Long-term changes in dysnatremia incidence in the ICU: a shift from hyponatremia to hypernatremia

Deranged plasma sodium concentrations expose all cells to hypotonic or hypertonic stress. Clinical manifestations of dysnatremia are primarily neurological and rapid changes in plasma sodium in either direction can cause severe, permanent, and sometimes even lethal brain injury [1]. The reported prevalence of dysnatremia in the intensive care unit (ICU) ranges between 6.9 and 17.7 % and varies according to the time of onset (i.e., on admission or later during ICU stay), the threshold for diagnosis, and the population being assessed [2].

Patients in the ICU are at risk of developing both hyponatremia and hypernatremia. Critical illness may result in increased or reduced activity of the antidiuretic hormone [3, 4]. Additional factors that predispose to hypernatremia include a reduced urinary concentrating ability, the inability to express thirst, no free access to water, and increased insensible losses [5, 6]. In addition to critical illness per se, factors contributing to hyponatremia include excess use of hypotonic fluids and drugs stimulating antidiuretic hormone secretion [7].

The severity of hyponatremia on ICU admission is a demonstrated predictor of mortality [8]. Even slightly abnormal sodium levels on ICU admission are independently associated with poor outcome [2, 9]. Although ICU-acquired hyponatremia is less prevalent, it is also associated with an increased risk of hospital mortality [5]. ICU-acquired hypernatremia is also an independent risk factor for mortality and associated with increased ICU length of stay [10–13]. The relation between sodium derangement and mortality has been reported in medical, surgical, mixed, cardiac, cardiovascular surgery, trauma, and neurological ICUs [5, 10–15]. Finally, comparable to variability in serum glucose [16] or potassium [17], the magnitude of changes in sodium has also been associated with a higher risk of death in ICU patients [14, 15].

Based on our impression that hypernatremia has nowadays become more prevalent than hyponatremia in the ICU, we hypothesized that a shift in the incidence of hyponatremia and hypernatremia occurred during the past two decades. Therefore, the aim of this study was to analyze the long-term changes in the incidences of hyponatremia and hypernatremia in the ICU. Furthermore, we studied the association between dysnatremia and mortality.

This retrospective study was performed in two cohorts of adult ICU patients obtained from the two largest ICUs in The Netherlands, including the University Medical Center Groningen (44 bed unit) and the Erasmus Medical Center (48 bed unit). From the ICU of the University Medical Center in Groningen, all patients admitted between 1992 and 2011 were analyzed, and from the ICU of the Erasmus Medical Center in Rotterdam, all patients admitted between 1998 and 2012 were analyzed. The 21-year study period was divided into five periods to detect shift in time: 1992–1996, 1997–2000, 2001–2004, 2005–2008, and 2009–2012. Data on the type of admission (surgical, medical, etc.) were available for patients from Groningen but not from Rotterdam. Patients aged <15 years were excluded. Mortality was determined at 90 days after ICU admission. The anonymized data analysis in this study was performed in accordance with the guidelines and Dutch legislation, and it was approved by the medical ethical committees of our institutions (Medisch Ethische Commissie, UMC Groningen, METc 2014.264, MEC Erasmus MC MEC-2015-401). Since this concerned a retrospective study on routinely collected data, informed consent was not required by the ethical committees.

In this large retrospective dual-center study, we observed a consistent and marked shift in the incidence of dysnatremia from hyponatremia to hypernatremia over a two-decade observation period. The incidence of hyponatremia nearly halved over the study period, whereas the incidence of hypernatremia almost doubled. The trend toward higher serum sodium levels was consistently observed in both centers and seemed to be more important in Groningen center. To our knowledge, this clear shift from hyponatremia to hypernatremia has not been reported before. The increased number of studies that address hypernatremia instead of hyponatremia may reflect increased awareness of this problem in other centers as well [5, 9, 10, 13–15]. Our observation that most hypernatremia typically developed after ICU admission strongly suggests that changes in therapy are involved in this trend.

Although Fig. 1 shows that hypernatremia at ICU admission has also increased over time, it is in particular the increase of hypernatremia after ICU admission that is striking. Although we do not have the data to evaluate the etiology of the dysnatremia, we do want to speculate on several factors which might have played a role. Two treatment-related factors that may have contributed to the shift from hyponatremia to hypernatremia are the less liberal use of intravenous fluids in combination with wider use of diuretic treatment and the increased use of steroids and in particular hydrocortisone. In 2006 the ARDS clinical network performed a study comparing a conservative strategy (mostly accomplished by administration of furosemide) with a liberal strategy of fluid management in patients with acute lung injury [18]. This trial provided evidence that more restrictive fluid management in critically ill patients results in improved lung function and shortened duration of mechanical ventilation and intensive care stay [19]. Fluid restriction may have contributed indirectly to the rising incidence of hypernatremia. Nevertheless, we have shown previously that more than one-third of the patients with ICU-acquired hypernatremia are actually still volume overloaded [11]. This phenomenon is explained by the combination of large volumes of (approximately) isotonic fluids and a reduced urinary concentrating ability. In line with these observations, it was recently shown that NaCl 0.9 % used to dilute drugs and keep catheters open contributes to the occurrence of ICU-acquired hypernatremia [20]. In the group of patients with ICU-acquired hypernatremia, the plasma creatinine and dose of furosemide were also higher, again suggesting compromised urinary concentrating ability as an important contributor to hypernatremia.

