181Quantitative assessment of myocardial stiffness using shear wave imaging in normal and hypertrophic isolated rat hearts

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Purpose: Shear Wave Imaging (SWI) is a novel ultrasound-based technique for quantitative, local and non-invasive mapping of soft tissues elastic properties. Myocardial stiffness can be measured in real time over the cardiac cycle using SWI, allowing quantification of stiffness variation between systole and diastole. In a recent study on normal isolated rat hearts, we have shown that SWI can be used to provide an index of myocardial contractility [pernot et al. JACC 58, 1 2011:65–72]. The goal of the present study is to evaluate the performance of SWI on hypertrophic isolated rat hearts.Methods: SWI was performed in normal and hypertrophic Langendorff perfused isolated rat hearts (Heart/body weight ratio: 3.7±0.2 mg/g in shams and 6.1±0.9 mg/g in hypertrophy, n=6 each). Left ventricular hypertrophy was induced at 3 weeks of age (body weight <60 g): the thorax was opened and a stainless steel hemoclip of 0.6 mm ID was placed on the ascending aorta. Age-matched control animals (sham-operated) underwent the same procedure without placement of the clip. Studies were performed 4 months after the surgery. Shear wave was generated and imaged in the anterior wall of the left ventricle using a conventional ultrasonic probe connected to an ultrafast scanner (12,000 frames/s). The local myocardial stiffness was derived from the shear wave velocity every 7.5 ms during one single cardiac cycle.Results: Myocardial stiffness was measured in all hearts with a good reproducibility all over the cardiac cycle (s.d.<6%). In normal hearts, the average myocardial stiffness was 2.1±0.8 kPa in diastole and 9.2±1.5 kPa in systole. In hypertrophic hearts, myocardium was found significantly stiffer in diastole (3.8±0.9 kPa, p<0.01, n=6) but not in systole (10.7±2 kPa, p=0.17, n=6). Myocardial stiffness was also measured during administration of isoproterenol (10-9, 10-8 and 10-7 M, 5 min each). Systolic myocardial stiffness was found to increase strongly up to 23.4±3.4 kPa in normal hearts whereas no significant change was found for hypertrophic hearts. In contrast, diastolic stiffness did not change in both normal and hypertrophic hearts during isoproterenol stimulation.Conclusions: Passive and active elastic properties of the myocardium can be investigated locally and quantitatively using SWI. The application of SWI to an hypertrophied heart exhibits a strong increase of stiffness in diastole, which can be interpreted as reflecting underlying fibrosis within the myocardium. The failure of hypertrophied heart to increase its contractility under a inotropic stimulation is also highlighted by SWI when performed in systole.

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