Pulmonary arterial wall distensibility assessed by intravascular ultrasound in children with congenital heart disease: an indicator for pulmonary vascular disease?

Noninvasive evaluation of pulmonary artery stiffness in heart failure patients via cardiovascular magnetic resonance

Heart failure (HF) presents manifestations in both cardiac and vascular abnormalities. Pulmonary hypertension (PH) is prevalent in up 50% of HF patients. While pulmonary arterial hypertension (PAH) is closely associated with pulmonary artery (PA) stiffness, the association of HF caused, post-capillary PH and PA stiffness is unknown. We aimed to assess and compare PA stiffness and blood flow hemodynamics noninvasively across HF entities and control subjects without HF using CMR. We analyzed data of a prospectively conducted study with 74 adults, including 55 patients with HF across the spectrum (20 HF with preserved ejection fraction [HFpEF], 18 HF with mildly-reduced ejection fraction [HFmrEF] and 17 HF with reduced ejection fraction [HFrEF]) as well as 19 control subjects without HF. PA stiffness was defined as reduced vascular compliance, indicated primarily by the relative area change (RAC), altered flow hemodynamics were detected by increased flow velocities, mainly by pulse wave velocity (PWV). Correlations between the variables were explored using correlation and linear regression analysis. PA stiffness was significantly increased in HF patients compared to controls (RAC 30.92 ± 8.47 vs. 50.08 ± 9.08%, p < 0.001). PA blood flow parameters were significantly altered in HF patients (PWV 3.03 ± 0.53 vs. 2.11 ± 0.48, p < 0.001). These results were consistent in all three HF groups (HFrEF, HFmrEF and HFpEF) compared to the control group. Furthermore, PA stiffness was associated with higher NT-proBNP levels and a reduced functional status. PA stiffness can be assessed non-invasively by CMR. PA stiffness is increased in HFrEF, HFmrEF and HFpEF patients when compared to control subjects.Trial registration The study was registered at the German Clinical Trials Register (DRKS, registration number: DRKS00015615).

Cardiac catheterization in pediatric pulmonary hypertension: a systematic and practical approach

Significant progress in the understanding of the etiology, epidemiology, pathobiology and prognosis of pulmonary hypertension (PH) has been made over the last years. Especially in the pediatric patient population the etiology of PH is very heterogeneous. Nevertheless, the most recent change of the definition of PH to a mean pulmonary artery pressure (mPAP) >20 mmHg has been accepted by pediatricians for uniformity and concordance with adult physicians. Based on the diverse underlying medical conditions leading to PH, a comprehensive and systematic approach for diagnosis and treatment is mandatory. Cardiac catheterization remains the gold standard for invasive assessment and acute vasoreactivity testing (AVT) additionally providing detailed information about nature of PH. In most patients repeat cardiac catheterization may be helpful for evaluation of response to targeted PH treatment, risk stratification and indication for lung transplantation. However, the information and results taken from cardiac catheterization should be interpreted by experienced investigators only who are familiar with confounding factors that may influence the results. Here we provide an overview of current recommendations for invasive hemodynamic evaluation in pediatric PH. We point out different patient scenarios and provide a structured approach for AVT and response interpretation.

Inverse modeling framework for characterizing patient-specific microstructural changes in the pulmonary arteries

Microstructural changes in the pulmonary arteries associated with pulmonary arterial hypertension (PAH) is not well understood and characterized in humans. To address this issue, we developed and applied a patient-specific inverse finite element (FE) modeling framework to characterize mechanical and structural changes of the micro-constituents in the proximal pulmonary arteries using in-vivo pressure measurements and magnetic resonance images. The framework was applied using data acquired from a pediatric PAH patient and a heart transplant patient with normal pulmonary arterial pressure, which serves as control. Parameters of a constrained mixture model that are associated with the structure and mechanical properties of elastin, collagen fibers and smooth muscle cells were optimized to fit the patient-specific pressure-diameter responses of the main pulmonary artery. Based on the optimized parameters, individual stress and linearized stiffness resultants of the three tissue constituents, as well as their aggregated values, were estimated in the pulmonary artery. Aggregated stress resultant and stiffness are, respectively, 4.6 and 3.4 times higher in the PAH patient than the control subject. Stress and stiffness resultants of each tissue constituent are also higher in the PAH patient. Specifically, the mean stress resultant is highest in elastin (PAH: 69.96, control: 14.42 kPa-mm), followed by those in smooth muscle cell (PAH: 13.95, control: 4.016 kPa-mm) and collagen fibers (PAH: 13.19, control: 2.908 kPa-mm) in both the PAH patient and the control subject. This result implies that elastin may be the key load-bearing constituent in the pulmonary arteries of the PAH patient and the control subject.

Aging influences pulmonary artery flow and stiffness in healthy individuals: non-invasive assessment using cardiac MRI

AIM

To investigate age-related changes of the pulmonary artery (PA) using cardiac magnetic resonance imaging (cMRI) in healthy subjects.

