The Cardiovascular Autonomic Nervous System and Anaesthesia

Correspondence: Dr W Pearce [email protected] Once awareness, our seat of deliberate action and movement, is temporarily extinguished by sleep or a loss of consciousness, we rely on a primordial, indispensable, and autonomous (“vegetative”) system that continues to sustain and control our vital organ systems. This vegetative system is never allowed to rest. The Cambridge physiologist J N Langley referred to it as the autonomic nervous system in a paper on the superior cervical ganglion in 1898. The term and concept did not receive immediate and overwhelming acceptance, but what they refer to has since been revealed as something astonishingly complex and nuanced in both its central organization, and its peripheral responses to external stimuli. Anaesthesia has as its sine qua non the reversible abolition of a patient’s so-called “nocifensive” movement response to potential, or inflicted, tissue damage. This abolition, as well as the subsequent surgery it allows, is always accompanied by a degree of physiological stress that leaves the patient’s autonomic nervous system neither unprovoked nor untouched. Successful anaesthesia paradoxically both perturbs and relies on the integrity of the autonomic nervous system. Anaesthesia is autonomic medicine. The greater part of our training and practice is spent acquiring skills in averting or utilizing the autonomic nervous system effects of anaesthetic drugs or surgical procedures under a variety of pathophysiological conditions. Moreover, many of these pathophysiological conditions may be associated with impaired preoperative autonomic function. Therefore, almost every aspect of clinical anaesthesia can be discussed with some reference to the autonomic nervous system. Considerations of space preclude an exhaustive, systematic treatment of the autonomic nervous system in anaesthesia. The approach here is, firstly, to assume that the reader has a fundamental grasp of the physiology and anatomy of the cardiovascular autonomic nervous system as they relate to anaesthesia. Secondly, the extrasynaptic non-innervated hormonal receptors that mediate part of the autonomic response and that are relied on whenever inotropes are administered, are not strictly part of the autonomic nervous system and will not be elaborated on. Similarly, a description of the comparative autonomic pharmacology of the anaesthetic agents belong more appropriately in a separate monograph. Autonomic dysfunction can manifest in protean ways, and involve many organ systems, that are decisively relevant to anaesthesia. Lists, for example, of the relevant autonomic manifestations of diabetic autonomic neuropathies (Table I), of those in multiple system atrophy (MSA – formerly known as Shy-Drager syndrome) (Table II), and in Parkinson’s disease (Table III) serve to illustrate this. This discussion will consider the assessment of the autonomic TABLE II: Autonomic Signs/Symptoms in MSA.