Design and implementation of a fluid-mechanical dynamic afterload for use in an isolated heart apparatus

An isolated heart attached to a fluid-mechanical impedance (afterload) provides a method for study of myocardial processes and pressure and flow mechanics within the heart. Afterloads currently available allow various impedance parameter settings, but they are not automatically or dynamically controlled. A dynamically controlled afterload was constructed and its suitability tested for implementation with an isolated heart apparatus.; Initial work was in development of a cardiovascular model to reveal trends for aortic pressure changes with afterload parameter adjustments. The LabVIEW(TM) model enables simulations with open-loop windkessel-type impedances and simulations with a closed-loop circulatory model. Cataloged trends were used to guide the dynamic afterload controls, and the open-loop impedances provided methods for modeling the fluid-mechanical system.; Following this work, a systems analysis tool was developed in LabVIEW(TM) and MatlabRTM to enable characterization of the fluid-mechanical afterload. The program contains time-domain and spectral analyses that incorporate equal variance algorithms for the correlation analyses and averaging methods for noise reduction in the spectral analyses for stationary signals. Auto- and cross-spectral analyses were used to generate system impedance spectra from dynamic afterload simulations.; The culmination of this project was construction of a fluid-mechanical dynamic afterload. The dynamic nature of the afterload involves controlled, automatic adjustment of mechanical resistance, compliance and volume elements. These adjustments in afterload cause input pulsatile pressure to match the mean and range of a reference pressure. Simulations were performed with a pulsatile pressure pump for ten reference pressures with physiologically realistic mean and range values. The dynamic afterload constrained input pressures to within +/- 5% of the reference values and typically settled to the targeted values in 45--50 cycles. Impedance spectra from the simulations provided consistent and physiologically realistic estimates of afterload parameters fitted to a four-element windkessel-type impedance. Effects of changing impedance on the mean, range and stroke volume followed anticipated trends. These tests demonstrate that the dynamic afterload exhibits the qualities necessary for implementation with an isolated heart apparatus. Furthermore, this system will enable studies both of transient behavior in the isolated heart with changing afterload and of controlled pressure characteristics from a changing input pressure source.