Recruitment of lung diffusing capacity with exercise before and after pneumonectomy in dogs

Is the Lung Built for Exercise? Advances and Unresolved Questions

ABSTRACT

Nearly 40 yr ago, Professor Dempsey delivered the 1985 ACSM Joseph B. Wolffe Memorial Lecture titled: "Is the lung built for exercise?" Since then, much experimental work has been directed at enhancing our understanding of the functional capacity of the respiratory system by applying complex methodologies to the study of exercise. This review summarizes a symposium entitled: "Revisiting 'Is the lung built for exercise?'" presented at the 2022 American College of Sports Medicine annual meeting, highlighting the progress made in the last three-plus decades and acknowledging new research questions that have arisen. We have chosen to subdivide our topic into four areas of active study: (i) the adaptability of lung structure to exercise training, (ii) the utilization of airway imaging to better understand how airway anatomy relates to exercising lung mechanics, (iii) measurement techniques of pulmonary gas exchange and their importance, and (iv) the interactions of the respiratory and cardiovascular system during exercise. Each of the four sections highlights gaps in our knowledge of the exercising lung. Addressing these areas that would benefit from further study will help us comprehend the intricacies of the lung that allow it to meet and adapt to the acute and chronic demands of exercise in health, aging, and disease.

What are appropriate values of relative krogh diffusion Constant of NO against CO and of theta-NO in alveolar septa?

OBJECTIVES

To propose new physical constants for NO and CO (Krogh diffusion constant ratio (KD_NO/CO) and specific blood conductance for NO (θ_NO)) for calculating DMCO and Vc, according to Roughton-Forster's equation (Roughton and Forster, J. Appl. Physiol. 11: 290-302, 1957) from simultaneous DLNO and DLCO measurements.

RESULTS AND CONCLUSIONS

(1) The Graham's law is unacceptable for determining KD_NO/CO because CO does not fulfil all the conditions of an "ideal" gas. We have re-estimated KD_NO/CO in a new way based on difference in molar volumes of two gases (molar volume theory). The KD_NO/CO thus decided is 2.34. (2) θ_NO measured with rapid-reaction, constant-flow method by Carlsen and Comroe (J. Gen. Physiol. 42: 83-107, 1958) may be underestimated by about 40 % due to unstirred water layer surrounding the erythrocyte. (3) Erythrocyte θ_O2 can be harvested from O₂ release kinetics in presence of high concentration of dithionite, which effectively removes the unstirred water layer-elicited effect. Multiplication of erythrocyte θ_O2 by erythrocyte KD_NO/O2 equals erythrocyte θ_NO, the value of which is 6.2 mL/min/mmHg/(mL⋅blood). According to the concepts of Kang et al. (RESPNB. 241: 62-71, 2017) and Borland et al. (RESPNB. 241: 58-61, 2017), in vitro θ_NO decided from rapid-mixing experiments may mirror bulk absorption of NO by erythrocytes. (4) In pulmonary capillaries, NO uptake takes place predominantly in the surface rim of the erythrocyte. This surface absorption of NO increases the θ_NO 10-fold versus bulk absorption of NO to about 60 mL/min/mmHg/(mL⋅blood).

Bioengineering of Pulmonary Epithelium With Preservation of the Vascular Niche

The shortage of transplantable donor organs directly affects patients with end-stage lung disease, for which transplantation remains the only definitive treatment. With the current acceptance rate of donor lungs of only 20%, rescuing even one half of the rejected donor lungs would increase the number of transplantable lungs threefold, to 60%. We review recent advances in lung bioengineering that have potential to repair the epithelial and vascular compartments of the lung. Our focus is on the long-term support and recovery of the lung ex vivo, and the replacement of defective epithelium with healthy therapeutic cells. To this end, we first review the roles of the lung epithelium and vasculature, with focus on the alveolar-capillary membrane, and then discuss the available and emerging technologies for ex vivo bioengineering of the lung by decellularization and recellularization. While there have been many meritorious advances in these technologies for recovering marginal quality lungs to the levels needed to meet the standards for transplantation – many challenges remain, motivating further studies of the extended ex vivo support and interventions in the lung. We propose that the repair of injured epithelium with preservation of quiescent vasculature will be critical for the immediate blood supply to the lung and the lung survival and function following transplantation.

