Abstract
The mechanical components of the heart, especially the valves and leaflets, are enormous stressed during lifetime. Therefore, those structures undergo different pathophysiological tissue transformations which affect cardiac output and in consequence living comfort of affected patients. These changes may lead to calcific aortic valve stenosis (AVS), the major heart valve disease in humans. The knowledge about changes of the dynamic behaviour during the course of this disease and the possibility of early stage diagnosis is of particular interest and could lead to the development of new treatment strategies and drug based options of prevention or therapy. 4D optical coherence tomography (OCT) in combination with high-speed video microscopy were applied to characterize dynamic behaviour of the murine aortic valve and to characterize dynamic properties during artificial stimulation. We present a promising tool to investigate the aortic valve dynamics in an ex vivo disease model with a high spatial and temporal resolution using a multimodal imaging setup.
1 Introduction
Calcific aortic valve stenosis (AVS) is the most common valve disease in modern industrial countries [1]. AVS is a major cause of morbidity and mortality in elderly patients and shares many risk factors with atherosclerosis like age, hypertension and smoking [2]. During a person’s lifetime, the valve tissue operates in a wearing hemodynamic environment and is strained by maintaining the unidirectional blood flow from the left ventricle to the aorta. Therefore, the valve tissue is influenced by remodelling and patho-physiological changes. Until now, little is known about the pathogenesis and progression of AVS because the clinical manifestation of the disease is preceded by a mostly undetected multi-year process of inflammation and fibrotic changes leading to thickened and stiff valve cusps with decreased flexibility and left ventricular outflow obstruction. Hence, the surgical replacement of the aortic valve is the only efficient treatment affecting annually 275,000 patients worldwide [3]. To fill the gap of knowledge, animal models provide a powerful tool to study valve biomechanics as well as the progression of AVS. Though mechanics of murine valves are difficult to test due to their small size, mouse models offer the advantages to demonstrate the progression of AVS and feature significant benefit providing genetic knockouts as an excellent tool to investigate potential key molecular modulators of heart valve disease. By using 4D optical coherence tomography (OCT) and high-speed video microscopy in an ex vivo murine heart model we present a novel approach for the investigation of the dynamic behaviour of artificially stimulated aortic valves with a high spatial and temporal resolution. The aim of this study is to show the feasibility of those imaging techniques to promote the understanding of tissue changes and biomechanical properties during AVS in mice.
2 Methods
2.1 Heart preparation and stimulation
All experiments were approved by the animal care and use committee of the local government authorities and were performed in accordance with the Guide for Care and Use of Laboratory Animals (Institute of Laboratory Animal Resources, 7th edition, 1996). In this study, 17-week-old C57BL/6J wildtype and 12-month-old ApoE knockout mice on C57BL/6J background were used.
The mice were anesthetized and heparinized. Afterwards, they were sacrificed and the hearts were removed immediately and rinsed in PBS to remove the entire blood. Because the aortic tissue is too thick for visualizing the entire valve structure with OCT, the aorta was shortened as much as possible to assure an optimal view onto the aortic valve cusps. Finally, the apex of the heart was cut off and a catheter was inserted into the left ventricle to mimic the normal flow direction.
As no suitable established heart model for dynamic visualization was found, we created an experimental setup shown in Figure 1 consisting of a custom-made pump for artificial stimulation of the aortic valve controlled by custom-made software and the image acquisition device. The device is a modified version of an experimental small animal ventilator [4] and stimulation procedure is described more in detail elsewhere [5]. In brief, each valve is stimulated with a stroke volume of about 40 µl at a frequency of 50 beats per minute. Therefore, two independent syringe pumps were used to insert and to withdraw fluid (PBS), respectively. Due to the shortening of the aorta, the closing of the valve is aided by applying negative pressure to the valve.
Experimental setup for investigation of aortic valve dynamics. Two independent syringe pumps perform the opening and closing of the valve by imitating a physiological stroke volume of 40 µl. Due to the pump setup and the OCT imaging speed the stimulation frequency was limited to 50 beats per minute.
2.2 Imaging setup
For imaging ex vivo aortic valve structure and dynamics, a spectral domain OCT system [6] with a center wavelength of 830 nm and a band width of 50 nm at half maximum was used. As the OCT system with an A-scan rate of 12 kHz is not fast enough to acquire 3D images of the moving aortic valve, a scanning algorithm making use of the repetitive tissue movement due to the artificial stimulation was applied. Therefore, the image acquisition of cross-sections is continuously performed without interruptions at the same position by deflecting just one galvanometer scanner. A trigger signal created from the pump device after each stimulation cycle sets the position of a second galvanometer scanner one step further and again cross-sections are continuously acquired at this new position.
After the measurement, the acquired OCT cross-sections were rearranged to form the different 3D image stacks showing the time resolved movement of the aortic valve cusps. High-speed video microscopy was performed with a Basler camera (acA2000-340kc) using the same beam path as the OCT separated via a dichroic mirror.
3 Results and discussion
The aortic valve of a 17-week-old and a 12-month-old ApoE knockout mouse was visualized under same conditions of artificial stimulation with 4D OCT and video microscopy (Figure 2).
Exemplary findings of the valve dynamics of a 17-week-old wildtype mouse and a 12-month-old female ApoE knockout mouse. The aorta tissue of the ApoE knockout mouse is thickened compared to the wildtype and the motility of the cusps is decreased. Furthermore, the measurement of the valve opening area (lower diagram) shows significant differences between both samples due to the progression of calcification and aortic valve stenosis.
Due to the advanced calcification, the movement of one cusp of the 12-month-old mouse is terminated (see purple arrow in Figure 2). Compared to the healthy tissue of the young mouse where all three cusps open widely the dynamic of the diseased valve is significantly decreased. Due to this late stage of the disease in the 12-month-old knockout mouse, the differences in the aortic structure are clearly visible between both mice regarding the thickness of the aortic tissue and the maximum opening area of the valve (diagram on the bottom of Figure 2). While these differences are difficult to see in the video microscopy images, OCT provides a clear insight into the tissue structure (not shown here) due to the three-dimensional image information. Therefore detailed analyses and measurements regarding the tissue morphology can be done without any staining or cutting of the samples. OCT enables the measurement and comparison of clinically relevant parameters like maximum opening area and valve thickness due to the 3D image information.
The presented results show that optical coherence tomography and high-speed video microscopy are promising tools for the investigation of dynamic behaviour and its changes in calcific aortic valve stenosis disease models in mice. OCT offers an easy access to the tissue morphology in 3D and the measurement of tissue parameters like thickness and flow channel area without any sample preparation like staining or cutting.
Those imaging techniques can be helpful tools to observe the progression of AVS during new developed drug therapies and novel approaches for the treatment of AVS in animal models. The high spatial and temporal resolution enables new insights into the course of this disease and may lead to a characterization and identification of early stages before significant hemodynamic obstruction has occurred. This first feasibility study reveals further steps to enhance the experimental setup and imaging techniques to increase the information content of the measurements and thereby rising comparability of the results to clinically relevant parameters.
Acknowledgments
This project was financially supported by the Else Kröner-Fresenius-Stiftung.
Author's Statement
Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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