Control of pulmonary vascular tone during exercise in health and pulmonary hypertension

Pathogenesis, Assessment, and Treatment of Coronary Microcirculation Dysfunction

Cardiovascular disease is the predominant cause of mortality on a global scale. Research indicates that women exhibit a greater likelihood of presenting with non-obstructive coronary artery disease (CAD) when experiencing symptoms of myocardial ischemia in comparison to men. Additionally, women tend to experience a higher burden of symptoms relative to men, and despite the presence of ischemic heart disease, they are frequently reassured erroneously due to the absence of obstructive CAD. In cases of ischemic heart disease accompanied by symptoms of myocardial ischemia but lacking obstructive CAD, it is imperative to consider coronary microvascular dysfunction as a potential underlying cause. Coronary microvascular dysfunction, characterized by impaired coronary flow reserve resulting from functional and/or structural abnormalities in the microcirculation, is linked to adverse cardiovascular outcomes. Lifestyle modifications and the use of anti-atherosclerotic and anti-anginal medications may offer potential benefits, although further clinical trials are necessary to inform treatment strategies. This review aims to explore the prevalence, underlying mechanisms, diagnostic approaches, and therapeutic interventions for coronary microvascular dysfunction.

Adults with small, unrepaired atrial septal defects have reduced cardiac index during exercise

OBJECTIVES

Small, unrepaired atrial septal defects are considered a benign lesion with good prognosis. Recently, clinical and register-based studies discovered increased long-term mortality and morbidity. The nature of these findings is not fully understood. Therefore, MRI was performed to evaluate cardiac function at rest and during exercise.

METHODS

Adults with open or spontaneously closed atrial septal defects and healthy, matched controls underwent MRI for evaluation of cardiac chamber volume. Quantitative flow scans measured blood flow in the ascending aorta and the proximal pulmonary artery at rest and during increasing supine exercise.

RESULTS

In total, 15 open defects (39 ± 11 years) and 15 matched controls (38 ± 12 years) were included, along with 20 spontaneously closed (36 ± 13 years) and 20 controls (36 ± 11 years). Cardiac chamber volumes and flow measurements at rest were comparable between groups, as were heart rates and workloads during exercise. At maximal exercise, open defects reached 31% lower cardiac index and had 38% higher retrograde flow in the pulmonary artery than their controls, p < 0.01. Shunt ratio remained unchanged during exercise, 1.2 ± 0.2. Closed defects reached 18% lower cardiac index, p = 0.02, with comparable pulmonary retrograde flow. Maximal cardiac index was inversely correlated with increasing age for patients only.

CONCLUSION

Adults with a small, open or spontaneously closed atrial septal defects exhibit markedly lower exercise capacity compared with healthy peers. Moreover, open defects exhibit higher retrograde flows with increasing exercise. Finally, increasing age is related to poorer results in patients but not healthy controls. Longitudinal studies are necessary in order to determine potential accelerated worsening of physical capacity along with age-related changes in patients.

Cannabinoids-A New Perspective in Adjuvant Therapy for Pulmonary Hypertension

Currently, no treatment can completely cure pulmonary hypertension (PH), which can lead to right ventricular failure and, consequently, death. Therefore, searching for new therapies remains important. Increased resistance in pulmonary circulation is mainly caused by the excessive contraction and proliferation of small pulmonary arteries. Cannabinoids, a group of lipophilic compounds that all interact with cannabinoid receptors, exert a pulmonary vasodilatory effect through several different mechanisms, including mechanisms that depend on vascular endothelium and/or receptor-based mechanisms, and may also have anti-proliferative and anti-inflammatory properties. The vasodilatory effect is important in regulating pulmonary resistance, which can improve patients’ quality of life. Moreover, experimental studies on the effects of cannabidiol (plant-derived, non-psychoactive cannabinoid) in animal PH models have shown that cannabidiol reduces right ventricular systolic pressure and excessive remodelling and decreases pulmonary vascular hypertrophy and pulmonary vascular resistance. Due to the potentially beneficial effects of cannabinoids on pulmonary circulation and PH, in this work, we review whether cannabinoids can be used as an adjunctive therapy for PH. However, clinical trials are still needed to recommend the use of cannabinoids in the treatment of PH.

Preoperative coupling between right ventricle and pulmonary vasculature is an important determinant of residual symptoms after the closure of atrial septal defect

PURPOSES

The closure of atrial septal defect (ASD) is associated with a significant reduction in right ventricular (RV) overload and an improvement in functional capacity in most adults with ASD. However, a subset of patients remains symptomatic even after closure due to therapeutic delay. To date, no clinically robust preoperative predictor of postoperative residual symptoms has been clearly identified.

METHODS

In this study, 120 adult patients with ASD and 39 controls were investigated. As an index of RV myocardial deformation, RV global longitudinal strain (RV-GLS) was evaluated. The degree of coupling between RV and pulmonary artery (PA) was quantified by the tricuspid annular plane systolic excursion (TAPSE) divided by the PA systolic pressure (PASP).

RESULTS

Compared to controls, baseline RV-GLS was significantly greater (- 27 ± 7 vs. - 23 ± 5%, P = 0.02) and TAPSE/PASP ratio was severely impaired (0.8 ± 0.3 vs. 2.1 ± 1.6 mm/mmHg, P < 0.01) in ASD patients. At 6 months after closure, 15 patients (12.5%) remained symptomatic. In patients without residual symptoms, TAPSE/PASP ratio significantly improved from 0.9 ± 0.3 to 1.0 ± 0.6 mm/mmHg (P = 0.02), and RV-GLS normalized (from - 28 ± 11 to - 24 ± 7%, P < 0.01) after closure. However, RV-GLS and TAPSE/PASP ratio showed no significant change in ASD patients with residual symptoms. On multivariate analysis, preoperative TAPSE/PASP ratio (odds ratio [OR] 0.034, 95% confidence interval [CI] 0.000-0.604, P = 0.03) and pulmonary vascular resistance index ([PVRI], OR 1.011, 95% CI 1.000-1.021, P < 0.05) were associated with the postoperative symptomatic status.

CONCLUSION

In terms of integrated assessment of the RV-PA unit, preoperative TAPSE/PASP ratio and PVRI were important determinants of residual symptoms after ASD closure.

Chronic Thromboembolic Pulmonary Hypertension - What Have We Learned From Large Animal Models

Chronic thrombo-embolic pulmonary hypertension (CTEPH) develops in a subset of patients after acute pulmonary embolism. In CTEPH, pulmonary vascular resistance, which is initially elevated due to the obstructions in the larger pulmonary arteries, is further increased by pulmonary microvascular remodeling. The increased afterload of the right ventricle (RV) leads to RV dilation and hypertrophy. This RV remodeling predisposes to arrhythmogenesis and RV failure. Yet, mechanisms involved in pulmonary microvascular remodeling, processes underlying the RV structural and functional adaptability in CTEPH as well as determinants of the susceptibility to arrhythmias such as atrial fibrillation in the context of CTEPH remain incompletely understood. Several large animal models with critical clinical features of human CTEPH and subsequent RV remodeling have relatively recently been developed in swine, sheep, and dogs. In this review we will discuss the current knowledge on the processes underlying development and progression of CTEPH, and on how animal models can help enlarge understanding of these processes.