Cournand's and Richards' Post-Nobel Challenge: "Do Anything on the Structure of the Lung That Is of Interest for Physiology"

In their Nobel Lectures of 1956, the two cardiopulmonary physiologists André Cournand and Dickinson Richards described the future projects that they considered important for rounding up their impressive scientific achievements. One of them called for work that brought together structure and function of the lung in a systematic way. They challenged a young anatomist to undertake this in their cardiopulmonary laboratory at Bellevue Hospital. The first steps established the need for quantitative information on the structures that form the pulmonary gas exchanger as well as the airway tree. This resulted in a new approach to lung anatomy: morphometry would provide the quantitative information for modeling structure-function relations in the lung, and eventually in the entire respiratory system, thus allowing a rational analysis of how structures affect or limit functions.

Pulmonary capillary surface area in supine exercising humans: demonstration of vascular recruitment

In exercising humans, cardiac output (CO) increases, with minor increases in pulmonary artery pressure (PAP). It is unknown if the CO is accommodated via distention of already perfused capillaries or via recruitment of nonconcomitantly perfused pulmonary capillaries. Ten subjects (9 female) performed symptom-limited exercise. Six had resting mean PAP (PAPm) <20 mmHg, and four had PAPm between 21 and 24 mmHg. The first-pass pulmonary circulatory metabolism of [³H]benzoyl-Phe-Ala-Pro (BPAP) was measured at rest and at peak exercise, and functional capillary surface area (FCSA) was calculated. Data are means ± SD. Mean pulmonary arterial pressure rose from 18.8 ± 3.3 SD mmHg to 28.5 ± 4.6 SD mmHg, CO from 6.4 ± 1.6 to 13.4 ± 2.9 L/min, and pulmonary artery wedge pressure from 14 ± 3.3 to 19.5 ± 5 mmHg (all P ≤ 0.001). Percent BPAP metabolism fell from 74.7 ± 0.1% to 67.1 ± 0.1%, and FCSA/body surface area (BSA) rose from 2,939 ± 640 to 5,018 ± 1,032 mL·min^-1·m^-2 (all P < 0.001). In nine subjects, the FCSA/BSA-to-CO relationship suggested principally capillary recruitment and not distention. In subject 10, a marathon runner, resting CO and FCSA/BSA were high, and increases with exercise suggested distention. Exercising humans demonstrate pulmonary capillary recruitment and distention. At moderate resting CO, increasing blood flow causes principally recruitment while, based on one subject, when exercise begins at high CO, further increases appear to cause distention. Our findings clarify an important physiologic question. The technique may provide a means for further understanding exercise physiology, its limitation in pulmonary hypertension, and responses to therapy.

Comparative analysis of the mechanical signals in lung development and compensatory growth

This review compares the manner in which physical stress imposed on the parenchyma, vasculature and thorax and the thoraco-pulmonary interactions, drive both developmental and compensatory lung growth. Re-initiation of anatomical lung growth in the mature lung is possible when the loss of functioning lung units renders the existing physiologic-structural reserves insufficient for maintaining adequate function and physical stress on the remaining units exceeds a critical threshold. The appropriate spatial and temporal mechanical interrelationships and the availability of intra-thoracic space, are crucial to growth initiation, follow-on remodeling and physiological outcome. While the endogenous potential for compensatory lung growth is retained and may be pharmacologically augmented, supra-optimal mechanical stimulation, unbalanced structural growth, or inadequate remodeling may limit functional gain. Finding ways to optimize the signal-response relationships and resolve structure-function discrepancies are major challenges that must be overcome before the innate compensatory ability could be fully realized. Partial pneumonectomy reproducibly removes a known fraction of functioning lung units and remains the most robust model for examining the adaptive mechanisms, structure-function consequences and plasticity of the remaining functioning lung units capable of regeneration. Fundamental mechanical stimulus-response relationships established in the pneumonectomy model directly inform the exploration of effective approaches to maximize compensatory growth and function in chronic destructive lung diseases, transplantation and bioengineered lungs.

Clinical Outcome following Pneumonectomy for Management of Chronic Pyothorax in Four Cats

Pneumonectomy is the resection of all lung lobes from one side of the thorax. The clinical findings, treatment and outcome of four cases of feline chronic pyothorax managed with exploratory thoracotomy and pneumonectomy are reported. All cases were initially medically managed with thoracic drain placement and antibiosis. However, resolution was not achieved with medical therapy and diagnostic imaging findings consistent with an area of abscessation or marked lung lobe consolidation were identified, supporting a decision for surgical management. Surgical exploration was performed via median sternotomy and, on the basis of gross inspection, non-functional lung was removed. A left-sided pneumonectomy was performed in three cats and a right-sided pneumonectomy in one. All cases survived to discharge and an excellent quality of life was reported on long-term follow-up. Pneumonectomy appears to be well tolerated in the cat.

