Peripheral pulmonary vascular and airway abnormalities in adolescents with rheumatic mitral stenosis

Pathophysiology of Congestion in Heart Failure: A Contemporary Review

Acutely decompensated heart failure is one of the leading causes of hospitalisation worldwide, with a significant majority of these cases attributed to congestion. Although congestion is commonly mistaken for volume overload, evidence suggests that decompensation can occur without significant water accumulation, being attributed to volume redistribution. Yet, the distinction between intravascular and extravascular congestion in heart failure often blurs, as patients frequently exhibit overlapping features of both, and as patients may transition between phenotypes over time. Considering that differentiation between intravascular and extravascular congestion can lead to different management strategies, the aim of this review was to delineate the pathophysiological nuances between the two, as well as their correlation with clinical, biochemical and imaging indices.

A 49-year-old man with ischemic cardiomyopathy and persistent hemoptysis for eighteen months

Idiopathic pulmonary hemosiderosis (IPH) is a rare cause of recurrent episodes of diffuse alveolar hemorrhage (DAH). IPH commonly manifests with hemoptysis, radiologic chest infiltrates and anemia. The etiology of IPH is unknown, but an immunologic mechanism is widely speculated. The definitive diagnosis of IPH requires a thorough exclusion of other causes of DAH, such as infections, inflammation, malignancy, cardiac diseases, drug and toxin exposure, and medications. Due to the rarity of the disease, a diagnosis is often delayed by years. We present the case of a 49-year-old man with ischemic cardiomyopathy who presented with hemoptysis for eighteen months. Serologic workup was negative for vasculitides and autoimmune diseases. Bronchoscopy revealed DAH. A surgical lung biopsy showed 'bland pulmonary hemorrhage.' A right heart catheterization ruled out cardiac causes of DAH. The patient was diagnosed with IPH and started on systemic corticosteroids with rapid improvement of hemoptysis.

Diffuse Alveolar Hemorrhage in Cardiac Diseases

Diffuse alveolar hemorrhage (DAH) is a rare condition with reported mortality ranging between 20 and 100%. There are many etiologies of DAH. Cardiac diseases are likely underreported causes of DAH. Heart failure and mitral valve diseases are the most common cardiac causes of DAH. The DAH results from pulmonary venous hypertension leading to stress failure of the pulmonary capillaries. There is also a contribution of the bronchial circulation. The Alveolar-capillary membrane or blood-gas barrier is an extremely thin structure that allows rapid and passive diffusion of oxygen from the inhaled air to the pulmonary capillaries while preventing pulmonary edema and DAH with chronic elevation of the transmural hydrostatic pressure. The purpose of this manuscript is to inform the clinician about this rare cause of DAH, which may be overlooked unless specifically sought after. We also discuss the pathophysiologic aspects of DAH and the safety mechanisms in place to prevent such occurrences.

Pulmonary Limitations in Heart Failure

The heart and lungs are intimately linked. Hence, impaired function of one organ may lead to changes in the other. Accordingly, heart failure is associated with airway obstruction, loss of lung volume, impaired gas exchange, and abnormal ventilatory control. Cardiopulmonary exercise testing is an excellent tool for evaluation of gas exchange and ventilatory control. Indeed, many parameters routinely measured during cardiopulmonary exercise testing, including the level of minute ventilation per unit of carbon dioxide production and the presence of exercise oscillatory ventilation, have been found to be strongly associated with prognosis in patients with heart failure.

Cardiogenic Pulmonary Edema

The initial events in cardiogenic pulmonary edema involve hemodynamic pulmonary congestion with high capillary pressures. This causes increased fluid transfer out of capillaries into the interstitium and alveolar spaces. High capillary pressures can also cause barrier disruption which increases permeability and fluid transfer into the interstitium and alveoli. Fluid in alveoli alters surfactant function and increases surface tension. This can lead to more edema formation and to atelectasis with impaired gas exchange. Patients with barrier disruption have increased levels of surfactant protein B in the circulation, and these levels often remain high after the initial clinical improvement. Routine clinical assessment may not identify patients with increased extravascular fluid in the lungs; pulmonary ultrasound can easily detect pulmonary edema in patients with acute decompensation and in patients at risk for decompensation. Studies using serial pulmonary ultrasound could help characterize patients with cardiogenic pulmonary edema and help identify subgroups who need alternative management. The conventional management of cardiogenic pulmonary edema usually involves diuresis, afterload reduction and in some cases noninvasive ventilation to reduce the work of breathing and improve oxygenation. Patients with persistent symptoms, abnormal chest x-rays and diuretic resistance might benefit from alternative approaches to management. These could include beta agonists and pentoxifylline which warrant more study in patients with cardiogenic pulmonary edema.

Lung morphology and surfactant function in cardiogenic pulmonary edema: a narrative review

The conventional analysis of acute cardiogenic pulmonary edema involves the development of high pulmonary capillary pressures resulting in hydrostatic gradients for fluid flux out of capillaries into the interstitial space and alveolar spaces. However, some patients respond poorly to diuretic management. The PubMed database was searched to identify experimental studies on pulmonary edema in animals, experimental studies on surfactant function, including patients with pulmonary edema, and clinical studies reporting barrier dysfunction and/or injury in patients with acute pulmonary edema. Studies with animal models demonstrate that high capillary pressures can cause barrier disruption in alveolar capillary units which increases permeability and the transfer of fluid and protein into lung parenchyma. Fluid in alveolar spaces alters surfactant function which increases fluid flux out of capillaries into the lung parenchyma secondary to larger transcapillary hydrostatic gradients. Patients with acute cardiogenic pulmonary edema have increased levels of surfactant protein B in their plasma which reflect barrier disruption and increased levels of tumor necrosis factor alpha which reflect acute tissue injury. Increased surfactant protein B plasma levels are associated with abnormal gas exchange in patients with chronic heart failure. Patients with exercise-induced left ventricular dysfunction have increased levels of surfactant protein B after short periods of exercise. Pathology studies in patients with chronic heart failure have found increased connective tissue in alveolar capillary units and increased numbers of type II alveolar cells, and these changes represent an adaptive response in these patients. Clinicians need to consider the possibility of barrier dysfunction and disruption in patients with both acute and chronic pulmonary edema and understand that diuresis may have a limited effect on symptoms in some patients.

