Principles of Cardiovascular Neural Regulation in Health and Disease
222 pp. Norwell, Mass: Kluwer Academic Publishers; 2000. $160.
Medicine and science today increasingly focus on a reductionist approach to solve their mysteries and provide greater understanding of the mechanisms of disease. Principles of Cardiovascular Neural Regulation in Health and Disease offers a refreshing perspective that emphasizes the whole organism, particularly human subjects, and gives researchers and physicians an integrative conception of the control mechanisms operative in normal and pathophysiological situations. The book concentrates on neural control of the cardiovascular system and assesses both negative and positive feedback mechanisms. The theme is sympathovagal balance using spectral analysis of heart rate and blood pressure variability, incorporating linear and nonlinear techniques. What differentiates this text from so many others is the philosophical and ethical approach that this author takes in providing an analysis of current, noninvasive methods used to assess function of the autonomic nervous system in regulating cardiovascular function.
In his long and distinguished career as a basic and clinical scientist, Dr Malliani has worked broadly in Europe and the United States to achieve an understanding of neural regulation of the cardiovascular system. His early studies focused on cardiac sympathetic afferent nerves, their mechanisms of activation, and their excitatory influence on the cardiovascular system. In the first chapter of this book, he gives a historical review of homeostasis and describes comprehensively the early work that set the stage for our modern understanding of the dual mechanisms of cardiovascular reflex control that originate from the heart, including vagal and sympathetic afferents. Little mention is made of more recent work that has refined and extended earlier studies on peripheral mechanisms of activation and central neural integration of these important cardiac sensory systems.
In the second chapter, the pathophysiological mechanisms that both stimulate and lead to disturbed function of cardiac afferents, including acute myocardial infarction, hypertension, congestive heart failure, and pain, are mentioned in some detail. Dr Malliani concludes that in pathophysiological circumstances the system is replaced by largely purposeless neuromechanisms. This controversial comment, although containing some truth, obscures the important function of cardiac afferents as a warning system and, importantly, their role in the organism’s attempt to restore homeostasis in disease states.
The focus of Dr Malliani’s treatise is contained in the third and fourth chapters, which describe the balance between the sympathetic and vagal systems and their imbalance in various pathophysiological circumstances. Dr Malliani introduces the concept of time and frequency domain studies to assess time-related periodicity, frequency, and amplitude, respectively. He also provides a lengthy discussion of the attributes of analysis by frequency domain in cardiac, respiratory, and 24-hour slow-wave function, including circadian rhythms and central oscillators. Of note, the author stresses the possibility that these rhythms provide information to organs that can alter their function profoundly.
The methodology used in frequency domain analysis is defined, including fast Fourier transform to measure sinusoidal components of amplitude, frequency, and phase, and the representation of high- and low-frequency rhythms, their ratio, and their amplitude in the assessment of heart rate and blood pressure variability. Dr Malliani assumes a somewhat defensive position in his interpretation of the high-frequency representation of vagal and the more controversial low-frequency representation of sympathetic activity in determining heart rate and blood pressure variability. The validity of high- and low-frequency variability is carefully discussed, albeit mostly from the author’s own perspective. One is left with the impression that high-frequency, and to a greater extent, low-frequency rhythms are not pure measures of vagal-sympathetic balance.
In pathophysiological disease states, heart rate variability (HRV) as a means of determining sympathetic-parasympathetic balance is limited by age, sex, and heredity. HRV does not provide a direct measure of sympathetic and vagal balance, nor does it provide an understanding of the mechanisms by which autonomic balance is altered; rather, it measures their modulation and interaction. HRV has been used to assess ischemia and infarction, malignant arrhythmias, hypertension, congestive heart failure, and vasovagal syncope, and in each of these situations, it has provided a useful noninvasive assessment of relative changes in cardiac autonomic activity. More importantly, HRV may predict outcome, particularly with regard to sudden death and malignant arrhythmias, which can accompany these cardiovascular conditions. However, effector organ responsiveness and confounding features of disease (for example, in heart failure and hypertension) can limit the utility of HRV.
Nonlinear dynamics, as assessed by chaos theory, is described in the fifth chapter. Regularity, synchronization, coordination, and pattern recognition as fundamental information in nonlinear dynamics are mentioned briefly.
Throughout this book, Dr Malliani injects a measure of his own sense of reasoning and understanding of the assessment of autonomic balance in cardiovascular regulation. Nowhere is this more evident than in the last chapter, which departs from the general theme to focus squarely on ethics in medicine and science today. Blunt but often insightful statements on scientific poverty, ignorance of ethics, conflicts of interest (especially with regard to the economic links that have become so prevalent), and the need for an international code of conduct serve as important reminders of problems that plague science today. Because scientists and physicians are respected as leaders who are knowledgeable about events that can pose great risk to human life, their role in providing a greater understanding of the body and its complex interactions and their need to seek truth in an objective and unbiased fashion are essential.
This monograph provides a wealth of information about cardiovascular neural regulation, focusing on spectral analysis of heart rate and blood pressure variability, its interpretation, and its meaning. Although the view provided is mostly unipolar in the interpretation of this methodology, and although the text is frequently philosophical and the discussions complex, readers interested in understanding neural cardiovascular interactions from an integrative viewpoint will derive significant intellectual benefit.