The human heart is an amazing hydraulic pump that pushes a non-Newtonian fluid in a system of non-rigid vessels millions of times over the course of a lifetime. The mechanic work of this pump is finely regulated by a complex electrical structure that operates with sophisticated feedback systems. These latter are equipped with peripheral sensors and central and peripheral effectors, in order to ensure that the blood flow to all the body systems is sufficient to satisfy the various basic metabolic needs of life, and to supply of adequate energy necessary for the completion of any external work.
The muscle activity required by physical exercise, which goes from simply get up from a chair in an elder to preserve the autonomy at home, to the performance of an ultra-marathon runner, finalized to complete a race against extreme environmental conditions, represents the most important physiologic stress for the heart. To cope with the exercise challenge, the cardiovascular system must be therefore able to adapt its performance ranging from a minimum activation level to the capacity of supporting for long time the maximum cardiac output. Amazingly, the physical work required by the exercise to the heart can be done in a cardiovascular continuum of performance that goes from the diseased heart (as in heart failure and primarily cardiac diseases) to the trained heart of the elite athlete. The five articles reported in this dissertation are linked by an important fil rouge: the ability of the cardiovascular system to cope with extreme conditions, in which the exceptional nature of the stimulus is due to very different factors: for a pathology that directly compromises the heart's response to exercise, the light muscle movements necessary to move an electric-wheelchair can be very "demanding" for the cardiovascular system, while for the trained heart the physical activity requested in conditions of extreme environment underlines an adaptation ability near to the tolerable limits of human possibilities.
Throughout this continuum of cardiovascular performance of the human heart, the interaction between the environment (simple daily relation life, microgravity, extreme ambient conditions) and the cardiac response to exercise highlights very close links between the heart and the main control system of its mechanical action, the autonomic nervous system, which represents one of the most ancestral (and thereby important) parts of the entire nervous system.
In all the conditions in which the cardiac performance was studied in these five articles, it was important to be able to measure as directly and simultaneously as possible both, the cardiac internal load during exercise and the efficiency of its main neural control factors. In these papers it is shown how, despite the extreme variability of environmental stimuli and the stressful factors that influence the cardiovascular system, HR and HRV represent two adequate tools for synchronously assessing cardiac performance on the one hand and the efficiency of his autonomic control during physical exercise on the other. Furthermore, in some study conditions depicted in these articles, physical stress was also added to mental/psychologic stress: again, the HRV tool proved to be adequate in describing the heart's response to these types of non-muscular stress on the heart. If one considers that this monitoring system is absolutely non-invasive, that it can be analyzed starting from a single biological signal (the electrical activity of the heart), that it easy to use, that does not require sophisticated and expensive instruments, its usefulness in evaluating the adaptability of the heart, both pathological and trained, to the continuous "challenges" that the cardiovascular system has to face is clear.
Nevertheless, some aspects are still to be evaluated and will require further studies about the use of these analysis systems: for example, i) finding more correct descriptors of the sympatho-vagal balance, ii) better exploring the non-linear components of the cardiac signal, aiming at evaluating the responses of the systems that do not depend on simple action-reaction feedback mechanisms, iii) assessing new ECG signal descriptors that are more directly correlated with the mechanical characteristics of the heart, iv) evaluating the efficiency of new HR and HRV monitoring systems that do not require the strict processing of the cardiac electrical signal (such as the vascular photoplethysmography), v) evaluating the determination capacity of these analysis tools with respect to the onset of fatigue during a continuous recording, vi) better focusing on the crucial relationships between cardiac and respiratory activity, vii) evaluating the effects of senescence both on the mechanical part and on the autonomic control of the heart, are just some of the several challenges that we will face in the next years.
However, I believe that the results obtained so far are very promising and I hope that this field of investigation can be deepened and enriched with new methods to evaluate the cardiovascular efficiency during exercise, especially by expanding the prognostic capabilities of these measurements in the field of cardiovascular pathologies, which is still one of the most important health issues to be addressed in the near future.
