Longitudinal distribution of vascular resistance in the pulmonary arteries, capillaries, and veins

Effects of nitric oxide inhalation on pulmonary arterial impedance: differences between normal and pulmonary hypertension male rats

Nitric oxide (NO) inhalation improves pulmonary hemodynamics in participants with pulmonary arterial hypertension (PAH). Although it can reduce pulmonary vascular resistance (PVR) in PAH, its impact on the dynamic mechanics of pulmonary arteries and its potential difference between control and participants with PAH remain unclear. PA impedance provides a comprehensive description of PA mechanics. With an arterial model, PA impedance can be parameterized into peripheral pulmonary resistance (Rp), arterial compliance (Cp), characteristic impedance of the proximal arteries (Zc), and transmission time from the main PA to the reflection site. This study investigated the effects of inhaled NO on PA impedance and its associated parameters in control and monocrotaline-induced pulmonary arterial hypertension (MCT-PAH) male rats (6/group). Measurements were obtained at baseline and during NO inhalation at 40 and 80 ppm. In both groups, NO inhalation decreased PVR and increased the left atrial pressure. Notably, its impact on PA impedance was frequency dependent, as revealed by reduced PA impedance modulus in the low-frequency range below 10 Hz, with little effect on the high-frequency range. Furthermore, NO inhalation attenuated Rp, increased Cp, and prolonged transmission time without affecting Zc. It reduced Rp more pronouncedly in MCT-PAH rats, whereas it increased Cp and delayed transmission time more effectively in control rats. In conclusion, the therapeutic effects of inhaled NO on PA impedance were frequency dependent and may differ between the control and MCT-PAH groups, suggesting that the effect on the mechanics differs depending on the pathological state.NEW & NOTEWORTHY Nitric oxide inhalation decreased pulmonary arterial impedance in the low-frequency range (<10 Hz) with little impact on the high-frequency range. It reduced peripheral pulmonary resistance more pronouncedly in pulmonary hypertension rats, whereas it increased arterial compliance and transmission time in control rats. Its effect on the mechanics of the pulmonary arteries may differ depending on the pathological status.

Effects of recombinant human soluble thrombomodulin on neutrophil extracellular traps in the kidney of a mouse model of endotoxin shock

OBJECTIVES

Sepsis is a life-threatening condition characterized by multi-organ dysfunction due to host immune system dysregulation in response to an infection. During sepsis, neutrophils release neutrophil extracellular traps (NETs) as part of the innate immune response. However, excessive NETs play a critical role in the development of organ failure during sepsis. Although recombinant human soluble thrombomodulin (rTM) can inhibit NET formation in the lungs and liver of a mouse model of endotoxin shock, its effects on the kidneys are unclear.

METHODS

The specific effects of NETs and rTM on the renal cortex and renal medulla were examined in a mouse model of endotoxin shock generated by intraperitoneal (i.p.) injection of lipopolysaccharide (LPS), followed by i.p. injection of rTM or an identical volume of saline 1 h later.

RESULTS

LPS injection increased serum creatinine, blood urea nitrogen, and histone H3 levels. However, rTM administration significantly decreased histone H3 and citrullinated histone H3 (citH3) levels. Immunohistochemical analysis revealed no significant changes in citH3 quantity in the renal cortex of any group. However, in the renal medulla, the increase in citH3 induced by LPS was abolished in the LPS+rTM group.

CONCLUSIONS

Our findings demonstrate that rTM can suppress NETs in the renal medulla of mice with endotoxin-induced acute kidney injury.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Hemodynamic function of the right ventricular-pulmonary vascular-left atrial unit: normal responses to exercise in healthy adults

With each heartbeat, the right ventricle (RV) inputs blood into the pulmonary vascular (PV) compartment, which conducts blood through the lungs at low pressure and concurrently fills the left atrium (LA) for output to the systemic circulation. This overall hemodynamic function of the integrated RV-PV-LA unit is determined by complex interactions between the components that vary over the cardiac cycle but are often assessed in terms of mean pressure and flow. Exercise challenges these hemodynamic interactions as cardiac filling increases, stroke volume augments, and cycle length decreases, with PV pressures ultimately increasing in association with cardiac output. Recent cardiopulmonary exercise hemodynamic studies have enriched the available data from healthy adults, yielded insight into the underlying mechanisms that modify the PV pressure-flow relationship, and better delineated the normal limits of healthy responses to exercise. This review will examine hemodynamic function of the RV-PV-LA unit using the two-element Windkessel model for the pulmonary circulation. It will focus on acute PV and LA responses that accommodate increased RV output during exercise, including PV recruitment and distension and LA reservoir expansion, and the integrated mean pressure-flow response to exercise in healthy adults. Finally, it will consider how these responses may be impacted by age-related remodeling and modified by sex-related cardiopulmonary differences. Studying the determinants and recognizing the normal limits of PV pressure-flow relations during exercise will improve our understanding of cardiopulmonary mechanisms that facilitate or limit exercise.

Lung disease and pulmonary hypertension in the premature infant

The premature infant is born into the world unprepared to naturally thrive in a foreign environment. Lung development entails immense growth, structural remodeling and differentiation of specialized cells during the normal term perinatal and postnatal periods. Thus, the premature infant presents with a lung deficient for appropriate respiration. Disruption of lung development seen in bronchopulmonary dysplasia (BPD) and chronic lung disease (CLD) results in not only impaired airway growth but also a deficiency in the accompanying vasculature including the capillary system required for gas exchange. Deficient vascular area can lead to elevated pulmonary vascular resistance and the development of pulmonary hypertension (PH). Unlike PH seen in children and adults with pulmonary arterial hypertension (PAH), treatment with conventional pulmonary vasodilators can be limited in developmental lung disease-associated PH because there are fewer blood vessels to dilate. In this brief review, we highlight some of the knowledge on PH in the premature infant presented at the Proceedings of the 22^nd Annual Update on Pediatric and Congenital Cardiovascular Disease.

