Background/Purpose: The prevalence
of hypertension is increased in patients with systemic lupus erythematosus
(SLE).  Sodium
(Na⁺) and potassium (K⁺) intake are modifiable
determinants of blood pressure in the general population and can be estimated
by measuring urinary Na⁺ and K⁺.  Higher Na⁺
and lower K⁺ intake, as well as a higher Na⁺: K⁺
ratio, are associated with elevated blood pressure. However, the contribution
of Na⁺ and K⁺ intake to hypertension in SLE is not
known.  We hypothesized that urinary excretion of Na⁺ and K⁺,
as an estimate of intake, are related to blood pressure in SLE.    Methods:
We
studied 178 patients with SLE and 86 control subjects frequency-matched for
age, sex, and race. First morning urine specimens were collected and urine Na⁺
and K⁺ concentrations measured by flame photometry. The Kawasaki
formula, a validated method that incorporates age, sex, height, weight, and
urinary creatinine, was used to estimate 24 hour urine Na⁺ and K⁺
excretion. Blood pressure was the average of two resting measurements. Fisher’s
exact and Mann-Whitney U tests were used to compare categorical and continuous
variables, respectively. The associations between systolic (SBP) and diastolic
blood pressures (DBP) with estimated 24 hour urinary Na⁺, K⁺,
and Na⁺ : K⁺ ratio were tested using Spearman
correlation and then modeled using linear regression adjusting for age, sex,
and race. Two-sided p values < 0.05 were significant.  

Results: Descriptive
variables including demographics, blood pressure, and urinary values for SLE
patients and controls are shown in Table 1. The estimated 24 hour urinary Na⁺
excretion was similar in SLE patients vs. controls, but the estimated 24 hour
urinary K⁺ excretion was significantly lower in SLE patients
compared to controls. The urinary Na⁺: K⁺ ratio
was higher in SLE patients than in controls, and this difference remained
significant when subjects taking anti-hypertensive drugs, including diuretics,
were excluded. In SLE patients, higher urinary Na⁺: K⁺
ratio was significantly associated with higher SBP [β coefficient (95% CI)
=4.01 (0.57-7.46), p=0.023] and DBP [β coefficient=4.41
(1.71-7.11), p=0.002] after adjustment for age, sex, and race. In
controls, there was no significant association with estimated 24 hour urinary Na⁺,
 K⁺, and Na⁺: K⁺ ratio
and SBP and DBP.   Conclusion:
SLE
patients had significantly lower estimated 24 hour urinary K⁺ and
higher estimated 24 hour urinary Na⁺: K⁺ ratio
than control subjects. The estimated 24 hour urinary Na⁺: K⁺
ratio was significantly associated with SBP and DBP in SLE patients but not in
controls.  Our studies suggest that diets with low Na⁺ and high K⁺
content may reduce the risk of hypertension in SLE patients.

  Table
1. Demographics of SLE cases and controls.