Hemodynamic and Autonomic Response to Different Salt Intakes in Normotensive Individuals

Abstract

Background: Even if sodium sensitivity represents a risk factor at any blood pressure (BP) level, limited evidence is available that it may influence cardiovascular control in normotensives, particularly in white individuals. Therefore, the aim of the study was to investigate whether sodium sensitivity alters hemodynamic or autonomic responses to salt in normotensives.

Methods and results: We evaluated the Sodium-Sensitivity Index (SS-Index) in 71 white normotensives after 5 days of high- and low-sodium diets. We measured BP continuously at the end of each period, estimating hemodynamic indices from BP waveform analysis, and autonomic indices from heart rate (HR) and BP variability. According to the SS-Index distribution, we defined 1 sodium-sensitive group (SS, with SS-Index >15 mm Hg/[mmol·day]), 1 sodium-resistant group, (unresponsive to sodium load with -15≤ SS-Index ≤+15), and 1 inverse sodium-sensitive group, responsive to sodium by decreasing BP, with SS-Index <-15). We compared the effects of the diets among groups, and correlated autonomic/hemodynamic indices with the SS-Index. After sodium loading, a significant decrease in systemic peripheral resistances, HR, spectral indices of BP modulation, and a significant increase of indices of HR vagal modulation were found in the inverse sodium-sensitive group but not in SS normotensives. Moreover, the highest SS-Indices were associated with the lesser vagal HR decelerations.

Conclusions: Our data suggest that salt sensitivity in white normotensive individuals is associated with impaired vasodilation and altered autonomic response to dietary salt. Such dysfunction may critically contribute to induce a BP response to dietary salt.

Keywords: autonomic function; baroreflex; blood pressure spectral analysis; heart rate variability; peripheral resistance; salt intake; salt‐sensitive.

Figure 1

Brachial measures of blood pressure (BP) and heart rate (HR). Mean (SEM) after low‐salt (open circles) and high‐salt (solid circles) diets in 3 groups of normotensive volunteers, defined on the basis of SS‐Index: a sodium‐sensitive group, SS, with SS‐Index >15 mm Hg/(mmol·day), and 2 sodium‐resistant groups: 1 unresponsive to sodium load, SR, with −15≤ SS‐Index ≤15; and 1 responding to sodium load with a BP decrease, ISS, with SS‐index <−15. ISS indicates inverse sodium sensitive; MAP, mean arterial pressure. The “*” marks significant differences between diets.

Figure 2

Hemodynamic parameters. Mean (SEM) after low‐salt and high‐salt diets in inverse sodium‐sensitive (ISS), sodium‐resistant (SR), and sodium‐sensitive (SS) individuals; SVR indicates systemic vascular resistances. The “*” marks significant differences between diets.

Figure 3

Autonomic indices of HR variability. Left: frequency‐ and complexity‐domain indices of cardiac sympatho/vagal balance (PI LF/HF and α₁); right: frequency‐ and time‐domain indices of vagal modulations of HR (PI HF and NN50+), in inverse sodium‐sensitive (ISS), sodium‐resistant (SR), and sodium‐sensitive (SS) individuals. α₁ indicates short‐term scale coefficient of PI; HF indicates high frequency; HR, heart rate; LF, low frequency; NN50+, number of PI increases per minute larger than 50 ms; PI, pulse interval. Spectral indices are represented in logarithmic scale. The “*” marks significant differences between diets.

Figure 4

Autonomic indices of BP variability. Indices of baroreflex sensitivity (BRS LF and BRS HF) and of sympathetic modulations of vascular tone (diastolic BP VLF and LF) represented in logarithmic scale, in inverse sodium sensitive (ISS), sodium resistant (SR), and sodium sensitive (SS) individuals. BP indicates blood pressure; HF, high frequency; LF, low frequency; VLF, very‐low frequency.

Figure 5

Dependence of SVR, cardiac output, and MAP on SS‐Index. Mean and SEM by quintiles of the SS‐Index distribution, plotted separately after the low‐salt diet (upper panels) and the high‐salt diet (lower panels). SS‐Index range for each quintile is reported on the lower right panel. MAP indicates mean arterial pressure; SVR, systemic vascular resistances.