MATERIALS AND METHODS

A cross-sectional observational study was conducted on apparently healthy subjects who underwent PA velocity-encoded cMRI. cMRI was used to determine PA stiffness parameters such as PA elasticity, relative area change (PA-RAC) and pulse-wave velocity (PA-PWV), and PA flow parameters by subtracting simultaneous forward flow (FF) and backward flow (BF) velocity across the PA cross-section. Data were presented in five age and sex matched groups.

RESULTS

One hundred and fifty subjects (20-70 years, 75 men) met the enrolment criteria. PA elasticity and PA-RAC significantly decreased with age (p<0.001), while PA-PWV, regurgitant volume (V_reg) and backward flow volume (V_BF) increased in the elderly (p<0.001). Linear regression analysis indicated that PA elasticity (r=-0.441, p<0.0001) and PA-RAC (r=-0.484, p<0.0001) were indirectly and negatively associated with advancing age, whereas PA_min (r=0.331, p<0.0001), PA-PWV (r=0.490, p<0.0001), V_Reg (r=0.335, p<0.0001) and V_BF (r=0.349, p<0.0001) were directly associated with age. Multivariate analysis indicated that age was independently associated with V_reg and V_BF, and the addition of PA_min and PA-PWV marginally increased its predictive capacity.

CONCLUSION

Aging significantly increases cMRI-based PA flow and stiffness parameters. These could become relevant markers of subclinical changes of the PA geometry in healthy subjects.

Assessment of reversibility in pulmonary arterial hypertension and congenital heart disease

Pulmonary arterial hypertension (PAH) in congenital heart disease (CHD) can be reversed by early shunt closure, but this potential is lost beyond a certain point of no return. Therefore, it is crucial to accurately assess the reversibility of this progressive pulmonary arteriopathy in an early stage. Reversibility assessment is currently based on a combination of clinical symptoms and haemodynamic variables such as pulmonary vascular resistance. These measures, however, are of limited predictive value and leave many patients in the grey zone. This review provides a concise overview of the mechanisms involved in flow-dependent progression of PAH in CHD and evaluates existing and future alternatives to more directly investigate the stage of the pulmonary arteriopathy. Structural quantification of the pulmonary arterial tree using fractal branching algorithms, functional imaging with intravascular ultrasound, nuclear imaging, putative new blood biomarkers, genetic testing and the potential for transcriptomic analysis of circulating endothelial cells and educated platelets are being reviewed.

Transcatheter Pulmonary Valve Replacement and Acute Increase in Diastolic Pressure are Associated with Increases in Both Systolic and Diastolic Pulmonary Artery Dimensions

Stable positioning of a transcatheter pulmonary valve (TPV) in native outflow tracts depends on a clear understanding of underlying anatomy and outflow tract dimensions. We hypothesized that restoration of pulmonary competence may acutely alter these dimensions. A retrospective single-center review of consecutive patients after TPV placement from 2007 to 2014 was performed. Patients with less than moderate pulmonary regurgitation were excluded. We reviewed acute catheterization data on 46 patients, most with tetralogy of Fallot (70%). Baseline and post-implant (7.5 ± 3 min post-deployment) measurements of central pulmonary arteries (PAs) were determined angiographically. The right PA diameter increased (20 ± 4-24 ± 6 mm systole*, 16 ± 4-21 ± 6 mm diastole*), as did the left PA (20 ± 6-24 ± 8 mm systole*, 16 ± 5-21 ± 7 mm diastole*). PA pressures increased from averages of 29.3/10.6 (17) to 29.8/15.1 (21) mmHg. We noted that pre-implant systolic PA diameter correlated with diastolic PA diameter post-implant (r = 0.9). On follow-up catheterization in seven patients [median 3 years; (1-8)], combined central PA diameter decreased an average of 20% (systole: 20% ± 12, diastole: 18% ± 11) as compared to post-implant measurements. Acute pulmonary valve competence in patients with at least moderate pulmonary regurgitation results in an immediate increase in PA diameter (20% systole and 30% diastole). The cause of this diameter change is unclear. This acute change may have implications for device and patient selection (*p < 0.001).

Intravascular Ultrasound Characterization of a Tissue-Engineered Vascular Graft in an Ovine Model

Patients who undergo implantation of a tissue-engineered vascular graft (TEVG) for congenital cardiac anomalies are monitored with echocardiography, followed by magnetic resonance imaging or angiography when indicated. While these methods provide data regarding the lumen, minimal information regarding neotissue formation is obtained. Intravascular ultrasound (IVUS) has previously been used in a variety of conditions to evaluate the vessel wall. The purpose of this study was to evaluate the utility of IVUS for evaluation of TEVGs in our ovine model. Eight sheep underwent implantation of TEVGs either unseeded or seeded with bone marrow-derived mononuclear cells. Angiography, IVUS, and histology were directly compared. Endothelium, tunica media, and graft were identifiable on IVUS and histology at multiple time points. There was strong agreement between IVUS and angiography for evaluation of luminal diameter. IVUS offers a valuable tool to evaluate the changes within TEVGs, and clinical translation of this application is warranted.

