Mechanisms of Orthostatic Tolerance and Age-Related Changes in Orthostatic Challenge

Abstract

Orthostatic tolerance is a term that defines the ability to prevent hypotension during gravity stress. First, maintaining postural balance and homeostasis requires the integration of sensory information through proprioception, visual and vestibular pathways. Regulation of blood pressure depends on the proper function of the muscle pump, cardiac, renal, neural (parasympathetic and sympathetic nervous systems, baroreflex) and endocrine systems. Baroreflex is a typical compensatory reflex mechanism to a reduction in stroke volume, which is manifested by increased heart rate, cardiac contractility, and peripheral vascular resistance. Skeletal muscles, especially the lower limb muscles, in the upright position, help protect the venous return by compressing the vessels. Renin-Angiotensin-Aldosterone System (RAAS) is activated when blood pressure drops below normal. All of these mechanisms maintain the mean arterial blood pressure in the normal range. All three phases of the orthostatic changes: 1) an initial heart rate rise and blood pressure drop, 2) an early phase of stabilization, and 3) a phase of prolonged stabilization are influenced by aging. The number of pacemakers in the sinoatrial node, beta-adrenoreceptor, cardiac compliance, vasoconstrictor response to alpha 1-adrenergic stimulation, RAAS activation decrease with aging. In the light of these terms, orthostatic intolerance is common in older adults.