrawn and held to measure the gas concentrations at 50% (Table 1).expired volume.All data were sampled and digitized at 20 Hz.

DLCO was adjusted for the VA (alveolar volume).
For analysis, the breath hold time was measured from 0.5 s from Both the DLCO and DLCO/VA decreased in response the start of inspiration to 30 cc of exhalation.The duration of inhalato induction of emphysema.The effect of LVRS in tion was rapid and peak concentrations were achieved within 1 s.
The initial helium and C 18 O concentrations were measured at their DLCO was evaluated as percentage change in DLCO respective concentration plateaus following gas insufflation.The fifrom preoperative to postoperative values.Sham nal gas concentrations were measured at 30 cc of exhalation.DLCO LVRS caused a further decrease in the average DLCO was calculated from the standard formula and corrected to STPD. and DLCO/VA (Table 1).As larger volumes of lung Adjustments were made for the rabbit body temperature and water vapor pressure.
were excised, the DLCO increased until an average of 2.8 g was resected.At progressively higher resections actual volume reduction showed declining diffusion capacity.In contrast, the diffusion capacity of the above 2.8 g, DLCO decreased again to below preoperative values (Fig. 2).
rabbits that underwent excision of 2 -3 g of lung tissue showed no deterioration, and therefore a benefi-The rabbits with emphysema that did not receive cial effect, in DLCO from LVRS.However, the diffu-improvement in spirometry following LVRS in animals with severe emphysema.However, beyond some point, sion capacity of the rabbits that underwent greater than 3 g showed a significant decrease in DLCO from lung function would be expected to deteriorate as greater resected tissue volumes would leave too little preoperative to postoperative values (Fig. 3).
residual lung parenchyma to function adequately.Analogous predictions would also be made for gas ex-DISCUSSION change properties and diffusing capabilities of the lung.This elastase-induced model of pulmonary emphy-A major issue requiring investigation is whether optisema has been shown to have increased static complimal volumes of tissue removal for spirometric improveance and decreased expiratory flows with development ment are similar to the optimal volume of tissue of emphysema [3,4].We have previously shown that removal for the best gas exchange measurement LVRS reverses these changes by decreasing compliance outcomes.Therefore, we examined the relationship beand improving expiratory flows in these animals [4].tween improvement in spirometry as a function of How LVRS affects diffusion capacity of the lungs has amount of lung tissue removed and compared this to not been previously studied in animal models.It is eschange in DLCO in this model.sential to evaluate the relationship between lung me-Measurement of diffusing capacity requires concurchanics and gas transfer functions following LVRS in rent measurement of alveolar lung volumes.In this order to optimize surgical approaches.study, multibreath helium dilution lung volume mea-It is predictable that, within limits, removal of increasing amounts of lung tissue would result in greater and greater than 3 g (n Å 10).02616.8/ 3125.4 x / 01352.7 x 2 / 252.17 x 3 / 017.2 x 4 , r Å 0.54).
g) had improvement in DLCO compared to controls (P õ 0.05).Animals with very large volume resections (ú3 g) had reduction in DLCO.
Given the small number of animals in our trials, there is a fair amount of variability in our data.Furthermore, predicting the optimal range of lung mass resection is probably a complex function of lung spirometry, oxygenation, pulmonary pressures, and other outcome variables.Although we controlled for the amount of elastase aerosolized, there was a variable amount of induced emphysema in individual rabbits which may have added further variability to our data.
Similarly clinical LVRS may be attributed to excision of a larger proportion of nonventilating lung volume in humans.Removal of lung tissue that does not participate in gas exchange does not decrease diffusion capacity, but can surements were used.Helium dilution lung volume may be inaccurate in severe obstructive lung disease actually increase diffusion capacity by increasing ventilatory flows to previously partially obstructed areas.due to localized air trapping.However, helium dilution appears to be accurate in this animal model as evi- The optimal resected lung volume of 2-3 g in this model for improvement in DLCO is in sharp contrast denced by flat helium dilution curves within 5 to 10 s of rebreathing.This rapid equilibration likely reflects to the optimal spirometry response to tissue removal.
In this same animal model, larger lung volume resec-the small size and rapid respiratory rate capabilities of rabbit lung (Fig. 4).
tions were associated with progressively greater improvements in elastic recoil, expiratory flows, and Diffusing capacity reflects the ability of alveolar membranes to exchange pulmonary gases.In obstruc-residual volumes.These findings suggest that gas exchange measures, rather than deterioration in spirom-tive lung disease, air trapping, airway disease, and ventilation perfusion inequality in lung units can effec-etry, may define the upper limits of optimal tissue resection for LVRS emphysema surgery.Such findings tively remove the involved regions from participating in gas exchange.This may lead to decreases in mea-will need to be confirmed in humans.
There are a number of limitations to the current sured gas diffusing capacity.We used single breath helium DLCO measurement techniques in this study study techniques.Single breath DLCO measures may still be limited in accuracy in this animal model of ob-analogous to those used in patients with emphysema undergoing LVRS.DLCO measurements were reliable structive lung disease due to regional inhomogeneity.
The lung tissue removed was measured by weight and reproducible in these studies.
There are several key points illustrated by the data rather than true volume.There is currently no method for measuring the exact volume of lung tissue removed.from this study.First, the elastase-mediated destruction of lung parenchyma is associated with a decrease Resected lung weight and volume may not correlate well, particularly in severely diseased, hyperinflated in DLCO.The coalescence of the air spaces in the lung decreases the surface area for gas exchange.Second, lung regions.The disease distribution patterns in rabbits may not be exactly analogous to those in humans.DLCO remains stable following reduction of appropriate lung tissue volumes.In marked contrast, the There is regional heterogeneity although the degree of localization of airtrapping as well as the overall extent control rabbits undergoing sternotomy with no lung resection (sham surgery) continue to experience deteri-of emphysema is not as extensive as seen in human disease.The emphysema is centri-lobular and diffusely orating diffusion capacity, probably due to progression of emphysema.This suggests that LVRS has a benefi-distributed.Rabbit lung function was measured in the lateral decubitus position in this study.Finally, chest cial effect in lung mechanics, and that this improvement in lung mechanics can compensate for the contin-wall compliance is considerably greater in rabbits than in humans and may affect the relationship between uing decrease in diffusing capacity.
This model also demonstrates possible critical end excised tissue and resultant changes in lung volume.Nonetheless, despite these limitations, this study eluci-points for lung volume reduction.Resection beyond 3 g of lung tissue resulted in further deterioration of dif-dates important issues regarding measurement end points to be followed when assessing optimal LVRS fusion capacity.As predicted, animals with sham surgery or small volume tissue resection (õ2 g) had de-techniques.
Lung volume reduction surgery has resulted in im-creases in DLCO following surgery.In contrast, animals with moderate-sized resected lung volumes (2-3 provement in physiologic and functional status in some
FIG. 4. Multibreath helium lung volume measurement.Helium 7].The improvements in diffusion capacity following reaches a stable final concentration within four to five breaths with no delayed mixing.

decrease in diffusion capacity. ᭧ 1998 Academic Press gery
are not known.Spirometric improvement with in-

Key Words: lung volume reduction surgery; rabbit creasing
lung volume resection may have to be weighed