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.

Disappearance of the shunt and lower cardiac index during exercise in small, unrepaired ventricular septal defects

OBJECTIVES

Clinical studies have revealed decreased exercise capacity in adults with small, unrepaired ventricular septal defects. Increasing shunt ratio and growing incompetence of the aortic and pulmonary valve with retrograde flow during exercise have been proposed as reasons for the previously found reduced exercise parameters. With MRI, haemodynamic shunt properties were measured during exercise in ventricular septal defects.

METHODS

Patients with small, unrepaired ventricular septal defects and healthy peers were examined with MRI during exercise. Quantitative flow scans measured blood flow through ascending aorta and pulmonary artery. Scans were analysed post hoc where cardiac index, retrograde flows, and shunt ratio were determined.

RESULTS

In total, 32 patients (26 ± 6 years) and 28 controls (27 ± 5 years) were included. The shunt ratio was 1.2 ± 0.2 at rest and decreased to 1.0 ± 0.2 at peak exercise, p < 0.01. Aortic cardiac index was lower at peak exercise in patients (7.5 ± 2 L/minute/m2) compared with controls (9.0±2L l/minute/m2), p<0.01. Aortic and pulmonary retrograde flow was larger in patients during exercise, p < 0.01. Positive correlation was demonstrated between aortic cardiac index at peak exercise and previously established exercise capacity for all patients (r = 0.5, p < 0.01).

CONCLUSIONS

Small, unrepaired ventricular septal defects revealed declining shunt ratio with increasing exercise and lower aortic cardiac index. Patients demonstrated larger retrograde flow both through the pulmonary artery and the aorta during exercise compared with controls. In conclusion, adults with unrepaired ventricular septal defects redistribute blood flow during exercise probably secondary to a more fixed pulmonary vascular resistance compared with age-matched peers.

Pulmonary hypertension: Pathophysiology beyond the lung

Pulmonary hypertension (PH) is classically considered a disease of pulmonary vasculature which has been the predominant target for drug development and PH therapy. Despite significant advancement in recent years in identification of new drug targets and innovative treatment strategies, the prognosis of PH remains poor, with median survival of 5 years. Recent studies have demonstrated involvement of neuroinflammation, altered autonomic and gastrointestinal functions and increased trafficking of bone marrow-derived cells in cardiopulmonary pathophysiology. This has led to the proposal that PH could be considered a systemic disease involving complex interactions among many organs. Our objectives in this review is to summarize evidence for the involvement of the brain, bone marrow and gut in PH pathophysiology. Then, to synthesize all evidence supporting a brain-gut-lung interaction hypothesis for consideration in PH pathophysiology and finally to summarize unanswered questions and future directions to move this novel concept forward. This forward-thinking view, if proven by further experiments, would provide new opportunities and novel targets for the control and treatment of PH.

Atrial arrhythmias in chronic lung disease-associated pulmonary hypertension

Atrial arrhythmias are common during episodes of acute respiratory failure in patients with chronic lung disease-associated pulmonary hypertension. Expert opinion suggests that management of atrial arrhythmias in patients with pulmonary hypertension should aim to restore sinus rhythm. This is clinically challenging in pulmonary hypertension patients with coexisting chronic lung disease, as there is controversy on the use of rhythm control agents; generally, in regard to either their pulmonary toxicity profile or the lack of evidence supporting their use. Rate control methods are largely focused on the use of beta blockers and calcium channel blockers. Concerns regarding their use involve their negative inotropic properties in cor pulmonale, the risk of bronchospasm associated with beta blockers, and the potential for ventilation/perfusion mismatching associated with calcium channel blockers. While digoxin has been associated with promising outcomes during acute right ventricular failure, there is limited evidence to suggest its routine use. Electrical cardioversion is associated with a high failure rate and it frequently requires multiple attempts. Radiofrequency catheter ablation is a more definitive approach, but concerns surrounding mechanical ventilation and sedation limit its applicability in decompensated pulmonary hypertension. Individual approaches are needed to address atrial arrhythmia management during acute episodes of respiratory failure.

Endogenously released adenosine causes pulmonary vasodilation during the acute phase of pulmonary embolization in dogs

Background

Endogenous adenosine levels increase under stress in various organs. Exogenously administered adenosine is a well-known pulmonary vasodilator. However, the physiology and therapeutic potential of endogenous adenosine during alteration in pulmonary hemodynamics such as pulmonary embolism is not elucidated. We hypothesized that the adenosine level increases following an acute elevation of pulmonary resistance, resulting in pulmonary vasodilation.

Methods

We induced acute pulmonary embolization by injecting plastic beads in anesthetized dogs. Plasma adenosine levels, defined as the product of plasma adenosine concentration and simultaneous cardiac output, were assessed from blood samples from the superior vena cava, main pulmonary artery (MPA), and ascending aorta 1 and 10 min following injection. Hemodynamics were assessed with (n = 3) and without (n = 8) administration of the adenosine receptor blocker, 8-(p-sulfophenyl)theophylline (8SPT).

Results

Mean pulmonary arterial pressure (PAP) increased from 11 ± 1 mmHg, peaking at 28 ± 4 mmHg at 52 ± 13 s after injection. During this period, total pulmonary resistance (TPR) elevated from 11 ± 1 to 33 ± 6 Wood unit. Plasma adenosine levels increased in the MPA from 14.5 ± 2 to 38.8 ± 7 nmol/min 1 min after injection. TPR showed greater elevation under 8SPT treatment, to 96 ± 12 Wood unit at PAP peak.

Conclusions

Endogenously released adenosine after acute pulmonary embolization is one of the initial pulmonary vasodilators. The immediate surge in plasma adenosine levels in the MPA could lead to a hypothesis that adenosine is released by the right heart in response to pressure overload.

•

Adenosine levels increased after experimental acute pulmonary embolization.

•

Plasma adenosine levels immediately rose in the main pulmonary artery.

•

Adenosine is one of the initial pulmonary vasodilators after embolization.

•

Released adenosine could originate from the right heart following pressure overload.

Serial pulmonary vascular resistance assessment in patients late after ventricular septal defect repair

BACKGROUND

The long-term evolution of pulmonary vascular resistance (PVR) after ventricular septal defect (VSD) repair is unknown. This study serially evaluated resting and exercise PVR after VSD repair in childhood.

METHODS

Patients were enrolled from the outpatient Adult Congenital Heart Disease clinic of the University Hospitals Leuven and compared to age- and gender-matched controls. Participants underwent resting and exercise echocardiography and cardiopulmonary exercise testing at baseline and follow-up. Total PVR was calculated as the ratio of mean pulmonary artery pressure (mPAP) to cardiac output (CO). The slope of the mPAP-CO curve (exercise PVR) was obtained using linear regression analysis.

