Bronchial artery revascularization restores peribronchial tissue oxygenation after lung transplantation

[The bronchial arteries: a small but vital contribution to lung perfusion after lung transplantation]

Blood supply to the lungs is carried out by the pulmonary and bronchial-arterial system. The bronchial-arterial vessels are involved in supplying the small airways all the way up to the terminal bronchioles. The bronchial-arterial system is also necessary for the regulation of airway temperature, humidity and mucociliary clearance. Chronic ischaemia of the small airways due to damage or injury to bronchial arterial supply increases the risk of fibrosis of the small airways (bronchiolitis obliteration), especially in lung transplantation (LTx). Although survival after LTx has improved over time, it is, with a 5-year survival rate of only 50 to 60%, still significantly worse than that of other organ transplants. It is likely that bronchial arterial revascularisation at the time of LTx plays an important transplant-preserving function.

[Airway complications after lung transplantation]

Lung transplantations have been regularly performed in the Russian Federation since 2010. Therefore, the number of lung transplant recipients, as well as the number of patients with airway complications following lung transplantation has been increasing. Treatment of these patients takes place not only in transplantation centers, but also in other hospitals. This review is devoted to risk factors, clinical manifestations, treatment and prevention of airway complications after lung transplantation. We analyzed literature data over the last 15 years. It was confirmed that bronchoscopy is a «gold standard» for diagnosis of airway complications while bronchoscopic interventions are preferred for treatment. Balloon and rigid bronchodilation and endoscopic airway stenting are the most effective interventions. Silicone stents are the most optimal. Antimicrobial prophylaxis and timely use of antiproliferative immunosuppressive drugs are important factors in prevention and treatment of airway complications after lung transplantation.

The Bronchial Arterial Circulation in Lung Transplantation: Bedside to Bench to Bedside, and Beyond

Chronic allograft dysfunction (CLAD) remains a major complication, causing the poor survival after lung transplantation (Tx). Although strenuous efforts have been made at preventing CLAD, surgical approaches for lung Tx have not been updated over the last 2 decades. The bronchial artery (BA), which supplies oxygenated blood to the airways and constitutes a functional microvasculature, has occasionally been revascularized during transplants, but this technique did not gain popularity and is not standard in current lung Tx protocols, despite the fact that a small number of studies have shown beneficial effects of BA revascularization on limiting CLAD. Also, recent basic and clinical evidence has demonstrated the relationship between microvasculature damage and CLAD. Thus, the protection of the bronchial circulation and microvasculature in lung grafts may be a key factor to overcome CLAD. This review revisits the history of BA revascularization, discusses the role of the bronchial circulation in lung Tx, and advocates for novel bronchial-arterial-circulation sparing approaches as a future direction for overcoming CLAD. Although there are some already published review articles summarizing the surgical techniques and their possible contribution to outcomes in lung Tx, to the best of our knowledge, this review is the first to elaborate on bronchial circulation that will contribute to prevent CLAD from both scientific and clinical perspectives: from bedside to bench to bedside, and beyond.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Penis Allotransplantation in Beagle Dog

This is an original research of penis allotransplantation. The paper presents an experiment allogenic penis transplantation model in Beagles, with a focus on recovery of blood supply and changes in tissue architecture. Twenty adult Beagles were allocated to 10 pairs for penile transplantation. After operation, the skin and glans were observed. If adverse symptoms occurred, the transplanted penis was resected and pathologically examined. Frequency of urination, urinary stream, and patency level were recorded 7 days after transplantation. Cystourethrography was performed on Day 10. The transplanted penises were resected on Day 14 for pathological examination. The research showed that transplanted penises survived after allotransplantation, and the dogs regained urination ability. Penis autotransplantation in Beagles is feasible. This preliminary study shows a potential for application of this new procedure for penis transplantation in humans.

