Application of cardiovascular models in comparative physiology and blood pressure variability

The usefulness of cardiovascular models is determined by their intended function with respect to elucidating underlying hemodynamic concepts and to enable simulations that will assist in understanding the effects of specific parameters. Models can take different forms, including mock circulatory constructs with physical components, mathematical representations of parameter space relations employing constitutive equations, or closed form representations of electrical circuit analogs described in the time or frequency domain. This investigation describes the use of cardiovascular models based on electrical analogs of mechanical hydrodynamic systems to elucidate two different physiologic concepts: (i) the use of distributed vascular impedance to investigate comparative physiology of optimal design and features related to body size across a broad range of animal species; (ii) use of lumped parameter models to assess the role of arterial stiffness in blood pressure variability. The impedance model shows that an allometric relationship between body weight and aortic effective length can be determined by using the frequency of minimum input impedance and aortic pulse wave velocity. This concept provides a background for optimal matching of body size and hemodynamic load on the heart. The lumped parameter model indicates that arterial stiffness, simulated by the total arterial compliance term, has a significant impact on variability of arterial pressure when changes are due to dynamic alterations of peripheral resistance. In addition, the known pressure dependency of arterial stiffness results in a curvilinear relationship between blood pressure variability and mean pressure. This has implications in hypertensive treatment where there are marked changes in arterial stiffness, as occurs with aging.