Pulmonary hypertension's variegated landscape: a snapshot

The many types of pulmonary hypertension (PH) are so protean in their biological origin, histological expression, and natural history that it is difficult to create a summary picture of the disease, or to easily compare and contrast characteristics of one type of PH with another. For newcomers to the field, however, such a picture would facilitate a broad understanding of PH. In this paper, we suggest that four characteristics are fundamental to describing the nature of various types of PH, and that taken together they define a number of patterns of PH expression. These characteristics are histopathology, developmental origin, associated clinical conditions, and potential for resolution. The “snapshot” is a way to concisely display the ways that these signal characteristics intersect in select specific types of PH, and is an effort to summarize these patterns in a way that facilitates a “big picture” comprehension of this disease.

Sleep apnoea in heart failure: To treat or not to treat?

Heart failure (HF) and sleep apnoea are common disorders which frequently coexist. Two main types of apnoea occur: one is obstructive which, through recurring episodes of snoring, hypoxaemia, large negative intra-thoracic pressures and arousals from sleep leading to downstream inflammatory and autonomic nervous system changes, is thought to be a causative factor to the development of systemic hypertension and HF. The other type of apnoea, Cheyne-Stokes respiration with central sleep apnoea (CSR-CSA), is characterized by an oscillatory pattern of ventilation with a prevailing hyperventilation-induced hypocapnia, often in the absence of significant hypoxaemia and snoring, and is thought to be a consequence of advanced HF-related low cardiac output, high sympathetic nervous system activation and pulmonary congestion. CSR-CSA may be a compensatory response to advanced HF. Rostral fluid shift during sleep may play an important role in the pathogenesis of both obstructive sleep apnoea (OSA) and CSA. Studies of positive airway pressure (PAP) treatment of OSA and CSA in HF have shown short-term improvements in cardiac and autonomic function; however, there is no evidence of improved survival. Loop gain may provide useful marker of continuous PAP (CPAP) responsiveness in patients with central apnoea. A greater understanding of the pathophysiology of the interaction between obstructive and central apnoea and the various types of HF, and the mechanisms of therapies, such as PAP, is required to develop new strategies to overcome the disabling symptoms, and perhaps improve the mortality, that accompany HF with sleep apnoea.

Ventilation heterogeneity is increased in patients with chronic heart failure

In the healthy lung, ventilation is distributed heterogeneously due to factors such as anatomical asymmetry and gravity. This ventilation heterogeneity increases pathologically in conditions such as asthma, chronic obstructive lung disease, and cystic fibrosis. In chronic heart failure, lung biopsy demonstrates evidence of peripheral lung fibrosis and small airways narrowing and distortion. We hypothesized that this would lead to increased ventilation heterogeneity. Furthermore, we proposed that rostral fluid shifts when seated patients lie supine would further increase ventilation heterogeneity. We recruited 30 ambulatory chronic heart failure patients (57 ± 10 years, 83% male, left ventricular ejection fraction 31 ± 12%) as well as 10 healthy controls (51 ± 13 years, 90% male). Heart failure patients were clinically euvolemic. Subjects underwent measurement of ventilation heterogeneity using the multiple-breath nitrogen washout technique in the seated position, followed by repeat measurements after 5 and 45 min in the supine position. Ventilation heterogeneity was calculated using the lung clearance index (LCI), Sacin and Scond which represent overall, acinar, and small conducting airway function, respectively. Lung clearance index (9.6 ± 1.2 vs. 8.6 ± 1.4 lung turnovers, P = 0.034) and Scond (0.029 ± 0.014 vs. 0.006 ± 0.016/L, P = 0.007) were higher in the heart failure patients. There was no difference in Sacin (0.197 ± 0.171 vs. 0.125 ± 0.081/L, P = 0.214). Measures of ventilation heterogeneity did not change in the supine position. This study confirms the presence of peripheral airway pathology in patients with chronic heart failure. This leads to subtle but detectable functional abnormalities which do not change after 45 min in the supine position.

Pulmonary hypertension caused by pulmonary venous hypertension

The effect of pulmonary venous hypertension (PVH) on the pulmonary circulation is extraordinarily variable, ranging from no impact on pulmonary vascular resistance (PVR) to a marked increase. The reasons for this are unknown. Both acutely reversible pulmonary vasoconstriction and pathological remodeling (especially medial hypertrophy and intimal hyperplasia) account for increased PVR when present. The mechanisms involved in vasoconstriction and remodeling are not clearly defined, but increased wall stress, especially in small pulmonary arteries, presumably plays an important role. Myogenic contraction may account for increased vascular tone and also indirectly stimulate remodeling of the vessel wall. Increased wall stress may also directly cause smooth muscle growth, migration, and intimal hyperplasia. Even long-standing and severe pulmonary hypertension (PH) usually abates with elimination of PVH, but PVH-PH is an important clinical problem, especially because PVH due to left ventricular noncompliance lacks definitive therapy. The role of targeted PH therapy in patients with PVH-PH is unclear at this time. Most prospective studies indicate that these medications are not helpful or worse, but there is ample reason to think that a subset of patients with PVH-PH may benefit from phosphodiesterase inhibitors or other agents. A different approach to evaluating possible pharmacologic therapy for PVH-PH may be required to better define its possible utility.