Multiscale Modeling of Superior Cavopulmonary Circulation: Hemi-Fontan and Bidirectional Glenn Are Equivalent

Superior cavopulmonary circulation (SCPC) can be achieved by either the Hemi-Fontan (hF) or Bidirectional Glenn (bG) connection. Debate remains as to which results in best hemodynamic results. Adopting patient-specific multiscale computational modeling, we examined both the local dynamics and global physiology to determine if surgical choice can lead to different hemodynamic outcomes. Six patients (age: 3-6 months) underwent cardiac magnetic resonance imaging and catheterization prior to SCPC surgery. For each patient: (1) a finite 3-dimensional (3D) volume model of the preoperative anatomy was constructed to include detailed definition of the distal branch pulmonary arteries, (2) virtual hF and bG operations were performed to create 2 SCPC 3D models, and (3) a specific lumped network representing each patient's entire cardiovascular circulation was developed from clinical data. Using a previously validated multiscale algorithm that couples the 3D models with lumped network, both local flow dynamics, that is, power loss, and global systemic physiology can be quantified. In 2 patients whose preoperative imaging demonstrated significant left pulmonary artery (LPA) stenosis, we performed virtual pulmonary arterioplasty to assess its effect. In one patient, the hF model showed higher power loss (107%) than the bG, while in 3, the power losses were higher in the bG models (18-35%). In the remaining 2 patients, the power loss differences were minor. Despite these variations, for all patients, there were no significant differences between the hF and bG models in hemodynamic or physiological outcomes, including cardiac output, superior vena cava pressure, right-left pulmonary flow distribution, and systemic oxygen delivery. In the 2 patients with LPA stenosis, arterioplasty led to better LPA flow (5-8%) while halving the power loss, but without important improvements in SVC pressure or cardiac output. Despite power loss differences, both hF and bG result in similar SCPC hemodynamics and physiology outcome. This suggests that for SCPC, the pre-existing patient-specific physiology and condition, such as pulmonary vascular resistance, are more deterministic in the hemodynamic performance than the type of surgical palliation. Multiscale modeling can be a decision-assist tool to assess whether an extensive LPA reconstruction is needed at the time of SCPC for LPA stenosis.

Synchrotron Imaging Shows Effect of Ventilator Settings on Intrabreath Cyclic Changes in Pulmonary Blood Volume

Despite the importance of dynamic changes in the regional distributions of gas and blood during the breathing cycle for lung function in the mechanically ventilated patient, no quantitative data on such cyclic changes are currently available. We used a novel gated synchrotron computed tomography imaging to quantitatively image regional lung gas volume (Vg), tissue density, and blood volume (Vb) in six anesthetized, paralyzed, and mechanically ventilated rabbits with normal lungs. Images were repeatedly collected during ventilation and steady-state inhalation of 50% xenon, or iodine infusion. Data were acquired in a dependent and nondependent image level, at zero end-expiratory pressure (ZEEP) and 9 cm H₂O (positive end-expiratory pressure), and a tidal volume (Vt) of 6 ml/kg (Vt1) or 9 ml/kg (Vt2) at an Inspiratory:Expiratory ratio of 0.5 or 1.7 by applying an end-inspiratory pause. A video showing dynamic decreases in Vb during inspiration is presented. Vb decreased with positive end-expiratory pressure (P = 0.006; P = 0.036 versus Vt1-ZEEP and Vt2-ZEEP, respectively), and showed larger oscillations at the dependent image level, whereas a 45% increase in Vt did not have a significant effect. End-inspiratory Vb minima were reduced by an end-inspiratory pause (P = 0.042, P = 0.006 at nondependent and dependent levels, respectively). Normalized regional Vg:Vb ratio increased upon inspiration. Our data demonstrate, for the first time, within-tidal cyclic variations in regional pulmonary Vb. The quantitative matching of regional Vg and Vb improved upon inspiration under ZEEP. Further study is underway to determine whether these phenomena affect intratidal gas exchange.

Re-endothelialization of rat lung scaffolds through passive, gravity-driven seeding of segment-specific pulmonary endothelial cells

Effective re-endothelialization is critical for the use of decellularized scaffolds for ex vivo lung engineering. Current approaches yield insufficiently re-endothelialized scaffolds that haemorrhage and become thrombogenic upon implantation. Herein, gravity-driven seeding coupled with bioreactor culture facilitated widespread distribution and engraftment of endothelial cells throughout rat lung scaffolds. Initially, human umbilical vein endothelial cells were seeded into the pulmonary artery by either gravity-driven, variable flow perfusion seeding or pump-driven, pulsatile flow perfusion seeding. Gravity seeding evenly distributed cells and supported cell survival and re-lining of the vascular walls while perfusion pump-driven seeding led to increased cell fragmentation and death. Using gravity seeding, rat pulmonary artery endothelial cells and rat pulmonary vein endothelial cells attached in intermediate and large vessels, while rat pulmonary microvascular endothelial cells deposited mostly in microvessels. Combination seeding of these cells led to positive vascular endothelial cadherin staining. In addition, combination seeding improved barrier function as assessed by serum albumin extravasation; however, leakage was observed in the distal portions of the re-endothelialized tissue suggesting that recellularization of the alveoli is necessary to complete barrier function of the capillary-alveolar network. Overall, these data indicate that vascular recellularization of rat lung scaffolds is achieved through gravity seeding. Copyright © 2016 John Wiley & Sons, Ltd.

Pulmonary embolism is more prevalent than deep vein thrombosis in cases of chronic obstructive pulmonary disease and interstitial lung diseases

BACKGROUND

Chronic lung diseases may have an influence on pulmonary vessel walls as well as on pulmonary haemodynamics. However, there is limited data on the occurrence of pulmonary embolism (PE) and deep vein thrombosis (DVT) in patients with chronic lung diseases, which have the potential to contribute to the development of pulmonary vascular abnormalities. We aimed to explore the prevalence of PE and DVT in patients with COPD and ILD.