RESULTS

Twenty-seven patients (mean age 31 ± 7 years, 70% male) and 18 controls were included. At baseline, patients had larger right ventricular (RV) end-diastolic areas (10 ± 2 vs 9 ± 1 cm²/m², p = 0.001) and lower tricuspid annular plane systolic excursion (TAPSE) (17 (17-19) vs 26 (22-28) mm, p < 0.001). After 1.1 (1.0-1.5) years follow-up, similar differences in RV areas and TAPSE were found. Patients reached lower peak workload and cardiac index compared to controls at each time point. Peak total PVR was higher (Baseline: 2.7 ± 0.8 vs 2.2 ± 0.3 mm Hg/L/min, p = 0.005; Follow-up: 2.9 ± 0.9 vs 2.1 ± 0.3 mm Hg/L/min, p < 0.001) and the mPAP-CO slope was steeper (Baseline: 2.2 ± 0.8 vs 1.7 ± 0.3 mm Hg/L/min, p = 0.008; Follow-up: 2.5 ± 0.9 vs 1.6 ± 0.3 mm Hg/L/min, p < 0.001) in patients. The mPAP-CO slope in patients correlated inversely with peak oxygen uptake (R = -0.41 and - 0.45, p = 0.036 and 0.022, baseline and follow-up, respectively).

CONCLUSION

Despite repair, VSD patients seem to show altered pulmonary hemodynamics and RV impairment at rest and exercise, supporting life-long follow-up.

Changes in the nitric oxide pathway of the pulmonary vasculature after exposure to hypoxia in swine model of neonatal pulmonary vascular disease

Neonatal pulmonary vascular disease (PVD) is increasingly recognized as a disease that complicates the cardiopulmonary adaptations after birth and predisposes to long-term cardiopulmonary disease. There is growing evidence that PVD is associated with disruptions in the nitric oxide (NO)-cGMP-phosphodiesterase 5 (PDE5) pathway. Examination of the functionality of different parts of this pathway is required for better understanding of the pathogenesis of neonatal PVD. For this purpose, the role of the NO-cGMP-PDE5 pathway in regulation of pulmonary vascular function was investigated in vivo, both at rest and during exercise, and in isolated pulmonary small arteries in vitro, in a neonatal swine model with hypoxia-induced PVD. Endothelium-dependent vasodilatation was impaired in piglets with hypoxia-induced PVD both in vivo at rest and in vitro. Moreover, the responsiveness to the NO-donor SNP was reduced in hypoxia-exposed piglets in vivo, while the relaxation to SNP and 8-bromo-cyclicGMP in vitro were unaltered. Finally, PDE5 inhibition-induced pulmonary vasodilatation was impaired in hypoxia-exposed piglets both in vitro and in vivo at rest. During exercise, however, the pulmonary vasodilator effect of PDE5 inhibition was significantly larger in hypoxia-exposed as compared to normoxia-exposed piglets. In conclusion, the impaired endothelium-dependent vasodilatation in piglets with hypoxia-induced PVD was accompanied by reduced responsiveness to NO, potentially caused by altered sensitivity and/or activity of soluble guanylyl cyclase (sGC), resulting in an impaired cGMP production. Our findings in a newborn animal model for neonatal PVD suggests that sGC stimulators/activators may be a novel treatment strategy to alleviate neonatal PVD.

Pulmonary microvascular remodeling in chronic thrombo-embolic pulmonary hypertension

Pulmonary vascular remodeling in pulmonary arterial hypertension involves perturbations in the nitric oxide (NO) and endothelin-1 (ET-1) pathways. However, the implications of pulmonary vascular remodeling and these pathways remain unclear in chronic thrombo-embolic pulmonary hypertension (CTEPH). The objective of the present study was to characterize changes in microvascular morphology and function, focussing on the ET-1 and NO pathways, in a CTEPH swine model. Swine were chronically instrumented and received up to five pulmonary embolizations by microsphere infusion, whereas endothelial dysfunction was induced by daily administration of the endothelial NO synthase inhibitor N^ω-nitro-l-arginine methyl ester until 2 wk before the end of study. Swine were subjected to exercise, and the pulmonary vasculature was investigated by hemodynamic, histological, quantitative PCR, and myograph experiments. In swine with CTEPH, the increased right-ventricular afterload, decreased cardiac index, and mild ventilation-perfusion-mismatch were exacerbated during exercise. Pulmonary microvascular remodeling was evidenced by increased muscularization, which was accompanied by an increased maximal vasoconstriction. Although ET-1-induced vasoconstriction was increased in CTEPH pulmonary small arteries, the ET-1 sensitivity was decreased. Moreover, the contribution of the ET_A receptor to ET-1 vasoconstriction was increased, whereas the contribution of the ET_B receptor was decreased and the contribution of Rho-kinase was lost. A reduction in endogenous NO production was compensated in part by a decreased phosphodiesterase 5 (PDE5) activity resulting in an apparent increased NO sensitivity in CTEPH pulmonary small arteries. These findings suggest that pulmonary microvascular remodeling with a reduced activity of PDE5 and Rho-kinase may contribute to the lack of therapeutic efficacy of PDE5 inhibitors and Rho-kinase inhibitors in CTEPH.

Transition from post-capillary pulmonary hypertension to combined pre- and post-capillary pulmonary hypertension in swine: a key role for endothelin

Key points

Passive, isolated post‐capillary pulmonary hypertension (PH) secondary to left heart disease may progress to combined pre‐ and post‐capillary or ‘active’ PH

This ‘activation’ of post‐capillary PH significantly increases morbidity and mortality, and is still incompletely understood.

In this study, pulmonary vein banding gradually produced post‐capillary PH with structural and functional microvascular remodelling in swine.

Ten weeks after banding, the pulmonary endothelin pathway was upregulated, likely contributing to pre‐capillary aspects in the initially isolated post‐capillary PH.

Inhibition of the endothelin pathway could potentially stop the progression of early stage post‐capillary PH.

Abstract

Passive, isolated post‐capillary pulmonary hypertension (IpcPH) secondary to left heart disease may progress to combined pre‐ and post‐capillary or ‘active’ PH (CpcPH) characterized by chronic pulmonary vascular constriction and remodelling. The mechanisms underlying this ‘activation’ of passive pulmonary hypertension (PH) remain incompletely understood. Here we investigated the role of the vasoconstrictor endothelin‐1 (ET) in the progression from IpcPH to CpcPH in a swine model for post‐capillary PH. Swine underwent pulmonary vein banding (PVB; n = 7) or sham‐surgery (Sham; n = 6) and were chronically instrumented 4 weeks later. Haemodynamics were assessed for 8 weeks, at rest and during exercise, before and after administration of the ET receptor antagonist tezosentan. After sacrifice, the pulmonary vasculature was investigated by histology, RT‐qPCR and myograph experiments. Pulmonary arterial pressure and resistance increased significantly over time. mRNA expression of prepro‐endothelin‐1 and endothelin converting enzyme‐1 in the lung was increased, while ET_A expression was unchanged and ET_B expression was downregulated. This was associated with increased plasma ET levels from week 10 onward and a more pronounced vasodilatation to in vivo administration of tezosentan at rest and during exercise. Myograph experiments showed decreased endothelium‐dependent vasodilatation to Substance P and increased vasoconstriction to KCl in PVB swine consistent with increased muscularization observed with histology. Moreover, maximal vasoconstriction to ET was increased whereas ET sensitivity was decreased. In conclusion, PVB swine gradually developed PH with structural and functional vascular remodelling. From week 10 onward, the pulmonary ET pathway was upregulated, likely contributing to pre‐capillary activation of the initially isolated post‐capillary PH. Inhibition of the ET pathway could thus potentially provide a pharmacotherapeutic target for early stage post‐capillary PH.