Study of Stress Induced Failure of the Blood-gas Barrier and the Epithelial-epithelial Cells Connections of the Lung of the Domestic Fowl, Gallus gallus Variant Domesticus after Vascular Perfusion

Complete blood-gas barrier breaks (BGBBs) and epithelial-epithelial cells connections breaks (E-ECCBs) were enumerated in the lungs of free range chickens, Gallus gallus variant domesticus after vascular perfusion at different pressures. The E-ECCBs surpassed the BGBBs by a factor of ~2. This showed that the former parts of the gas exchange tissue were structurally weaker or more vulnerable to failure than the latter. The differences in the numbers of BGBBs and E-ECCBs in the different regions of the lung supplied with blood by the 4 main branches of the pulmonary artery (PA) corresponded with the diameters of the blood vessels, the angles at which they bifurcated from the PA, and the positions along the PA where they branched off. Most of the BGBBs and the E-ECCBs occurred in the regions supplied by the accessory- and the caudomedial branches: the former is the narrowest branch and the first blood vessel to separate from the PA while the latter is the most direct extension of the PA and is the widest. The E-ECCBs appeared to separate and fail from tensing of the blood capillary walls, as the perfusion- and intramural pressures increased. Compared to the mammalian lungs on which data are available, i.e., those of the rabbit, the dog, and the horse, the blood-gas barrier of the lung of free range chickens appears to be substantially stronger for its thinness.

Cardiac asthma: new insights into an old disease

Cardiac asthma has been defined as wheezing, coughing and orthopnea due to congestive heart failure. The clinical distinction between bronchial asthma and cardiac asthma can be straight forward, except in patients with chronic lung disease coexisting with left heart disease. Pulmonary edema and pulmonary vascular congestion have been thought to be the primary causes of cardiac asthma but most patients have a poor response to diuretics. There appears to be limited effectiveness of classical asthma medications like bronchodilators or corticosteroids in treating cardiac asthma. Evidence suggests that circulating inflammatory factors and tissue growth factors also lead to airway obstruction suggesting the possibility of developing novel therapies.

Fragility of pulmonary capillaries

Although the pulmonary capillaries were discovered in 1661, the ultrastructure of the wall was not elucidated until 60 years ago. Electron micrographs then showed that only 0.2 μm of tissue separated the capillary endothelium from the alveolar space over much of the area. In retrospect this vanishingly small protective layer should have alerted physiologists to the potential fragility of the capillaries, but this was not appreciated until almost 40 years later. This predicament is unique to pulmonary capillaries. No other capillaries in the body are shielded from the outside environment by such a minute amount of tissue. Reasons why the fragility of the capillaries was not recognized earlier include an inappropriate comparison with the properties of systemic capillaries, the mistaken view that the pulmonary capillary pressure is always low, and a misleading use of the Laplace equation. Evidence for the fragility comes from physiological, pathological, and laboratory observations. As expected from evolutionary considerations, the fragility only becomes evident in the normal lung under exceptional conditions. These include elite human athletes at maximal exercise and animals that have developed the capacity for extreme aerobic activity. However, lung and heart diseases frequently cause capillary disruption. Remodeling of pulmonary capillaries occurs in humans in whom the capillary pressure rises over a long period. Neonatal capillaries are extremely fragile, presumably because they have never been exposed to increased transmural pressures. The capillaries conform to the general biological rule that tissue adapts its structure to carry out its required function.

Prevention of airway hyperresponsiveness induced by left ventricular dysfunction in rats

Background

The effectiveness of strategies for treatment of the altered static lung volume and against the development of bronchial hyperreactivity (BHR) following a left ventricular dysfunction (LVD) induced by myocardial ischaemia was investigated in a rat model of sustained postcapillary pulmonary hypertension.

Methods

Airway resistance (Raw) was identified from the respiratory system input impedance (Zrs) in four groups of rats. End-expiratory lung volume (EELV) was determined plethysmographically, and Zrs was measured under baseline conditions and following iv infusions of 2, 6 or 18 μg/kg/min methacholine. Sham surgery was performed in the rats in Group C, while the left interventricular coronary artery was ligated and Zrs and its changes following identical methacholine challenges were reassessed in the same rats 8 weeks later, during which no treatment was applied (Group I), or the animals were treated daily with a combination of an angiotensin enzyme converter inhibitor and a diuretic (enalapril and furosemide, Group IE), or a calcium channel blocker (diltiazem, Group ID). The equivalent dose of methacholine causing a 100% increase in Raw (ED₅₀) was determined in each group. Diastolic pulmonary arterial pressure (Pap_D) was assessed by introducing a catheter into the pulmonary artery.

Results

The sustained presence of a LVD increased Pap_D in all groups of rats, with variable but significant elevations in Groups I (p = 0.004), ID (p = 0.013) and IE (p = 0.006). A LVD for 8 weeks induced no changes in baseline Raw but elevated the EELV independently of the treatments. In Group I, BHR consistently developed following the LVD, with a significant decrease in ED₅₀ from 10.0 ± 2.5 to 6.9 ± 2.5 μg/kg/min (p = 0.006). The BHR was completely abolished in both Groups ID and IE, with no changes in ED₅₀ (9.5 ± 3.6 vs. 10.7 ± 4.7, p = 0.33 and 10.6 ± 2.1 vs. 9.8 ± 3.5 μg/kg/min p = 0.56, respectively).

Conclusions

These findings suggest that a LVD following coronary ischaemia consistently induces BHR. The more consistent efficacy of both treatment strategies in preventing BHR than in treating the adverse pulmonary vascular consequences suggests the benefit of both calcium channel blockade and ACE inhibition to counteract the airway susceptibility following a LVD.

Pulmonary blood flow and pulmonary hypertension: Is the pulmonary circulation flowophobic or flowophilic?