A novel dual ex vivo lung perfusion technique improves immediate outcomes in an experimental model of lung transplantation

The lungs are dually perfused by the pulmonary artery and the bronchial arteries. This study aimed to test the feasibility of dual-perfusion techniques with the bronchial artery circulation and pulmonary artery circulation synchronously perfused using ex vivo lung perfusion (EVLP) and evaluate the effects of dual-perfusion on posttransplant lung graft function. Using rat heart-lung blocks, we developed a dual-perfusion EVLP circuit (dual-EVLP), and compared cellular metabolism, expression of inflammatory mediators, and posttransplant graft function in lung allografts maintained with dual-EVLP, standard-EVLP, or cold static preservation. The microvasculature in lung grafts after transplant was objectively evaluated using microcomputed tomography angiography. Lung grafts subjected to dual-EVLP exhibited significantly better lung graft function with reduced proinflammatory profiles and more mitochondrial biogenesis, leading to better posttransplant function and compliance, as compared with standard-EVLP or static cold preservation. Interestingly, lung grafts maintained on dual-EVLP exhibited remarkably increased microvasculature and perfusion as compared with lungs maintained on standard-EVLP. Our results suggest that lung grafts can be perfused and preserved using dual-perfusion EVLP techniques that contribute to better graft function by reducing proinflammatory profiles and activating mitochondrial respiration. Dual-EVLP also yields better posttransplant graft function through increased microvasculature and better perfusion of the lung grafts after transplantation.

Experimental studies in the bronchial circulation. Which is the ideal animal model?

BACKGROUND

The importance of the role of bronchial arteries is notable in modern days thoracic surgery. The significance of their anastomoses with adjusted structures has not yet been sufficiently rated, especially in cases of haemoptysis, heart-lung transplantations and treatment of aneurysms of the thoracic aorta. The need of a thorough study is more relevant than ever and appropriate laboratory animals are required.

METHODS

We review the literature in order to highlight the ideal experimental animal for the implementation of pilot programs relative to the bronchial circulation. A comparative analysis of the anatomy of the bronchial arterial system in humans along with these of pigs, dogs, rats, and birds, as being the most commonly used laboratory animals, is presented in details.

RESULTS

The pig has the advantage that the broncho-oesophageal artery usually originates from the aorta as a single vessel, which makes the recognition and dissection of the artery easy to perform. In dogs, there is significant anatomical variation of the origin of the bronchial arteries. In rats, bronchial artery coming from the aorta is a rare event while in birds the pattern of the bronchial artery tree is clearly different from the human analog.

CONCLUSIONS

The pig is anatomically and physiologically suited for experimental studies on the bronchial circulation. The suitable bronchial anatomy and physiology along with the undeniable usefulness of the pig in experimental research and the low maintenance cost make the pig the ideal model for experiments in bronchial circulation.

Complement-mediated microvascular injury leads to chronic rejection

Microvascular loss may be an unappreciated root cause of chronic rejection for all solid organ transplants. As the only solid organ transplant that does not undergo primary systemic arterial revascularization at the time of surgery, lung transplants rely on the establishment of a microcirculation and are especially vulnerable to the effects of microvascular loss. Microangiopathy, with its attendant ischemia, can lead to tissue infarction and airway fibrosis. Maintaining healthy vasculature in lung allografts may be critical for preventing terminal airway fibrosis, also known as the bronchiolitis obliterans syndrome (BOS). BOS is the major obstacle to lung transplant success and affects up to 60% of patients surviving 5 years. The role of complement in causing acute microvascular loss and ischemia during rejection has recently been examined using the mouse orthotopic tracheal transplantation; this is an ideal model for parsing the role of airway vasculature in rejection. Prior to the development of airway fibrosis in rejecting tracheal allografts, C3 deposits on the vascular endothelium just as tissue hypoxia is first detected. With the eventual destruction of vessels, microvascular blood flow to the graft stops altogether for several days. Complement deficiency and complement inhibition lead to markedly improved tissue oxygenation in transplants, diminished airway remodeling, and accelerated vascular repair. CD4+ T cells and antibody-dependent complement activity independently mediate vascular destruction and sustained tissue ischemia during acute rejection. Consequently, interceding against complement-mediated microvascular injury with adjunctive therapy during acute rejection episodes, in addition to standard immunosuppression which targets CD4+ T cells, may help prevent the subsequent development of chronic rejection.

Tissue hypoxaemia causes oedema, inflammation and fibrosis in porcine bronchial transsection

OBJECTIVES

Bronchial artery revascularization in lung transplantation is disputed. This study examined the physiological consequences of porcine bronchial transsection and reanastomosis with and without bronchial artery blood supply with relation to lung transplantation.