The Critical Role of Pulmonary Arterial Compliance in Pulmonary Hypertension

The normal pulmonary circulation is a low-pressure, high-compliance system. Pulmonary arterial compliance decreases in the presence of pulmonary hypertension because of increased extracellular matrix/collagen deposition in the pulmonary arteries. Loss of pulmonary arterial compliance has been consistently shown to be a predictor of increased mortality in patients with pulmonary hypertension, even more so than pulmonary vascular resistance in some studies. Decreased pulmonary arterial compliance causes premature reflection of waves from the distal pulmonary vasculature, leading to increased pulsatile right ventricular afterload and eventually right ventricular failure. Evidence suggests that decreased pulmonary arterial compliance is a cause rather than a consequence of distal small vessel proliferative vasculopathy. Pulmonary arterial compliance decreases early in the disease process even when pulmonary artery pressure and pulmonary vascular resistance are normal, potentially enabling early diagnosis of pulmonary vascular disease, especially in high-risk populations. With the recognition of the prognostic importance of pulmonary arterial compliance, its impact on right ventricular function, and its contributory role in the development and progression of distal small-vessel proliferative vasculopathy, pulmonary arterial compliance is an attractive target for the treatment of pulmonary hypertension.

Cardiac catheterization in children with pulmonary hypertensive vascular disease: consensus statement from the Pulmonary Vascular Research Institute, Pediatric and Congenital Heart Disease Task Forces

Cardiac catheterization is important in the diagnosis and risk stratification of pulmonary hypertensive vascular disease (PHVD) in children. Acute vasoreactivity testing provides key information about management, prognosis, therapeutic strategies, and efficacy. Data obtained at cardiac catheterization continue to play an important role in determining the surgical options for children with congenital heart disease and clinical evidence of increased pulmonary vascular resistance. The Pediatric and Congenital Heart Disease Task Forces of the Pulmonary Vascular Research Institute met to develop a consensus statement regarding indications for, conduct of, acute vasoreactivity testing with, and pitfalls and risks of cardiac catheterization in children with PHVD. This document contains the essentials of those discussions to provide a rationale for the hemodynamic assessment by cardiac catheterization of children with PHVD.

Wall shear stress is decreased in the pulmonary arteries of patients with pulmonary arterial hypertension: An image-based, computational fluid dynamics study

Previous clinical studies in pulmonary arterial hypertension (PAH) have concentrated predominantly on distal pulmonary vascular resistance, its contribution to the disease process, and response to therapy. However, it is well known that biomechanical factors such as shear stress have an impact on endothelial health and dysfunction in other parts of the vasculature. This study tested the hypothesis that wall shear stress is reduced in the proximal pulmonary arteries of PAH patients with the belief that reduced shear stress may contribute to pulmonary endothelial cell dysfunction and as a result, PAH progression. A combined MRI and computational fluid dynamics (CFD) approach was used to construct subject-specific pulmonary artery models and quantify flow features and wall shear stress (WSS) in five PAH patients with moderate-to-severe disease and five age- and sex-matched controls. Three-dimensional model reconstruction showed PAH patients have significantly larger main, right, and left pulmonary artery diameters (3.5 ± 0.4 vs. 2.7 ± 0.1 cm, P = 0.01; 2.5 ± 0.4 vs. 1.9 ± 0.2 cm, P = 0.04; and 2.6 ± 0.4 vs. 2.0 ± 0.2 cm, P = 0.01, respectively), and lower cardiac output (3.7 ± 1.2 vs. 5.8 ± 0.6 L/min, P = 0.02.). CFD showed significantly lower time-averaged central pulmonary artery WSS in PAH patients compared to controls (4.3 ± 2.8 vs. 20.5 ± 4.0 dynes/cm(2), P = 0.0004). Distal WSS was not significantly different. A novel method of measuring WSS was utilized to demonstrate for the first time that WSS is altered in some patients with PAH. Using computational modeling in patient-specific models, WSS was found to be significantly lower in the proximal pulmonary arteries of PAH patients compared to controls. Reduced WSS in proximal pulmonary arteries may play a role in the pathogenesis and progression of PAH. This data may serve as a basis for future in vitro studies of, for example, effects of WSS on gene expression.

Pulsatile haemodynamic parameters are predictors of survival in paediatric pulmonary arterial hypertension

BACKGROUND

There is a need for reliable prognostic parameters in pulmonary arterial hypertension (PAH), especially in children. Pulsatile components of the right ventricular afterload, represented by pulmonary arterial compliance (PACi) and pulmonary stroke volume (PSVi), may provide important additional prognostic information to conventional static haemodynamic parameters. The aim of this study was to determine the prognostic value of PACi and PSVi in paediatric PAH.

METHODS

Right heart catheterization data of 52 consecutive paediatric idiopathic/hereditary PAH and PAH associated with congenital heart disease patients with full haemodynamic evaluation seen at the Dutch national referral centre for paediatric pulmonary hypertension between 1993 and 2010 were reviewed. A control group was composed of patients with normal pulmonary vascular resistance. PSVi and PACi were calculated and tested for predictive value for transplant-free survival.

RESULTS

PAH patients had significantly lower PSVi and PACi compared to control patients. PSVi and PACi were lower in patients with higher WHO-functional class compared to those with lower functional classes. Higher PSVi, PACi and mSAP and lower mPAP/mSAP and heart rate were associated with improved survival, independent from WHO-functional class and PAH-targeted therapy. In multivariate analyses PSVi, heart rate and mSAP emerged as the strongest haemodynamic predictors of survival. The effect of vasodilator challenge on the haemodynamic variables did not provide additional prognostic information.