Passive, isolated post‐capillary pulmonary hypertension (PH) secondary to left heart disease may progress to combined pre‐ and post‐capillary or ‘active’ PH

This ‘activation’ of post‐capillary PH significantly increases morbidity and mortality, and is still incompletely understood.

In this study, pulmonary vein banding gradually produced post‐capillary PH with structural and functional microvascular remodelling in swine.

Ten weeks after banding, the pulmonary endothelin pathway was upregulated, likely contributing to pre‐capillary aspects in the initially isolated post‐capillary PH.

Inhibition of the endothelin pathway could potentially stop the progression of early stage post‐capillary PH.

Measuring pulmonary arterial compliance: mission impossible? Insights from a novel in vivo continuous-flow based experimental model

Arterial compliance (C) is related to the elasticity, size, and geometrical distribution of arteries. Compliance is a determinant of the load that impedes ventricular ejection. Measuring compliance is difficult, particularly in the pulmonary circulation in which resistive and compliant vessels overlap. Comparing different methods for quantification of compliance to a method that involves a continuous flow might help to identify the optimal method. Pulmonary arterial compliance was computed in six pigs based on the stroke volume to pulse pressure ratio, diastolic decay exponential fitting, area method, and the pulse pressure method (PPM). Compliance measurements were compared to those obtained under continuous flow conditions through a right ventricular bypass (Heartware Inc., Miami Lakes, FL, USA). Compliance was computed for various flows using diastolic decay exponential fitting after an abrupt interruption of the pump. Under the continuous flow conditions, resistance (R) was a decreasing function of the flow, and the fitting to P = e^-t/RC yielded a pulmonary time constant (RC) of 2.06 s ( ± 0.48). Compliance was an increasing function of flow. Steady flow inter-method comparisons of compliance under pulsatile flow conditions showed large discrepancies and values (7.23 ± 4.47 mL/mmHg) which were lower than those obtained under continuous flow conditions (10.19 ± 1 0.31 mL/mmHg). Best agreement with steady flow measurements is obtained with the diastolic decay method. Resistance and compliance are both flow-dependent and are inversely related in the pulmonary circulation. The dynamic nature of the pulsatile flow may induce a non-uniformly distributed compliance, with an influence on the methods of measurement.

Sex differences in pulmonary vascular control: focus on the nitric oxide pathway

Although the incidence of pulmonary hypertension is higher in females, the severity and prognosis of pulmonary vascular disease in both neonates and adults have been shown to be worse in male subjects. Studies of sex differences in pulmonary hypertension have mainly focused on the role of sex hormones. However, the contribution of sex differences in terms of vascular signaling pathways regulating pulmonary vascular function remains incompletely understood. Consequently, we investigated pulmonary vascular function of male and female swine in vivo, both at rest and during exercise, and in isolated small pulmonary arteries in vitro, with a particular focus on the NO‐cGMP‐PDE5 pathway. Pulmonary hemodynamics at rest and during exercise were virtually identical in male and female swine. Moreover, NO synthase inhibition resulted in a similar degree of pulmonary vasoconstriction in male and female swine. However, NO synthase inhibition blunted bradykinin‐induced vasodilation in pulmonary small arteries to a greater extent in male than in female swine. PDE5 inhibition resulted in a similar degree of vasodilation in male and female swine at rest, while during exercise there was a trend towards a larger effect in male swine. In small pulmonary arteries, PDE5 inhibition failed to augment bradykinin‐induced vasodilation in either sex. Finally, in the presence of NO synthase inhibition, the pulmonary vasodilator effect of PDE5 inhibition was significantly larger in female swine both in vivo and in vitro. In conclusion, the present study demonstrated significant sex differences in the regulation of pulmonary vascular tone, which may contribute to understanding sex differences in incidence, treatment response, and prognosis of pulmonary vascular disease.

Structural and functional changes of the pulmonary vasculature after hypoxia exposure in the neonatal period: a new swine model of pulmonary vascular disease

Pulmonary vascular disease (PVD) represents an underestimated and increasing clinical burden not only in the neonatal period but also later in life, when exercise tolerance is decreased. Animal models performing long-term followup after a perinatal insult are lacking. This study aimed to develop and characterize a neonatal swine model with hypoxia-induced PVD during long-term followup after reexposure to normoxia and to investigate the exercise response in this model. Piglets were exposed to a normoxic ( n = 10) or hypoxic environment ( n = 9) for 4 wk. Neonatal hypoxia exposure resulted in pulmonary hypertension. Mean pulmonary artery pressure was elevated 1 day after reexposure to normoxia (30.2 ± 3.3 vs. 14.3 ± 0.9 mmHg) and remained significantly higher in the second week (32.8 ± 3.8 vs. 21.4 ± 1.2 mmHg), accompanied by decreased exercise tolerance. Exercise resulted in a trend toward an exaggerated increase of pulmonary artery pressure in hypoxia-exposed animals ( week 6, P = 0.086). Although pulmonary hypertension was transient, thickening of pulmonary arterioles was found at the end of followup. Furthermore, right ventricular dilation, lower right ventricular fractional area change ( week 8, 40.0 ± 2.7% vs. 29.5 ± 4.7%), and tricuspid annular plane systolic excursion ( week 8, 27.0 ± 2.5 vs. 22.9 ± 2.1 mm) persisted during followup. Male animals showed more severe PVD than female animals. In conclusion, we developed a neonatal swine model that allows examination of the long-term sequelae of damage to the developing neonatal lung, the course of the disease and the effect of therapy on long-term outcome. NEW & NOTEWORTHY The swine model of neonatal pulmonary vascular disease developed in the present study is the first that allows exercise testing and examination of long-term sequelae of a perinatal hypoxic insult, the course of the disease, and the effect of therapy on long-term outcome.