Outcome After Pneumonectomy in 17 Dogs and 10 Cats: A Veterinary Society of Surgical Oncology Case Series

OBJECTIVE

To report the signalment, presenting clinical signs, surgical complications, histologic diagnosis, postoperative complications, and outcome of dogs and cats undergoing pneumonectomy.

STUDY DESIGN

Retrospective case series; multicenter study.

ANIMALS

Client-owned dogs (n=17) and cats (n=10).

METHODS

Signalment, clinical signs, side affected, surgical data, preoperative diagnostic tests (including complete blood count, serum biochemistry, cytologic diagnosis, chest radiographs, and computed tomography), histologic diagnosis, surgical complications, adjunctive therapy, and date and cause of death were collected from records of dogs and cats that underwent pneumonectomy. Survival estimates and complication were assessed.

RESULTS

Seventeen animals had a left-sided pneumonectomy performed (12 dogs, 5 cats) and 10 animals had a right-sided pneumonectomy (5 dogs, 5 cats). Fourteen animals were diagnosed with neoplasia (52%). The overall incidence of complications for dogs and cats were 76 and 80%, respectively, with major complications in 41 and 50%, respectively. Respiratory complications (persistent pleural effusion, oxygen dependence, persistent increased respiratory rate, or coughing) were the most frequent complications. No animals died or were euthanatized intraoperative or within the first 24 hours postoperative. One dog (6%) and 2 cats (20%) died, or were euthanatized in the first 2 weeks postoperative.

CONCLUSION

Based on this case series, right and left pneumonectomy can be performed with low perioperative mortality in dogs and cats, with some animals experiencing prolonged survival.

Perfusion-related stimuli for compensatory lung growth following pneumonectomy

Following pneumonectomy (PNX), two separate mechanical forces act on the remaining lung: parenchymal stress caused by lung expansion, and microvascular distension and shear caused by increased perfusion. We previously showed that parenchymal stress and strain explain approximately one-half of overall compensation; the remainder was presumptively attributed to perfusion-related factors. In this study, we directly tested the hypothesis that perturbation of regional pulmonary perfusion modulates post-PNX lung growth. Adult canines underwent banding of the pulmonary artery (PAB) to the left caudal (LCa) lobe, which caused a reduction in basal perfusion to LCa lobe without preventing the subsequent increase in its perfusion following right PNX while simultaneously exaggerating the post-PNX increase in perfusion to the unbanded lobes, thereby creating differential perfusion changes between banded and unbanded lobes. Control animals underwent sham pulmonary artery banding followed by right PNX. Pulmonary function, regional pulmonary perfusion, and high-resolution computed tomography of the chest were analyzed pre-PNX and 3-mo post-PNX. Terminally, the remaining lobes were fixed for detailed morphometric analysis. Results were compared with corresponding lobes in two control (Sham banding and normal unoperated) groups. PAB impaired the indices of post-PNX extravascular alveolar tissue growth by up to 50% in all remaining lobes. PAB enhanced the expected post-PNX increase in alveolar capillary formation, measured by the prevalence of double-capillary profiles, in both unbanded and banded lobes. We conclude that perfusion distribution provides major stimuli for post-PNX compensatory lung growth independent of the stimuli provided by lung expansion and parenchymal stress and strain.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.

Lung regeneration and translational implications of the postpneumonectomy model

Lung regeneration research is yielding data with increasing translational value. The classical models of lung development, postnatal alveolarization, and postpneumonectomy alveolarization have contributed to a broader understanding of the cellular participants including stem-progenitor cells, cell-cell signaling pathways, and the roles of mechanical deformation and other physiologic factors that have the potential to be modulated in human and animal patients. Although recent information is available describing the lineage fate of lung fibroblasts, genetic fate mapping, and clonal studies are lacking in the study of lung regeneration and deserve further examination. In addition to increasing knowledge concerning classical alveolarization (postnatal, postpneumonectomy), there is increasing evidence for remodeling of the adult lung after partial pneumonectomy. Though limited in scope, compelling data have emerged describing restoration of lung tissue mass in the adult human and in large animal models. The basis for this long-term adaptation to pneumonectomy is poorly understood, but investigations into mechanisms of lung regeneration in older animals that have lost their capacity for rapid re-alveolarization are warranted, as there would be great translational value in modulating these mechanisms. In addition, quantitative morphometric analysis has progressed in conjunction with developments in advanced imaging, which allow for longitudinal and nonterminal evaluation of pulmonary regenerative responses in animals and humans. This review focuses on the cellular and molecular events that have been observed in animals and humans after pneumonectomy because this model is closest to classical regeneration in other mammalian systems and has revealed several new fronts of translational research that deserve consideration.

