European Journal of Cardio-Thoracic Surgery Advance Access published May 24, 2016 European Journal of Cardio-Thoracic Surgery (2016) 1–8 doi:10.1093/ejcts/ezw132
ORIGINAL ARTICLE
Cite this article as: Ayaon-Albarran A, Fernandez-Jimenez R, Silva-Guisasola J, Agüero J, Sanchez-Gonzalez J, Galan-Arriola C et al. Systolic flow displacement using 3D magnetic resonance imaging in an experimental model of ascending aorta aneurysm: impact of rheological factors. Eur J Cardiothorac Surg 2016; doi:10.1093/ejcts/ezw132.
Systolic flow displacement using 3D magnetic resonance imaging in an experimental model of ascending aorta aneurysm: impact of rheological factors† Ali Ayaon-Albarrana,b,‡, Rodrigo Fernandez-Jimeneza,b,‡, Jacobo Silva-Guisasolaa,c, Jaume Agüeroa, Javier Sanchez-Gonzaleza,d, Carlos Galan-Arriolaa, Fernando Reguillo-Lacruzb, Luis C. Maroto Castellanosa,b and Borja Ibaneza,e,* a b c d e
Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC), Madrid, Spain Cardiovascular Institute, Hospital Clínico San Carlos, Madrid, Spain Hospital Central de Asturias, Oviedo, Spain Philips Healthcare, Madrid, Spain IIS-Fundación Jiménez Díaz, Madrid, Spain
* Corresponding author. Translational Laboratory for Cardiovascular Imaging and Therapy, Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC), Melchor Fernández Almagro 3, 28029 Madrid, Spain; Department of Cardiology, Instituto de Investigación Sanitaria, Fundación Jiménez Díaz, Madrid, Spain. Tel: +34-91-4531200; fax: +34-91-4531245; e-mail:
[email protected] (B. Ibanez). Received 14 September 2015; received in revised form 5 March 2016; accepted 8 March 2016
Abstract OBJECTIVES: The impact of systolic flow displacement on the development and progression of ascending aorta dilatation in aortic valve disease is a matter of controversy. Our objective was to study the association between rheological stimuli and development of aortic dilatation in a large animal model of supravalvular aortic stenosis and eccentric flow. METHODS: Twenty-four pigs weighing 10–14 kg were randomly allocated (ratio 2:1) to either restrictive ascending aortic banding or sham operation. Aortic diameter and systolic flow displacement were assessed by three-dimensional phase-contrast magnetic resonance imaging at 6 and 18 weeks after surgery. Twenty pigs (n = 14, banded vs n = 6, sham) completed full imaging protocol and were included in the analysis. After the last follow-up, a subset of 14 animals was sacrificed for histological analysis. RESULTS: All banded animals developed significant progressive aortic dilatation both at 6 and 18 weeks, compared with sham-operated pigs: 34.3 ± 4.8 vs 21.4 ± 2.7 mm at 6 weeks (P < 0.001); and 50.0 ± 8.4 vs 38.0 ± 8.3 mm at 18 weeks (P = 0.002). The peak gradient at 6 weeks showed a trend to positively correlate with aortic diameter at 18 weeks (R = 0.50, P = 0.06), whereas the systolic flow displacement at 6 weeks correlated better with aortic diameter at 18 weeks (R = 0.59, P = 0.02). The aortic wall thickness was significantly decreased in the anterior aortic section in banded, compared with sham-operated, pigs (1.5 ± 0.4 vs 2.0 ± 0.1 mm, respectively; P = 0.03). In addition, banded pigs showed a higher degree of cystic medial necrosis and elastin fibre fragmentation, compared with sham-operated animals. CONCLUSIONS: In this preclinical model of supravalvular aortic stenosis and eccentric flow, we found that systolic flow displacement at earlier stages is positively correlated with the degree of aortic dilatation during follow-up as assessed by three-dimensional phase-contrast magnetic resonance imaging. If our findings are confirmed in further studies, this imaging parameter might be useful to identify those subjects with aortic valve disease who are at risk of developing aortic dilatation at a later stage. Keywords: Animal model • Aortic valve disease • 3D magnetic resonance imaging • Flow displacement • Thoracic aneurysm • Aortic dilatation
† ‡
Presented at the 29th Annual Meeting of the European Association for Cardio-Thoracic Surgery, Amsterdam, Netherlands, 3–7 October 2015. The first two authors contributed equally to this work.
