Relationship between haemodynamics and morphology in pulmonary hypertension. A quantitative intravascular ultrasound study

Influence of coupled hemodynamics-arterial wall interaction on compliance in a realistic pulmonary artery with variable intravascular wall properties

Pulmonary hypertension is characterized by elevation of pulmonary artery (PA) pressure (p) and structural remodeling of the PA wall, leading to reduction in arterial compliance (c). As a step towards improving diagnosis of pulmonary disease, we use the PA branch geometry (main pulmonary artery (MPA) branching into left (LPA) and right (RPA) pulmonary arteries) obtained from MRI in conjunction with an inverse algorithm to obtain the pre-stress level in the artery walls. Next, a coupled blood-wall interaction (BWI) calculation provides hemodynamic information as well as compliance of the PA walls. We show that the computed load-free geometry from the inverse algorithm exhibits a 27.8% lower inner diameter (d) and 18.5% lower outer d compared to the in vivo geometry from MRI. Further, the mean p computed from the BWI computation in the main PA (p_MPA-n) is within 4% of the mean p_MPA-e (n-numerical; e-experimental). Also, the mean Q computed in the left PA (Q_LPA-n) is within 10% of the mean Q_LPA-e. Finally, the compliance c_MPA-n is computed to be 27% lower than c_MPA-e, while the c_LPA-n is computed to be 20.4% lower than c_LPA-e. Importantly, the PA shows significant intra-vascular variation in compliance, with the MPA showing higher overall compliance compared to the LPA (3.5-4 times).

Non-invasive diagnosis of pulmonary hypertension from lung Doppler signal: a proof of concept study

Transthoracic Parametric Doppler (TPD) is a novel ultrasound technique recently developed for the investigation of pulmonary blood vessels. Lung Doppler Signals (LDS) recorded from TPD provide information regarding the functional mechanical characteristics of pulmonary blood vessels. We aimed to define the specific profile of LDS generated from TPD imaging in patients with pulmonary hypertension (PH), and to evaluate the diagnostic performance of LDS to detect PH using right heart catheterization (RHC) as gold standard reference. Seventy nine PH patients and 79 healthy controls matched for age, gender and BMI were recruited in a prospective case-control multicenter study. LDS recordings were performed by TPD consisting of a pulsed Doppler with a 2 MHz single element transducer. LDS were recorded within 24 h of RHC. Following LDS extraction, classification and performance evaluation were performed offline using a support vector machine (k-fold cross validation method). The best LDS parameters for PH detection were (1) peak velocity of the systolic (S) and diastolic (D) signals, (2) the rise slope of the S and D signals, and (3) time to peak of the S signal. Overall, the sensitivity and specificity of TPD for detection of PH were 82.7 % (95 % CI 81.3-84.1) and 87.4 % (95 % CI 86.3-88.5), respectively, with an area under the receiver operating curve of 0.95 (95 % CI 0.94-0.96). Detection rate of PH increased progressively with the level of mean pulmonary artery pressure. LDS recorded by TPD display a specific profile in PH and appears to be a promising and reliable tool for PH diagnosis. Further studies are required to confirm the clinical usefulness of LDS.

Validation of Cardiac Output as Reported by a Permanently Implanted Wireless Sensor

The CardioMEMS heart failure (HF) system was tested for cardiac output (CO) measurement accuracy using an in vitro mock circulatory system. A software algorithm calculates CO based on analysis of the pressure waveform as measured from the pulmonary artery, where the CardioMEMS system resides. Calculated CO was compared to that from reference flow probe in the circulatory system model. CO measurements were compared over a clinically relevant range of stroke volumes and heart rates with normal, pulmonary hypertension (PH), decompensated left heart failure (DLHF), and combined DHLF + PH hemodynamic conditions. The CardioMEMS CO exhibited minimal fixed and proportional bias.

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.

Abnormal pulmonary artery stiffness in pulmonary arterial hypertension: in vivo study with intravascular ultrasound

Background

There is increasing recognition that pulmonary artery stiffness is an important determinant of right ventricular (RV) afterload in pulmonary arterial hypertension (PAH). We used intravascular ultrasound (IVUS) to evaluate the mechanical properties of the elastic pulmonary arteries (PA) in subjects with PAH, and assessed the effects of PAH-specific therapy on indices of arterial stiffness.

Method

Using IVUS and simultaneous right heart catheterisation, 20 pulmonary segments in 8 PAH subjects and 12 pulmonary segments in 8 controls were studied to determine their compliance, distensibility, elastic modulus and stiffness index β. PAH subjects underwent repeat IVUS examinations after 6-months of bosentan therapy.