Defining a stimuli-response relationship in compensatory lung growth following major resection

Major lung resection is a robust model that mimics the consequences of loss-of-functioning lung units. We previously observed in adult canines, following 42% and 58% lung resection, a critical threshold of stimuli intensity for the initiation of compensatory lung growth. To define the range and limits of this stimuli-response relationship, we performed morphometric analysis on the remaining lobes of adult dogs, 2-3 years after surgical removal of ∼ 70% of lung units in the presence or absence of mediastinal shift. Results were expressed as ratios to that in corresponding control lobes. Lobar expansion and extravascular tissue growth (∼ 3.8- and ∼ 2.0-fold of normal, respectively) were heterogeneous; the lobes remaining next to the diaphragm exhibited a greater response. Tissue growth and capillary formation, indexed by double-capillary profiles, increased, regardless of mediastinal shift. Septal collagen fibers increased up to 2.7-fold, suggesting a greater need for structural support. Compared with previous cohorts following less-extensive resection, tissue volume and gas-exchange surface areas increased significantly only in the infracardiac lobe following 42% resection, exceeded two- to threefold in all lobes following 58% resection, and then exhibited diminished gains following ∼ 70% resection. In contrast, alveolar-capillary formation increased with incremental resection without reaching an upper limit. Overall structural regrowth was most vigorous and uniform following 58% resection. The diminishment of gains in tissue growth, following ∼ 70% resection, could reflect excessive or maldistributed mechanical stress that threatens septal integrity. Results also suggest additional independent stimuli of alveolar-capillary formation, possibly related to the postresection augmentation of regional perfusion.

Highly athletic terrestrial mammals: horses and dogs

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems--pulmonary-cardiovascular-muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise-induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood-gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities.

Predicting diffusive alveolar oxygen transfer from carbon monoxide-diffusing capacity in exercising foxhounds

Although lung diffusing capacity for carbon monoxide (DL(CO)) is a widely used test of diffusive O2 transfer, few studies have directly related DL(CO) to O2-diffusing capacity (DL(O2)); none has used the components of Dl(CO), i.e., conductance of alveolar membrane and capillary blood, to predict DL(O2) from rest to exercise. To understand the relationship between DL(CO) and DL(O2) at matched levels of cardiac output, we analyzed cumulative data from rest to heavy exercise in 43 adult dogs, with normal lungs or reduced lung capacity following lung resection, that were studied by two techniques. 1) A rebreathing (RB) technique was used to measure Dl(CO) and pulmonary blood flow at two O2 tensions, independent of O2 exchange. DL(CO) was partitioned into CO-diffusing capacity of alveolar membrane and pulmonary capillary blood volume using the Roughton-Forster equation and converted into an equivalent DL(O2), [DL(O2)(RB)]. 2) A multiple inert-gas elimination technique (MIGET) was used to measure ventilation-perfusion distributions, O2 and CO2 exchange under hypoxia, to derive DL(O2) [DL(O2)(MIGET)] by the Lilienthal-Riley technique and Bohr integration. For direct comparisons, DL(O2)(RB) was interpolated to the cardiac output measured by the Fick principle corresponding to DL(O2)(MIGET). The DL(O2)-to-DL(CO) ratio averaged 1.61. Correlation between DL(O2)(RB) and DL(O2)(MIGET) was similar in normal and post-resection groups. Overall, DL(O2)(MIGET) = 0.975 DL(O2)(RB); mean difference between the two techniques was under 5% for both animal groups. We conclude that, despite various uncertainties inherent in these two disparate methods, the Roughton-Forster equation adequately predicts diffusive O2 transfer from rest to heavy exercise in canines with normal, as well as reduced, lung capacities.