© The Author 2016. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved.
EXPERIMENTAL
Winner of the 2015 EACTS Young Investigator’s Award – Vascular
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INTRODUCTION Tricuspid aortic valve (TAV) stenosis is the commonest valve heart disease, affecting up to 4% of the elderly population [1]. Although aortic root dilatation is a frequent occurrence, it does not seem to be related to the severity of the stenosis [1]. Conversely, bicuspid aortic valve (BAV), which is the commonest congenital heart disorder with a prevalence of 1–2%, is more frequently associated with an ascending aortic aneurysm [2]. There is a considerable debate on the pathogenesis of aortic root dilatation in aortic valve stenosis, especially in the case of BAV patients. Two main theories have been proposed to explain the high incidence of aortopathy in BAV disease: (i) the genetic theory, whereby the presence of aortic wall fragility is a consequence of a common developmental defect involving the aortic valve and the aortic wall; and (ii) the haemodynamic theory, whereby the abnormal haemodynamic stress on the aortic wall induced by an eccentric turbulent flow through the BAV leads to subsequent aortopathy [3]. The conformation of leaflet fusion in the BAV is associated with different patterns in the regional wall shear stress distribution and systolic flow eccentricity, and this is feasible to identify with the use of 3D phase-contrast magnetic resonance imaging (MRI) [4]. Systolic flow displacement, a measure of the systolic flow eccentricity, has been shown to be associated with larger ascending aorta diameters; however, the impact of these rheological factors on ascending aorta dilatation and tissue anatomical changes has not been explored in detail [5–7]. Indeed, the presence of abnormal displacement is not necessarily accompanied by changes in regional wall shear stress distribution. Interestingly, aneurysms developed in individuals with TAV and aortic stenosis and/or regurgitation manifest abnormal flow patterns, which are similar to those of BAV individuals [8]. The objective of this preclinical study was to use the state-of-the-art 3D phase-contrast MRI to study the impact of rheological stimuli on the development and progression of ascending aorta dilatation in a large animal model of experimentally induced supravalvular aortic stenosis and eccentric flow. The absence of a BAV genetic background allowed us to study the sole impact of rheology itself.
MATERIALS AND METHODS Study design Castrated male large white piglets were randomly allocated (ratio 2 : 1) to either restrictive ascending aortic banding (n = 16) or sham operation (n = 8). We performed a sample size calculation on the basis of a pilot study we previously conducted in six pigs to set up the model. In this preliminary study, according to imaging at 18 weeks follow-up, the mean aortic diameter was 50 ± 20 and 33 ± 8 mm for banded versus sham pigs, respectively. The estimated sample size, taking into account a power of 0.80, a significance level of 0.05 and a ratio of sample sizes of 2.0 (banded/ sham), was 24 animals (16 banded vs 8 sham). We used a 2:1 ratio, since higher mortality (approximately doubled) was initially expected in the banded group. Given the very rapid growth rate of these farm pigs, a relative decrease in the aortic lumen area at the time of surgery allowed a very high periprocedural survival rate and the subsequent development of progressive supravalvular aortic stenosis paralleling the pigs’ growth. The study protocol was approved by the institutional animal research committee and was
conducted in accordance with recommendations of the Guide for the Care and Use of Laboratory Animals.