CONCLUSIONS

The parameters of both the pulsatile and static pulmonary circulations are strong independent predictors for transplant-free survival, and therefore can be of complementary value in assessing disease severity, predicting survival and guiding treatment in paediatric PAH.

In vivo and in vitro measurements of pulmonary arterial stiffness: A brief review

During the progression of pulmonary hypertension (PH), proximal pulmonary arteries (PAs) undergo remodeling such that they become thicker and the elastic modulus increases. Both of these changes increase the vascular stiffness. The increase in pulmonary vascular stiffness contributes to increased right ventricular (RV) afterload, which causes RV hypertrophy and eventually failure. Studies have found that proximal PA stiffness or its inverse, compliance, is strongly related to morbidity and mortality in patients with PH. Therefore, accurate in vivo measurement of PA stiffness is useful for prognoses in patients with PH. It is also important to understand the structural changes in PAs that occur with PH that are responsible for stiffening. Here, we briefly review the most common parameters used to quantify stiffness and in vivo and in vitro methods for measuring PA stiffness in human and animal models. For in vivo approaches, we review invasive and noninvasive approaches that are based on measurements of pressure and inner or outer diameter or cross-sectional area. For in vitro techniques, we review several different testing methods that mimic one, two or several aspects of physiological loading (e.g., uniaxial and biaxial testing, dynamic inflation-force testing). Many in vivo and in vitro measurement methods exist in the literature, and it is important to carefully choose an appropriate method to measure PA stiffness accurately. Therefore, advantages and disadvantages of each approach are discussed.

Pulmonary arterial hypertension: an imaging review comparing MR pulmonary angiography and perfusion with multidetector CT angiography

Pulmonary hypertension (PH) is a progressive disease that leads to substantial morbidity and eventual death. Pulmonary multidetector CT angiography (MDCTA), pulmonary MR angiography (MRA) and MR-derived pulmonary perfusion (MRPP) imaging are non-invasive imaging techniques for the differential diagnosis of PH. MDCTA is considered the gold standard for the diagnosis of pulmonary embolism, one of the most common causes of PH. MRA and MRPP are promising techniques that do not require the use of ionising radiation or iodinated contrast material, and can be useful for patients for whom such material cannot be used. This review compares the imaging aspects of pulmonary MRA and 64-row MDCTA in patients with chronic thromboembolic or idiopathic PH.

A new flow co-culture system for studying mechanobiology effects of pulse flow waves

Artery stiffening is known as an important pathological change that precedes small vessel dysfunction, but underlying cellular mechanisms are still elusive. This paper reports the development of a flow co-culture system that imposes a range of arterial-like pulse flow waves, with similar mean flow rate but varied pulsatility controlled by upstream stiffness, onto a 3-D endothelial-smooth muscle cell co-culture. Computational fluid dynamics results identified a uniform flow area critical for cell mechanobiology studies. For validation, experimentally measured flow profiles were compared to computationally simulated flow profiles, which revealed percentage difference in the maximum flow to be <10, <5, or <1% for a high, medium, or low pulse flow wave, respectively. This comparison indicated that the computational model accurately demonstrated experimental conditions. The results from endothelial expression of proinflammatory genes and from determination of proliferating smooth muscle cell percentage both showed that cell activities did not vary within the identified uniform flow region, but were upregulated by high pulse flow compared to steady flow. The flow system developed and characterized here provides an important tool to enhance the understanding of vascular cell remodeling under flow environments regulated by stiffening.

Pulmonary vascular wall stiffness: An important contributor to the increased right ventricular afterload with pulmonary hypertension

Pulmonary hypertension (PH) is associated with structural and mechanical changes in the pulmonary vascular bed that increase right ventricular (RV) afterload. These changes, characterized by narrowing and stiffening, occur in both proximal and distal pulmonary arteries (PAs). An important consequence of arterial narrowing is increased pulmonary vascular resistance (PVR). Arterial stiffening, which can occur in both the proximal and distal pulmonary arteries, is an important index of disease progression and is a significant contributor to increased RV afterload in PH. In particular, arterial narrowing and stiffening increase the RV afterload by increasing steady and oscillatory RV work, respectively. Here we review the current state of knowledge of the causes and consequences of pulmonary arterial stiffening in PH and its impact on RV function. We review direct and indirect techniques for measuring proximal and distal pulmonary arterial stiffness, measures of arterial stiffness including elastic modulus, incremental elastic modulus, stiffness coefficient β and others, the changes in cellular function and the extracellular matrix proteins that contribute to pulmonary arterial stiffening, the consequences of PA stiffening for RV function and the clinical implications of pulmonary vascular stiffening for PH progression. Future investigation of the relationship between PA stiffening and RV dysfunction may facilitate new therapies aimed at improving RV function and thus ultimately reducing mortality in PH.

