Objectives: Plasma norepinephrine displays a limited ability to reflect the enhanced sympathetic drive characterizing essential hypertension. Among the factors responsible, an important one is the reduced reproducibility of the norepinephrine approach. The present study aimed to determine whether increasing the number of blood samples on which norepinephrine assay is based improves norepinephrine reproducibility. This was done by taking muscle sympathetic nerve traffic recording as 'gold standard', which is characterized by an elevated short- and long-term reproducibility.
Methods: In 14 untreated mild-to-moderate essential hypertensive patients, we evaluated, in two experimental sessions spaced each other by a 10-14-day interval, blood pressure (Finapres), heart rate (ECG), plasma norepinephrine (HPLC) and muscle sympathetic nerve traffic (microneurography, peroneal nerve). In the two sessions, three norepinephrine samples were obtained at 30-min intervals between each. Norepinephrine reproducibility between sessions was assessed on a single norepinephrine sample or averaging together the values obtained from second or third samples. Reproducibility of norepinephrine data was then compared with the microneurographic one.
Results: Although muscle sympathetic nerve traffic values showed a highly significant correlation between sessions (r=0.79, P<0.001), norepinephrine values derived from a single blood sample did not correlate with each other (r=0.42, P=NS). Norepinephrine correlation coefficients were consistently improved and achieved statistical significance when average data from second or third blood samples were examined (r=0.63, P<0.03).
Conclusion: In essential hypertension, the reproducibility of plasma norepinephrine as an adrenergic marker can be substantially improved by performing a norepinephrine assay on multiple blood samples.