[Impaired urinary flow rate during the day: a new factor possibly involved in hypertension and in the lack of nocturnal dipping]

Abstract

Previous studies suggest that a low urinary flow rate may reduce the capacity of the kidney to excrete sodium and could thus favour hypertension. In the present study, we evaluated the relationships between urinary flow rate (V) during day (D) and night (N), blood pressure (BP), and the day-night BP difference (delta in % of day BP) in 65 diabetic patients (glycosuria < 90 mmol/24 h) (35 F and 30 M, age 59 +/- 2 y) hospitalized for a 24 h urine collection and mean ambulatory blood pressure recording (AMBP). Urine was collected as two separate samples during D (8:00 am to 10:00 pm) and N (10:00 pm to 8:00 am). V, sodium excretion (NaEx) and mean systolic and diastolic BP (SBP and DBP, respectively) were calculated for the two periods. Patients were a posteriori classified according to the D/N ratio of V, and the mean values of 3 tertiles (T1, T2, T3, n = 22, 22, and 21 subjects) were calculated and compared by ANOVA (see table; *: p < 0.05; **: p < 0.01; ***: p < 0.001). [table: see text] Although total 24 h urine volume was similar in the three tertiles (1,670, 1,927 and 2,007 mL, NS), the fraction of urine excreted during D and N differed widely, with parallel differences in NaEx. With lower V during the day, BP tended to be higher and the fall in nocturnal BP to be reduced.

Conclusions: This study shows the advantages of separate day and night urine collections in relation with AMBP. For a similar total diuresis, some subjects exhibit a too low urine flow rate and too low NaEx during the day, and a compensatory rise during the night. This low diurnal diuresis is associated with a higher blood pressure and a lower nocturnal fall (= nocturnal pressure diuresis). The factors responsible for a too low diuresis during the day need further investigation.