Real-time model-based control of afterload for in vitro cardiac tissue experimentation

The performance of mechanical work by isolated cardiac muscle samples has typically been studied by subjecting their tissues to an isotonic shortening protocol, which results in "flat-topped" work-loop profiles. In order to better replicate the forces experienced by these tissues in vivo, we have developed a system for imposing a model-based, time-varying, load on isolated cardiac tissue preparations. A model of systemic afterload was developed from the combination of a Windkessel-type model of vascular fluid impedance, and the Laplace law of the heart, and encoded into a hardware-based control system. The model-predicted length change was then imposed on an isolated cardiac trabecula in a work-loop calorimeter, giving rise to force-length work-loops that more closely resemble those experienced by these tissues in vivo.