BACKGROUND Myocardial contractility, a significant determinant of cardiac function, is valuable for diagnosis and evaluation of treatment in cardiovascular disorders including heart failure. Shear Wave Elasticity Imaging (SWEI) is a newly developed ultrasound-based elastography technique that can directly assess the stiffness of cardiac tissue. The aim of the study was to verify the ability of technique to quantify contractility changes in the myocardium. METHODS In 12 isolated rabbit hearts we made SWEI measurements of systolic stiffness at 5 different coronary perfusion pressures from 0 to 92 mmHg. The changes in coronary perfusion were used to induce acute stepwise reversible changes in cardiac contractility via the Gregg effect. The Gregg effect is the dependency of contractility on coronary perfusion. In 4 of the hearts, we repeated the measurements after delivery of Gadolinium, which is known to block the Gregg effect. RESULTS Systolic stiffness measured by SWEI changed linearly with coronary perfusion pressure with a slope of 0.27 kPa/mmHg (mean of 95% CI, R2=0.73). As expected the change in contractility due to the Gregg effect was blocked by Gadolinium with a significant reduction of the slope to 0.08 kPa/mmHg. CONCLUSION SWEI measurements of systolic stiffness provide an index of contractility in the unloaded isolated rabbit heart. While this study was done under ideal imaging conditions and with non-physiological loading conditions, it reinforces the concept that this ultrasound technique has the potential to provide a direct and noninvasive index of cardiac contractility.

Shear wave elasticity imaging (SWEI) is a novel ultrasound elastography technique for assessing tissue stiffness. In this study, we investigate the potential of SWEI for providing diastolic functional assessment. In 11 isolated rabbit hearts, pressure-volume (PV) measurements were recorded simultaneously with SWEI recordings from the left ventricle free wall before and after induction of global ischemia. PV-based end diastolic stiffness increased by 100% after ischemia (p <0.05), and SWEI stiffness showed an increase of 103% (p <0.05). The relaxation time constant (τ) before and after ischemia derived from pressure and SWEI curves showed increases of 79% and 76%, respectively (p <0.05). A linear regression between pressure-derived and SWEI-based (τ) showed a slope of 1.164 with R = 0.80, indicating the near equivalence of the two assessments. SWEI can be used to derive (τ) values and myocardial end diastolic stiffness. In global conditions, these measurements are consistent with PV measurements of diastolic function.

We have shown that SWEI measurements are linearly related to IVP; therefore, pressure-based indices could potentially be derived from SWEI ultrasound elastography. The feasibility of using SWEI to estimate IVP with a single point calibration was also shown in this study.