Endothelin-1 Regulation of Exercise-Induced Changes in Flow: Dynamic Regulation of Vascular Tone

Although endothelin (ET)-1 is a highly potent vasoconstrictor with considerable efficacy in numerous vascular beds, the role of endogenous ET-1 in the regulation of vascular tone remains unclear. The perspective that ET-1 plays little role in the on-going regulation of vascular tone at least under physiologic conditions is supported by findings that potential ET-1 constriction is minimized by the release of the vasodilator and ET-1 synthesis inhibitor, nitric oxide (NO). Indeed, ET-1 release and constriction is self-limited by ET-1-induced, endothelial ET_B receptor-mediated release of NO. Moreover, even if the balance between ET-1 and NO were reversed as the result of lowered NO activity, as occurs in a number of pathophysiologies associated with endothelial dysfunction, the well-known resistance of ET-1 constriction to reversal (as determined with exogenous ET-1) precludes ET-1 in the dynamic, i.e., moment-to-moment, regulation of vascular tone. On the other hand, and as presently reviewed, findings of ET-1-dependent modulation of organ blood flow with exercise under physiologic conditions demonstrate the dynamic regulation of vascular tone by ET-1. We speculate that this regulation is mediated at least in part through changes in ET-1 synthesis/release caused by pulsatile flow-induced shear stress and NO.

Pulmonary artery wave propagation and reservoir function in conscious man: impact of pulmonary vascular disease, respiration and dynamic stress tests

KEY POINTS

Wave travel plays an important role in cardiovascular physiology. However, many aspects of pulmonary arterial wave behaviour remain unclear. Wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery in subjects with and without pulmonary hypertension during spontaneous respiration and dynamic stress tests. Arterial wave energy decreased during expiration and Valsalva manoeuvre due to decreased ventricular preload. Wave energy also decreased during handgrip exercise due to increased heart rate. In pulmonary hypertension patients, the asymptotic pressure at which the microvascular flow ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by waves increased. The reservoir and excess pressures decreased during Valsalva manoeuvre but remained unchanged during handgrip exercise. This study provides insights into the influence of pulmonary vascular disease, spontaneous respiration and dynamic stress tests on pulmonary artery wave propagation and reservoir function.

ABSTRACT

Detailed haemodynamic analysis may provide novel insights into the pulmonary circulation. Therefore, wave intensity and reservoir-excess pressure analyses were applied in the pulmonary artery to characterize changes in wave propagation and reservoir function during spontaneous respiration and dynamic stress tests. Right heart catheterization was performed using a pressure and Doppler flow sensor tipped guidewire to obtain simultaneous pressure and flow velocity measurements in the pulmonary artery in control subjects and patients with pulmonary arterial hypertension (PAH) at rest. In controls, recordings were also obtained during Valsalva manoeuvre and handgrip exercise. The asymptotic pressure at which the flow through the microcirculation ceases, the reservoir pressure related to arterial compliance and the excess pressure caused by arterial waves increased in PAH patients compared to controls. The systolic and diastolic rate constants also increased, while the diastolic time constant decreased. The forward compression wave energy decreased by ∼8% in controls and ∼6% in PAH patients during expiration compared to inspiration, while the wave speed remained unchanged throughout the respiratory cycle. Wave energy decreased during Valsalva manoeuvre (by ∼45%) and handgrip exercise (by ∼27%) with unaffected wave speed. Moreover, the reservoir and excess pressures decreased during Valsalva manoeuvre but remained unaltered during handgrip exercise. In conclusion, reservoir-excess pressure analysis applied to the pulmonary artery revealed distinctive differences between controls and PAH patients. Variations in the ventricular preload and afterload influence pulmonary arterial wave propagation as demonstrated by changes in wave energy during spontaneous respiration and dynamic stress tests.

Regional heterogeneity in the reactivity of equine small pulmonary blood vessels

Regional differences in large equine pulmonary artery reactivity exist. It is not known if this heterogeneity extends into small vessels. The hypothesis that there is regional heterogeneity in small pulmonary artery and vein reactivity to sympathomimetics (phenylephrine and isoproterenol) and a parasympathomimetic (methacholine) was tested using wire myography on small vessels from caudodorsal (CD) and cranioventral (CV) lung of 12 horses [9 mares, 3 geldings, 8.67 ± 0.81 (age ± SE) yr, of various breeds that had never raced]. To study relaxation, vessels were precontracted with U46619 (10(-6) M). Methacholine mechanism of action was investigated using L-nitroarginine methylester (L-NAME, 100 μM) and indomethacin (10 μM). Phenylephrine did not contract any vessels. Isoproterenol relaxed CD arteries more than CV arteries (maximum relaxation 28.18% and 48.67%; Log IC50 ± SE -7.975 ± 0.1327 and -8.033 ± 0.1635 for CD and CV, respectively, P < 0.0001), but not veins. Methacholine caused contraction of CD arteries (maximum contraction 245.4%, Log EC50 ± SE -6.475 ± 0.3341), and relaxation of CV arteries (maximum relaxation 40.14%, Log IC50 ± SE -6.791 ± 0.1954) and all veins (maximum relaxation 50.62%, Log IC50 ± SE -6.932 ± 0.1986) in a nonregion-dependent manner. L-NAME (n = 8, P < 0.0001) and indomethacin (n = 7, P < 0.0001) inhibited methacholine-induced relaxation of CV arteries, whereas indomethacin augmented CD artery contraction (n = 8, P < 0.0001). Our data demonstrate significant regional heterogeneity in small blood vessel reactivity when comparing the CD to the CV region of the equine lung.

Pulmonary vascular tone is dependent on the central modulation of sympathetic nerve activity following chronic intermittent hypoxia

Chronic intermittent hypoxia (IH) provokes a centrally mediated increase in sympathetic nerve activity (SNA). Although this sympathetic hyperexcitation has been linked to systemic hypertension, its effect on the pulmonary vasculature is unclear. This study aimed to assess IH-mediated sympathetic excitation in modulating pulmonary vasculature tone, particularly acute hypoxia vasoconstrictor response (HPV), and the central β-adrenergic signaling pathway for facilitating the increase in SNA. Sprague-Dawley rats were exposed to IH (cycle of 4% O2 for 90 s/air for 90 s) for 8 h/day for 6 weeks. Subsequently, rats were anesthetized and either pulmonary SNA was recorded (electrophysiology), or the pulmonary vasculature was visualized using microangiography. Pulmonary sympathetic and vascular responses to acute hypoxia were assessed before and after central β1-adrenergic receptor blockade (Metoprolol, 200 nmol i.c.v.). Chronic IH increased baseline SNA (110% increase), and exacerbated the sympathetic response to acute hypoxia. Moreover, the magnitude of HPV in IH rats was blunted compared to control rats (e.g., 10 and 20% vasoconstriction, respectively). In only the IH rats, β1-receptor blockade with metoprolol attenuated the hypoxia-induced increase in pSNA and exacerbated the magnitude of acute HPV, so that both sympathetic and HPV responses were similar to that of control rats. Interestingly, the expression of β1-receptors within the brainstem was similar between both control and IH rats. These results suggest that the centrally mediated increase in SNA following IH acts to blunt the local vasoconstrictor effect of acute hypoxia, which reflects an inherent difference between vasodilator and vasoconstrictor actions of SNA in pulmonary and systemic circulations.

