How useful is measurement of plasma norepinephrine in the evaluation of dysautonomia?
The main source of plasma norepinephrine is the sympathetic network around blood vessels.
However, most of the released norepinephrine is metabolized before spilling over to the plasma, and only a small proportion enters the bloodstream unchanged.
An elevated plasma norepinephrine level could suggest either a high rate of sympathetic activity or a decreased rate of clearance from plasma.
Measurement of both norepinephrine and its metabolite DHPG levels can provide further information about the mechanism involved: an increased plasma level of norepinephrine mediated by increased sympathetic activity is associated with elevated DHPG.
On the other hand, if the diminished norepinephrine reuptake is the cause of increased NE level, an increase in plasma DHPG would not be observed.
In normal subjects, the plasma level of norepinephrine is 150 to 170 pg/mL after 30 minutes in the supine position; it increases 50% to 100% above supine values after 5 minutes of standing and remains constant after 10 minutes of standing.