Surgical procedure Supravalvular aortic stenosis was induced in 4-week-old large white pigs (weighing 10–14 kg) by surgical banding of the ascending aorta, as previously described [9]. Sedation was induced by intramuscular injection of ketamine (20 mg/kg), xylazine (2 mg/ kg) and midazolam (0.5 mg/kg), and then maintained with sevoflurane. Continuous intravenous infusion of fentanyl served as an analgesic during surgery, and a single dose of cefuroxime as antibiotic prophylaxis was administered just before the procedure. Mechanical endotracheal ventilation was controlled by an external respirator. A minimally invasive right lateral thoracotomy was performed in the fourth intercostal space. Through a small pericardiotomy, the ascending aorta was constricted to 3 cm above the aortic valve with a 10-mm Dacron band, adjusted to 70% of the measured perimeter of the aorta and tied to the adventitial aorta with a single 4-0 prolene stitch. In all cases, the band was tied to form a tight fit around the aorta without giving rise to a palpable thrill. Since the banding is tied with a single stitch that penetrates the adventitial-media layer in only one side of the vessel, this procedure creates a small region where the circular lumen stenosis becomes more elliptical, perhaps even tear shaped, coinciding with the tied stich. This slight asymmetrical banding increases in severity with time and growth of the animals, therefore creating a model of supravalvular aortic stenosis and eccentric flow (Video 1). The small pericardiotomy was left open, the pneumothorax evacuated with a chest tube and the chest wall and skin incisions were closed. After surgery and before the animals recovered from anaesthesia, postoperative analgesia was provided by the intramuscular administration of buprenorphine (0.01 mg/kg). The same procedure was performed in six sham-operated animals, but without tightening of the band.
Magnetic resonance imaging protocol Follow-up MRI studies were performed at 6 and 18 weeks after surgery. Only those animals that completed the entire protocol were included in the final study analysis. The pigs were anaesthetized by intramuscular injection of ketamine, xylazine and midazolam, as described above, and anaesthesia was maintained by continuous intravenous infusion of midazolam (0.2 mg/kg/h). All
Video 1: Sagittal view; phase-contrast magnetic resonance imaging in banded animal recorded at 18 weeks. 462 at 18 weeks.
A. Ayaon-Albarran et al. / European Journal of Cardio-Thoracic Surgery
Cardiac function and left ventricular mass. To evaluate the global left ventricular (LV) motion, segmented cine steady-state free precession sequences were performed to acquire 13–15 contiguous short-axis slices covering the heart from the base to the apex field of view of 280 × 280 mm; slice thickness of 8 mm without gap; repetition time (TR) 2.8 ms; echo time (TE) 1.4 ms; flip angle (FA) 45°; cardiac phases 25; voxel size 1.8 × 1.8 mm; and three number of excitations.
Aortic dimensions. Single-shot balanced 2D multislice turbo field echo (TFE) was used to measure aortic dimensions. Pulse sequences were TR/TE/FA 2.9 ms/1.47 ms/60°, with a factor of two parallel MRI images in plane resolution 1.7 × 1.7 and 48 slices of 2.5-mm thickness, with a 0-mm gap between them covering an acquired volume of 280 × 280 × 112.5 mm3.
Aortic blood flow. For the assessment of aortic blood flow, timeresolved 3D phase-contrast MRI with three-directional velocity encoding (3D flow MRI) was used to measure 3D blood flow velocities with full volumetric coverage of the thoracic aorta (280 × 280 × 40 mm3), with an acquired resolution of 2.0 × 2.0 × 4.0 mm3 reconstructed to 0.88 × 0.88 × 2.0 mm3. The 3D flow MRI was acquired using retrospective gating in a sagittal oblique 3D volume of the ascending aorta, aortic arch and descending thoracic aorta. Acquisition sequence was based on spoiled TFE sequence (TR/TE/ FA 3.9 ms/2.3 s/10°) parameters with a TFE factor of 3, radial 3D k-space filling and a temporal resolution of 66 ms. Three velocityencoding directions were applied in the foot-to-head (FH), left-to-right (LR) and anterior-to-posterior (AP) directions, with a temporal resolution with a velocity encoding of 200 cm/s. The 3D images were acquired with a SENSE factor of 1.6 in an AP direction. The total acquisition time was 284–350 s.