Perinatal nitric oxide therapy prevents adverse effects of perinatal hypoxia on the adult pulmonary circulation

Adverse events in utero are associated with the occurrence of chronic diseases in adulthood. We previously demonstrated in mice that perinatal hypoxia resulted in altered pulmonary circulation in adulthood, with a decreased endothelium-dependent relaxation of pulmonary arteries, associated with long-term alterations in the nitric oxide (NO)/cyclic GMP pathway. The present study investigated whether inhaled NO (iNO) administered simultaneously to perinatal hypoxia could have potential beneficial effects on the adult pulmonary circulation. Indeed, iNO is the therapy of choice in humans presenting neonatal pulmonary hypertension. Long-term effects of neonatal iNO therapy on adult pulmonary circulation have not yet been investigated. Pregnant mice were placed in hypoxia (13% O₂) with simultaneous administration of iNO 5 days before delivery until 5 days after birth. Pups were then raised in normoxia until adulthood. Perinatal iNO administration completely restored acetylcholine-induced relaxation, as well as endothelial nitric oxide synthase protein content, in isolated pulmonary arteries of adult mice born in hypoxia. Right ventricular hypertrophy observed in old mice born in hypoxia compared to controls was also prevented by perinatal iNO treatment. Therefore, simultaneous administration of iNO during perinatal hypoxic exposure seems able to prevent adverse effects of perinatal hypoxia on the adult pulmonary circulation.

Impairment of pulmonary vascular reserve and right ventricular systolic reserve in pulmonary arterial hypertension

Background

Exercise capacity is impaired in pulmonary arterial hypertension (PAH). We hypothesized that cardiovascular reserve abnormalities would be associated with impaired hemodynamic response to pharmacological stress and worse outcome in PAH.

Methods

Eighteen PAH patients (p) group 1 NYHA class II/III and ten controls underwent simultaneous right cardiac catheterization and intravascular ultrasound at rest and during low dose-dobutamine (10 mcg/kg/min) with trendelenburg (DST). We estimated cardiac output (CO), pulmonary vascular resistance (PVR) and capacitance (PC), and PA elastic modulus (EM). We concomitantly measured tricuspid annular plane systolic excursion (TAPSE), RV myocardial peak systolic velocity (Sm) and isovolumic myocardial acceleration (IVA) in PAH patients. Based on the rounded mean + 2 SD of the increase in mPAP in our healthy control group during DST (2.8 + 1.8 mm Hg), PAH p were divided into two groups according to mean PA pressure (mPAP) response during DST, 1: ΔmPAP > 5 mm Hg and 2: ΔmPAP ≤ 5 mm Hg. Cardiovascular reserve was estimated as the change (delta, Δ) during DST compared with rest, including ΔmPAP with respect to ΔCO (ΔmPAP/ΔCO). All patients were prospectively followed up for 2 years.

Results

PAH p showed significant lower heart rate and CO increase than controls during DST, with a significant mPAP and pulse PAP increase and higher ΔmPAP/ΔCO (p < 0.05). Neither hemodynamic, IVUS and echocardiographic data were different between both PAH groups at rest. In group 1, DST caused a higher ΔEM, ΔmPAP/ΔCO, ΔPVR, and ΔTAPSE than group 2, with a lower IVA increase and a negative ΔSV (p < 0.05). TAPSE correlated with mPAP and RVP (p < 0.05) and, IVA and Sm correlated with CO (p < 0.05). ΔEM correlated with ΔmPAP and ΔIVA with ΔCO (p < 0.05). There were two deaths/pulmonary transplantations in group 1 and one death in group 2 during the follow-up (p > 0.05).

Conclusions

Pulmonary vascular reserve and RV systolic reserve are significantly impaired in patients with PAH. The lower recruitable cardiovascular reserve is significantly related to a worse hemodynamic response to DST and it could be associated with a poor clinical outcome.

Pulmonary vasoconstrictor influence of endothelin in exercising swine depends critically on phosphodiesterase 5 activity

Both phosphodiesterase 5 (PDE5) inhibition and endothelin (ET) receptor blockade have been shown to induce pulmonary vasodilation. However, little is known about the effect of combined blockade of these two vasoconstrictor pathways. Since nitric oxide (NO) exerts its pulmonary vasodilator influence via production of cyclic guanosine monophosphate (cGMP) as well as through inhibition of ET, we hypothesized that interaction between the respective signaling pathways precludes an additive vasodilator effect. We tested this hypothesis in chronically instrumented swine exercising on a treadmill by comparing the vasodilator effect of the PDE5 inhibitor EMD360527, the ETA/ETB antagonist tezosentan, and combined EMD360527 and tezosentan. In the systemic circulation, vasodilation by tezosentan and EMD360527 was additive, both at rest and during exercise, resulting in a 17 ± 2% drop in blood pressure. In the pulmonary circulation, both EMD360527 and tezosentan produced vasodilation. However, tezosentan produced no additional pulmonary vasodilation in the presence of EMD360527, either at rest or during exercise. Moreover, in isolated preconstricted porcine pulmonary small arteries (∼300 μm) EMD360527 (1 nM-10 μM) induced dose-dependent vasodilation, whereas tezosentan (1 nM-10 μM) failed to elicit vasodilation irrespective of the presence of EMD360527. However, both PDE5 inhibition and 8Br-cGMP, but not 8Br-cAMP, blunted pulmonary small artery contraction to ET and its precursor Big ET in vitro. In conclusion, in healthy swine, either at rest or during exercise, PDE5 inhibition and the associated increase in cGMP produce pulmonary vasodilation that is mediated in part through inhibition of the ET pathway, thereby precluding an additional vasodilator effect of ETA/ETB receptor blockade in the presence of PDE5 inhibition.

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.

Hypoxic pulmonary vasoconstriction in the absence of pretone: essential role for intracellular Ca2+ release

