Abstract 4856: Nighttime Sodium Excretion Following Acute Sodium Loading Predicts Blood Pressure Dipping and Aortic Augmentation Index

Abstract

Background: In subjects who are in sodium (Na) balance, chronically increased night:day urinary Na excretion ratio is associated with arterial stiffening and nocturnal blood pressure non-dipping; less is known about these relationships following acute Na loading. Methods: We studied 11 young (24±5 yrs, 8 male/3 female), non obese, otherwise healthy prehypertensives over 6 days. Subjects ate a 20 mMol Na diet for 4 days. On day 5, they received 2 liters of 0.9% saline (308 mMol Na) over 2 hours, then an additional 130 mMol Na over 2 meals. Between days 4 and 6, subjects performed complete urine collections and ambulatory blood pressure monitoring, and underwent SphygmoCor assessment of aortic augmentation index (AIx75; in %, standardized to heart rate 75) on days 4 and 6. We defined the increase in nighttime urinary Na excretion following Na loading as: Nighttime-Na Excretion Ratio (NNER) = mMol Na/hr (rate night 5, post-Na load)/mMol Na/hr (rate night 4, pre-Na load). We also defined the change in nocturnal blood pressure dipping between night 4 (pre-Na load) and night 5 (post-Na load) as: ChgDip = nocturnal decrease in mean arterial pressure [mmHg, night 4] - nocturnal decrease in mean arterial pressure [mmHg, night 5] We then explored how the NNER related to ChgDip and AIx75 post-Na loading with linear regression. Results: The nighttime Na excretion rate increased following Na loading (.9±.9 to 8±4 mMol/hr, p < .001); the mean NNER was 13±9. After stepwise regression including age, gender, body mass index, daytime Na excretion, post-Na mean arterial pressure, and pre-Na blood pressure dip, higher NNER predicted less effect of Na loading on ChgDip ( β = −0.4, p = .005). In a stepwise regression model including age, gender, height, mean arterial pressure, left ventricular mass, aortic augmentation index in low-Na balance, and daytime Na excretion, the NNER was the only significant predictor of post-Na loading AIx75 ( β = −1.3, p= .009). Blood pressure dipping and AIx75 did not correlate with daytime Na excretion. Conclusions : Following an acute Na load, prehypertensive subjects who increase their nighttime Na excretion rate the most preserve nocturnal blood pressure dipping and have lower aortic augmentation index.