Modelling the cardiac response to a mechanical stimulation using a low-order model of the heart

A new synergistic model for simulating exercise incorporating control mechanisms at cellular and organ scales

The physiological response of the cardio-vascular system (CVS) to physical activity is of great importance to those working in sporting research and has profound consequences for the health and well-being of people. Coronary vasodilation and the physiological mechanisms involved in exercise have frequently been the focus of numerical models for simulating exercise. This is partly achieved using the time-varying-elastance (TVE) theory, which prescribes the pressure-volume relationship of the ventricle as a periodic function of time, tuned using empirical data. The empirical foundations of the TVE method however, and its suitability for CVS modelling are frequently questioned. To overcome this challenge, we adopt a different synergistic approach in which a model for the microscale heart muscle (myofibers) activity is embedded within a macro organ-scale CVS model. We developed such a synergistic model by including the coronary flow and various control mechanisms at the circulation level through feedback and feedforward means, and at the microscale (contractile) through the regulation of ATP availability and myofiber force depending on exercise intensity or heart rate. The coronary flow produced by the model displays the well-known 2-phase character of the flow, which is preserved under exercise. The model is tested by simulating reactive hyperemia, which is a transient occlusion of the coronary flow, successfully reproducing the additional coronary flow following the block removal. On-transient exercise results reveal a rise in both cardiac output and mean ventricle pressure as expected. The stroke volume increases initially, but then declines during the latter period of HR rise, corresponding with one of the main physiological responses to exercise. The pressure-volume loop expands during exercise, as systolic pressure rises. The Myocardial oxygen demand increases during exercise and the coronary blood supply increases in response, causing an excess of oxygen supply to the heart. Off-transient exercise recovery is largely a reverse of this response, although the behaviour is slightly more varied, with sudden spikes in coronary resistance. Different levels of fitness and exercise intensity are tested and reveal that the stroke volume rises until a level of myocardial oxygen demand is reached at which point it declines. This level of demand is independent of fitness or exercise intensity. An advantage of our model is demonstrated in the correspondence between the micro and organ scale mechanics so that cellular pathologies can be traced from exercise performance with relatively little computational or experimental expense.

Development of a novel low-order model for atrial function and a study of atrial mechano-electric feedback

Numerical models of the cardiovascular system have largely focused on the function of the ventricles, with atrial function often neglected. Furthermore, the time-varying elastance method that prescribes the pressure-volume relationship rather than calculating it consistently is frequently used for the ventricles and atrium. This method has yet to be validated however, so its applicability for cardiac modelling is frequently questioned. To overcome this challenge, we propose a synergistic model of left atrium (LA) and left ventricle (LV) by self-consistently integrating various feedback mechanisms among the electro-mechanical and chemical functions of the micro-scale myofiber, the macro-scale dynamics of the LA and LV, the atrioventricular node (AV), and circulation. The model is tested and shown to reproduce the essential features of the atrium cycling, such as the characteristic figure of eight pressure-volume loops. Our model is further developed to investigate the effect of dysfunctions of the mechanical-electric feedback (MEF) in the atrium. Our model not only successfully reproduces key experimental MEF observations such as prolonged action-potential and increases in action-potential magnitude induced by atrial stretch but also shows how MEF and arrhythmia of the atrium lead to a degradation of cardiac output and pumping power with significant consequences. In particular, MEF reproduces arrhythmia such as ectopic and erratic cycling, missed heart beats and restricted function.

An Investigation of Left Ventricular Valve Disorders and the Mechano-Electric Feedback Using a Synergistic Lumped Parameter Cardiovascular Numerical Model

Cardiac diseases and failure make up one of largest contributions to global mortality and significantly detriment the quality of life for millions of others. Disorders in the valves of the left ventricle are a prominent example of heart disease, with prolapse, regurgitation, and stenoses—the three main valve disorders. It is widely known that mitral valve prolapse increases the susceptibility to cardiac arrhythmia. Here, we investigate stenoses and regurgitation of the mitral and aortic valves in the left ventricle using a synergistic low-order numerical model. The model synergy derives from the incorporation of the mechanical, chemical, and electrical elements. As an alternative framework to the time-varying elastance (TVE) method, it allows feedback mechanisms at work in the heart to be considered. The TVE model imposes the ventricular pressure–volume relationship using a periodic function rather than calculating it consistently. Using our synergistic approach, the effects of valve disorders on the mechano-electric-feedback (MEF) are investigated. The MEF is the influence of cellular mechanics on the electrical activity, and significantly contributes to the generation of arrhythmia. We further investigate stenoses and regurgitation of the mitral and aortic valves and their relationship with the MEF and generation of arrhythmia. Mitral valve stenosis is found to increase the sensitivity to arrhythmia-stimulating systolic stretch, and reduces the sensitivity to diastolic stretch. Aortic valve stenosis does not change the sensitivity to arrhythmia-stimulating stretch, and regurgitation reduces it. A key result is found when valve regurgitation is accompanied by diastolic stretch. In the presence of MEF disorder, ectopic beats become far more frequent when accompanied by valve regurgitation. Therefore, arrhythmia resulting from a disorder in the MEF will be more severe when valve regurgitation is present.