METHODS

We evaluated the venous thromboembolism prevalence associated with COPD and ILD using Korean Health Insurance Review and Assessment Service (HIRA) data from January 2011 to December 2011. This database (HIRA-NPS-2011-0001) was created using random sampling of outpatients; 1,375,842 sample cases were collected, and 670,258 (age ≥40) cases were evaluated. Patients with COPD, ILDs, or CTD were identified using the International Classification of Disease-10 diagnostic codes.

RESULTS

The PE prevalence rates per 100,000 persons for the study population with COPD, ILD, CTD, and the general population were 1185, 1746, 412, and 113, respectively, while the DVT prevalence for each group was 637, 582, 563, and 138, respectively.

CONCLUSIONS

PE prevalence was significantly higher than that of DVT in patients with COPD or ILDs, while the prevalence of PE was lower than that for DVT in the general population or in patients with CTD.

The vital role of the right ventricle in the pathogenesis of acute pulmonary edema

The development of acute pulmonary edema involves a complex interplay between the capillary hydrostatic, interstitial hydrostatic, and oncotic pressures and the capillary permeability. We review the pathophysiological processes involved and illustrate the concepts in a number of common clinical situations including heart failure with normal and reduced ejection fractions, mitral regurgitation, and arrhythmias. We also describe other rarer causes including exercise, swimming, and diving-induced acute pulmonary edema. We suggest a unifying framework in which the critical abnormality is a mismatch or imbalance between the right and left ventricular stroke volumes. In conclusion, we hypothesize that increased right ventricular contraction is an important contributor to the sudden increase in capillary hydrostatic pressure, and therefore, a central mechanism involved in the development of alveolar edema.

Patient-specific assessment of cardiovascular function by combination of clinical data and computational model with applications to patients undergoing Fontan operation

The assessment of cardiovascular function is becoming increasingly important for the care of patients with single-ventricle defects. However, most measurement methods available in the clinical setting cannot provide a separate measure of cardiac function and loading conditions. In the present study, a numerical method has been proposed to compensate for the limitations of clinical measurements. The main idea was to estimate the parameters of a cardiovascular model by fitting model simulations to patient-specific clinical data via parameter optimization. Several strategies have been taken to establish a well-posed parameter optimization problem, including clinical data-matched model development, parameter selection based on an extensive sensitivity analysis, and proper choice of parameter optimization algorithm. The numerical experiments confirmed the ability of the proposed parameter optimization method to uniquely determine the model parameters given an arbitrary set of clinical data. The method was further tested in four patients undergoing the Fontan operation. Obtained results revealed a prevalence of ventricular abnormalities in the patient cohort and at the same time demonstrated the presence of marked inter-patient differences and preoperative to postoperative changes in cardiovascular function. Because the method allows a quick assessment and makes use of clinical data available in clinical practice, its clinical application is promising.

Hemodynamic performance of the Fontan circulation compared with a normal biventricular circulation: a computational model study

The physiological limitations of the Fontan circulation have been extensively addressed in the literature. Many studies emphasized the importance of pulmonary vascular resistance in determining cardiac output (CO) but gave little attention to other cardiovascular properties that may play considerable roles as well. The present study was aimed to systemically investigate the effects of various cardiovascular properties on clinically relevant hemodynamic variables (e.g., CO and central venous pressure). To this aim, a computational modeling method was employed. The constructed models provided a useful tool for quantifying the hemodynamic effects of any cardiovascular property of interest by varying the corresponding model parameters in model-based simulations. Herein, the Fontan circulation was studied compared with a normal biventricular circulation so as to highlight the unique characteristics of the Fontan circulation. Based on a series of numerical experiments, it was found that 1) pulmonary vascular resistance, ventricular diastolic function, and systemic vascular compliance play a major role, while heart rate, ventricular contractility, and systemic vascular resistance play a secondary role in the regulation of CO in the Fontan circulation; 2) CO is nonlinearly related to any single cardiovascular property, with their relationship being simultaneously influenced by other cardiovascular properties; and 3) the stability of central venous pressure is significantly reduced in the Fontan circulation. The findings suggest that the hemodynamic performance of the Fontan circulation is codetermined by various cardiovascular properties and hence a full understanding of patient-specific cardiovascular conditions is necessary to optimize the treatment of Fontan patients.

Radius exponent in elastic and rigid arterial models optimized by the least energy principle

It was analyzed in normal physiological arteries whether the least energy principle would suffice to account for the radius exponent x. The mammalian arterial system was modeled as two types, the elastic or the rigid, to which Bernoulli's and Hagen-Poiseuille's equations were applied, respectively. We minimized the total energy function E, which was defined as the sum of kinetic, pressure, metabolic and thermal energies, and loss of each per unit time in a single artery transporting viscous incompressible blood. Assuming a scaling exponent α between the vessel radius (r) and length (l) to be 1.0, x resulted in 2.33 in the elastic model. The rigid model provided a continuously changing x from 2.33 to 3.0, which corresponded to Uylings' and Murray's theories, respectively, through a function combining Reynolds number with a proportional coefficient of the l - r relationship. These results were expanded to an asymmetric arterial fractal tree with the blood flow preservation rule. While x in the optimal elastic model accounted for around 2.3 in proximal systemic (r >1 mm) and whole pulmonary arteries (r ≥0.004 mm), optimal x in the rigid model explained 2.7 in elastic-muscular (0.1 < r ≤1 mm) and 3.0 in peripheral resistive systemic arteries (0.004 ≤ r ≤0.1 mm), in agreement with data obtained from angiographic, cast-morphometric, and in vivo experimental studies in the literature. The least energy principle on the total energy basis provides an alternate concept of optimality relating to mammalian arterial fractal dimensions under α = 1.0.

