Renal resistive index and nocturnal non-dipping: is there an association in essential hypertension?

Renal Doppler ultrasonography (RDU) is a useful method to determine renal resistive index (RRI). The RRI has been used to evaluate target organ damage (TOD) in essential hypertension. Nocturnal non-dipping of blood pressure (BP) in essential hypertension was also associated with TOD. The relationship between increased RRI and non-dipping has not been specifically studied before. Patients with newly diagnosed essential hypertension underwent 24-h ambulatory BP monitoring, biochemistry analysis, 24-h urine testing, and RDU. Totally, 198 patients (137 women, 61 men, aged 53.8 +/- 11.4 years) were included. Sixty-two patients were non-dippers, and 56 patients had increased RRI. RRI was increased in 32.3% of non-dipper patients and in 26.5% of dipper patients (P = 0.402). The RRIs of dippers were lower than the RRIs of non-dippers (0.65 +/- 0.06 vs. 0.68 +/- 0.07, P = 0.036). Multivariate logistic regression analysis of potential factors predicting increased RRI disclosed that advanced age (OR 1.090, CI 1.042-1.140, P < 0.0001) and increased pulse pressure (OR 1.037, CI 1.012-1.062, P = 0.004) were independently associated with increased RRI. In multivariate linear regression analysis, using the same independent variables, we found that square root-transformed RRI was independently associated with age (Beta + 0.366, P < 0.0001) and pulse pressure (Beta + 0.222, P = 0.001). Increased RRI and nocturnal non-dipping are not independently associated with each other in newly diagnosed essential hypertensive patients. Possible different mechanisms, or the same mechanisms but with different activation levels, may be responsible for the increased RRI and non-dipping as discrete pathologies.