Abstract
The current surge of interest in postural orthostatic tachycardia syndrome commonly known as POTS requires good knowledge of the very complex physiology involved, but this is currently lacking. The often overlooked normal physiology of orthostasis is reviewed including the definition of normal postural orthostatic tachycardia. An illustrated functional anatomy that embeds orthostatic tachycardia within the learned and skilful motor functions in the human population is presented. The four physiological phases of orthostasis and the role of tachycardia are described in a laboratory-controlled and progressive orthostatic stress in normal human volunteers. Standardized surrogate measures of autonomic control were used to quantify the trigger level for excessive tachycardia and the minimum autonomic control required to sustain viable arterial blood pressure during severe orthostatic stress in normal human volunteers. Tachycardia during orthostasis is part of a "democratic" contribution by four cardiovascular parameters of which the chronotropic function of the heart is just one of the parameters contributing toward cardiovascular compensation. It is adjusted during orthostasis in proportion to contributions from the other three parameters, namely inotropic function of the heart, windkessel vascular resistance and venous vascular capacitance. The physiological effects of the two stressors during orthostasis, gravity and isometric contraction of skeletal muscles are reviewed. A model of how the four cardiovascular parameters are regulated during orthostasis to achieve proportionate contributions is proposed emphasizing the necessity to quantify individual contributions from all these four parameters. Any one or more of these parameters may be compromised due to disease requiring disproportionate contribution of the prevailing magnitude of orthostatic tachycardia in an individual. It therefore requires neurophysiological assessment of the autonomic regulation of all the four cardiovascular parameters to assess the condition fully. We recommend here some current and novel neurophysiological methods that use modern medical technology to quantify laboratory standardized surrogate measures of some of these cardiovascular parameters including central parasympathetic regulation in postural orthostatic tachycardia syndrome.