Effect of salt intake on beat-to-beat blood pressure nonlinear dynamics and entropy in salt-sensitive versus salt-protected rats

Abstract

Blood pressure exhibits substantial short- and long-term variability (BPV). We assessed the hypothesis that the complexity of beat-to-beat BPV will be differentially altered in salt-sensitive hypertensive Dahl rats (SS) versus rats protected from salt-induced hypertension (SSBN13) maintained on high-salt versus low-salt diet. Beat-to-beat systolic and diastolic BP series from nine SS and six SSBN13 rats (http://www.physionet.org) were analyzed following 9 weeks on low salt and repeated after 2 weeks on high salt. BP complexity was quantified by detrended fluctuation analysis (DFA), short- and long-range scaling exponents (αS and αL), sample entropy (SampEn), and traditional standard deviation (SD) and coefficient of variation (CV(%)). Mean systolic and diastolic BP increased on high-salt diet (P < 0.01) particularly for SS rats. SD and CV(%) were similar across groups irrespective of diet. Salt-sensitive and -protected rats exhibited similar complexity indices on low-salt diet. On high salt, (1) SS rats showed increased scaling exponents or smoother, systolic (P = 0.007 [αL]) and diastolic (P = 0.008 [αL]) BP series; (2) salt-protected rats showed lower SampEn (less complex) systolic and diastolic BP (P = 0.046); and (3) compared to protected SSBN13 rats, SS showed higher αL for systolic (P = 0.01) and diastolic (P = 0.005) BP Hypertensive SS rats are more susceptible to high salt with a greater rise in mean BP and reduced complexity. Comparable mean pressures in sensitive and protective rats when on low-salt diet coupled with similar BPV dynamics suggest a protective role of low-salt intake in hypertensive rats. This effect likely reflects better coupling of biologic oscillators.

Keywords: Beat‐to‐beat variability; complexity; detrended fluctuation analysis.

Figure 1

Example of beat‐to‐beat systolic (SBP) and diastolic (DBP) blood pressure time series derived from 2‐min data epoch recordings: (A) salt‐sensitive rat on low‐salt diet; (B) same salt‐sensitive rat on high‐salt diet; (C) protected rat on low‐salt diet; and (D) same protected rat on high‐salt diet.

Figure 2

Log–log plot of DFA‐derived F(n) versus n derived for systolic and diastolic BP time series datasets shown in Figure 1, for example, salt‐sensitive and ‐protected rats while on low‐salt diet compared to high‐salt diet: (A) SBP, salt‐sensitive rat; (B) DBP, same salt‐sensitive rat; (C) SBP, protected rat; and (D) DBP, same protected rat. Log–log plots of all rats are shown in Figures S1–S4.

Figure 3

Effect of high‐salt (HS) diet following low‐salt (LS) diet on entropy and detrended fluctuation properties of the systolic and diastolic BP time series shown for all study salt‐sensitive (n = 9) and ‐protected (n = 6) rats individually: (A) α _S of SBP; (B) α _L of SBP; (C) SampEn of SBP; (D) α _S of DBP; (E) α _L of DBP; and (F) SampEn of DBP. Comparisons were done using the paired t test and the signed‐rank test as appropriate.