Pulmonary vascular stiffness: measurement, modeling, and implications in normal and hypertensive pulmonary circulations

This article introduces the concept of pulmonary vascular stiffness, discusses its increasingly recognized importance as a diagnostic marker in the evaluation of pulmonary vascular disease, and describes methods to measure and model it clinically, experimentally, and computationally. It begins with a description of systems-level methods to evaluate pulmonary vascular compliance and recent clinical efforts in applying such techniques to better predict patient outcomes in pulmonary arterial hypertension. It then progresses from the systems-level to the local level, discusses proposed methods by which upstream pulmonary vessels increase in stiffness, introduces concepts around vascular mechanics, and concludes by describing recent work incorporating advanced numerical methods to more thoroughly evaluate changes in local mechanical properties of pulmonary arteries.

Over-the-wire intravascular ultrasound in an animal model of pulmonary hypertension

BACKGROUND

Intravascular ultrasound (IVUS) measures prognostically important pulsatile flow indexes in patients with pulmonary hypertension (PH). IVUS catheters traditionally require a guiding catheter for placement which can impact hemodynamics in small infants because the guiding catheter renders the atrioventricular valve incompetent.

METHODS

Domestic swine (1.4-2.2 kg) were raised in isobaric normoxia (n = 4) or hypoxia (n = 3, FiO(2) 10-12%) for 72 hr for induction of PH. Cardiac catheterization and intravascular imaging was performed using a 3.5-Fr 20-MHz Eagle Eye Gold catheter (Volcano Corp., CA, USA) over a 0.014'' guide wire. Intima-media thickness (IMT) was measured and relative area change and vascular pulsatility were calculated.

RESULTS

The IVUS probe was easily manipulated over a 0.014'' wire without hemodynamic compromise in all animals. The IMT was thicker in the hypoxic group than the normoxic group (0.19 ± 0.03 mm vs. 0.31 ± 0.04 mm, p = .067). Hypoxic animals had systolic PH (39.66 ± 2.51 vs. 21.75 ± 2.87 mmHg, p = .02). Systemic arterial pressures between the groups were the same (hypoxic 68 ± 10.44 vs. normoxic 79.75 ± 14.84 mmHg, p = .26). Vascular pulsatility was similar (hypoxic 24 ± 2.64 vs. 20.25 ± 0.57%, p = .18). However, the arterial wall distensibility was significantly different (0.98 ± 0.2 vs. 2.01 ± 1.38 %/mmHg, p = .04).

CONCLUSIONS

Monorail IVUS imaging without a guide catheter overcomes a major limitation for use in infants and small animal experimental models by avoiding hemodynamic compromise. This would be a valuable tool for assessment of PH in the research and clinical setting.

Assessment of pulmonary artery stiffness using velocity-encoding magnetic resonance imaging: evaluation of techniques

The loss of pulmonary artery (PA) compliance has significant pathophysiological effect on the right ventricle. Noninvasive and reliable assessment of PA wall stiffness would be an essential determiner of right heart load and a clinically useful factor to assess cardiovascular risk. Two MRI techniques have been proposed for assessing PA stiffness by measuring pulse wave velocity (PWV): transit time (TT) and flow area (QA). However, no data are available that compares the two techniques and evaluates their performance, especially over a wide range of PWV values or at 3.0-T, which is the purpose of the present study. Thirty-three patients with different heart conditions were imaged using optimized high-temporal resolution and high-spatial resolution velocity-encoding MRI sequences. Statistical analysis was conducted to study intermethod, interobserver and intraobserver variabilities. The PWV measurements using TT and QA techniques showed good agreement (P>0.1). The Bland-Altman analysis showed negligible differences between the two methods (mean±S.D.=0.11±0.35 m/s, correlation coefficient r=0.94). The repeated measurements showed low interobserver and intraobserver variabilities, although the S.D. of the differences was larger in the QA technique. The mean±S.D. of the TT/QA measurement differences were -0.05±0.2/0.0±0.36 m/s and 0.02±0.26/0.02±0.39 m/s for the interobserver and intraobserver differences, respectively. In conclusion, each technique has its own advantages and disadvantages. The two techniques result in similar measurements, although the QA method is more subjective due to its dependency on operator intervention.

In vivo measurement of proximal pulmonary artery elastic modulus in the neonatal calf model of pulmonary hypertension: development and ex vivo validation

Developing clinical work suggests that vascular stiffening plays a role in the progression of pulmonary hypertension (PH), while recent studies in animal models of hypoxic PH have found significant proximal vascular stiffening in the diseased population. Here, we develop and validate a minimally invasive, clinically realizable method to estimate the local elastic modulus of the proximal pulmonary arteries from pressure-diameter (PD) data. PD measurements were made in the main pulmonary arteries of 16 calves; lumen diameter was assessed using color M-mode tissue Doppler imaging ultrasound, while pressure was measured via catheter. Two methods corresponding to thin-walled pressure vessel theory ("thin") and Lame's equation for a thick-walled cylinder ("thick") were used to approximate the artery elastic modulus from PD measurements. The harvested arteries were tested independently to determine their "true" ex vivo elastic modulus and stiffness. Both approximations displayed excellent correlation with ex vivo elastic modulus of the calf main pulmonary artery (thin r(2) = 0.811; thick r(2) = 0.844; both P < 0.01). Bland-Altman analysis indicated that the thick-walled approximation has better overall agreement with ex vivo modulus. The approximations displayed quantitatively distinct regression slopes that were statistically different (P = 0.02). The elastic modulus of the main pulmonary artery can be reasonably estimated from combined color M-mode tissue Doppler imaging ultrasound and catheter pressure measurements in calves. Such measurements may be a valuable tool in the diagnosis and treatment of human PH.

