PubMed2001
Increased sympathetic activity and plasma levels of norepinephrine, parasympathetic withdrawal and impaired baroreflex gain have been reported in patients with chronic heart failure (CHF). Since excessive neurohormonal activation is implicated in the progression of heart failure, it is crucial to measure and quantify it in clinical practice. Some techniques that may be suitable for this purpose such as catecholamine plasma level assessment or direct recording of the neural efferent activity to the peripheral muscles, require expertise and are very unattractive in terms of cost-effectiveness. On the contrary, the measures that test the reflex or tonic responses of the sinus node activity have been extensively used in both experimental and clinical studies. However, consensus regarding their true relevance in the clinical scenario has not yet been reached. Heart rate variability is one of the easiest techniques available for physicians but the identification of the RR intervals in CHF is often very difficult owing to the presence of rhythm disorders or of sino-atrial or atrioventricular blocks and the analytical softwares included in the commercially available Holter systems are not reliable enough. Similarly, for all methods currently employed in clinical practice, the assessment of baroreflex sensitivity in CHF has some limitations or is not applicable for all patients. In this article, the main clinical methods used to assess the autonomic nervous system in CHF are reviewed focusing on the clinical applicability of the obtained parameters. It is underlined that, using these techniques, many investigators have obtained important results in the pathophysiological comprehension of the disease, but to date their use in the clinical setting is very modest. Indeed, before the measurement of the autonomic nervous system can be applied to clinical practice and used to guide therapy, more precise insights into the link between the autonomic indexes and sympathetic or parasympathetic activity and between them and mortality are needed. Moreover, no standardization has been established for the various commercial systems and the development of their software has not been guided by any research committee. For many other techniques there are no commercially available devices and the software has been developed in the physiological laboratories as prototypes. These limitations will be overcome if the researchers succeed in convincing the companies about the importance and the clinical applicability of the evaluation of the autonomic nervous system. With regard to cardiac transplantation, as the donor heart is completely denervated, it constitutes an important physiological model for the study of autonomic activity. The more interesting research field has been the assessment of a possible post-transplant reinnervation both at the experimental and clinical levels. It has been shown that in animal models reinnervation occurs within the first year of heart transplantation. Several authors have demonstrated a time-dependent restoration of myocardial norepinephrine content as well as an increase in the heart rate and myocardial contractility in response to neural sympathetic stimulation. Parasympathetic efferent reinnervation has also been demonstrated in a canine transplanted model within 12 months of surgery. Despite evidence in favor of both sympathetic and parasympathetic reinnervation in animals, the problem of whether such reinnervation also occurs in humans is still debated, especially as far as parasympathetic reinnervation is concerned. The presence, in humans, of sympathetic reinnervation has been documented by different methods. However, there are significant differences regarding the degree of regeneration and the frequency of occurrence. It has also been assumed that the regeneration of sympathetic fibers takes place slowly over time. In contrast, regrowth of parasympathetic nerve fibers in patients after heart transplantation has not been unequivocally confirmed. Although various investigators have used the same method, their results differ and are indeed often contradictory. Recently it has been suggested that the type of surgery may have a major influence on neural and particularly parasympathetic regeneration. If the patients undergo heart transplantation by bicaval techniques the nerves are resected and they are more prone to regenerate towards the donor heart. This is not true if the traditional technique is employed.
Autonomic Nervous System↗