Increased pulmonary blood flow (PBF) is widely thought to provoke pulmonary vascular obstructive disease (PVO), but the impact of wall shear stress in the lung is actually poorly defined. We examined information from patients having cardiac lesions which impact the pulmonary circulation in distinct ways, as well as experimental studies, asking how altered hemodynamics impact the risk of developing PVO. Our results are as follows: (1) with atrial septal defect (ASD; increased PBF but low PAP), shear stress may be increased but there is little tendency to develop PVO; (2) with normal PBF but increased pulmonary vascular resistance (PVR; mitral valve disease) shear stress may also be increased but risk of PVO still low; (3) with high PVR and PBF (e.g., large ventricular septal defect), wall shear stress is markedly increased and the likelihood of developing PVO is much higher than with high PBF or PAP only; and (4) with ASD, experimental and clinical observations suggest that increased PBF plus another stimulus (e.g., endothelial inflammation) may be required for PVO. We conclude that modestly increased wall shear stress (e.g., ASD) infrequently provokes PVO, and likely requires other factors to be harmful. Likewise, increased PAP seldom causes PVO. Markedly increased wall shear stress may greatly increase the likelihood of PVO, but we cannot discriminate its effect from the combined effects of increased PAP and PBF. Finally, the age of onset of increased PAP may critically impact the risk of PVO. Some implications of these observations for future investigations are discussed.

Left ventricular failure produces profound lung remodeling and pulmonary hypertension in mice: heart failure causes severe lung disease

Chronic left ventricular failure causes pulmonary congestion with increased lung weight and type 2 pulmonary hypertension. Understanding the molecular mechanisms for type 2 pulmonary hypertension and the development of novel treatments for this condition requires a robust experimental animal model and a good understanding of the nature of the resultant pulmonary remodeling. Here we demonstrate that chronic transverse aortic constriction causes massive pulmonary fibrosis and remodeling, as well as type 2 pulmonary hypertension, in mice. Thus, aortic constriction-induced left ventricular dysfunction and increased left ventricular end-diastolic pressure are associated with a ≤5.3-fold increase in lung wet weight and dry weight, pulmonary hypertension, and right ventricular hypertrophy. Interestingly, the aortic constriction-induced increase in lung weight was not associated with pulmonary edema but resulted from profound pulmonary remodeling with a dramatic increase in the percentage of fully muscularized lung vessels, marked vascular and lung fibrosis, myofibroblast proliferation, and leukocyte infiltration. The aortic constriction-induced left ventricular dysfunction was also associated with right ventricular hypertrophy, increased right ventricular end-diastolic pressure, and right atrial hypertrophy. The massive lung fibrosis, leukocyte infiltration, and pulmonary hypertension in mice after transverse aortic constriction clearly indicate that congestive heart failure also causes severe lung disease. The lung fibrosis and leukocyte infiltration may be important mechanisms in the poor clinical outcome in patients with end-stage heart failure. Thus, the effective treatment of left ventricular failure may require additional efforts to reduce lung fibrosis and the inflammatory response.

The impact of pulmonary venous hypertension on the pulmonary circulation in the young

OBJECTIVE AND DESIGN

Pulmonary venous hypertension is a well-characterized cause of pulmonary hypertension in adults, but little is known regarding the relationship between left atrial pressure and pulmonary arteriolar resistance in the young. Also, in adults relief of pulmonary venous hypertension results in a marked fall in pulmonary arteriolar resistance, but this could be different in children because vascular changes are more severe in young patients than adults with mitral stenosis. We inspected records of children at Children's Hospital Boston having mitral balloon valvuloplasty, and patients ≤5 years old having mitral valve replacement, to determine (1) the relationship between left atrial pressure and pulmonary arterial pressure and resistance (n = 94 children, median age 17.8 months) and (2) how pulmonary arteriolar resistance changes after mitral valve replacement.

RESULTS

The average indexed pulmonary arteriolar resistance was 7.8 ± 5.9 units and was unrelated to age but was positively related to left atrial pressure. There was great variability in pulmonary arteriolar resistance for any given left atrial pressure. Pulmonary arterial pressure (n = 16) and pulmonary arterial resistance (n = 9) were measured before and after mitral valve replacement (median = 29.4 months old). Despite preoperative indexed pulmonary arterial resistance of ≥5 units in 11 of 15 patients, postoperative pulmonary arterial pressure was substantially lower in all save three, and two patients with high pulmonary arterial pressure still had high left atrial pressure postoperatively (25 mmHg).

CONCLUSIONS

We conclude that in young children, as in adults, pulmonary arterial resistance generally falls greatly with reduction in left atrial pressure.

Heart failure and the lung

Heart failure (HF) is a highly prevalent disease that leads to significant morbidity and mortality. There is increasing evidence that the symptoms of HF are exacerbated by its deleterious effects on lung function. HF appears to cause airway obstruction acutely and leads to impaired gas diffusing capacity and pulmonary hypertension in the longer term. It is postulated that this is the result of recurrent episodes of elevated pulmonary capillary pressure leading to pulmonary oedema and pulmonary capillary stress fracture, which produces lung fibrosis. It is likely that impaired lung function impairs the functional status of HF patients and makes them more prone to central sleep apnoea.

Mediastinal lymphadenopathy in patients undergoing cardiac transplant evaluation

BACKGROUND

We evaluated the association between hemodynamic parameters of chronic congestive heart failure (CHF) and mediastinal lymphadenopathy (MLA) in heart transplantation (HT) candidates and the effect of HT on MLA. We also described the results of lymph node (LN) biopsies of MLA in the patients.

METHODS

Patients who underwent HT evaluation over an 8-year period and had chest CT scans were evaluated retrospectively. Data collected included LN sizes pre-HT and post-HT, echocardiographic measurements, radionuclide-derived ejection fraction, and right-sided heart catheterization hemodynamics. MLA was defined as LNs > 1 cm in smallest dimension.

RESULTS

Of 118 patients, 53 patients had MLA. MLA had weak statistically significant correlations with elevated mean pulmonary artery pressure (MPAP), mitral regurgitation (MR), tricuspid regurgitation (TR), right atrial pressure (RAP), and pulmonary capillary wedge pressure (PCWP). Thirty-six patients with MLA underwent HT, and nine of the 36 had post-HT chest CT scans. All nine patients showed a decrease in LN size post-HT (mean LN diameter pre-HT = 1.16 ± 0.137 cm, post-HT = 0.75 ± 0.32 cm). Seven of 53 patients with MLA underwent biopsies. Four had benign LNs, one had sarcoidosis, and two had lung cancer.

