Low normal sodium a cardiovascular risk

Serum sodium levels are influenced by a number of factors linked to cardiovascular health including dysglycemia (blood glucose disorders), hypothyroid, adrenal dysregulation and others. Research just published in Nutrition, Metabolism & Cardiovascular Diseases identifies low serum sodium within the normal range as a risk predictor for cardiovascular disease and stroke. The authors state:

"Hyponatremia, usually defined as serum sodium concentration <136 mEq/L, is one of the most common electrolyte abnormalities observed in hospitalised patients and in patients with chronic kidney disease (CKD), coronary heart disease (CHD) and heart failure (HF). Several clinical and epidemiological studies have shown hyponatremia to be associated with increased total mortality in these patients. More recently, attention has turned to the possibility that mild hyponatremia, may be associated with adverse outcomes in the general population...in the three population studies that have examined the relationship between hyponatremia and mortality in community based subjects, there is evidence that hyponatremia is associated with increased mortality and even a level of sodium concentration in the lower normal range (serum sodium 135-137 mEq/L), a level usually considered benign, is associated with increased mortality."

Less is known about the association between serum sodium and potassium and the risk of cardiovascular disease and stroke in older men withoutmthese disorders. So the authors examined data for 3099 men aged 60-79 years without a history of cardiovascular disease for an average 11 years. During that time there were 528 major CVD events (fatal coronary heart disease [CHD] and non-fatal MI, stroke and CVD death) and 873 total deaths.

Mildly low sodium is not benign

 Although no association was seen between serum potassium and cardiovascular disease, low serum sodium within the normal range as well as high serum sodium is associated with increased risk of stroke and cardiovascular disease mortality:

"A U shaped relationship was seen between serum sodium concentration and major CVD events and mortality. Hyponatremia (<136 mEq/L) and low sodium within the normal range (136-138 mEq/L) showed significantly increased risk of major CVD events and total mortality compared to men within the upper normal range (139-143 mEq/L) after adjustment for a wide range of confounders and traditional risk factors [adjusted HRs 1.55 and 1.40 for major CVD events respectively and 1.30 and 1.30 respectively for total mortality]. Hyponatremia was associated with inflammation, NT-proBNP, low muscle mass and alkaline phosphatase; these factors contributed to the increased total mortality associated with hyponatremia but did not explain the increased risk of CVD events associated with hyponatremia or low normal sodium concentration. Hypernatremia (>145 mEq/L) was associated with significantly increased risk of CVD events and mortality due to CVD causes."

In other words, both low normal and high sodium are a significant risk factor for cardiovascular disease and mortality.

Clinical Implications

Practitioner's should bear in mind the authors' conclusions and not dismiss low normal serum sodium:

"Hyponatremia and hypernatremia are both associated with increased risk of CVD incidence and mortality. Low sodium within the normal range is associated with significantly increased CVD events and total mortality in older men without major CVD or HF even in the absence of diuretic use and renal dysfunction. The data lends further evidence to the suggestion that the presence of mild hyponatremia is not benign. The findings may have important implications for the monitoring of sodium levels in clinical practice in older adults. The presence of mild hyponatremia in the absence of known causes such as renal dysfunction and diuretics may warrant further investigation in these men to assess CVD risk factors or possible underlying ill-health such as chronic inflammation. Further large studies are required to confirm and elucidate the nature of the association between low normal sodium and risk of incident CVD."