Physiological and pathological angiogenesis in the adult pulmonary circulation

Angiogenesis occurs during growth and physiological adaptation in many systemic organs, for example, exercise-induced skeletal and cardiac muscle hypertrophy, ovulation, and tissue repair. Disordered angiogenesis contributes to chronic inflammatory disease processes and to tumor growth and metastasis. Although it was previously thought that the adult pulmonary circulation was incapable of supporting new vessel growth, over that past 10 years new data have shown that angiogenesis within this circulation occurs both during physiological adaptive processes and as part of the pathogenic mechanisms of lung diseases. Here we review the expression of vascular growth factors in the adult lung, their essential role in pulmonary vascular homeostasis and the changes in their expression that occur in response to physiological challenges and in disease. We consider the evidence for adaptive neovascularization in the pulmonary circulation in response to alveolar hypoxia and during lung growth following pneumonectomy in the adult lung. In addition, we review the role of disordered angiogenesis in specific lung diseases including idiopathic pulmonary fibrosis, acute adult distress syndrome and both primary and metastatic tumors of the lung. Finally, we examine recent experimental data showing that therapeutic enhancement of pulmonary angiogenesis has the potential to treat lung diseases characterized by vessel loss.

Partitioning pulmonary vascular resistance using the reservoir-wave model

The conventional determination of pulmonary vascular resistance does not indicate which vascular segments contribute to the total resistance of the pulmonary circulation. Using measurements of pressure and flow, the reservoir-wave model can be used to partition total pulmonary vascular resistance into arterial, microcirculation, and venous components. Changes to these resistance components are investigated during hypoxia and inhaled nitric oxide, volume loading, and positive end-expiratory pressure. The reservoir-wave model defines the pressure of a volume-related reservoir and the asymptotic pressure. The mean values of arterial and venous reservoir pressures and arterial and venous asymptotic pressures define a series of resistances between the main pulmonary artery and the pulmonary veins: the resistance of large and small arteries, the microcirculation, and veins. In 11 anaesthetized, open-chest dogs, pressure and flow were measured in the main pulmonary artery and a single pulmonary vein. Volume loading reduced each vascular resistance component, whereas positive end-expiratory pressure only increased microcirculation resistance. Hypoxia increased the resistance of small arteries and veins, whereas nitric oxide only decreased small-artery resistance significantly. The reservoir-wave model provides a novel method to deconstruct total pulmonary vascular resistance. The results are consistent with the expected physiological responses of the pulmonary circulation and provide additional information regarding which segments of the pulmonary circulation react to hypoxia and nitric oxide.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Distribution of perfusion

Local driving pressures and resistances within the pulmonary vascular tree determine the distribution of perfusion in the lung. Unlike other organs, these local determinants are significantly influenced by regional hydrostatic and alveolar pressures. Those effects on blood flow distribution are further magnified by the large vertical height of the human lung and the relatively low intravascular pressures in the pulmonary circulation. While the distribution of perfusion is largely due to passive determinants such as vascular geometry and hydrostatic pressures, active mechanisms such as vasoconstriction induced by local hypoxia can also redistribute blood flow. This chapter reviews the determinants of regional lung perfusion with a focus on vascular tree geometry, vertical gradients induced by gravity, the interactions between vascular and surrounding alveolar pressures, and hypoxic pulmonary vasoconstriction. While each of these determinants of perfusion distribution can be examined in isolation, the distribution of blood flow is dynamically determined and each component interacts with the others so that a change in one region of the lung influences the distribution of blood flow in other lung regions.

(18)FDG PET imaging can quantify increased cellular metabolism in pulmonary arterial hypertension: A proof-of-principle study

The past decade has seen increased application of 18-flurodeoxyglucose positron emission tomography (¹⁸FDG-PET) imaging to help diagnose and monitor disease, particularly in oncology, vasculitis and atherosclerosis. Disordered glycolytic metabolism and infiltration of plexiform lesions by inflammatory cells has been described in idiopathic pulmonary arterial hypertension (IPAH). We hypothesized that increased ¹⁸FDG uptake may be present in the lungs, large pulmonary arteries and right ventricle of patients with pulmonary hypertension, and that this uptake would be related to markers of immune activation. We imaged the thorax of 14 patients with pulmonary hypertension (idiopathic and chronic thromboembolic) and six controls by ¹⁸FDG-PET/computed tomography (CT) and measured uptake in the lung parenchyma, large pulmonary arteries and right ventricle. ¹⁸FDG uptake in the lungs and pulmonary arteries was normalized for venous blood activity to give a target-to-background ratio (TBR). Blood was contemporaneously drawn for high-sensitivity CRP - C-reactive protein (CRP) (hsCRP), N-Terminal Probrain natriuteric peptide (NT-ProBNP) and other inflammatory cytokines. IPAH patients had significantly higher lung parenchymal TBR (P=0.034) and right ventricle FDG uptake (P=0.007) than controls. Uptake in the main pulmonary arteries was similar in chronic thromboembolic pulmonary hypertension, IPAH and controls. There were no correlations between ¹⁸FDG uptake and hsCRP or inflammatory cytokine levels. NT-ProBNP correlated with RV uptake in those with pulmonary hypertension (r=0.55, P=0.04). In this pilot study, we found increased ¹⁸FDG uptake in the lung parenchyma and right ventricle of subjects with IPAH. The lung uptake might be useful as a surrogate marker of increased cellular metabolism and immune activation as underlying mechanisms in this disease. Further evaluation of the impact of targeted therapies in treatment-naïve patients and the significance of right ventricular uptake is suggested.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Model combining hydrodynamics and fractal theory for analysis of in vivo peripheral pulmonary and systemic resistance of shunt cardiac defects

The fractal state of the arterial vascular tree is considered to have a universal dimension related to the principle of minimum work rate, but can demonstrate the capacity to adapt to other dimensions in disease states such as congenital high-flow pulmonary hypertension (PH) by a process that is incompletely understood. To document and interpret fractal adaptation in patients with different degrees of PH, pulmonary and systemic vascular resistance was analyzed by a model that evaluated the fractal dimension, x, of the Poiseuille resistance contribution of the arterial vessel radius between 10 and 100μm, via the proportionality Q∝(R(peri)/BL)(-x/4), with Q, R(peri), and BL clinically observed variables representing total pulmonary or systemic blood flow, its peripheral arterial resistance, and body length, respectively. Identification of x in the pulmonary (P) and systemic (S) beds was evaluated from hemodynamic data of 213 patients, categorized into 7 groups by PH grade. In controls without PH, x(P)=2.2 while the dimension increased to 3.0, with the systemic dimension constant at x(S)=3.1. Our model predicts that severe grades of PH are associated with: a more elongated and hindered vessel in the periphery, and reductions in vessel numbers, as unit pulmonary resistive arterial trees (N(1)) and their component intra-acinar arteries (N(W)). These model network changes suggest a complex adaptive process of arterial network reorganization in the pulmonary circulation to minimize the work rate of high-flow congenital heart defects.