Evaluation of pulmonary artery stiffness in pulmonary hypertension with cardiac magnetic resonance

OBJECTIVES

This study sought to evaluate indexes of pulmonary artery (PA) stiffness in patients with pulmonary hypertension (PH) using same-day cardiac magnetic resonance (CMR) and right heart catheterization (RHC).

BACKGROUND

Pulmonary artery stiffness is increased in the presence of PH, although the relationship to PH severity has not been fully characterized.

METHODS

Both CMR and RHC were performed on the same day in 94 patients with known or suspected PH. According to the RHC, patients were classified as having no PH (n = 13), exercise-induced PH (EIPH) only (n = 6), or PH at rest (n = 75). On CMR, phase-contrast images were obtained perpendicular to the pulmonary trunk. From CMR and RHC data, PA areas and indexes of stiffness (pulsatility, compliance, capacitance, distensibility, elastic modulus, and the pressure-independent stiffness index beta) were measured at rest.

RESULTS

All quantified indexes showed increased PA stiffness in patients with PH at rest in comparison with those with EIPH or no PH. Despite the absence of significant differences in baseline pressures, patients with EIPH had lower median compliance and capacitance than patients with no PH: 15 (interquartile range: 9 to 19.8) mm2/mm Hg versus 8.4 (interquartile range: 6 to 10.3) mm2/mm Hg, and 5.2 (interquartile range: 4.4 to 6.3) mm3/mm Hg versus 3.7 (interquartile range: 3.1 to 4.1) mm3/mm Hg, respectively (p < 0.05). The different measurements of PA stiffness, including stiffness index beta, showed significant correlations with PA pressures (r2 = 0.27 to 0.73). Reduced PA pulsatility (<40%) detected the presence of PH at rest with a sensitivity of 93% and a specificity of 63%.

CONCLUSIONS

Pulmonary artery stiffness increases early in the course of PH (even when PH is detectable only with exercise and before overt pressure elevations occur at rest). These observations suggest a potential contributory role of PA stiffness in the development and progression of PH.

Vascular injury after whole thoracic x-ray irradiation in the rat

PURPOSE

To study vascular injury after whole thoracic irradiation with single sublethal doses of X-rays in the rat and to develop markers that might predict the severity of injury.

METHODS AND MATERIALS

Rats that received 5- or 10-Gy thorax-only irradiation and age-matched controls were studied at 3 days, 2 weeks, and 1, 2, 5, and 12 months. Several pulmonary vascular parameters were evaluated, including hemodynamics, vessel density, total lung angiotensin-converting enzyme activity, and right ventricular hypertrophy.

RESULTS

By 1 month, the rats in the 10-Gy group had pulmonary vascular dropout, right ventricular hypertrophy, increased pulmonary vascular resistance, increased dry lung weights, and decreases in total lung angiotensin-converting enzyme activity, as well as pulmonary artery distensibility. In contrast, irradiation with 5 Gy resulted in only a modest increase in right ventricular weight and a reduction in lung angiotensin-converting enzyme activity.

CONCLUSION

In a previous investigation using the same model, we observed that recovery from radiation-induced attenuation of pulmonary vascular reactivity occurred. In the present study, we report that deterioration results in several vascular parameters for </=1 year after 10 Gy, suggesting sustained remodeling of the pulmonary vasculature. Our data support clinically relevant injuries that appear in a time- and dose-related manner after exposure to relatively low radiation doses.

New concepts in the invasive and non invasive evaluation of remodelling of the right ventricle and pulmonary vasculature in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare fatal disease defined as a sustained elevation of pulmonary arterial pressure to more than 25 mmHg at rest, with a mean pulmonary-capillary wedge pressure and left ventricular enddiastolic pressure of less than 15 mmHg at rest. Histopathology of PAH is founded on structural modifications on the vascular wall of small pulmonary arteries characterized by thickening of all its layers. These changes, named as vascular remodelling, include vascular proliferation, fibrosis, and vessel obstruction. In clinical practice the diagnosis of PAH relies on measurements of pulmonary vascular pressure and cardiac output, and calculation of pulmonary vascular resistances. Direct evaluation of pulmonary vascular structure is not routinely performed in pulmonary hypertension since current imaging techniques are limited and since little is known about the relationship between structural changes and functional characteristics of the pulmonary vasculature. Intravascular ultrasound studies in patients with pulmonary hypertension have shown a thicker middle layer, increased wall-thickness ratio and diminished pulsatility than in control patients. Optical Coherence Tomography, a new high resolution imaging modality that has proven its superiority over intravascular ultrasound (IVUS) for the detection and characterization of coronary atherosclerotic plaque composition, may potentially be a useful technique for the in vivo study of the pulmonary arterial wall. In addition current progress in Echo Doppler technique will quantify right ventricular function with parameters independent of loading conditions and not requiring volumetric approximations of the complex geometry of the right ventricle. This would allow the in vivo study of right ventricular and pulmonary artery remodelling in PAH.