Shifting sources of functional limitation following extensive (70%) lung resection

We previously found that, following surgical resection of approximately 58% of lung units by right pneumonectomy (PNX) in adult canines, oxygen-diffusing capacity (Dl(O(2))) fell sufficiently to become a major factor limiting exercise capacity, although the decline was mitigated by recruitment, remodeling, and growth of the remaining lung units. To determine whether an upper limit of compensation is reached following the loss of even more lung units, we measured pulmonary gas exchange, hemodynamics, and ventilatory power requirements in adult canines during treadmill exercise following two-stage resection of approximately 70% of lung units in the presence or absence of mediastinal distortion. Results were compared with that in control animals following right PNX or thoracotomy without resection (Sham). Following 70% lung resection, peak O(2) uptake was 45% below normal. Ventilation-perfusion mismatch developed, and pulmonary arterial pressure and ventilatory power requirements became markedly elevated. In contrast, the relationship of Dl(O(2)) to cardiac output remained normal, indicating preservation of Dl(O(2))-to-cardiac output ratio and alveolar-capillary recruitment up to peak exercise. The impairment in airway and vascular function exceeded the impairment in gas exchange and imposed the major limitation to exercise following 70% resection. Mediastinal distortion further reduced air and blood flow conductance, resulting in CO(2) retention. Results suggest that adaptation of extra-acinar airways and blood vessels lagged behind that of acinar tissue. As more lung units were lost, functional compensation became limited by the disproportionately reduced convective conductance rather than by alveolar diffusion disequilibrium.

Splenectomy impairs diffusive oxygen transport in the lung of dogs

The spleen acts as an erythrocyte reservoir in highly aerobic species such as the dog and horse. Sympathetic-mediated splenic contraction during exercise reversibly enhances convective O2 transport by increasing hematocrit, blood volume, and O2-carrying capacity. Based on theoretical interactions between erythrocytes and capillary membrane (Hsia CCW, Johnson RL Jr, and Shah D. J Appl Physiol 86: 1460-1467, 1999) and experimental findings in horses of a postsplenectomy reduction in peripheral O2-diffusing capacity (Wagner PD, Erickson BK, Kubo K, Hiraga A, Kai M, Yamaya Y, Richardson R, and Seaman J. Equine Vet J 18, Suppl: 82-89, 1995), we hypothesized that splenic contraction also augments diffusive O2 transport in the lung. Therefore, we have measured lung diffusing capacity (DL(CO)) and its components during exercise by a rebreathing technique in six adult foxhounds before and after splenectomy. Splenectomy eliminated exercise-induced polycythemia, associated with a 30% reduction in maximal O2 uptake. At any given pulmonary blood flow, DL(CO) was significantly lower after splenectomy owing to a lower membrane diffusing capacity, whereas pulmonary capillary blood volume changed variably; microvascular recruitment, indicated by the slope of the increase in DL(CO) with respect to pulmonary blood flow, was also reduced. We conclude that splenic contraction enhances both convective and diffusive O2 transport and provides another compensatory mechanism for maintaining alveolar O2 transport in the presence of restrictive lung disease or ambient hypoxia.

Further examination of alveolar septal adaptation to left pneumonectomy in the adult lung

Recent data from our laboratory are presented concerning alveolar septal adaptation following 42-45% lung resection by left pneumonectomy (PNX) in adult foxhounds compared to sham-operated control animals. Results confirm our previous conclusion that compensation in the remaining lung occurs without a net growth of additional alveolar septal tissue. The major ultrastructural responses are (a) alveolar capillary distention, which recruits capillary blood volume and surface area, leading to a 30-50% increase in lung diffusing capacity estimated by morphometry, a magnitude similar to that measured by physiologic methods; (b) a selectively increased volume of type 2 alveolar epithelial cells. These data, taken together with the balanced compensatory growth of alveolar septal cells observed in adult dogs following 55-58% lung resection by right PNX, support a graded alveolar cellular response to chronic mechanical strain with the alveolar epithelial cells being activated first; as strain increases further with greater lung resection other alveolar cells also become activated leading to an overt increase in septal tissue volume. The spatial distribution of lobar mechanical strain and lobar tissue volume assessed by high resolution computed tomography was markedly non-uniform after PNX, suggesting possible non-uniform distribution of alveolar cellular response. The sequential activation of physiologic recruitment and cellular adaptation confer additive functional benefits that optimize long-term exercise performance after PNX.

