Abstract
In the last decades cardiovascular diseases greatly increased worldwide, and bioengineering provided new technologies and cardiovascular prostheses to medical doctors and surgeons. The design of active and passive devices aroused notable interests becoming more and more challenging as well as crucial. In this framework, it is important to faithfully reproduce the interaction between the prostheses and the cardiovascular system when in-vitro experiments are performed. For this reason, a new and improved kind of test benches becomes necessary. Purely hydraulic mock circulatory systems showed low flexibility to allow tests of different cardiovascular devices and low precision when a reference mathematical model must be reproduced. In this chapter a new bench is described. It combines the computer model of the cardiovascular system and its real-time interaction with the device to be tested. The solution adopted can be exploited in a Grid environment to allow remote experimentation.
TopBackground
In this section the main MCSs available at the present time for use in functional experiments are briefly described.
Key Terms in this Chapter
Pulmonary Circulation: It is the portion of the cardiovascular system comprised between right ventricle and left atrium. Its task is to transport blood poor of oxygen throughout the lungs and to return oxygenated blood to the heart.
Afterload: In cardiovascular mechanics, is the myocardial wall stress during systolic ejection. If the muscle is allowed to shorten only after developing a certain force, this additional force is called afterload. Arterial input impedance is the ventricular afterload system. It influences, but is independent of, cardiac ejection of blood into the arterial vasculature and the resultant instantaneous pressure and volume of the ventricle. Instantaneous ventricular pressure and flow are the ventriculo-arterial coupling variables. The arterial input impedance is defined as the ratio of arterial pressure to arterial flow, both of which are measured at the root of the ascending aorta and expressed as a complex variable in the frequency domain. Mean arterial pressures can be considered as appropriate measures of ventricular afterload.
Preload: In cardiovascular mechanics, is the myocardial wall stress at the end of diastole. As a measure of ventricular preload, end-diastolic (end-filling) volume is preferable to end-diastolic pressure for its closer relationship to muscle length, and thus to sarcomere length, but end-diastolic pressure can be used as preload as well. Mean atrial pressure can be considered as an appropriate measure of ventricular preload.
Mock Circulatory System: (MCS): is a hydraulic test bench used to test cardiovascular prostheses. By mimicking the cardiovascular system, MCSs allow analyzing the functionality of each prostheses, their design, and their interactions with the mimicked environment.
Compliance: It is the tendency of the vessels to stretch in response to pressure variations. It has a large effect on perfusion and blood pressure. Compliance is computed as:, where ?V and ?P are the volume and pressure variations, respectively. It is the reciprocal of elastance.
Systemic Circulation: It is the portion of the cardiovascular system comprised between left ventricle and right atrium. Its task is to transport oxygenated blood throughout the peripheral circulation (except lungs) and to return deoxygenated blood to the heart.
Elastance: (Pressure-Volume Relationship, and Time-Varying Elastance): A powerful tool for studying cardiac mechanics and the interactions between ventricle and circulatory system is the pressure-volume (PV) relationship. Cardiac contractions can be viewed “through the window of the pressure-volume diagram”. Thus, it is possible to link the instantaneous pressure Plv(t) and the instantaneous blood volume Vlv(t) inside the ventricle by means of the following pressure-volume relationship: where P0 is the external pressure, V0 is the ventricular volume at zero transmural pressure [P(t)-P0], and E(t) is the time-varying ventricular elastance. By neglecting the term P0, the ratio: states that in one cardiac cycle the relationship between the instantaneous pressure Plv(t) and the instantaneous volume Vlv(t) inside the ventricle is a time-dependent parameter, which is the time-varying elastance E(t). By connecting isochronous sets of instantaneous pressure-volume data points, a family of curves whose slope rise and fall can be plotted. This results in an increase and decrease of the elastance. All the elastance curves E(t), if normalized with respect to the peak value Emax and the time to peak Tmax would reduce a single curve which has unique shape for any heart, loading conditions, contractile state, or heart rate. As a consequence, Emax and Tmax can serve as a reliable index of ventricular contractility and are independent of preload (end diastolic ventricular volume/atrial pressure) and afterload (aortic pressure/arterial impedance). A more accurate formulation of the pressure-volume relationship is the non-linear time-varying elastance model, where a non-linear time varying function is used instead of the linear one, E(t). Nevertheless, the classical linear formulation of the elastance is still meaningful and much easier to use in most of the cases.