  Hypoxic pulmonary vasoconstriction (HPV) maintains blood oxygenation during acute hypoxia but contributes to pulmonary hypertension during chronic hypoxia. The mechanisms of HPV remain controversial, in part because HPV is usually studied in the presence of agonist-induced preconstriction ('pretone'). This potentiates HPV but may obscure and distort its underlying mechanisms. We therefore carried out an extensive assessment of proposed mechanisms contributing to HPV in isolated intrapulmonary arteries (IPAs) in the absence of pretone by using a conventional small vessel myograph. Hypoxia elicited a biphasic constriction consisting of a small transient (phase 1) superimposed upon a sustained (phase 2) component. Neither phase was affected by the L-type Ca2+ channel antagonists diltiazem (10 and 30 μm) or nifedipine (3 μm). Application of the store-operated Ca2+ entry (SOCE) blockers BTP2 (10 μm) or SKF96365 (50 μm) attenuated phase 2 but not phase 1, whereas a lengthy (30 min) incubation in Ca2+-free physiological saline solution similarly reduced phase 2 but abolished phase 1. No further effect of inhibition of HPV was observed if the sarco/endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (30 μm) was also applied during the 30 min incubation in Ca2+-free physiological saline solution. Pretreatment with 10 μm ryanodine and 15 mm caffeine abolished both phases, whereas treatment with 100 μm ryanodine attenuated both phases. The two-pore channel blocker NED-19 (1 μm) and the nicotinic acid adenine dinucleotide phosphate (NAADP) antagonist BZ194 (200 μm) had no effect on either phase of HPV. The lysosomal Ca2+-depleting agent concanamycin (1 μm) enhanced HPV if applied during hypoxia, but had no effect on HPV during a subsequent hypoxic challenge. The cyclic ADP ribose antagonist 8-bromo-cyclic ADP ribose (30 μm) had no effect on either phase of HPV. Neither the Ca2+-sensing receptor (CaSR) blocker NPS2390 (0.1 and 10 μm) nor FK506 (10 μm), a drug which displaces FKBP12.6 from ryanodine receptor 2 (RyR2), had any effect on HPV. HPV was virtually abolished by the rho kinase blocker Y-27632 (1 μm) and attenuated by the protein kinase C inhibitor Gö6983 (3 μm). Hypoxia for 45 min caused a significant increase in the ratio of oxidised to reduced glutathione (GSSG/GSH). HPV was unaffected by the NADPH oxidase inhibitor VAS2870 (10 μm), whereas phase 2 was inhibited but phase 1 was unaffected by the antioxidants ebselen (100 μm) and TEMPOL (3 mm). We conclude that both phases of HPV in this model are mainly dependent on [Ca2+]i release from the sarcoplasmic reticulum. Neither phase of HPV requires voltage-gated Ca2+ entry, but SOCE contributes to phase 2. We can detect no requirement for cyclic ADP ribose, NAADP-dependent lysosomal Ca2+ release, activation of the CaSR, or displacement of FKBP12.6 from RyR2 for either phase of HPV. Sustained HPV is associated with an oxidising shift in the GSSG/GSH redox potential and is inhibited by the antioxidants ebselen and TEMPOL, consistent with the concept that it requires an oxidising shift in the cell redox state or the generation of reactive oxygen species.

A process-based review of mouse models of pulmonary hypertension

Genetically modified mouse models have unparalleled power to determine the mechanisms behind different processes involved in the molecular and physiologic etiology of various classes of human pulmonary hypertension (PH). Processes known to be involved in PH for which there are extensive mouse models available include the following: (1) Regulation of vascular tone through secreted vasoactive factors; (2) regulation of vascular tone through potassium and calcium channels; (3) regulation of vascular remodeling through alteration in metabolic processes, either through alteration in substrate usage or through circulating factors; (4) spontaneous vascular remodeling either before or after development of elevated pulmonary pressures; and (5) models in which changes in tone and remodeling are primarily driven by inflammation. PH development in mice is of necessity faster and with different physiologic ramifications than found in human disease, and so mice make poor models of natural history of PH. However, transgenic mouse models are a perfect tool for studying the processes involved in pulmonary vascular function and disease, and can effectively be used to test interventions designed against particular molecular pathways and processes involved in disease.

Pulmonary blood flow and its distribution in highly trained endurance athletes and healthy control subjects

Pulmonary blood flow (PBF) is an important determinant of endurance sports performance, yet studies investigating adaptations of the pulmonary circulation in athletes are scarce. In the present study, we investigated PBF, its distribution, and heterogeneity at baseline and during intravenous systemic adenosine infusion in 10 highly trained male endurance athletes and 10 untrained but fit healthy controls, using positron emission tomography and [15O]water at rest and during adenosine infusion at supine body posture. Our results indicate that PBF at rest and during adenosine stimulation was similar in both groups (213 ± 55 and 563 ± 138 ml·100 ml−1·min−1 in athletes and 206 ± 83 and 473 ± 212 ml·100 ml−1·min−1 in controls, respectively). Although the PBF response to adenosine was thus unchanged in athletes, overall PBF heterogeneity was reduced from rest to adenosine infusion (from 84 ± 18 to 70 ± 19%, P < 0.05), while remaining unchanged in healthy controls (77 ± 16 to 85 ± 33%, P = 0.4). Additionally, there was a marked gravitational influence on general PBF distribution so that clear dorsal dominance was observed both at rest and during adenosine infusion, but training status did not have an effect on this distribution. Regional blood flow heterogeneity was markedly lower in the high-perfusion dorsal areas, both at rest and during adenosine, in all subjects, but flow heterogeneity in dorsal area tended to further decrease in response to adenosine in athletes. In conclusion, reduced blood flow heterogeneity in response to adenosine in endurance athletes may be a reflection of capillary reserve, which is more extensively recruitable in athletes than in matched healthy control subjects.

The pulmonary vascular response to combined activation of the muscle metaboreflex and mechanoreflex

Muscle metabo- and mechanoreflexes are known to influence systemic cardiovascular responses to exercise. Whether interplay between these reflexes is operant in the control of the pulmonary vascular response to exercise is unknown. The aim of this study was to assess the pulmonary vascular response to the combined activation of the two muscle reflexes. Nine healthy subjects performed a bout of isometric calf plantarflexion exercise during local circulatory occlusion, which was continued for 9 min postexercise (PECO). At 5 min into PECO the calf muscle was passively stretched for 180 s. A control (no exercise) protocol was also undertaken. Heart rate, blood pressure measurements and echocardiographically determined estimates of systolic pulmonary artery pressure (SPAP) and cardiac output ( ) were obtained at intervals throughout the two protocols. Elevations in SPAP (by 22.51 ± 2.61%), (by 26.92 ± 2.99%) and mean arterial pressure (by 15.38 ± 2.29%) were noted during isometric exercise in comparison to baseline (all P < 0.05). Increases in SPAP and mean arterial pressure persisted during PECO (All P < 0.05), whereas returned to resting levels. These increases in mean arterial pressure and SPAP were sustained during stretch which significantly elevated (All P < 0.05). These data suggest that activation of the muscle mechanoreflex attenuated the increases in pulmonary vascular resistance caused by metaboreflex activation. This finding has important implications for the regulation of pulmonary haemodynamics during human exercise.

Endothelial dysfunction enhances the pulmonary and systemic vasodilator effects of phosphodiesterase-5 inhibition in awake swine at rest and during treadmill exercise

Cardiovascular disease is characterized by impaired exercise capacity and endothelial dysfunction, i.e. reduced bioavailability of nitric oxide (NO). Phosphodiesterase-5 (PDE5) inhibition is a promising vasodilator therapy, but its effects on pulmonary and systemic hemodynamic responses to exercise in the absence, and particularly in the presence, of endothelial dysfunction have not been studied. We investigated the effects of PDE5 inhibitor EMD360527 in chronically instrumented swine at rest and during exercise with and without NO synthase inhibition (N(ω)-nitro-l-arginine; NLA). PDE5 inhibition caused a 19 ± 3% decrease in systemic vascular resistance (SVR) and a 24 ± 4% decrease in pulmonary vascular resistance (PVR) at rest. At maximal exercise, PDE5 inhibition caused a 13 ± 1% decrease in SVR and a 29 ± 3% decrease in PVR. NLA enhanced PDE5-inhibition-induced pulmonary (decrease in PVR 32 ± 12% at rest and 41 ± 3% during exercise) and systemic (decrease in SVR 24 ± 5% at rest and 18 ± 3% during exercise) vasodilation. Similarly, NLA increased the pulmonary and systemic vasodilation to nitroprusside and 8-bromo-cyclic guanosine monophosphate (cGMP), indicating that inhibition of NO synthase increases responsiveness to stimulation of the NO/cGMP pathway. Thus, PDE5 inhibition causes pulmonary and systemic vasodilation that is, respectively, maintained and slightly blunted during exercise. The degree of dilation in both the pulmonary and systemic beds were paradoxically enhanced in the presence of reduced bioavailability of NO, suggesting that this vasodilator therapy is most effective in patients with cardiovascular disease.