Lack of response to all-trans retinoic acid supplementation in adult dogs following left pneumonectomy

We showed previously that removing 55-58% of the lung by right pneumonectomy (R-PNX) in adult dogs triggers compensatory growth of the remaining lung, but removing 42-45% of the lung by left PNX (L-PNX) does not. We also showed that, following R-PNX, supplemental all-trans retinoic acid (RA) selectively enhances alveolar capillary endothelial cell volume (Yan X, Bellotto DJ, Foster DJ, Johnson RL, Jr., Hagler HH, Estrera AS, and Hsia CC. J Appl Physiol 96: 1080-1089, 2004). We hypothesized that RA supplementation might enhance compensation following L-PNX and tested this hypothesis by administering RA (2 mg.kg(-1).day(-1), 4 days/wk) or placebo orally to litter-matched adult foxhounds for 4 mo following L-PNX. Resting lung function was measured under anesthesia. Air and tissue volumes of the remaining lung were assessed by high-resolution computed tomography scan and by detailed postmortem morphometric analysis of the fixed lung. There was no significant difference in resting lung function, lung volume, alveolar structure, or septal ultrastructure between RA and placebo treatment groups. We conclude that RA supplementation does not induce post-PNX compensatory lung growth in the absence of existing cellular growth activities initiated by other primary signals.

Signals and mechanisms of compensatory lung growth

Growth of the lung involves unique structure-function interactions not seen in solid organs. Mechanical feedback between the lung and thorax constitutes a major signal that sustains developmental as well as compensatory lung growth. After the loss of lung units as by pneumonectomy (PNX), increased mechanical stress and strain on the remaining units induce adaptive responses to augment oxygen transport, including 1) recruitment of alveolar-capillary reserves, 2) remodeling of existing tissue, and 3) regenerative growth of acinar tissue when strain exceeds a critical threshold. Alveolar hypoxia, hormones, and growth factors may feed into the mechanical feedback system to modify an existing growth response but are unlikely to initiate compensatory growth in the absence of sufficient mechanical signals. Whereas endogenous post-PNX alveolar growth preserves normal structure-function relationships, experimental manipulation of selected metabolic pathways can distort these relationships. Finally, PNX widens the disparity between the rapidly adapting acini and slowly adapting conducting airways and blood vessels, leading to disproportionate airflow and hemodynamic dysfunction and secondary hypertrophy of the right ventricle and respiratory muscles that limits overall organ function despite regeneration of gas exchange tissue. These are key concepts to consider when formulating approaches to stimulate or augment compensatory growth in chronic lung disease.

Pneumonectomy: four case studies and a comparative review

Pneumonectomy is the resection of all lung lobes in either the left or right lung field. The surgical technique and postoperative results of pneumonectomy for clinical disease have not been reported in companion animals. Pneumonectomy was performed in three dogs and one cat to treat pulmonary or pleural disease, and the postoperative outcome compared with the complications and results reported in the human literature. One dog died immediately postoperatively due to suspected respiratory insufficiency and the remaining three animals survived the perioperative period. Postoperative complications were reported in two animals. Cardiac complications occurred in the cat, with perioperative arrhythmias and progressive congestive heart failure. Gastrointestinal complications were diagnosed in one dog, with mediastinal shift and oesophageal dysfunction. Left- and right-sided pneumonectomy is feasible in companion animals, and the postoperative outcome and complications encountered in this series were similar to those reported in humans.

Reducing lung strain after pneumonectomy impairs oxygen diffusing capacity but not ventilation-perfusion matching

After pneumonectomy (Pnx), mechanical strain on the remaining lung is an important signal for adaptation. To examine how mechanical lung strain alters gas exchange adaptation after Pnx, we replaced the right lung of adult dogs with a custom-shaped inflatable silicone prosthesis. The prosthesis was kept 1) inflated (Inf) to reduce mechanical strain of the remaining lung and maintain the mediastinum in the midline, or 2) deflated (Def) to allow lung strain and mediastinal shift. Gas exchange was studied 4-7 mo later at rest and during treadmill exercise by the multiple inert gas elimination technique while animals breathed 21 and 14% O2 in balanced order. In the Inf group compared with Def group during hypoxic exercise, arterial O2 saturation was lower and alveolar-arterial O2 tension difference higher, whereas O2 diffusing capacity was lower at any given cardiac output. Dispersion of the perfusion distribution was similar between groups at rest and during exercise. Dispersion of the ventilation distribution was lower in the Inf group at rest, associated with a much higher respiratory rate, but rose to similar levels in both groups during hypoxic exercise. Mean pulmonary arterial pressure at a given cardiac output was higher in the Inf group, whereas peak cardiac output was similar between groups. Thus creating lung strain by post-Pnx mediastinal shift primarily enhances diffusive gas exchange with only minor effects on ventilation-perfusion matching, consistent with the generation of additional alveolar-capillary surfaces but not conducting airways and blood vessels.