DESIGN

Translational, controlled animal study. Twelve pigs were operated through a left lateral thoracotomy. The left bronchus was transsected and reanastomosed. In the control group (n = 6), the bronchial arteries were preserved and in the study group (n = 6) they were severed. Bronchial mucosa blood flow (BMBF) was measured with laser-Doppler velocimetry and bronchial mucosa haemoglobin saturation and concentration with diffuse reflectance spectrophotometry. Measurements were made preoperatively, postoperatively and after 1 week.

RESULTS

In the study group, left postoperative BMBF was significantly lower than preoperatively (115 vs. 210 PU/s, p = 0.0001) and lower than in the control group (115 vs. 205 PU/s, p = 0.002). Repeated measurement ANOVA showed a significant treatment effect depending on time (p = 0.0034). The left mucosal haemoglobin saturation in the study group was significantly reduced postoperatively, 92% versus 61%, with a treatment effect depending on time (p = 0.0080). The reduction in left/right ratio of the mucosal haemoglobin concentration 1 week postoperatively in the study group was insignificant.

CONCLUSION

Bronchial transsection and reanastomosis without bronchial artery blood supply was followed by significant decrease in mucosal blood flow and saturation postoperatively, and also in tissue haemoglobin concentration at section, and provides a physiologic explanation of histological changes.

A friend to the airways: a review of the emerging clinical importance of the bronchial arterial circulation

Lungs receive the bulk of their blood supply through the pulmonary arteries. The bronchial arteries, on the other hand, vascularize the bronchi and their surroundings. These two arteries anastomose near the alveolar ducts. Contrary to the pulmonary circulation which is fairly well studied, the bronchial arteries have been appreciated more by their absence, and in some cases, by an interruption in the pulmonary arterial flow. Therefore, a more accurate anatomical and functional knowledge of these atherosclerosis-resistant vessels is needed to help surgeons and clinicians to avoid iatrogenic injuries during pulmonary interventions. In this review, we have revisited the anatomy and pathophysiology of the bronchial arteries in humans, considering the recent advances in imaging techniques. We have also elaborated on the known clinical applications of these arteries in both the pathogenesis and management of common pulmonary conditions.

Chronic lung allograft rejection and airway microvasculature: is HIF-1 the missing link?

Chronic lung allograft rejection, known as obliterative bronchiolitis (OB), is the leading cause of death in lung transplant patients. Although OB pathogenesis is not fully understood, in this issue of the JCI, Jiang and colleagues report that tissue hypoxia resulting in dysfunctional airway microvasculature precedes the airway fibrosis characteristic of OB. In addition, a relative deficiency of allograft endothelial cell-derived HIF-1α contributes to this process. Data showing that overexpressing HIF-1α restores the microvascular airway normoxia and prevents airway fibrosis highlight a novel role for vascular biology in OB pathogenesis.

Bronchial blood supply after lung transplantation without bronchial artery revascularization

PURPOSE OF REVIEW

This review discusses how the bronchial artery circulation is interrupted following lung transplantation and what may be the long-term complications of compromising systemic blood flow to allograft airways.

RECENT FINDINGS

Preclinical and clinical studies have shown that the loss of airway microcirculations is highly associated with the development of airway hypoxia and an increased susceptibility to chronic rejection.

SUMMARY

The bronchial artery circulation has been highly conserved through evolution. Current evidence suggests that the failure to routinely perform bronchial artery revascularization at the time of lung transplantation may predispose patients to develop the bronchiolitis obliterans syndrome.

Lung transplant airway hypoxia: a diathesis to fibrosis?

RATIONALE

Chronic rejection, manifested pathologically as airway fibrosis, is the major problem limiting long-term survival in lung transplant recipients. Airway hypoxia and ischemia, resulting from a failure to restore the bronchial artery (BA) circulation at the time of transplantation, may predispose patients to chronic rejection. To address this possibility, clinical information is needed describing the status of lung perfusion and airway oxygenation after transplantation.

