Effects of Salt Sensitivity on Neural Cardiovascular Regulation in Essential Hypertension

Abstract

Abstract-Salt-sensitive hypertensive subjects, as defined by conventional categorical classification, exhibit alterations of autonomic cardiovascular control. The aim of our study was to explore whether, in hypertensive subjects, the degree of autonomic dysfunction and the level of salt sensitivity are correlated even when the latter is only mildly elevated and displays under-threshold values. Salt sensitivity of 34 essential hypertensive subjects was assessed on a continuous basis by the salt sensitivity index after low-and high-sodium diet. Beat-by-beat finger blood pressure was recorded after each diet period. Autonomic cardiovascular control was evaluated by spectral analysis of blood pressure and pulse interval and by assessment of spontaneous baroreflex sensitivity (sequence technique). Salt sensitivity and baroreflex sensitivity showed a negative relationship during low and high sodium intake, starting from low values of the salt sensitivity index. All spectral indexes of pulse interval, except the ratio between low-and high-frequency powers, were inversely related to salt sensitivity index after high sodium intake. In subjects with lower salt sensitivity, baroreflex sensitivity and pulse interval power in the high-frequency band were higher after high sodium intake than after low sodium intake. In contrast, subjects with a higher salt sensitivity index showed lower values of baroreflex sensitivity and pulse interval power in the high-frequency band, uninfluenced by salt intake. Our results provide the first demonstration of an impairment of parasympathetic cardiac control in parallel with the increase in the degree of salt sensitivity, also in subjects who were not ranked as salt-sensitive by the conventional categorical classification. Key Words: autonomic nervous system Ⅲ heart rate Ⅲ baroreflex Ⅲ blood pressure Ⅲ sodium, dietary T he higher rate of cardiovascular events occurring in salt-sensitive subjects, either normotensive or hypertensive, has led to the suggestion that a salt sensitivity condition might represent an independent cardiovascular risk factor. 1,2 An enhanced sympathetic cardiovascular drive has been suggested to play a role in determining the increased cardiovascular risk associated with salt sensitivity. In fact, an impaired sympathetic inhibition has been described during high sodium intake in salt-sensitive hypertensive humans, 3 and a deranged reflex cardiovascular control has been proposed as one of the responsible mechanisms. 4,5 Usually, salt sensitivity is considered a categorical parameter, with salt-sensitive individuals being defined as those with a difference in mean arterial pressure (MAP) between low-and high-sodium diet Ͼ10%, and salt-resistant subjects those in whom MAP does not increase or shows an increase Ͻ5% under sodium loading. 6 -8 However, adoption of this categorical approach does not allow exploring whether an impairment of cardiovascular autonomic regulation may occur also in subjects with salt-related blood pressure (BP) changes smaller than those defining...

Hemodynamic and Autonomic Response to Different Salt Intakes in Normotensive Individuals

“…The effects of sodium diets we reported in normotensive individuals are similar to changes described in hypertensive patients 4, 5. Increased BRS, increased cardiac vagal index, and decreased HR and cardiac sympatho/vagal balance have been reported in hypertensive patients after a high‐salt diet, with these changes being blunted in those hypertensive patients with the more severe degree of sodium sensitivity.…”

“…Sodium loading/depletion maneuvers may unveil alterations in cardiovascular control associated with sodium sensitivity possibly involved in the increased rate of cardiovascular events. In this regard, while it has been shown that salt‐sensitive hypertensive patients respond to sodium loading with either a blunted sympathetic deactivation3 or an impaired cardiac parasympathetic activation,4, 5 limited evidence of an altered autonomic cardiovascular modulation is available in normotensive subjects.…”

Dietary sodium effects on heart rate variability in salt sensitivity of blood pressure

“…One previous study with hypertensive patients showed that high salt intake increased the HF and decreased the LF/HF components of HRV only in SI subjects (Minami, Kawano, Ishimitsu & Takishita, 1997). Moreover, SS patients with mild hypertension showed an impaired baroreceptor reflex after high salt intake (Coruzzi, et al, 2005), indicating a deficit in cardiac parasympathetic control. Structural changes in the cardiovascular system associated with the hypertensive adaptation could contribute to quantitative or qualitative differences from the HRV responses to salt observed in healthy individuals.…”

“…Medical personnel performed a physical examination, a comprehensive medical history, electrocardiogram (EKG) and obtained blood and urine samples to ensure that the participants met the study entry criteria. Each qualifying subject participated in a 12-day experiment consisting of six days on a low sodium diet followed immediately by six days on a high sodium diet, similar to those in previous studies laboratories (Sullivan, Prewitt, Ratts, Josephs, & Connor 1988;Anderson et al, 1998;Coruzzi, et al, 2005). Participants were given low sodium-low potassium meals in insulated containers, and provided with dietary logs to record food intake to augment adherence to the diet.…”

Neurogenic Mechanisms and Salt Sensitivity

“…In addition, the risk for the occurrence of cardiovascular events is Ͼ3-fold higher in saltsensitive than in salt-resistant hypertensive subjects. 3 In this issue of Hypertension, Coruzzi et al 4 show that in individuals with essential hypertension, even modest levels of salt sensitivity are associated with alterations in autonomic cardiovascular control. Their study strongly supports the idea that neurogenic mechanisms are involved in the pathophysiology of salt sensitivity in essential hypertension.…”