Resting sympathetic outflow does not predict the morning blood pressure surge in hypertension

OBJECTIVE: The blood pressure (BP) rise on awakening (morning surge) might be a predictor of hypertension-related cardiovascular complications. Previous studies suggest that the autonomic nervous system may contribute to the early morning BP increase. We tested the hypothesis that resting sympathetic outflow [assessed by direct measures of intraneural sympathetic nerve activity (SNA)] may help predict the morning BP surge in hypertension. METHODS: We measured muscle SNA (MSNA), heart rate (HR) and BP during undisturbed supine rest in 68 newly diagnosed untreated hypertensive patients (53 men and 15 women, age 40 ± 3 years, BMI 27 ± 1 kg/m, mean ± SEM). The morning BP surge was defined as the difference between the morning BP and the pre-awake BP. RESULTS: SBP averaged 143 ± 3 mmHg for daytime and 126 ± 2 mmHg for night-time. Mean HR was 81 ± 2 beats/min for daytime and 69 ± 2 beats/min for night-time. Average MSNA was 32 ± 2 bursts/min., SBP morning surge 19 ± 2 mmHg and HR morning surge 14 ± 2 beats/min. In univariate analysis, MSNA correlated with daytime SBP (r=0.28, P=0.02); night-time SBP (r=0.26, P=0.03); daytime HR (r=0.28, P=0.02); and night-time HR (r=0.26, P=0.03). Multivariate analysis, taking into consideration age, BMI and sex, revealed that MSNA was independently related to both daytime (P=0.006) and night-time HR (P=0.02), but not to ambulatory SBP. The morning surge of SBP and HR was not related to MSNA (r=0.01 and r=0.07, respectively, P=NS). CONCLUSION: In patients with essential hypertension, MSNA is related to both daytime and night-time HR, but not to the morning BP surge.