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Serum potassium in patients with chronic heart failure: once we make a U-turn where should we go?

There has been an increased focus on the association between serum potassium (Kþ) and mortality over the past few years. Several studies have pointed out a U-shaped relationship with an increased risk of death both below and above the normal range of serum Kþ. While there appears to be agreement that there is a U-shaped relationship between serum Kþ and death, the actual serum Kþ values at which the risk becomes evident appears to vary depending upon whether or not various comorbidities such as chronic kidney disease (CKD), diabetes mellitus (DM), heart failure (HF), and hypertension are or are not present, as well as age. For example, a recent analysis of a relatively large US insurance database showed that the U-shaped relationship of serum Kþ with death was much flatter in normal individuals than in those with comorbidities and age >65 years. However, there remains controversy as to the exact level of serum Kþ in a patient with HF at which the clinician should become concerned and consider altering therapy. Some clinicians become concerned for a patient with HF when the serum Kþ is < 3.5 or > 5.5 mmol/L, whereas others become concerned and consider altering cardiovascular drug therapy at a serum Kþ <4.0 or > 5.0 mmol/L. Investigators from the Danish National registries had previously analyzed the relationship between serum Kþ and death in patients with hypertension and in patients with acute HF, noting a U-shaped relationship. In this issue of European Heart Journal, these investigators now report the relationship between serum Kþ and mortality in patients with chronic HF. They identified 19 549 patients who had a measurement of serum Kþ within a minimum of 30 days after initiating medical therapy with a loop diuretic and a renin angiotensin system inhibitor (RAAS-I). Similar to their study in patients with acute HF, they found that deviations in serum Kþ above and below ‘normal’ were associated with an increased risk of death: a U-shaped relationship. Of particular interest in this relatively large and well characterized group of patients with chronic HF was that the lowest risk for death occurred with a serum Kþ between 4.2–4.4 mmol/L. At a serum Kþ between 3.9 and 4.1 mmol/L, a value many clinicians would believe to be within the normal range, the hazard ratio (HR) for death was 1.29 [confidence interval (CI) 1.08–1.55]. Similarly, a slight increase in serum Kþ between 4.8–5.0 mmol/L, also considered by many clinicians to be within the normal range (or at least a range that would not cause concern), the HR for death was 1.34 (CI 1.10–1.63). At a serum Kþ <3.9 or greater than 5.1 mmol/L there were further increases in the risk of death, reaching a HR of 3.16 (CI 2.43–4.11) for a Kþ between 2.8–3.4 mmol/L and 3.31 (CI 2.61–4.20) for a Kþ between 5.6–7.4 mmol/L. However, it should be pointed out that they provide no information as to whether patients had a reduced or preserved left ventricular ejection fraction. This may be important since the therapeutic options may differ between them (Figure 1). The finding in the present study that patients with chronic HF in whom there is even a slight deviation in serum Kþ <4.2 or >4.4 mmol/L are at increased risk of death should prompt a reconsideration of our current approach to patients with chronic HF. One should no longer feel complacent with a patient with chronic HF and a serum Kþ between 3.5–4.0 mmol/L or between 5.0–5.5 mmol/L. However, while the present study has pointed out the increased risk of both mild hypoand hyperkalaemia, the question as to how we can avoid these levels, what should be done when one encounters these values, and how to blunt this U-shaped relationship is less certain and warrants future evaluation (Figure 1). The risk factors for hypokalaemia are well known and include the use of loop diuretics. While diuretics have not been conclusively shown to reduce cardiovascular death in patients with chronic HF there is general agreement that their use is essential to relieve congestion. Thus, it may be difficult to avoid mild hypokalaemia in a patient with HF. One commonly