Estimation of pulse wave velocity in main pulmonary artery with phase contrast MRI: Preliminary investigation

PURPOSE

To assess the feasibility and reproducibility of a noninvasive MRI method to measure pulse wave velocity (PWV) in the main pulmonary artery (MPA).

MATERIALS AND METHODS

A total of 17 subjects without history of pulmonary diseases (38.2 +/- 18.4 years) participated in this study. Series of MR velocity maps of the MPA were acquired at 2 cm above the pulmonary valves using a two-dimensional phase-contrast sequence. Effective temporal resolution was 11 msec after interleaving two dynamic series with different values of electrocardiograph (ECG) trigger delay. PWV was derived as the rate of MPA flow variations per unit change in MPA cross-sectional area, during early systole. Seven healthy subjects underwent three repetitive examinations to investigate intrascan and interscan reproducibility.

RESULTS

Flow vs. area was highly linear in the MPA during early systole, with Pearson's coefficients ranging from 0.982 to 0.999, rendering derivation of PWV with little difficulty. Average value of PWV in MPA was 1.96 +/- 0.27 m/second, in good agreement with literature values measured using invasive means. The percentage intra- and interscan differences were 5.46% and -10.86%, respectively.

CONCLUSION

Phase-contrast MRI to noninvasively measure PWV in the MPA is feasible with good reproducibility.

Measurement of total pulmonary arterial compliance using invasive pressure monitoring and MR flow quantification during MR-guided cardiac catheterization

Pulmonary hypertensive disease is assessed by quantification of pulmonary vascular resistance. Pulmonary total arterial compliance is also an indicator of pulmonary hypertensive disease. However, because of difficulties in measuring compliance, it is rarely used. We describe a method of measuring pulmonary arterial compliance utilizing magnetic resonance (MR) flow data and invasive pressure measurements. Seventeen patients with suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment underwent MR-guided cardiac catheterization. Invasive manometry was used to measure pulmonary arterial pressure, and phase-contrast MR was used to measure flow at baseline and at 20 ppm nitric oxide (NO). Total arterial compliance was calculated using the pulse pressure method (parameter optimization of the 2-element windkessel model) and the ratio of stroke volume to pulse pressure. There was good agreement between the two estimates of compliance ( r = 0.98, P < 0.001). However, there was a systematic bias between the ratio of stroke volume to pulse pressure and the pulse pressure method (bias = 61%, upper level of agreement = 84%, lower level of agreement = 38%). In response to 20 ppm NO, there was a statistically significant fall in resistance, systolic pressure, and pulse pressure. In seven patients, total arterial compliance increased >10% in response to 20 ppm NO. As a population, the increase did not reach statistical significance. There was an inverse relation between compliance and resistance ( r = 0.89, P < 0.001) and between compliance and mean pulmonary arterial pressure ( r = 0.72, P < 0.001). We have demonstrated the feasibility of quantifying total arterial compliance using an MR method.

Pulmonary artery smooth muscle activation attenuates arterial dysfunction during acute pulmonary hypertension

Acute pulmonary hypertension (PH) may arise with or without an increase in vascular smooth muscle (VSM) tone. Our objective was to determine how VSM activation affects both the conduit (CF) and wall buffering (BF) functions of the pulmonary artery (PA) during acute PH states. PA instantaneous flow, pressure, and diameter of six sheep were recorded during normal pressure (CTL) and different states of acute PH: 1) passively induced by PA mechanical occlusion (PPH); 2) actively induced by intravenous administration of phenylephrine (APH); and 3) a combination of both (APPH). To evaluate the direct effect of VSM activation, isobaric (PPH vs. APH) and isometric (CTL vs. APPH) analyses were performed. We calculated the local BF from the elastic (EPD) and viscous (ηPD) indexes as ηPD/EPD and the characteristic impedance (ZC) from pressure and flow to evaluate CF as 1/ZC. We also calculated the absolute and normalized cross-sectional pulsatility (PCS and NPCS, respectively), the dynamic compliance (CDYN), the cross-sectional distensibility (DCS), and the pressure-strain elastic modulus (EP). The isobaric analysis showed increase of CF, BF, and ηPD ( P < 0.01) and decrease of EPD ( P < 0.05) during APH in respect to PPH (concomitant with isobaric VSM activation-induced vasoconstriction, P < 0.01). The isometric analysis showed increase of EPD and ηPD ( P < 0.01), nonsignificant difference in BF (even in the presence of a significant mean PA pressure rise, from 14 (SD 6) to 25 (SD 8) mmHg, P < 0.01), and decrease in CF ( P < 0.01) during APPH respect to CTL. Mechanical occlusions (PPH and APPH) reduced BF ( P < 0.01) and increased EPD ( P < 0.05) with regard to their previous steady states (CTL and APH). Nonsignificant differences were found in EPD between PPH and APPH. VSM activation (APH and APPH) increased ηPD ( P < 0.01) respect to their previous passive states (CTL and PPH), but no significant differences were found within similar levels of VSM activation. In conclusion, VSM plays a relevant role in main pulmonary artery function during acute pulmonary hypertension, because isobaric vasoconstriction induced by VSM activation improves both BF and CF, mainly due to the increase in ηPD concomitant with the arterial compliance. CDYN and DCS were the more pertinent clinical indexes of arterial elasticity. Additionally, the ηPD-mediated preservation of the BF could be evaluated by the geometric related indexes (PCS and NPCS), which appear to be qualitative markers of arterial wall viscosity status.