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).

Cellular and molecular mechanisms of pulmonary vascular remodeling: role in the development of pulmonary hypertension

Pulmonary artery vasoconstriction and vascular remodeling greatly contribute to a sustained elevation of pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) in patients with pulmonary arterial hypertension (PAH). The development of PAH involves a complex and heterogeneous constellation of multiple genetic, molecular, and humoral abnormalities, which interact in a complicated manner, presenting a final manifestation of vascular remodeling in which fibroblasts, smooth muscle and endothelial cells, and platelets all play a role. Vascular remodeling is characterized largely by medial hypertrophy due to enhanced vascular smooth muscle cell proliferation or attenuated apoptosis and to endothelial cell over-proliferation, which can result in lumen obliteration. In addition to other factors, cytoplasmic Ca2+ in particular seems to play a central role as it is involved in both the generation of force through its effects on the contractile machinery, and the initiation and propagation of cell proliferation via its effects on transcription factors, mitogens, and cell cycle components. This review focuses on the role played by cellular factors, circulating factors, and genetic molecular signaling factors that promote a proliferative, antiapoptotic, and vasoconstrictive physiological milieu leading to vascular remodeling.

Evidence for specific regional patterns of responses to different vasoconstrictors and vasodilators in sheep isolated pulmonary arteries and veins

1. Responses of large (5-7 mm in diameter) and medium sized (3-4 mm in diameter) branches of sheep isolated intrapulmonary arteries and veins and three groups of small pulmonary arteries (200, 500 and 1000 microns diameter) to the vasoconstrictors endothelin-1, 5-hydroxytryptamine (5-HT), noradrenaline and the thromboxane A2 mimetic, U46619, were examined. Also, relaxation responses to the endothelium-dependent vasodilators, acetylcholine (ACh), bradykinin and ionomycin and the endothelium-independent vasodilator, sodium nitroprusside (SNP), were studied to determine their predominant site of action within the pulmonary vasculature. 2. Endothelin-1 was the most potent vasoconstrictor tested in all vessels. The maximum response to endothelin-1, expressed as a percentage of the maximum contraction to KC1 depolarization, did not differ significantly between the different vessels. By contrast, pulmonary arteries greater than 200 microns in diameter failed to contract to U46619, whereas U46619 was a potent constrictor of large and medium-sized veins. 3. 5-HT caused similar contractions in all arteries > 200 microns in diameter, but the maximum response was significantly diminished in smaller arteries. By contrast, the maximum response to noradrenaline was progressively attenuated with decreasing arterial diameter. Both 5-HT and noradrenaline caused poor contractions in veins. Pulmonary veins were less sensitive to 5-HT than arteries and at low concentrations 5-HT caused relaxation. No change in sensitivity to noradrenaline was noted between the arteries and veins. 4. Relaxation responses to bradykinin and ionomycin decreased progressively along the pulmonary vascular tree and were nearly absent in large veins. Also, ACh was a poor relaxing agent of large and medium-sized arteries and failed to mediate any relaxation response in other vessel segments. Surprisingly the smallest arteries examined (approximately 200 microns in diameter) failed to relax to ionomycin, bradykinin and SNP. However, both the sensitivity and maximum relaxation to SNP were similar in all other arterial and venous segments. 5. In conclusion, marked regional differences in reactivity to both vasoconstrictors and vasodilators occur in arterial and venous segments of the sheep isolated pulmonary vasculature. Such specialization may have important implications for the regulation of resistance in this low tone vascular bed.

Pulmonary capillary pressure. Clinical implications

Pulmonary capillary pressure (Pcap) is the true edema-forming pressure within the pulmonary vascular bed. Pulmonary artery occlusion pressure has long been used to approximate Pcap. These two pressures may not always be well correlated, which has significant implications for fluid resuscitation and the evolution of pulmonary edema. This article reviews the technique for bedside measurement of Pcap.

Elevated pulmonary artery systolic storage volume associated with improved ventilation-to-perfusion ratios in acute respiratory failure

The possibility that an elevated pulmonary artery systolic storage volume (PASSV) correlates with improved overall ratios of ventilation-to-perfusion and hence benefits gas exchange in acute respiratory failure was examined. We examined this by assessing the correlation between PASSV and both the physiologic dead space to tidal volume ratio (VD/VT) and intrapulmonary shunt fraction (Qsp/Qt). The VD/VT and Qsp/Qt were used as an index of distribution of ventilation-to-perfusion as well as efficiency of pulmonary gas exchange. Twenty-eight patients suffering from acute respiratory failure were included. All required mechanical ventilation. PASSV was calculated from the pulmonary artery (PA) compliance and mean PA systolic distending pressure. Pulmonary arteriolar pressures were computed by Fourier analysis. PA compliance was derived from the PA time constant and the PA resistance. Storage volume fraction of stroke volume index (PASSV/SVI) was used to compare individual variations. There were inverse linear relationships between PASSV/SVI and VD/VT (r = 0.693, p less than 0.0001), and between PASSV/SVI and Qsp/Qt (r = 0.427, p = 0.012). Also, a direct correlation was found between VD/VT and PA time constant (r = 0.503, p = 0.002). The patients were divided into two groups based on PASSV/SVI to evaluate the effect of other hemodynamic data on PASSV. Comparison of the two groups revealed that VD/VT and Qsp/Qt were lower (p less than 0.0001, and p = 0.018, respectively), PA time constant was higher (p less than 0.001), and right ventricular stroke-work index was higher (p = 0.005) in the group with a high PASSV/SVI. There were no differences in other hemodynamic data between the two groups. These data suggest that an elevated PASSV may indeed benefit gas exchange in acute respiratory failure.

