High dietary sodium intake is a risk factor for hypertension, and heart rate variability (HRV) is decreased in hypertension. Effects of dietary sodium intake on HRV of normotensive persons have not, however, been investigated to date. The present study examined effects of low and high sodium diets on blood pressure, heart rate, and HRV in 36 healthy, normotensive women, ages 40-70. Each was placed on a low sodium diet for six days followed by a high sodium diet for six days. High salt diet increased mean systolic blood pressure, decreased heart rate, and increased high frequency HRV (HF). Cardiac vagal tone, estimated at baseline from heart period and a time domain estimate of respiratory sinus arrhythmia, was higher in salt-sensitive than salt-insensitive subjects. The finding of increased vagal tone in normotensive persons on high salt intake indicates that dietary sodium status should be considered in behavioral studies of HRV.
Index termsblood pressure; heart rate variability; sodium Behavioral interactions can influence the development of cardiovascular disease via effects on autonomic regulation of the heart. Heart rate variability (HRV) is a noninvasive window into autonomic control of cardiac function, and can be analyzed via time and frequency domain measures (Task Force of the European Society, 1996). Power spectral density methodology (Kay & Marple, 1981) identifies periodic components of HRV and estimates their frequency and power. The frequency domain includes a low frequency component (LF), which reflects a combination of sympathetic and parasympathetic influences, and a high frequency component (HF), which reflects parasympathetic influence (Berntson, Cacioppo & Quigley 1993). An estimate of sympathovagal balance is derived from the ratio of low frequency to high frequency power (LF/HF).Some forms of cardiovascular disease, including hypertension are characterized by decreased HRV (Mortara, et al., 1994;Piccirillo, et al. 1996;Stein & Kleiger, 1999;Virtanen, Jula, Kuusela, Helenius & Voipio-Pulkki, 2003;Lucini, De Vede, Parati & Pagani, 2005). Job strain and low decision latitude have also been associated with decreased HF (e.g. Gallo, Bogart, Vranceanu & Wait, 2004;Collins, Karasek & Costas, 2005).Several studies have shown that behavioral procedures that increase blood pressure and heart rate acutely also alter HRV consistent with decreases in parasympathetic activity (Madden & Savard, 1995;Reims et al, 2005;Guasti et al, 2005). In addition, decreases in HF of (Falkner & Kushner, 1990) found that mental stress generated larger increases in blood pressure and heart rate in SS than SI subjects. These studies also showed that salt-sensitivity was associated with greater anxiety and irritability (Deter, et al., 2006) and reduced vagal activity in response to mental stress (Buchholz et al 2003). The potential importance of the environment in cardiovascular response to salt-sensitivity was investigated in a study with salt-loaded medical students, who showed larger increases in resting blood ...