6 - Noradrenergic Responses in Postural Hypotension: Implications for Therapy

Abstract

This chapter focuses on noradrenergic responses in postural hypotension and their implications for therapy. Plasma norepinephrine (NE) levels provide a quantitative biochemical index of sympathetic nervous activity. Because hypotensive disorders inevitably involve diseases of the sympathetic nervous system (SNS) or compensatory response of SNS to a disease, these disorders can be conveniently divided according to NE levels of patients. Sympathetic tone normally doubles on standing in response to an effective decrease in blood volume. Standing causes an average 5 mm Hg rise in diastolic blood pressure. Postural hypotension occurs when there is a significant drop in diastolic blood pressure accompanied by symptoms attributable to inadequate perfusion of the brain. A drop in systolic blood pressure is a less reliable marker of postural hypotension because in people above 50 years of age, systolic pressure normally decreases on standing. Blood pressure can be maintained only in the presence of an adequate blood volume. The extracellular fluid volume, including blood volume, is governed by the amount of sodium retained by the body.