Permeability of the endothelium and partitioning of the pulmonary blood flow resistance in isolated perfused pig lungs: effects of breed and age

The right and left lungs of 5 healthy Minipigs and of 13 healthy Landrace piglets were isolated, perfused at constant pressure and maintained in an isogravimetric state under zone III conditions (pulmonary venous pressure greater than alveolar pressure). By applying the double, arterial and venous, occlusion technique, the total blood flow resistance (R) was partitioned into four components: arterial (Ra), pre- (Ra') and post-capillary (Rv') and venous (Rv). The capillary filtration coefficient (Kf,c) was evaluated by measuring the weight gained by the lungs when the arterial and venous pressures were suddenly increased. In the youngest Landrace piglets (5 weeks old), there was an uncontrolled vasoconstriction which sometimes prevented perfusion of the lungs and induced a large increase in Rt. These high values of Rt were decreased by tolazoline administration. The values of Rt recorded in older pigs (12-13 weeks old) were lower in Minipigs (33.66 +/- 3.77 cmH2O min L-1 per 100 g of lungs; n = 5) than in Landrace piglets (55.20 +/- 6.18 cmH2O min L-1 per 100 g; n = 5). This breed difference was due to the differences in Ra' and Rv'. The mean values of Kf,c were 0.193 +/- 0.015 and 0.202 +/- 0.029 ml min (cmH2O)-1 per 100 g of the lungs in Minipigs and Landrace piglets respectively. All these parameters were stable for the 3 hours following the equilibrium period. It was concluded that: (1) There is an age-related maturation of the control of the vasomotor tone in porcine lungs. (2) Pulmonary microvascular haemodynamics are influenced by the breed of the pigs. (3) There was no difference in the Kf,c values between both the breeds. (4) A comparison of the values reported for dogs and rabbits with our data shows that the pre- and post-capillary resistances and, to a lesser extent, the arterial and venous resistances are relatively high in pigs.

Pulmonary vascular pressure profile in adult ferrets: measurements in vivo and in isolated lungs

We have determined the vascular pressure profile in lungs of adult ferrets utilizing an anaesthetized open chested preparation and have compared the pressure profile in vivo with that in isolated, perfused lungs. Ten adult ferrets, mean body weight 980 +/- 108 g, were studied. For in vivo measurements, five ferrets were anaesthetized, mechanically ventilated and the left chest wall resected. Pressures were measured in the pulmonary artery, left atrium and by micropuncture, in 20-50 microns diameter subpleural arterioles and venules. During micropuncture, ventilation was stopped for 1-2 min and the lungs kept distended at an airway pressure of 6 cmH2O. Left atrial pressure was raised to approximately 8 cmH2O with saline infusion so that lungs were in Zone 3. Cardiac output was measured by thermodilution. Lungs of five other ferrets were isolated and perfused with a steady flow roller pump. In these lungs blood flow was adjusted so that pulmonary artery pressure was similar to that in anaesthetized ferrets, with airway and left atrial pressures at 6 and 8 cmH2O respectively (Zone 3). Blood haematocrit (35 +/- 7%) was similar in the two groups. In lungs of anaesthetized ferrets total arteriovenous pressure drop was 12.1 +/- 1.9 cmH2O, with cardiac output being 210 +/- 80 ml kg body weight-1 min-1. Fractional resistance in arteries was 37%, 37% in microvessels and 26% in veins. In isolated ferret lungs, though blood flow was only 48 +/- 10 ml kg body wt-1 min-1 for the same total arteriovenous pressure drop as in vivo, the longitudinal distribution of vascular resistance was similar to that in live ferrets.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary vascular pressure profile in 2-3-week-old, 5-6-week-old and adult ferrets

We have previously reported age-related differences in the pattern of vascular pressure drop in rabbit lungs (Raj et al., Pediatric Res. 20:1107-1111, 1986). The purpose of this study was to investigate if there were similar age related differences in the ferret lung. To determine the profile of vascular pressures and the longitudinal distribution of vascular resistance in arteries, microvessels and veins, we isolated and blood perfused lungs of 20 ferrets (8 adults greater than 6 months of age; six 5-6-week-old and six 2-3-week-old). All lungs were perfused under similar experimental conditions with blood flow adjusted to keep pulmonary artery pressure constant at approximately 20 cmH2O, when left atrial pressure was 8 cmH2O. Lungs were inflated to 6 cmH2O airway pressure with a gas mixture so that blood Po2 was greater than 100 Torr. Microvascular pressure measurements were obtained in each lung in subpleural 20-50 microns diameter arterioles and 20-50 microns diameter venules using the micropipette servonull method. Blood flow achieved in all three groups of lungs was similar. We found that in adult ferret lungs, 40% of total vascular resistance was in arteries, 40% in microvessels and 20% in veins. In 2-3-week-old and 5-6-week-old ferret lungs, the fractional distribution of resistance in arteries, microvessels and veins was similar to that in adult ferret lungs. We conclude that, unlike in the rabbit, the pulmonary vascular pressure profile does not change with age in the ferret.

Time shift in ventilation-induced density fluctuation of arterial blood

In an artificially ventilated dog, the varying tracheal pressure causes a density fluctuation in the blood sampled from the aorta. We cross-correlated the tracheal pressure with the density to determine the time shift or delay of the latter from the former waveform for a ventilation frequency in the range of 6-30 CPM. The delay time was found to be 29% of the mean transit time (MTT) of the pulmonary vasculature and independent of the ventilation frequency. A comparison of this percentage with the reported arterial-to-capillary-to-venous fractional volumes of the lung suggested that the delay time may be the MTT time for blood flowing through the venous network of the lung and the cross-correlation may serve as an in vivo means to partition the MTT of the pulmonary vasculature at its capillaries. These results and an analysis on the deformation of the viscoelastic, pulmonary capillaries indicated that the tracheal pressure, acting primarily through the viscous part of the viscoelasticity, deforms the capillaries to produce the density fluctuation in blood outflowing from the lung.