Pulmonary circulation at exercise

The pulmonary circulation is a high-flow and low-pressure circuit, with an average resistance of 1 mmHg/min/L in young adults, increasing to 2.5 mmHg/min/L over four to six decades of life. Pulmonary vascular mechanics at exercise are best described by distensible models. Exercise does not appear to affect the time constant of the pulmonary circulation or the longitudinal distribution of resistances. Very high flows are associated with high capillary pressures, up to a 20 to 25 mmHg threshold associated with interstitial lung edema and altered ventilation/perfusion relationships. Pulmonary artery pressures of 40 to 50 mmHg, which can be achieved at maximal exercise, may correspond to the extreme of tolerable right ventricular afterload. Distension of capillaries that decrease resistance may be of adaptative value during exercise, but this is limited by hypoxemia from altered diffusion/perfusion relationships. Exercise in hypoxia is associated with higher pulmonary vascular pressures and lower maximal cardiac output, with increased likelihood of right ventricular function limitation and altered gas exchange by interstitial lung edema. Pharmacological interventions aimed at the reduction of pulmonary vascular tone have little effect on pulmonary vascular pressure-flow relationships in normoxia, but may decrease resistance in hypoxia, unloading the right ventricle and thereby improving exercise capacity. Exercise in patients with pulmonary hypertension is associated with sharp increases in pulmonary artery pressure and a right ventricular limitation of aerobic capacity. Exercise stress testing to determine multipoint pulmonary vascular pressures-flow relationships may uncover early stage pulmonary vascular disease.

Integrated control of pulmonary vascular tone by endothelin and angiotensin II in exercising swine depends on gender

The lungs are now recognized as an active metabolic organ that is a major determinant of the plasma concentrations of the vasoconstrictors endothelin (ET) and ANG II. Several studies have suggested a complex interaction between ET and ANG II in the systemic and coronary vascular beds that is different at rest and during exercise. To date, the interaction between these vasoconstrictor peptides has barely been investigated in relation to the pulmonary vascular bed. Consequently, we investigated the integrated control of pulmonary vasomotor tone by ET and ANG II in 24 chronically instrumented swine (15 female and 9 male) at rest and during graded treadmill exercise. In the systemic circulation, ANG II type 1 (AT(1)) receptor blockade with irbesartan and mixed ET(A)/ET(B) blockade with tezosentan each produced vasodilation. The systemic vasodilator effect of ET(A)/ET(B) blockade was enhanced after AT(1) blockade in female swine, whereas a trend toward an increase was observed in male swine. In the pulmonary circulation, AT(1) receptor blockade had no effect on pulmonary vascular tone in male swine, whereas it resulted in an unexpected increase in pulmonary vasomotor tone in female swine. ET(A)/ET(B) receptor blockade did not result in a decrease in pulmonary vasomotor tone at rest but produced a decrease in vasomotor tone during exercise in both genders. This pulmonary vasodilation by ET(A)/ET(B) receptor blockade was enhanced after prior AT(1) blockade in female swine but not in male swine. In conclusion, in both the systemic and pulmonary circulation of female swine, ANG II inhibits the vasoconstrictor influence of ET. This interaction is gender specific. The observation that plasma ET levels were not altered by AT(1) blockade in either gender suggests that the interaction between these vasoconstrictors occurs locally in the vasculature.

Use of bicycle exercise echocardiography for unexplained exertional dyspnea

Unexplained exertional dyspnea is a common and perplexing clinical problem. Myocardial ischemia and left ventricular systolic dysfunction are important cardiac causes, but are often not detected in these patients. Recently, exercise-induced left ventricular diastolic dysfunction and exercised-induced pulmonary hypertension have emerged as common alternative mechanisms. While conventional exercise treadmill echocardiography effectively diagnoses left ventricular systolic dysfunction and myocardial ischemia, it has limited ability to detect exercise-induced diastolic dysfunction or pulmonary hypertension. The latest advances in exercise echocardiography, including utilization of tissue Doppler imaging and harmonic imaging, make noninvasive evaluation of both conventional and alternative cardiac causes of exertional dyspnea possible. These advancements, when coupled with newly designed supine exercise platforms for bicycle exercise echocardiography (BE), facilitate the detection of exercise-induced diastolic dysfunction and pulmonary hypertension. Moreover, BE using supine ergometry additionally permits the dynamic evaluation of valvular function and interatrial shunting and detection of pulmonary arteriovenous fistula, uncommon but important causes of unexplained exertional dyspnea. Therefore, we propose that because of its superior diagnostic capabilities, BE should be included as part of a comprehensive cardiac evaluation of patients with unexplained exertional dyspnea.

Prognostic significance of sympathetic nervous system activation in pulmonary arterial hypertension

RATIONALE

The sympathetic nervous system has been reported to be activated in pulmonary arterial hypertension (PAH).

OBJECTIVES

We investigated the prognostic significance of muscle sympathetic nervous system activity (MSNA) in PAH.

METHODS

Thirty-two patients with PAH were included in the study and underwent a measurement of MSNA over a 6-year period of time. They had undergone a concomitant evaluation of New York Heart Association (NYHA) functional class, a 6-minute walk distance (6MWD), an echocardiographic examination, and a right heart catheterization for diagnostic or reevaluation purposes. The median follow-up time was 20.6 months (interquartile range, 45.8 mo). Clinical deterioration was defined by listing for transplantation or death.

MEASUREMENTS AND MAIN RESULTS

Seventeen patients presented with clinical deterioration. As compared with the 15 others, they had an increased MSNA (80 +/- 12 vs. 52 +/- 18 bursts/min; P < 0.001) and heart rate (88 +/- 17 vs. 74 +/- 12 bpm; P = 0.01), a lower 6MWD (324 +/- 119 vs. 434 +/- 88 m; P < 0.01) and a deteriorated NYHA functional class (3.6 +/- 0.5 vs. 2.9 +/- 0.8; P < 0.001). The hemodynamic variables were not different. MSNA was directly related to heart rate and inversely to 6MWD. A univariate analysis revealed that increased MSNA and heart rate, NYHA class IV, lower 6MWD, and pericardial effusion were associated with subsequent clinical deterioration. A multivariate analysis showed that MSNA was an independent predictor of clinical deterioration. For every increase of 1 burst/minute, the risk of clinical deterioration during follow-up increased by 6%.

CONCLUSIONS

Sympathetic nervous system activation is an independent predictor of clinical deterioration in pulmonary arterial hypertension.