CONCLUSIONS

MPAP, MR, TR, RAP, and PCWP had weak statistically significant correlations with MLA. HT led to regression of MLA in patients who underwent CT scans post-HT, implying that MLA is related to CHF. However, we also identified clinically important causes of MLA; therefore, biopsy should be considered if enlarged LNs fail to regress after maximal medical management of CHF.

Does the bronchial circulation contribute to congestion in heart failure?

Pulmonary congestion is a hallmark feature of heart failure and is a major reason for hospital admissions in this patient population. Heart failure patients often demonstrate restrictive and obstructive pulmonary function abnormalities; however, the mechanisms of these changes remain controversial. It has been suggested that the bronchial circulation may play an important role in the development of these pulmonary abnormalities and in the symptoms associated with pulmonary congestion. Congestion may occur in the bronchial circulation from either a marked increase in flow or an increase in blood volume but with a reduction in flow due to high cardiac filling pressures and high pulmonary vascular pressures (a stasis like condition). Either may lead to thickened bronchial mucosal and submucosal tissues and reduced airway compliance resulting in airway obstruction and restriction and a lack of airway distensibility. These structural changes may contribute to "cardiac asthma" and dyspnea, characteristic features common in HF patients. Thus the bronchial circulation may be a potential target for therapeutic interventions. The aim of this paper is to review factors governing the control of the bronchial circulation, how bronchial vascular conductance may change with HF and to pose arguments, both supporting and in opposition to the bronchial circulation contributing to congestion and altered pulmonary function in HF. We ultimately hypothesize that the engorgement of the bronchial circulatory bed may play a role in pulmonary function abnormalities that occur in HF patients and contribute to symptoms such as orthopnea and exertional dyspnea.

Regulation of alveologenesis: clinical implications of impaired growth

During its development that begins in intrauterine life, the lung is transformed from a simple epithelial lined sac that emerges from the foregut into a complex arrangement of blood vessels, airways, and alveoli that make up the mature lung structure. This remarkable transformation that continues for several years postnatally, is achieved by the influence of several genes, transcription factors, growth factors and hormones upon the cells and proteins of the lung bud. A seminal event in this process is the formation of the air-blood barrier within the alveolar wall, an evolutionary modification that permits independent air-breathing existence in mammals. Molecular biological techniques have enabled elucidation of the mechanistic pathways contributing to alveologenesis and have provided probable molecular bases for examples of impaired alveologenesis encountered by the paediatric pathologist.

Thin and strong! The bioengineering dilemma in the structural and functional design of the blood-gas barrier

In gas exchangers, the tissue barrier, the partition that separates the respiratory media (water/air and hemolymph/blood), is exceptional for its remarkable thinness, striking strength, and vast surface area. These properties formed to meet conflicting roles: thinness was essential for efficient flux of oxygen by passive diffusion, and strength was crucial for maintaining structural integrity. What we have designated as "three-ply" or "laminated tripartite" architecture of the barrier appeared very early in the evolution of the vertebrate gas exchanger. The design is conspicuous in the water-blood barrier of the fish gills through the lungs of air-breathing vertebrates, where the plan first appeared in lungfishes (Dipnoi) some 400 million years ago. The similarity of the structural design of the barrier in respiratory organs of animals that remarkably differ phylogenetically, behaviorally, and ecologically shows that the construction has been highly conserved both vertically and horizontally, i.e., along and across the evolutionary continuum. It is conceivable that the blueprint may have been the only practical construction that could simultaneously grant satisfactory strength and promote gas exchange. In view of the very narrow allometric range of the thickness of the blood-gas barrier in the lungs of different-sized vertebrate groups, the measurement has seemingly been optimized. There is convincing, though indirect, evidence that the extracellular matrix and particularly the type IV collagen in the lamina densa of the basement membrane is the main stress-bearing component of the blood-gas barrier. Under extreme conditions of operation and in some disease states, the barrier fails with serious consequences. The lamina densa which in many parts of the blood-gas barrier is <50 nm thin is a lifeline in the true sense of the word.

The pulmonary manifestations of left heart failure

Determining whether a patient's symptoms are the result of heart or lung disease requires an understanding of the influence of pulmonary venous hypertension on lung function. Herein, we describe the effects of acute and chronic elevations of pulmonary venous pressure on the mechanical and gas-exchanging properties of the lung. The mechanisms responsible for various symptoms of congestive heart failure are described, and the significance of sleep-disordered breathing in patients with heart disease is considered. While the initial clinical evaluation of patients with dyspnea is imprecise, measurement of B-type natriuretic peptide levels may prove useful in this setting.

Dobutamine stress echocardiography for noninvasive assessment and risk stratification of patients with rheumatic mitral stenosis

OBJECTIVES

We sought to evaluate the impact of dobutamine stress echocardiography (DSE) in patients with known rheumatic mitral stenosis (MS) in order to assess its safety, feasibility, and prognostic correlation to well-known clinical outcomes.

BACKGROUND

Noninvasive prognostic assessment of MS still represents an unresolved task in patients with clinically challenging disease.

METHODS

Dobutamine stress echocardiography was performed in 53 patients with MS (8 males; age 37.4 +/- 11.3 years) with no major complications.

RESULTS

During follow-up (60.5 +/- 11.0 months), 29 patients presented with clinical events: 16 hospitalizations, seven cases of acute pulmonary edema, and six symptomatic supraventricular arrhythmias. On multivariate analysis, the diastolic mitral valve mean gradient at peak DSE (DSE-MG) was the best predictor of clinical events (p < 0.008), especially in patients with moderate disease (p < 0.001). The best performance of DSE for the detection of clinical events was obtained at a cut-off value of 18 mm Hg DSE-MG (sensitivity 90%, specificity 87%, and accuracy 90%). The addition of DSE to the conventional cardiology work-up would allow a 17% increment for the detection of high-risk patients in the entire population and a 40% increment in patients with presumed moderate disease.