Nitroglycerin therapy in the management of pulmonary hypertensive disorders

Vasodilator therapy has not been effective in patients with pulmonary hypertension because most of the drugs that have been utilized in treating this disorder do not exert selective effects on the pulmonary circulation. Nonselective agents may cause predominant systemic vasodilation and lead to severe hypotension; they may elicit reflex activation of the sympathetic nervous system and further elevate pulmonary artery pressures; or they may exert depressant effects on right ventricular function and aggravate right-sided heart failure. Nitroglycerin has theoretic appeal as a vasodilator drug in patients with pulmonary hypertension because it exerts a direct effect on the pulmonary circulation in doses that do not affect systemic resistance vessels or the myocardium and do not activate neurohumoral reflexes. Furthermore, the drug uniquely reduces pulmonary artery pressures in addition to pulmonary vascular resistance due to its ability to dilate venous capacitance vessels. Preliminary studies with sublingual and intravenous nitroglycerin in patients with pulmonary hypertension have shown that the drug produces marked hemodynamic improvement and that clinical benefits follow long-term therapy with transcutaneous or oral nitrates. However, treatment may provoke hypotensive events in some patients and systemic hypoxemia in others; still others may fail to benefit because the pulmonary vasculature is unresponsive to any vasodilator stimulus. Further work is needed to define the benefits and risks of nitroglycerin therapy in patients with pulmonary hypertension.

Determinants of pulmonary blood volume. Effects of acute changes in pulmonary vascular pressures and flow

To examine the effects of pulmonary vascular pressures and flow on pulmonary blood volume (PBV), experiments were performed at constant heart rate and zone 3 conditions (mean left atrial pressure (LAP) above airway pressure) in six anesthetized, open-chest dogs. PBV was calculated as the product of electromagnetic aortic flow and pulmonary mean transit time for ascorbate, obtained without blood withdrawal by polarographic recording of aortic ascorbate changes. In three series of experiments LAP was raised similarly in three steps, from 4.5 to 14.8 mmHg: by mitral constriction which reduced pulmonary blood flow, by blood volume expansion which more than doubled pulmonary blood flow, or by a combination of the two procedures which kept pulmonary blood flow constant. In all three series, LAP and mean pulmonary arterial pressure (PAP) rose in proportion, but PBV was better correlated to PAP (r = 0.87 +/- 0.02) than to LAP (r = 0.66 +/- 0.09). These experiments suggest that PAP is the most important factor in determining PBV under zone 3 conditions, whether PAP is raised by increasing pulmonary blood flow or by mitral constriction.

Influence of perfusate PO2 on hypoxic pulmonary vasoconstriction in rats

The purpose of these studies was to evaluate the influence of perfusate oxygen tension on hypoxic pulmonary vasoconstriction and to identify the site at which both alveolar and perfusate gas tensions stimulate hypoxic pulmonary vasoconstriction. Lungs from adult rats were ventilated and perfused in vitro at constant temperature, PCO2, and pH, with a perfusion circuit incorporating a membrane oxygenator that allowed independent control of the alveolar and perfusate gas tensions. Blood flow to the lung was constant (0.06 ml per g body weight per min), and pulmonary vascular resistance was therefore proportional to pulmonary artery pressure. In study 1, the pulmonary artery pressor response to zero or 22 mm Hg alveolar oxygen was measured when the perfusate oxygen tensions were approximately 8, 26, 41, 64, or 128 mm Hg. The pressor response as a percent of the maximum pressure change was progressively reduced as perfusate oxygen tension increased. For alveolar oxygen tension of zero; the pressor response = 128 -39 (Log PPO2) and r = 0.8 (P less than 0.01), the effect of perfusate gas tension on the response to alveolar gas tension of 22 mm Hg was similar. These results demonstrate that the stimulus for hypoxic pulmonary vasoconstriction is a function of both alveolar and perfusate oxygen tension. In study 2, the response to alveolar oxygen tension of 42 mm Hg was measured with mean perfusate oxygen tensions of 130, 52, and 17 mm Hg. In six animals with forward perfusion, the responses decreased with increasing perfusate oxygen tension, as in study 1. In another six animals, with retrograde perfusion, the responses to alveolar hypoxia were not altered when perfusate oxygen tension was increased. These results demonstrate that the sensor region for hypoxic pulmonary vasoconstriction is precapillary. These studies confirm and extend previous hypotheses that alveolar and perfusate oxygen tensions together, determine the PO2 at a precapillary site to stimulate hypoxic pulmonary vasoconstriction.

Microvascular pressure distribution and responses of pulmonary allografts and cheek pouch arterioles in the hamster to oxygen

Despite extensive investigations of the pulmonary circulation using both in vitro and in vitro preparations, few direct microcirculatory studies have been made. Consequently, the mechanisms involved in the response of the pulmonary microvasculature to changes in oxygen tension remain unclear. The present study represents the first direct observation of the responses in pulmonary microvessels to alterations in oxygen tension. Neonatal lung tissue was transplanted into the hamster cheek pouch using a chamber technique. Both tissues were characterized with respect to their microvascular pressure profile and vascular response to hypoxia. The results showed the two tissues to be remarkably different. Small pulmonary and cheek pouch arterioles exhibited opposite responses to changes in oxygen environment; hypoxia elicited a constriction of pulmonary arterioles, but a dilation of cheek pouch arterioles. Pulmonary capillary pressure, although comparable to that measured in the intact lung (13 mm Hg), was substantially lower than cheek pouch capillary pressure, which was within the range of that described for several systemic vascular beds. The microcirculatory effects of oxygen on both tissues were confined to the arteriolar segments. The characteristics of this pulmonary microcirculation are such that it is a unique model for further physiological and pharmacological studies.