Results

At baseline, PAH subjects demonstrated greater stiffness in all measured indices compared to controls: compliance (1.50±0.11×10^–2 mm^2/mmHg vs 4.49±0.43×10^–2 mm^2/mmHg, p<0.0001), distensibility (0.32±0.03%/mmHg vs 1.18±0.13%/mmHg, p<0.0001), elastic modulus (720±64 mmHg vs 198±19 mmHg, p<0.0001), and stiffness index β (15.0±1.4 vs 11.0±0.7, p = 0.046). Strong inverse exponential associations existed between mean pulmonary artery pressure and compliance (r² = 0.82, p<0.0001), and also between mean PAP and distensibility (r² = 0.79, p = 0.002). Bosentan therapy, for 6-months, was not associated with any significant changes in all indices of PA stiffness.

Conclusion

Increased stiffness occurs in the proximal elastic PA in patients with PAH and contributes to the pathogenesis RV failure. Bosentan therapy may not be effective at improving PA stiffness.

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.

Magnetic resonance imaging guided catheterisation for assessment of pulmonary vascular resistance: in vivo validation and clinical application in patients with pulmonary hypertension

OBJECTIVES

To validate in vivo a magnetic resonance imaging (MRI) method for measurement of pulmonary vascular resistance (PVR) and subsequently to apply this technique to patients with pulmonary hypertension (PHT).

METHODS AND RESULTS

PVR was assessed from velocity encoded cine MRI derived pulmonary artery (PA) flow volumes and simultaneously determined invasive PA pressures. For pressure measurements flow directed catheters were guided under magnetic resonance fluoroscopy at 1.5 T into the PA. In preliminary validation studies (eight swine) PVR was determined with the thermodilution technique and compared with PVR obtained by MRI (0.9 (0.5) v 1.1 (0.3) Wood units.m2, p = 0.7). Bland-Altman test showed agreement between both methods. Inter-examination variability was high for thermodilution (6.2 (2.2)%) but low for MRI measurements (2.1 (0.3)%). After validation, the MRI method was applied in 10 patients with PHT and five controls. In patients with PHT PVR was measured at baseline and during inhalation of nitric oxide. Compared with the control group, PVR was significantly increased in the PHT group (1.2 (0.8) v 13.1 (5.6) Wood units.m2, p < 0.001) but decreased significantly to 10.3 (4.6) Wood units.m2 during inhalation of nitric oxide (p < 0.05). Inter-examination variability of MRI derived PVR measurements was 2.6 (0.6)%. In all experiments (in vivo and clinical) flow directed catheters were guided successfully into the PA under MRI control.

CONCLUSIONS

Guidance of flow directed catheters into the PA is feasible under MRI control. PVR can be determined with high measurement precision with the proposed MRI technique, which is a promising tool to assess PVR in the clinical setting.

Intravascular ultrasound of the elastic pulmonary arteries: a new approach for the evaluation of primary pulmonary hypertension

OBJECTIVE

To assess the structural and functional characteristics of pulmonary arteries by intravascular ultrasound (IVUS) in the setting of primary pulmonary hypertension, and to correlate the ultrasound findings with haemodynamic variables and mortality at follow up.

DESIGN

Prospective observational study.

SETTING

University hospital (tertiary referral centre).

PATIENTS

20 consecutive patients with primary pulmonary hypertension (16 female; mean (SD) age, 39 (14) years).

METHODS

Cardiac catheterisation and simultaneous IVUS of pulmonary artery branches at baseline and after infusion of epoprostenol.

RESULTS

33 pulmonary arteries with a mean diameter of 3.91 (0.80) mm were imaged, and wall thickening was observed in all cases, 64% being eccentric. Mean wall thickness was 0.37 (0.13) mm, percentage wall area 31.0 (9.3)%, pulsatility 14.6 (4.8)%, and pulmonary/elastic strain index 449 (174) mm Hg. No correlation was observed between IVUS findings and haemodynamic variables. Epoprostenol infusion increased pulsatility by 53% and decreased the pulmonary/elastic strain index by 41% (p = 0.0001), irrespective of haemodynamic changes. At 18 (12) months follow up, nine patients had died. A reduced pulsatility and an increased pulmonary/elastic strain index were associated with increased mortality at follow up (12.0 (4.4)% v 16.4 (4.4)%, p = 0.03; 369 (67) v 546 (216) mm Hg, p = 0.02).

CONCLUSIONS

IVUS demonstrated pulmonary artery wall abnormalities in all patients with primary pulmonary hypertension, mostly eccentric. The severity of the changes did not correlate with haemodynamic variables, and epoprostenol improved pulmonary vessel stiffness. There was an association between impaired pulmonary artery functional state as determined by IVUS and mortality at follow up.

Intravascular ultrasound assessment of pulmonary vascular disease in patients with pulmonary hypertension

BACKGROUND

Measurements of pulmonary pressure and resistance are still considered to be the "gold standard" in the evaluation of pulmonary hypertension (PH), despite their limitations in predicting irreversible disease. Hemodynamic assessment also only provides a global evaluation of the pulmonary vascular bed, whereas PH is an inhomogeneous disease of the vessel wall.