CONCLUSIONS

In patients with MS, DSE is a safe and highly feasible stress test. A DSE-MG > or =18 mm Hg identifies a subgroup of high-risk patients in whom a more aggressive approach may be warranted; on the other hand, patients with a DSE-MG <18 mm Hg predicts an uneventful clinical course and may justify a more conservative strategy.

Thoughts on the pulmonary blood-gas barrier

The pulmonary blood-gas barrier is an extraordinary structure because of its extreme thinness, immense strength, and enormous area. The essential components of the barrier were determined early in evolution and have been highly conserved. For example, the barriers of the African, Australian, and South American lungfish that date from as much as 400 million years ago have essentially the same structure as in the modern mammal or bird. In the evolution of vertebrates from bony fishes through amphibia, reptiles, and ultimately mammals and birds, changes in the pulmonary circulation occurred to limit the stresses in the blood-gas barrier. Only in mammals and birds is there a complete separation of the pulmonary and systemic circulations, which is essential to protect the extremely thin barrier from the necessary high-vascular pressures. To provide the blood-gas barrier with its required strength, evolution has exploited the high ultimate tensile strength of type IV collagen in basement membrane. Nevertheless, stress failure of the barrier occurs under physiological conditions in galloping Thoroughbred racehorses and also apparently in elite human athletes at maximal exercise. The human blood-gas barrier maintains its integrity during all but the most extreme physiological conditions. However, many pathological conditions cause stress failure. The structure of the blood-gas barrier is apparently continually regulated in response to wall stress, and this regulation is essential to maintain the extreme thinness but adequate strength. The mechanisms of this regulation remain to be elucidated and constitute one of the fundamental problems in lung biology.

Invited review: pulmonary capillary stress failure

The pulmonary blood-gas barrier is an extraordinary bioengineering structure because of its vast area but extreme thinness. Despite this, almost no attention has been given to its mechanical properties. The remarkable area and thinness come about because gas exchange occurs by passive diffusion. However, the barrier also needs to be immensely strong to withstand the very high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the thin region of the barrier comes from type IV collagen in the basement membranes. When the stresses in the capillary walls rise to high levels, ultrastructural changes occur in the barrier, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include severe exercise in elite human athletes. Animals that have been selectively bred for high aerobic activity, such as Thoroughbred racehorses, consistently break their pulmonary capillaries during galloping. Pathophysiological conditions causing stress failure include high-altitude pulmonary edema and overinflation of the lung, which frequently occurs with mechanical ventilation. Remodeling of the capillary wall occurs in response to increased wall stress in diseases such as mitral stenosis. The barrier is able to maintain its extreme thickness with sufficient strength as a result of continual regulation of its wall structure. How it does this is a central problem in lung biology.

Structure, strength, failure, and remodeling of the pulmonary blood-gas barrier

The pulmonary blood-gas barrier needs to satisfy two conflicting requirements. It must be extremely thin for efficient gas exchange, but also immensely strong to withstand the extremely high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the blood-gas barrier on the thin side is attributable to the type IV collagen in the basement membranes. However, when the wall stresses rise to very high levels, ultrastructural changes in the barrier occur, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include intense exercise in elite human athletes. Some animals, such as Thoroughbred racehorses, consistently break their alveolar capillaries during galloping, causing hemorrhage. Pathophysiological conditions causing stress failure include neurogenic pulmonary edema, high-altitude pulmonary edema, left heart failure, and overinflation of the lung. Remodeling of the capillary wall occurs in response to increased wall stress, a good example being the thickening of the capillary basement membrane in diseases such as mitral stenosis. The blood-gas barrier is able to maintain its extreme thinness with sufficient strength only through continual regulation of its wall structure. Recent experimental work suggests that rapid changes in gene expression for extracellular matrix proteins and growth factors occur in response to increases in capillary wall stress. How the blood-gas barrier is regulated to be extremely thin but sufficiently strong is a central issue in lung biology.

Pulmonary vasoconstriction induced by mitral valve obstruction in sheep

We hypothesized that left atrial hypertension results in pulmonary vasoconstriction, which is obscured by the expected passive decrease in pulmonary vascular resistance. The objectives of this study were to demonstrate and quantify the vasoconstrictive changes that occur in the pulmonary circulation during experimental left atrial hypertension, to determine the site of vasoconstriction, and to explore its mechanism. Sheep were instrumented for measurement of pulmonary arterial (Ppa), left atrial (Pla), and systemic arterial pressures (Psa) with a Foley balloon catheter to variably obstruct the mitral valve. Distal pulmonary arterial wedge pressure (Ppaw) was determined by using a 5-Fr Swan-Ganz catheter that was advanced until it wedged with the balloon deflated. Cardiac output (CO) was estimated by thermodilution; pulmonary vascular resistances (PVR) were calculated as mean (Ppa - Pla)/CO = total PVR, (Ppa - Ppaw)/CO = upstream PVR, and (Ppaw - Pla)/CO = downstream PVR. We studied 15 awake sheep at baseline and during increases in Pla of 10 and 20 cmH2O, with and without inhalation of approximately 36 parts per million of nitric oxide. Left atrial hypertension resulted in elevation of Ppa. CO decreased only slightly at both levels of Pla elevation. Nitric oxide inhalation caused a significant decrease in PVR, which was greater as Pla increased. This vasodilator effect was most striking in downstream vessels. Experiments with phentolamine, atropine, and ibuprofen failed to reveal the mechanism of the reactive pulmonary vasoconstriction.

Inhaled corticosteroid improves bronchial reactivity and decreases symptoms in patients with mitral stenosis

STUDY OBJECTIVE

To determine if treatment with inhaled budesonide forte can diminish increased bronchial hyperreactivity and improve symptoms in patients with mitral valve stenosis.