The role of hydrocortisone in the treatment of septic shock has evolved over the last two decades. In 2002 a French multicenter RCT of patients in vasopressor-unresponsive septic shock showed significant shock reversal and reduction of mortality rate in patients with relative adrenal insufficiency [21]. This led to a more prominent role of hydrocortisone in guidelines for patients with septic shock. In 2008 the CORTICUS trial, a large European multicenter study failed to show a mortality benefit with steroid therapy [22]. Nevertheless, a recent study in a university ICU in The Netherlands showed that hydrocortisone was the seventh most frequently administered drug [20]. Even after several systematic analyses, the role of hydrocortisone in septic shock is still not settled [23, 24], and a systematic review identified a greater risk of hypernatremia with the use of corticosteroids [23]. We have no data on renal function and the use of renal replacement therapy (RRT) over this period, but RRT did not change in this respect, namely continuous veno-venous hemofiltration with a substitution fluid with a sodium content of 140 mmol/L. The widespread use of dopamine in past decades may have helped to avoid hypernatremia, since dopamine is a natriuretic agent.

The clinical implication of our observations and those of others is that early identification of hypernatremia or preferably impending hypernatremia may help to reduce the incidence, severity, and duration of hypernatremia. It has been proposed to consider the development of hypernatremia during ICU stay as an indicator of quality of care, because ICU patients depend fully on the competence of the medical staff for prescribing fluids and these patients are frequently monitored with sampling of blood [25]. The impact from dysnatremia on morbidity and mortality leads to extensive burden on healthcare resources [26]. It is sobering to note that this proposal to prevent hypernatremia [25] was made more than 15 years ago, and yet its incidence has only increased. Nevertheless, we still believe it may be possible to achieve the best of both worlds, i.e., combine the low hypernatremia levels observed in the 1990s and the low hyponatremia levels observed today. Since hypernatremia is a condition that takes several days to develop and also takes a relatively long time to correct (Fig. 1), prevention would be the most desirable strategy. Our own data (Additional file 1: Table 1) show that although the total infused volume decreased, unfortunately the sodium concentration of infused fluids has only increased over the years. A strategy of timely administration of infusion fluids with lower sodium content in the face of imminent hypernatremia seems reasonable to pursue. In this regard, the trend to use balanced fluids with lower sodium concentrations than the 154 mmol/L in NaCl 0.9 % may help [27, 28]. Since patients more often arrive at the ICU with hypernatremia, this is also relevant for the emergency department and operating room. When other risk factors for hypernatremia such as the administration of furosemide [20] or hydrocortisone are present, an even earlier switch to infusions with minimal sodium concentrations may be desirable. Monitoring of sodium concentrations is facilitated with modern point-of-care equipment. In the slipstream of glucose control, we have demonstrated that careful computer-guided potassium control is feasible with a clear reduction of abnormal potassium levels [17]. In fact, integration of potassium regulation with glucose control was very effective with marginal extra costs or time spent [29]. But implementing computerized sodium control will be considerably more challenging since more variables have to be taken into account. Although no study has shown that treatment of dysnatremia reduces mortality, a large multicenter observational study recently showed that successful rapid correction of dysnatremia was independently associated with survival [30]. The 28-day mortality in patients whose dysnatremia was corrected within 48 h was not significantly different from that in patients with normal serum sodium concentrations on ICU admission [30]. This finding suggests that the association between dysnatremia and mortality may be causal and could be improved by timely correction, although conclusive evidence should come from a randomized trial.

This study has a number of limitations. First, the identified trends in our two ICUs (large ICUs in university hospitals in The Netherlands) may not apply to other types of ICUs. Second, in addition to treatment-related factors, patient-related factors may also have played a role. For example, admissions for traumatic brain injury may have changed over the years, but this was not clearly reflected in our analysis of type of ICU admissions (Table 1). Also hypernatremia in patients with severe traumatic brain injury typically develops within the first three days [31] and not at the considerably later time as shown in Fig. 1. We do not have the data on shifts in medication usage in our two centers to support out speculation on the etiology of the dysnatremia. Data on sodium concentration in mmol/L of the administered infusions in the Groningen ICU show a significant increase over time (Additional file 1: Table 1). Data on hyperosmolar dye contrast administration, the use of citrate anticoagulation on RRT, or (par)enteral nutrition were also not analyzed. We were also not able to identify complications of dysnatremia. Likewise we could not reliably determine how often a disastrous complication such as central pontine myelinolysis occurred [32]. Finally, historical comparisons are vulnerable to all kinds of system changes that inevitably happen over time. This may be the case for sodium measurement. One study showed that the discrepancy between the direct assay and the indirect assay (which includes a dilution step) became larger as plasma albumin decreased [33], [34]. Because patients in the ICU are often hypoalbuminemic, this may predispose to pseudohypernatremia. However, the point-of-care measurements that use a direct sodium assay became more prevalent during the study period. Thus, pseudohypernatremia may have been more common in the past, making the incidence of true hypernatremia in earlier time periods even lower.

In two large cohorts of ICU patients, we found a shift in the incidence of dysnatremias. The incidence of hyponatremia decreased over the study period, whereas the incidence of hypernatremia increased. We suggest this shift is related to the increased use of diuretics and hydrocortisone. As ICU-acquired hypernatremia is often iatrogenic, it thus may be—to an important extent—preventable, and its incidence may be considered as a quality indicator. The relation of dysnatremia with mortality remained unchanged over the 21-year study period; therefore, we recommend that further study should be focused on interventions to prevent the occurrence of dysnatremias during ICU stay.