Reduced Elastic Mismatch Achieved by Interposing Vein Cuff in Expanded Polytetrafluoroethylene Femoral Bypass Decreases Intimal Hyperplasia

BACKGROUND

Wall shear stress, arterial wall elasticity, and intimal hyperplasia are related. The aim of this study was to investigate the in vitro mechanical properties of ovine femoral arteries, jugular veins, and expanded polytetrafluoroethylene conduits, and to evaluate postoperative intimal hyperplasia.

METHODS

Arterial, venous, and ePTFE mechanical properties were studied in a circulating loop at isobaric systemic pressures. Histological studies of intimal hyperplasia in ePTFE-bypassed femoral arteries with and without Miller cuffs were performed at the 40th and 120th day.

RESULTS

The incremental elastic modulus of veins was significantly higher than that of femoral arteries (P < 0.05), but significantly lower than that of ePTFE graft conduits (P < 0.05). Intimal hyperplasia was significantly less in Miller-cuff-bypassed arteries both at the 40th and 120th day (P < 0.01).

CONCLUSIONS

The Miller cuff acts as a mechanical adapter enhancing wall shear stress and the elastic matching between ePTFE and the native artery, resulting in an early decrease of intimal hyperplasia.

Pulmonary Hypertension in Patients With Chronic Renal Failure

RATIONALE

The aim of this work was to study the association of pulmonary hypertension (PH) with pulmonary artery calcifications (PACs) and hyperparathyroidism in patients with chronic renal failure (CRF) receiving regular hemodialysis.

BACKGROUND

Scarce data are available regarding the prevalence and the predictors of PH in patients with CRF. Abnormal (99m)Tc diphosphonate lung uptake was reported in these patients, suggesting a role of PACs.

METHODS

We studied 51 patients (28 men and 23 women) with end-stage renal disease, who were receiving regular hemodialysis. Patients underwent two-dimensional, Doppler echocardiographic imaging. Laboratory investigations included BUN, serum creatinine, calcium, phosphorus, alkaline phosphatase, and intact molecule parathormone. PH was defined as pulmonary artery systolic pressure > 35 mm Hg as determined by Doppler echocardiographic evaluation.

RESULTS

PH was detected in 15 patients (29%). Women had a higher prevalence of PH (48% vs 14%, p = 0.01). There was no significant differences between patients with PH and those without PH with regards to age, duration of dialysis, serum calcium (9.6 +/- 2 mg/dL vs 10 +/- 2 mg/dL), phosphorus (6 +/- 1.4 mg/L vs 6.2 +/- 1.9 mg/L), alkaline phosphatase (609 +/- 768 U/L vs 473 +/- 574 U/L), parathyroid hormone (PTH) [420 +/- 512 pg/mL vs 354 +/- 519 pg/mL] or the prevalence of an abnormal (99m)Tc diphosphate lung scan result (60% vs 73%, respectively [+/- SD]).

CONCLUSIONS

This study demonstrated that 29% of patients with CRF receiving regular hemodialysis have PH. The presence of PH was not related to the level of PTH or the severity of other metabolic abnormalities. There was no relation between PH and the presence or the severity of PAC. PH is detected more frequently in women. This study does not support a role of secondary hyperparathyroidism and subsequent PAC as the etiology of PH in patients with CRF.

Recent advances in the treatment of pulmonary hypertension

PURPOSE OF REVIEW

Pulmonary hypertension is a debilitating life-threatening disease of all ages. The long-term prognosis can be dismal despite maximal medical therapy. There have been significant advances in our understanding of the pathobiology and genetics of this disease, and novel pharmacological approaches appear to offer promising alternatives to conventional therapy. Anesthesiologists have been instrumental in the development and widespread clinical introduction of inhaled nitric oxide. Unfortunately, despite early optimism, inhaled nitric oxide has several significant limitations related to its cost, toxicity, required complex technology, and occasional therapeutic failure. Therefore, there is a need for an effective alternative pulmonary vasodilator. The early diagnosis and treatment of pulmonary hypertension are crucial if improvements are to be realized. This review will present recent work in this field in an attempt to increase anesthesiologists' awareness of potential new treatment options.

RECENT FINDINGS

Emerging data concerning the genetics of certain pulmonary hypertension variants have provided insight into the pathobiology of this disease and may lead to advances in the early detection or new treatment options. New pharmacological approaches include drugs such as nitric oxide donors, phosphodiesterase inhibitors, endothelin antagonists, and prostacyclin analogues. Attention has also been focused on the use of combinations of drugs of different classes.

SUMMARY

The clinical outcome of pulmonary hypertension is dependent upon early detection and therapy. Increased awareness of current therapeutic options will facilitate earlier effective treatment.