DESIGN

The study was randomized, double blind, and placebo controlled.

SETTING

Outpatient/university hospital.

PATIENTS

Twelve subjects, 8 female and 4 male, who qualified for mitral valve replacement. All subjects presented with increased bronchial reactivity to histamine at the time of the study.

INTERVENTIONS

Patients received placebo or budesonide forte twice a day (1,200 mg/d) for 6 weeks. During the study, patients were treated with the same doses of diuretics and other medications that could affect bronchial reactivity.

MEASUREMENTS

Spirometry, provocative concentration of histamine causing a 20% fall in the FEV1 (PC20H), symptom scores.

RESULTS

In the treated group, the initial PC20H was 0.82+/-0.72 mg/mL; in the placebo group 1.39+/-1.3 mg/mL. After 6 weeks of treatment, PC20H was significantly higher (3.07+/-2.28 mg/mL; p > 0.01) in the budesonide-treated group and remained unchanged in the placebo group (1.49+/-0.91). Symptom scores were significantly lower after administration of budesonide forte (mean change, 4.0+/-2.6).

CONCLUSIONS

Six weeks of treatment with budesonide forte significantly decreased bronchial reactivity to histamine and improved symptoms in patients with mitral valve stenosis.

Stress failure of the blood-gas barrier

The blood-gas barrier must be extremely thin because oxygen and carbon dioxide cross the alveolar-capillary membrane by passive diffusion, and the diffusion resistance is proportional to thickness. Despite its remarkable size (harmonic mean thickness approximately 0.6 microm) the membrane must be immensely strong, because maintenance of its integrity is fundamental for pulmonary gas exchange. The basement membrane is probably the principal anatomical structure providing the strength of the blood-gas barrier. Experimental studies have demonstrated that wall stress of the capillaries can become very high when perfusion pressure is increased to 5.2 kPa (39 mmHg) or more, which was associated with breaks of the capillary endothelium, the alveolar epithelium, or both. These values are potentially reached or exceeded in different cardiac or pulmonary diseases, or in healthy humans subjected to heavy exercise. Stress failure of pulmonary capillaries may play a role in neurogenic pulmonary oedema, high-altitude pulmonary oedema, re-expansion pulmonary oedema, and some forms of the adult respiratory distress syndrome. Increased alveolar pressure due to lung inflation potentiates damage of the blood-gas barrier, suggesting that increases in capillary transmural pressure and transpulmonary pressure are equivalent in terms of their effects on capillary wall stress. These data may have importance for the management of patients with acute respiratory failure requiring mechanical ventilation.

CHANGES IN PULMONARY VASCULATURE IN LUNG DISEASES WHICH LEAD TO PULMONARY HYPERTENSION

The pulmonary vasculature was studied in lung biopsy/autopsy specimens of 20 cases, in conditions likely to lead to pulmonary hypertension. The changes were classified as per Edward and Heath classification and morphometric measurements to gauge medial and intimai hypertrophy were done. Medial hypertrophy was found to be the earliest and commonest change in all cases, irrespective of the pathogenic mechanism of pulmonary hypertension. Variable changes specific to various arteriopathies/vasculopathies were noted.

The sensation of dyspnea during exercise is not determined by the work of breathing in patients with heart failure

OBJECTIVES

The present study sought to investigate whether the work of breathing was reduced after heart transplantation. Accordingly, the tension time index of the diaphragm was measured in patients with heart failure and in transplant recipients.

BACKGROUND

Patients with heart failure are frequently limited by exertional dyspnea that may be due to the increased work of breathing. After heart transplantation, exertional dyspnea is markedly diminished. Whether work of breathing is reduced in posttransplant recipients is unknown.

METHODS

Nine patients with heart failure, six normal subjects and six heart transplant recipients were studied. Transdiaphragmatic pressure was measured throughout exercise. Accessory respiratory muscle oxygenation was assessed using near-infrared spectroscopy. Peak oxygen consumption, time in inspiration, time per breath and maximal inspiratory and expiratory pressures were measured in all subjects.

RESULTS

The tension time index remained markedly abnormal after heart transplantation both at rest ([mean +/- SD] normal group 0.01 +/- 0.006, heart failure group 0.026 +/- 0.018, transplant group 0.058 +/- 0.015, p < 0.004) and at peak exercise (normal group 0.03 +/- 0.02, heart failure group 0.10 +/- 0.03, transplant group 0.10 +/- 0.04, p < 0.0001). Accessory respiratory muscle deoxygenation was present only in patients with heart failure (near-infrared absorbency changes [arbitrary units]: normal group -3 +/- 6, heart failure group 28 +/- 5, transplant group -3.5 +/- 4.4, p < 0.0001).

CONCLUSIONS

Although heart transplantation alleviates dyspnea in patients with heart failure, the work of breathing as assessed by the tension time index of the diaphragm is not decreased. Amelioration of exertional dyspnea is achieved by other mechanisms, such as improved respiratory muscle perfusion.

Vulnerability of pulmonary capillaries in heart disease

The pulmonary blood-gas barrier presents a dilemma. It must be extremely thin for efficient gas exchange. However, it also needs to be immensely strong because the stresses in the pulmonary capillary wall become extremely high when the capillary pressure rises. Stress failure of the capillaries occurs in several pathological conditions. It causes high-permeability edema as in neurogenic pulmonary edema or high-altitude pulmonary edema; alveolar hemorrhage, which occurs in all galloping racehorses; or a combination of the two as in severe congestive heart failure. The vulnerability of the capillary wall to increased mechanical stress has not previously been sufficiently appreciated.

Pulmonary factors limiting exercise capacity in patients with heart failure

Patients with heart failure are frequently limited by exertional dyspnea. The mechanisms underlying dyspnea in these patients remain unclear. In this review, the pathologic changes that occur in the lung as a consequence of chronic pulmonary venous hypertension, pulmonary function test abnormalities, and potential mechanisms for dyspnea including airflow obstruction and/or respiratory muscle dysfunction are discussed.