OBJECTIVES

To determine the relative pulmonary arterial blood flow, airway tissue oxygenation and BA anatomy in the transplanted lung was compared with the contralateral native lung in lung allograft recipients.

METHODS

Routine perfusion scans were evaluated at 3 and 12 months after transplantation in 15 single transplant recipients. Next, airway tissue oximetry was performed in 12 patients during surveillance bronchoscopies in the first year after transplant and in 4 control subjects. Finally, computed tomography (CT)-angiography studies on 11 recipients were reconstructed to evaluate the post-transplant anatomy of the BAs.

MEASUREMENTS AND MAIN RESULTS

By 3 months after transplantation, deoxygenated pulmonary arterial blood is shunted away from the native lung to the transplanted lung. In the first year, healthy lung transplant recipients exhibit significant airway hypoxia distal to the graft anastomosis. CT-angiography studies demonstrate that BAs are abbreviated, generally stopping at or before the anastomosis, in transplant airways.

CONCLUSIONS

Despite pulmonary artery blood being shunted to transplanted lungs after transplantation, grafts are hypoxic compared with both native (diseased) and control airways. Airway hypoxia may be due to the lack of radiologically demonstrable BAs after lung transplantation.

Model of pulmonary fluid traffic homeostasis based on respiratory cycle pressure, bidirectional bronchiolo-pulmonar shunting and water evaporation

The main puzzle of the pulmonary circulation is how the alveolar spaces remain dry over a wide range of pulmonary vascular pressures and blood flows. Although normal hydrostatic pressure in pulmonary capillaries is probably always below 10 mmHg, well bellow plasma colloid pressure of 25 mmHg, most textbooks state that some fluid filtration through capillary walls does occur, while the increased lymph drainage prevents alveolar fluid accumulation. The lack of a measurable pressure drop along pulmonary capillaries makes the classic description of Starling forces unsuitable to the low pressure, low resistance pulmonary circulation. Here presented model of pulmonary fluid traffic describes lungs as a matrix of small vascular units, each consisting of alveoli whose capillaries are anastomotically linked to the bronchiolar capillaries perfused by a single bronchiolar arteriole. It proposes that filtration and absorption in pulmonary and in bronchiolar capillaries happen as alternating periods of low and of increased perfusion pressures. The model is based on three levels of filtration control: short filtration phases due to respiratory cycle of the whole lung are modulated by bidirectional bronchiolo-pulmonar shunting independently in each small vascular unit, while fluid evaporation from alveolar groups further tunes local filtration. These mechanisms are used to describe a self-sustaining regulator that allows optimal fluid traffic in different settings. The proposed concept is used to describe development of pulmonary edema in several clinical entities (exercise in wet or dry climate, left heart failure, people who rapidly move to high altitudes, acute cyanide and carbon monoxide poisoning, large pulmonary embolisms).

Obliterative bronchiolitis in lung allografts removed at retransplant for intractable airway problems

BACKGROUND AND OBJECTIVE

The role of large airway ischaemia, with resultant airway narrowing, in the development of post-lung transplant bronchiolitis obliterans has not been defined. A determination of clinical bronchiolitis obliterans syndrome (BOS), which is defined as a decline in FEV(1) from a stable post-transplant baseline, is difficult in the setting of airway complications. The aim of this study was to assess the evidence for histological bronchiolitis obliterans in lung allografts removed during retransplantation for severe recurrent airway narrowing.

METHODS

Case records and histological findings in allograft lungs removed at retransplantation were retrospectively reviewed.

RESULTS

Five lung transplant recipients, who had undergone retransplantation because of severe recalcitrant airway stenosis, were identified. In each case, explant allograft lung pathology revealed evidence of bronchiolitis obliterans.

CONCLUSIONS

There is a possible link between airway ischaemia, large airway stenosis and the development of bronchiolitis obliterans, which is the most common cause of death in lung transplant recipients after the first year. These findings may provide an impetus for evaluation of the role of bronchial artery revascularization techniques in the prevention of bronchiolitis obliterans.