Direct measurement of microvascular pressures in the isolated perfused dog lung

Microvascular pressures in the pulmonary circulation were measured under the pleural surface of the isolated perfused dog lung by the servo-null technique. Strong glass micropipettes with short beveled tips were used, with a suction ring to stabilize the lung's surface. Of the total vascular resistance, 45 percent was in the alveolar wall capillaries themselves. Most of the remaining resistance was in the arterioles. There was negligible pressure drop in venules with diameters larger than 20 micrometers.

Distribution of extravascular fluid volumes in isolated perfused lungs measured with H215O

The distributions per unit volume of extravascular water (EVLW), blood volume, and blood flow were measured in isolated perfused vertical dog lungs. A steady-state tracer technique was employed using oxygen-15, carbon-11, and nitrogen-13 isotopes and external scintillation counting of the 511-KeV annihilation radiation common to all three radionuclides. EVLW, and blood volume and flow increased from apex to base in all preparations, but the gradient of increasing flow exceeded that for blood and EVLW volumes. The regional distributions of EVLW and blood volume were almost identical. With increasing edema, lower-zone EVLW increased slightly relative to that in the upper zone. There was no change in the distribution of blood volume or flow until gross edema (100% wt gain) occurred when lower zone values were reduced. In four lungs the distribution of EVLW was compared with wet-to-dry ratios from lung biopsies taken immediately afterwards. Whereas the isotopically measured EVLW increased from apex to base, the wet-to-dry weight ratios remained essentially uniform. We concluded that isotopic methods measure only an "exchangeable" water pool whose volume is dependent on regional blood flow and capillary recruitment. Second, the isolated perfused lung can accommodate up to 60% wt gain without much change in the regional distribution of EVLW, volume, or flow.

Quantitative structural analysis of pulmonary vessels in isolated ventricular septal defect in infancy

Structural changes in the pulmonary circulation were studied in the lungs of 5 infants dying with ventricular septal defect. Applying precise quantitative morphological techniques to the pulmonary vessels, it was possible to correlate pathological change with clinical and haemodynamic findings, and to identify two patterns of response. Three of the infants (group I) ppresnted in cardiac failure with a large pulmonary blood flow, dilated and tortuous pulmonary arteries, and fewer intra-acinar vessels than normal. Medial hypertrophy was moderate and affected chiefly the larger arteries, i.e. those with a diameter greater than 200 mum. The other 2 infants (group 2) had a high pulmonary vascular resistance with an intermittent right-to-left shunt. The pulmonary arteries were of normal size and the reduction in the number of the arteries was less striking. Medial hypertrophy was greater than in the first group and affected all sizes of artery including those less than 200 mum in diameter. In both groups, muscle extended further along the axial pathway. Muscular hypertrophy was found also in the vein wall in most cases and, as with the arteries, was more severe in those with a higher pulmonary vascular resistance. The findings illustrate the variation in pulmonary vascular response in infants with a ventricular septal defect. It is suggested that in patients with a ventricular septal defect, arterial muscularity usually regresses after birth and a left-to-right shunt develops; secondary hypertrophy of the media then develops in reaponse to the shunt. Our findings also suggest, however, that in some infants arterial muscle fails to regress postnatally so that pulmonary blood flow is never high and a right-to-left shunt develops soon after birth.

Effect of increased vascular pressure on lung fluid balance in unanesthetized sheep

In 20 unanesthetized sheep, we measured lung lymph flow and lymph and plasma protein concentrations during steady-state base-line conditions and during steady-state elevations of pulmonary microvascular hydrostatic pressure (range 3 to 23 cm H2O). In every sheep there was a base-line lung lymph flow (average 5.7 +/- 2.5 (SD) ml/hour), demonstrating that net fluid filtration occurred. The base-line lymph-plasma total protein ratio averaged 0.69 +/- 0.05, indicating a high protein osmotic pressure in the interstitial fluid at the filtration site. Lymph flow increased and lymph protein concentration decreased approximately linearly whenever hydrostatic pressure rose. A new steady-state condition was reached in 1-2 hours. The difference in plasma-to-lymph protein osmotic pressure increased by half the hydrostatic pressure increment (50% negative feedback regulation). Extravascular lung water content, measured post-mortem, did not change significantly until microvascular hydrostatic pressure more than doubled, indicating a large safety factor that protects the lungs against fluid accumulation normally. The major contributions to the safety factor appeared to be a sensitive and efficient lymph pump coupled to a washout of interstitial protein. The fluid filtration coefficient, whose calculation required many assumptions, averaged 1.64 +/- 2.65 ml/(cm H2O times hour) in the base-line condition and did not change significantly over the pressure range studied.

The cite of action of nerves in the pulmonary vascular bed in the dog

1. The effects of stimulation of the thoracic vagosympathetic nerve or upper thoracic sympathetic chain on the pulmonary vascular resistance have been studied in atropinized, isolated, ventilated lung lobes under various conditions of pulmonary circulation perfusion. Throughout the nerve-stimulation tests bronchial circulation perfusion was maintained or temporarily interrupted.2. The pulmonary vascular resistance increase evoked by nerve stimulation (a) occurred in the absence of tidal air changes; (b) did not consistently differ during predominantly ;sluice' and ;non-sluice' conditions of pulmonary circulation perfusion; (c) was approximately one and a half times greater during constant pressure than during constant volume inflow perfusion of the pulmonary circulation; and (d) was greater during reverse than during forward perfusion.3. In lung lobes perfused in either direction at constant volume inflow nerve stimulation produced an increase in inflow pressure and a diminution in total lung blood volume reflected by a temporary increase in blood outflow.4. In lung lobes in which neither the pulmonary nor the bronchial circulations were perfused and the capillaries were completely blocked by high intratracheal pressures, thus isolating the pulmonary arterial system from the venous system, nerve stimulation produced a diminution in the blood volume of both systems.5. Nerve stimulation produced a rise in bronchial arterial pressure in the absence of pulmonary circulation perfusion.6. Further evidence is adduced that pulmonary vasomotor nerve responses do not depend upon the transfer of transmitter substances from the bronchial to the pulmonary circulation.7. The possible significance of these observations in relation to the site of action of pulmonary vasomotor nerves is discussed.