Implications of post-pneumonectomy compensatory lung growth in pulmonary physiology and disease

In a number of species, partial pneumonectomy initiates hormonally regulated compensatory growth of the remaining lung lobes that restores normal mass, structure and function. Compensation is qualitatively similar across species, but differs with gender, age and hormonal status. Although the biology of response is best characterized in rats, dogs have proven valuable in defining post-operative physiological adaptations. Most recently, mice were recognized to offer unique opportunities to explore the genetic basis of the response, as well as to evaluate associated detrimental effects of pathophysiological significance in animals exposed to carcinogens. The pneumonectomy model thus offers powerful insight concerning adaptive organ growth.

Preventing mediastinal shift after pneumonectomy does not abolish physiological compensation

To determine the role of mediastinal shift after pneumonectomy (PNX) on compensatory responses, we performed right PNX in adult dogs and replaced the resected lung with a custom-shaped inflatable silicone prosthesis. Prosthesis was inflated (Inf) to prevent mediastinal shift, or deflated (Def), allowing mediastinal shift to occur. Thoracic, lung air, and tissue volumes were measured by computerized tomography scan. Lung diffusing capacities for carbon monoxide (DL(CO)) and its components, membrane diffusing capacity for carbon monoxide (Dm(CO)) and capillary blood volume (Vc), were measured at rest and during exercise by a rebreathing technique. In the Inf group, lung air volume was significantly smaller than in Def group; however, the lung became elongated and expanded by 20% via caudal displacement of the left hemidiaphragm. Consequently, rib cage volume was similar, but total thoracic volume was higher in the Inf group. Extravascular septal tissue volume was not different between groups. At a given pulmonary blood flow, DL(CO) and Dm(CO) were significantly lower in the Inf group, but Vc was similar. In one dog, delayed mediastinal shift occurred 9 mo after PNX; both lung volume and DL(CO) progressively increased over the subsequent 3 mo. We conclude that preventing mediastinal shift after PNX impairs recruitment of diffusing capacity but does not abolish expansion of the remaining lung or the compensatory increase in extravascular septal tissue volume.

Postpneumonectomy alveolar growth does not normalize hemodynamic and mechanical function

Immature foxhounds underwent 55% lung resection by right pneumonectomy (n = 5) or thoracotomy without pneumonectomy (Sham, n = 6) at 2 mo of age. Cardiopulmonary function was measured during treadmill exercise on reaching maturity 1 yr later. In pneumonectomized animals compared with Sham animals, maximal oxygen uptake, ventilatory response, and cardiac output during exercise were normal. Arterial and mixed venous blood gases and arteriovenous oxygen extraction during exercise were also normal. Mean pulmonary arterial pressure and resistance were elevated at a given cardiac output. Dynamic ventilatory power requirement was also significantly elevated at a given minute ventilation. These long-term hemodynamic and mechanical abnormalities are in direct contrast to the normal pulmonary gas exchange during exercise in these same pneumonectomized animals reported elsewhere (S. Takeda, C. C. W. Hsia, E. Wagner, M. Ramanathan, A. S. Estrera, and E. R. Weibel. J. Appl. Physiol. 86: 1301-1310, 1999). Functional compensation was superior in animals pneumonectomized as puppies than as adults. These data indicate a limited structural response of conducting airways and extra-alveolar pulmonary blood vessels to pneumonectomy and suggest the development of other sources of adaptation such as those involving the heart and respiratory muscles.

Compensatory alveolar growth normalizes gas-exchange function in immature dogs after pneumonectomy

To determine the extent and sources of adaptive response in gas-exchange to major lung resection during somatic maturation, immature male foxhounds underwent right pneumonectomy (R-Pnx, n = 5) or right thoracotomy without pneumonectomy (Sham, n = 6) at 2 mo of age. One year after surgery, exercise capacity and pulmonary gas-exchange were determined during treadmill exercise. Lung diffusing capacity (DL) and cardiac output were measured by a rebreathing technique. In animals after R-Pnx, maximal O2 uptake, lung volume, arterial blood gases, and DL during exercise were completely normal. Postmortem morphometric analysis 18 mo after R-Pnx (n = 3) showed a vigorous compensatory increase in alveolar septal tissue volume involving all cellular compartments of the septum compared with the control lung; as a result, alveolar-capillary surface areas and DL estimated by morphometry were restored to normal. In both groups, estimates of DL by the morphometric method agreed closely with estimates obtained by the physiological method during peak exercise. These data show that extensive lung resection in immature dogs stimulates a vigorous compensatory growth of alveolar tissue in excess of maturational lung growth, resulting in complete normalization of aerobic capacity and gas-exchange function at maturity.