Microvascular destruction identifies murine allografts that cannot be rescued from airway fibrosis

Small airway fibrosis (bronchiolitis obliterans syndrome) is the primary obstacle to long-term survival following lung transplantation. Here, we show the importance of functional microvasculature in the prevention of epithelial loss and fibrosis due to rejection and for the first time, relate allograft microvascular injury and loss of tissue perfusion to immunotherapy-resistant rejection. To explore the role of alloimmune rejection and airway ischemia in the development of fibroproliferation, we used a murine orthotopic tracheal transplant model. We determined that transplants were reperfused by connection of recipient vessels to donor vessels at the surgical anastomosis site. Microcirculation through the newly formed vascular anastomoses appeared partially dependent on VEGFR2 and CXCR2 pathways. In the absence of immunosuppression, the microvasculature in rejecting allografts exhibited vascular complement deposition, diminished endothelial CD31 expression, and absent perfusion prior to the onset of fibroproliferation. Rejecting grafts with extensive endothelial cell injury were refractory to immunotherapy. After early microvascular loss, neovascularization was eventually observed in the membranous trachea, indicating a reestablishment of graft perfusion in established fibrosis. One implication of this study is that bronchial artery revascularization at the time of lung transplantation may decrease the risk of subsequent airway fibrosis.

Bronchial circulation angiogenesis in the rat quantified with SPECT and micro-CT

INTRODUCTION

As pulmonary artery obstruction results in proliferation of the bronchial circulation in a variety of species, we investigated this angiogenic response using single photon emission computed tomography (SPECT) and micro-CT.

MATERIALS AND METHODS

After surgical ligation of the left pulmonary artery of rats, they were imaged at 10, 20, or 40 days post-ligation. Before imaging, technetium-labeled macroaggregated albumin ((99m)Tc MAA) was injected into the aortic arch (IA) labeling the systemic circulation. SPECT/micro-CT imaging was performed, the image volumes were registered, and activity in the left lung via the bronchial circulation was used as a marker of bronchial blood flow. To calibrate and to verify successful ligation, (99m)Tc MAA was subsequently injected into the left femoral vein (IV), resulting in accumulation within the pulmonary circulation. The rats were reimaged, and the ratio of the IA to the IV measurements reflected the fraction of cardiac output (CO) to the left lung via the bronchial circulation. Control and sham-operated rats were studied similarly.

RESULTS

The left lung bronchial circulation of the control group was 2.5% of CO. The sham-operated rats showed no significant difference from the control. However, 20 and 40 days post-ligation, the bronchial circulation blood flow had increased to 7.9 and 13.9%, respectively, of CO. Excised lungs examined after barium filling of the systemic vasculature confirmed neovascularization as evidenced by tortuous vessels arising from the mediastinum and bronchial circulation.

CONCLUSION

Thus, we conclude that SPECT/micro-CT imaging is a valuable methodology for monitoring angiogenesis in the lung and, potentially, for evaluating the effects of pro- or anti-angiogenic treatments using a similar approach.

Abnormalities of the bronchial arteries in asthma

STUDY OBJECTIVES

The bronchial arteries supply systemic blood to the airways, tracheobronchial lymph nodes, and nerves. Their structure has not been studied in patients with asthma.

DESIGN

Case-control study of pathologic changes of bronchial arteries in asthma.

PARTICIPANTS AND METHODS

Postmortem lungs were examined from three case groups: (1) fatal asthma (n = 12), death due to asthma; (2) nonfatal asthma (n = 12), asthmatic and death due to nonrespiratory causes; and (3) nonasthmatic control subjects (n = 12), no history of asthma and death due to nonrespiratory causes. In bronchial arteries with outer diameters of 0.1 to 1.0 mm, the areas of lumen, intima, and media were measured and compared between case groups.

RESULTS

There were no significant differences in artery size (outer diameter) or in medial area between the three groups. In the two asthma groups, the intimal area was increased (p < 0.05), with a corresponding decrease in luminal area compared with the control group. There was a significant effect of gender, age, and smoking on intimal area. In the asthma cases, the area of bronchial artery intima was related to duration of asthma (p < 0.05), and this increase was associated with smooth muscle proliferation, reduplication, and calcification of the elastica, but not with inflammatory cell infiltration.

CONCLUSIONS

While the pathophysiologic significance of these changes is uncertain, the relation to duration of asthma, age, and smoking suggests a secondary response to chronic airway disease.