Pulmonary intravascular ultrasound in infants and children with congenital heart disease

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.

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.

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.

Use of intravascular ultrasound to measure local compliance of the pediatric pulmonary artery: in vitro studies

BACKGROUND

The accurate measurement of local pulmonary artery compliance in pediatric pulmonary hypertension is an important step toward further understanding the biomechanical and hemodynamic aspects of the disease. The emergence of intravascular ultrasound (IVUS) imaging techniques promises the ability to make such measurements clinically. However, the use of IVUS for compliance measurements has not been validated. Furthermore, confusion exists regarding the most appropriate method to measure compliance.

METHODS

This study validated IVUS measurements against a laser micrometer standard for 4 elastic tubes of varying compliance. Two methods of quantifying local compliance were explored: The pressure-strain modulus (E(p)), (E(p)(g/cm(2)) = DeltaP x R(d)/DeltaR (Where DeltaP is pulse pressure, R(d) is diastolic radius, and DeltaR is systolic minus diastolic radii) and the dynamic compliance (C(dyn)), (C(dyn)(%/100 mm Hg) = [DeltaD/(DeltaP x D(d))] x 10(4) Where DeltaD is systolic minus diastolic diameters and D(d) is diastolic diameter.

RESULTS

IVUS diameter measurements agreed well with laser micrometer data although slight overestimation (mean = 3.67% +/- 2.78%) was present. Mean values of E(p) ranged from 353.3 g/cm(2) to 2676.0 g/cm(2); mean C(dyn) values ranged from 5.7% diametric change/100 mm Hg to 39.5% diametric change/100 mm Hg for all tube models. Although mean values of E(p) and C(dyn) could be distinguished among the various tubes, the extremely large measurement uncertainty for E(p) precluded statistical differentiation. The uncertainty in E(p) increased inversely with the diametric change, indicating a potential limitation of E(p) associated with stiffening arteries.

CONCLUSIONS

We conclude that C(dyn) is a more robust mean of quantifying pediatric pulmonary artery compliance, especially as arteries stiffen with chronic pulmonary hypertension.

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.

Intravascular ultrasonic characteristics and vasoreactivity of the pulmonary vasculature in children with pulmonary hypertension

We sought to describe the morphologic characteristics of pulmonary arteries by intravascular ultrasound (IVUS) in children with and without pulmonary hypertension to compare these anatomic findings with those of pulmonary wedge angiography, and to determine the relation between these structural findings and functional reactivity to pulmonary vasodilators. Direct evaluation of pulmonary vascular structure in children with pulmonary hypertension with current imaging techniques has been limited and little is known about the relation between structural and functional characteristics of the pulmonary vasculature. In 23 children undergoing cardiac catheterization (15 with pulmonary hypertension and 8 controls) we performed IVUS and pulmonary wedge angiography of the distal pulmonary arteries in the same lobe. IVUS was performed in 44 pulmonary arteries measuring 2.5 to 5.0 mm internal diameter with a 3.5Fr 30-MHz IVUS catheter. We assessed vasoreactivity to inhaled nitric oxide (NO) and oxygen in 13 of 15 children with pulmonary hypertension. Baseline pulmonary vascular resistance (PVR) was greater in the 15 children with pulmonary hypertension than in the 8 controls (9.5+/-1.9 vs 1.5+/-0.3 U x m2, p <0.05). NO lowered PVR in patients with pulmonary hypertension (p <0.05). IVUS studies in patients with pulmonary hypertension showed a thicker middle layer, wall thickness ratio, and diminished pulsatility than did those in controls (p <0.05). The inner layer was not visualized by IVUS in any control patient, but was seen in 9 of 15 patients with pulmonary hypertension. Pulmonary artery wedge angiography correlated with baseline mean pulmonary artery pressure and PVR as well as with IVUS findings of wall thickness ratio and inner layer thickness. The inner layer was not visualized by IVUS in any patient with grade 1 wedge angiograms or in 86% of patients with grade 2 wedge angiograms. All patients with grade 4 and 80% of patients with grade 3 wedge angiograms had a visible inner layer. Vasoreactivity to NO and oxygen did not correlate with structural assessment of the pulmonary vasculature by IVUS. Structural changes in the pulmonary arteries in children with pulmonary hypertension can be directly visualized by IVUS, but are not predictive of NO-induced pulmonary vasodilation. IVUS examination of pulmonary arteries may complement current techniques utilized in the evaluation of children with pulmonary hypertension.