Magnetic resonance imaging data analysis All MRI images were analysed by two observers experienced in MRI analysis and blinded to the aortic banding procedure.
Cardiac function and left ventricular mass. MRI images for cardiac function were analysed using dedicated software (QMass MR 7.5; Medis, Leiden, Netherlands). LV cardiac borders were traced in each cine image to obtain the LV end-diastolic volume (LVEDV) and LV end-systolic volume (LVESV). In the tracing convention used, the papillary muscles were included as part of the LV cavity volume. The LVEDV and LVESV were determined using a summation of discs with the ‘Simpson’s rule method’. The LV ejection fraction (LVEF) was computed as (LVEDV—LVESV)/ LVEDV. LV epicardial borders were also traced on the enddiastolic images, with the LV mass computed as the end-diastolic myocardial volume (i.e. the difference between the epicardial and endocardial volumes), multiplied by the myocardial density (1.05 g/ml). Values of the LV volume and LV mass normalized to the body surface area were calculated using Brody’s formula [10].
Aortic dimensions. Aortic dimensions were assessed between the banding and the origin of the right brachiocephalic trunk in single-shot balanced 2D multislice TFE sequences. In non-banded animals, maximal cross-sectional diameters were registered at 10 mm proximally to the right brachiocephalic trunk, coinciding with the location of maximal cross-sectional diameters seen in banded animals. Measurements were normalized to the body surface area and calculated using Brody’s formula [10]. Systolic flow displacement and peak gradient. Segmentation of the aortic lumen was performed at peak systole (i.e. time of maximum flow). A 3D phase-contrast MRI angiogram was derived from 4D flow data and used to manually position six equidistant consecutive analysis planes (S1–S6) in 3D visualization software (Extended MR WorkSpace 2.6.3.5, Philips Medical Systems Nederland B.V., 2013) at defined anatomical landmarks in the ascending aorta, covering from distal to the aortic banding (S1) to the origin of the right brachiocephalic trunk (S6) (Fig. 1). To quantify the flow eccentricity, we used the Sigovan method of normalized flow displacement [5]. This parameter was calculated by
Figure 1: Anatomic landmarks at a sagittal view from a 3D phase-contrast magnetic resonance imaging. Sagittal view from a 3D phase-contrast magnetic resonance imaging. (A) White arrow indicates surgically induced aortic stenosis. (B) Anatomical landmarks in the ascending aorta were manually positioned, covering from distal to the aortic banding (S1) to the origin of the right brachycephalic trunk (S6). Black arrow shows the location of the aortic banding. AscAo: ascending aorta; BCTs: brachycephalic trunks; RBCT: right brachycephalic trunk; LBCT: left brachycephalic trunk; TDA: thoracic descending aorta; RV: right ventricle; LV: left ventricle.
EXPERIMENTAL
MRI studies were performed using a Philips 3-Tesla Achieva TX whole-body scanner (Philips Medical Systems, Best, Netherlands) equipped with a 32-element cardiac phased-array surface coil.
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Table 1: General characteristics and cardiac function at 6 and 18 weeks Parameter
Weight (kg) BSA (m2) LVEDV (ml/m2) LVESV (ml/m2) LV mass (g/m2) LVEF (%)
6 weeks
18 weeks
Sham (n = 6)
Banded (n = 14)
P-value
Sham (n = 6)
Banded (n = 14)
P-value
27.3 (3.2) 0.78 (0.05) 103 (13.7) 49.4 (9.0) 52.4 (10.3) 55.1 (3.9)
26.9 (5.5) 0.77 (0.06) 102 (13.8) 46 (8.6) 69.8 (14.4) 58.9 (5.0)
0.09 0.18 0.93 0.22 0.01 0.06
64.6 (6.4) 1.36 (0.08) 113.5 (9.2) 50.5 (10.7) 50.6 (2.9) 55.8 (6.7)
64.2 (5.6) 1.35 (0.07) 113.2 (13.6) 39.4 (7.9) 67.1 (20.1) 65.2 (5.1)
0.43 0.44 0.48