Comparative aspects of the strength of pulmonary capillaries in rabbit, dog, and horse

In previous studies of rabbit and dog lung, we demonstrated stress failure of pulmonary capillaries at high transmural pressures (Ptm). The Ptm necessary to elicit stress failure was 40 cmH2O higher in dog than rabbit, and the total blood-gas barrier (BGB) thickness was greater in dog than rabbit. This suggests that stress failure may be related to BGB thickness, and is consistent with the Laplace relationship which states that wall stress is proportional to capillary radius but inversely proportional to wall thickness. In the present studies, we compared BGB thickness and an index of capillary radius in lungs from 3 rabbits, 3 dogs, and 2 horses perfusion fixed at a Ptm of approximately 30 cmH2O. Thicknesses of the BGB were measured at right angles to the barrier at random points on the capillary wall determined by test line intersections. Capillary radius was determined from the mean of major and minor axes measured on electron micrographs. Capillary pressure for failure in the horse was taken to be the mean of pulmonary arterial and left atrial pressures observed in galloping thoroughbreds known to develop exercise-induced pulmonary hemorrhage, although the actual pressure required for failure may be less than this. Average capillary radii were 3.6, 3.4, and 3.2 microns for rabbits, dogs, and horses, respectively. We found that the BGB was thinnest in the rabbit, intermediate in the dog, and thickest in the horse. Calculated capillary wall stress values for the median total BGB thickness at a nominal Ptm of 30 cmH2O were 2.5 x 10(4), 1.7 x 10(4), and 1.5 x 10(4) N.m-2 for rabbits, dogs, and horses, respectively. This species ranking fits with the pressures required to cause stress failure which are approximately 50, 90, and 130 cmH2O in rabbit, dog, and horse, respectively. We conclude that the differences in capillary radius of curvature and BGB thickness account for some of the observed differences in Ptm necessary to cause stress failure. However, other factors may also be important in determining the strength of the BGB.

Bronchial hyperreactivity in patients with moderate pulmonary circulation overload

The clinical course of congestive heart failure (CHF) and mitral valve stenosis (MVS) is accompanied by episodes of dyspnea, wheezing, and cough, symptoms also observed in patients with bronchial hyperreactivity. However, it is still controversial whether bronchial hyperreactivity is demonstrable in patients with chronic overload of the pulmonary circulation. In order to examine the effects of CHF on the respiratory function, we performed pulmonary function tests, titrated bronchial acetylcholine provocations, and left and right heart catheterization in 21 patients with impaired left ventricular function (mean ejection fraction, 37 percent, NYHA class 3), 5 patients with MVS, and 17 control patients with coronary artery disease (mean ejection fraction, 63 percent). Bronchial hyperresponsiveness was defined as an obstructive response to increased doses of inhaled acetylcholine. A 20 percent fall in forced expiratory volume in the first second (FEV1), a 100 percent increase in total airway resistance (Rtot), and a 60 percent reduction of pulmonary conductance (SGtot) were considered positive. Patients with impaired left ventricular function showed significantly higher airway resistance, and lower airway conductance at the maximal tolerated acetylcholine dose compared with control patients. Patients with MVS had a significant lower airway conductance. The induced bronchial obstruction was completely reversible upon inhalation of a beta 2-mimetic. We conclude that chronic overload of the pulmonary circulation is accompanied by bronchial hyperreactivity that may augment the symptoms of dyspnea in patients with CHF and MVS.

Bronchial hyperreactivity in patients with mitral valve disease

To elicit the mechanism of bronchial hyperreactivity (BHR) in chronic heart failure (CHF), a methacholine inhalation test, pulmonary function test, and cardiac catheterization were performed in 19 patients with mitral valve disease (MVD), and the change of severity of BHR before and after mitral valve replacement (MVR) was also examined in seven of 19 patients with MVD. Sixteen of 19 patients with MVD showed significant increase in respiratory resistance in methacholine inhalation test, while all normal subjects did not. The maximal expiratory flow at 25 percent of vital capacity (Vmax25), a parameter of small airway disease, correlated significantly with log cumulative dose producing a 35 percent decrease in respiratory conductance (PD35Grs) (r = 0.536) and the duration of symptoms (r = -0.682). There was a significant correlation between log PD35Grs and mean pulmonary artery wedge pressure (r = -0.466). After MVR, log PD35Grs was significantly improved in all seven operated-on patients, although six patients retained BHR. We conclude that patients with long-term MVD have marked BHR and that BHR in long-term MVD is related to peripheral airway narrowing with organic remodeling, which was not ameliorated with MVR procedure, in addition to pulmonary congestion.

Changes in pulmonary circulation in severe bronchopulmonary dysplasia

Eight patients with severe bronchopulmonary dysplasia underwent cardiac catheterisation. Seven had a pulmonary vascular resistance greater than 3 mm Hg.l-1 min.m2 (mean 8.9, range 2.2-13.8). All had raised intrapulmonary shunts (mean 25.6%, range 5.4-50%, normal less than 5%). Two had a high alveolar dead space, and two had unsuspected congenital heart disease. Epoprostenol (prostacyclin), but not 100% oxygen, caused a significant fall in pulmonary vascular resistance. Death was associated with a high pulmonary vascular resistance and a high shunt. Morphometric studies in three cases showed normal numbers of airways, but increased thickness of bronchial muscle. The numbers of alveoli were reduced and the walls thickened. There was increased medial thickness in small pulmonary arteries with distal extension of muscle. In the oldest child some vessels were obliterated by fibrosis. We speculate that measurements of pulmonary vascular resistance and shunt may have prognostic value; that a trial of pulmonary vasodilators other than oxygen might be worthwhile in patients with poor prognosis; and that abnormalities of the pulmonary circulation contribute to the difficulties of managing patients with bronchopulmonary dysplasia.