METHODS AND RESULTS

We assessed the value of intravascular ultrasound (IVUS) in 30 patients with suspected PH and correlated the structural changes in distal pulmonary arteries found on IVUS with conventional hemodynamic data. Plasma endothelin (ET)-1 levels and pulmonary ET-1 extraction also were measured as markers of the severity of PH. The anatomic abnormalities revealed by IVUS were more severe in the lower lobes than in the upper lobes, as evidenced by the greater percentage of wall thickness (WT), the smaller lumen diameter/WT and lumen area/total vessel area (p < 0.05 for each). IVUS anatomic indexes correlated directly with hemodynamic data (eg, with pulmonary arterial systolic pressure; r = 0.56; p < 0.001) and ET-1 levels but inversely with pulmonary ET-1 extraction.

CONCLUSION

Patients with PH have greater pulmonary arterial WT that is more severe in the lower lobes than in the upper lobes. The severity of structural abnormalities found on IVUS is directly correlated with hemodynamic findings and ET-1 levels. IVUS may provide useful additional information in the assessment of patients with PH.

Acetylcholine but not sodium nitroprusside exerts vasodilation in pulmonary hypertension secondary to chronic congestive heart failure

BACKGROUND

Reduced endothelium-dependent vasodilation contributes to the development of pulmonary hypertension in chronic congestive heart failure (CHF). We investigated pulmonary endothelium-dependent and independent vasodilation in patients with CHF.

METHODS

We studied 42 patients with CHF (age 55 +/- 10, NYHA Classes II-III, left ventricular ejection fraction 27 +/- 10%, mean PAP 29 +/- 12 mmHg). The endothelial vasodilator capacity of pulmonary resistance vessels was assessed by the infusion of acetylcholine into a pulmonary artery branch while measuring the blood flow velocity with a Doppler flow wire. For comparison endothelium-independent vasodilation was measured with the response to sodium nitroprusside. The conductance vessel diameter (4.4 +/- 0.2 mm) was determined by intravascular ultrasound. Acetylcholine was administered at concentrations of 10(-6) to 10(-4) mol/l, sodium nitroprusside was administered at concentrations of 0.125 and 0.25 microg/kg per min. The effects on conductance vessel diameter were investigated in 12 patients by the measurement of diameter and flow velocity following the administration of acetylcholine and sodium nitroprusside.

RESULTS

Acetylcholine markedly increased blood flow velocity (+39 +/- 7% at 10(-4) mol/l; p < .05). This correlated with the baseline PAP (r = 0.58; p < .05) and pulmonary vascular resistance (r = 0.58; p < .05). Sodium nitroprusside caused a small increase in the flow velocity (5 +/- 2% at 0.125, 12 +/- 4% at 0.25 microg/kg per minute; p < .05) that was accompanied by systemic vasodilation. The conductance vessel diameter was unchanged after acetylcholine was administered and was only marginally decreased after the administration of sodium nitroprusside.

CONCLUSIONS

In CHF acetylcholine reveals preserved receptor-mediated endothelial vasodilation, that is positively correlated to pulmonary hypertension, and cannot be reproduced by sodium nitroprusside.

L-arginine and substance P reverse the pulmonary endothelial dysfunction caused by congenital heart surgery

BACKGROUND

The increase in pulmonary vascular resistance (PVR) seen in children after cardiopulmonary bypass has been attributed to transient pulmonary endothelial dysfunction (PED). We therefore examined PED in children with congenital heart disease by assessing the L-arginine-nitric oxide (NO) pathway in terms of substrate supplementation (L-arginine [L-Arg]), stimulation of endogenous NO release (substance P [Sub-P]), and end-product provision (inhaled NO) before and after open heart surgery.

METHODS AND RESULTS

Ten patients (aged 0.62+/-0.27 years) with pulmonary hypertension undergoing cardiac catheterization who had not had surgery and 10 patients (aged 0.65+/-0.73 years) who had recently undergone cardiopulmonary bypass were examined. All were sedated and paralyzed and received positive-pressure ventilation. Blood samples and pressure measurements were taken from catheters in the pulmonary artery and the pulmonary vein or left atrium. Respiratory mass spectrometry was used to measure oxygen uptake, and cardiac output was determined by the direct Fick method. PVR was calculated during steady state at ventilation with room air, during FIO(2) of 0.65, then during additional intravenous infusion of L-Arg (15 mg. kg(-1). min(-1)) and Sub-P (1 pmol. kg(-1). min(-1)), and finally during inhalation of NO (20 ppm). In preoperative patients, the lack of an additional significant change of PVR with L-Arg, Sub-P, and inhaled NO suggests little preexisting PED. Postoperative PVR was higher, with an additional pulmonary endothelial contribution that was restorable with L-Arg and Sub-P.

CONCLUSIONS

Postoperatively, the rise in PVR suggested PED, which was restorable by L-Arg and Sub-P, with no additional effect of inhaled NO. These results may indicate important new treatment strategies for these patients.