In vivo assessment of changes in air and tissue volumes after pneumonectomy

We examined the progression and topographical distribution of postpneumonectomy volume changes in immature foxhounds undergoing right pneumonectomy (R-Pnx, n = 5) or sham pneumonectomy (Sham, n = 6) at 2 mo of age and subsequently raised to maturity. Volumes of lung air (Vair) and tissue (Vti) were estimated by computerized tomography (CT) scan at 7, 22, and 52 wk after surgery at a transpulmonary pressure of 20 cmH2O. Estimates of Vti by CT scan included both septal tissue as well as nonseptal tissue (small- and medium-sized airways and blood vessels); these were compared with estimates of septal Vti by an acetylene rebreathing (Rb) method. We found significant correlations between these techniques (Vair(CT) = 0.83 Vair(Rb) + 275, R = 0.97; Vti(CT) = 1.62 Vti(Rb) - 30, R = 0.81). Extravascular septal Vti returned to normal 7 wk after R-Pnx and remained normal up to maturity. Nonseptal Vti remained significantly below normal. The greatest increase in Vti occurred in the midlung region just cephalad and caudal to the heart. After an early period of accelerated tissue growth after R-Pnx, the rate of septal tissue growth matched that of somatic growth, whereas nonseptal tissue growth lagged behind. Compensatory growth of the remaining left lung was not associated with selective alterations in thoracic development.

Compensatory lung growth occurs in adult dogs after right pneumonectomy

We investigated the structural changes in the left lung of five adult male foxhounds 5 mo (n = 2) or 16 mo (n = 3) after right pneumonectomy (approximately 54% of lung resected) and five sex- and age-matched foxhounds 15-16 mo after right thoracotomy without pneumonectomy. Lungs were fixed by intratracheal instillation of glutaraldehyde and analyzed by standard morphometric techniques. After right pneumonectomy, volume of the left lung increased by 72%. Volumes of all septal structures increased significantly and were more pronounced at 5 than at 16 mo after pneumonectomy. At 16 mo, the relative increases in volume with respect to the control left lung were as follows: epithelium 73%, interstitium 100%, endothelium 55%, and capillary blood volume 43%. Surface areas of alveoli and capillary increased significantly by 52% and 34%, respectively. At 5 mo after pneumonectomy, harmonic mean thickness of the tissue-plasma barrier was significantly greater but at 16 mo it was not different from controls. There was a significant increase in diffusing capacity for oxygen (33% above controls) at 16 mo after pneumonectomy. These data suggest that, in contrast to previous findings after left pneumonectomy, compensatory lung growth does occur in adult dogs after resection of > 50% of lung.

Structural changes underlying compensatory increase of diffusing capacity after left pneumonectomy in adult dogs

To determine if the functional compensation in diffusing capacity of the remaining lung following pneumonectomy is due to structural growth, we performed morphometric analysis of the right lung in three adult foxhounds approximately 2 yr after left pneumonectomy (removal of 42% of lung) and compared the results to those in normal adult dogs previously studied by the same techniques. Diffusing capacity was calculated by an established morphometric model and compared to physiologic estimates at peak exercise in the same dogs after pneumonectomy. The major structural changes after left pneumonectomy are hyperinflation of the right lung, alveolar enlargement, and thinning of the alveolar-capillary tissue barrier. These changes confer significant functional compensation for gas exchange by reducing the overall resistance to O2 diffusion. The magnitude of compensation in diffusing capacity estimated either morphometrically or physiologically is similar. In spite of morphometric and physiologic evidence of functional compensation, there is no evidence of significant growth of structural components. After pneumonectomy, morphometric estimates of diffusing capacity are on average 23% higher than physiologic estimates in the same dogs at peak exercise. We conclude that the previously reported large differences between morphometric and physiologic estimates of diffusing capacity reflects the presence of large physiologic reserves available for recruitment.