European Journal of Cardiovascular Medicine

Hyponatremia, defined as a serum sodium level below 135 mEq/L, is the most common electrolyte disorder, especially in critically ill ICU patients. It results from an imbalance between water intake and excretion, influenced by underlying diseases, medications, and critical illness factors.1,2 Sodium and water balance is mainly regulated by antidiuretic hormone (ADH), renal function, and hormonal mechanisms. Disruptions in these systems can lead to hypovolemic, euvolemic, or hypervolemic hyponatremia, each with distinct pathophysiological mechanisms and clinical implications.3,4

In the Medicine ICU, hyponatremia is particularly common due to the complex interplay of critical illnesses that disturb normal fluid and electrolyte balance. Conditions such as sepsis, heart failure, liver cirrhosis, renal disease, and endocrine disorders like adrenal insufficiency, hypothyroidism, and SIADH are frequent contributors. Iatrogenic causes, including the use of hypotonic fluids, diuretics, and other medications, further aggravate this imbalance.5,6 Clinically, hyponatremia has serious implications, ranging from mild cognitive changes to severe neurological complications like seizures and coma, and is associated with prolonged ICU stay, increased ventilator use, and higher mortality. The association of hyponatremia with adverse clinical outcomes underscores the importance of timely identification, accurate classification, and appropriate management strategies to reduce complications.7-9

Although numerous studies have examined hyponatremia in ICU settings, data from Indian Medicine ICUs remain limited, especially those assessing both at-admission and in-hospital cases with respect to etiology, severity, and outcomes. Hence, this study aims to determine the prevalence, severity, and etiological profile of hyponatremia in Medicine ICU (MICU) patients and to assess its association with clinical outcomes.

After obtaining Institutional Ethical Committee approval, this hospital-based prospective observational study was conducted in the Medicine Intensive Care Unit (ICU) of a tertiary care centre. A total of 177 critically ill patients aged 18 years and above admitted to the Medicine ICU were enrolled. Patients with hypernatremia (serum sodium >145 mEq/L) and those with pseudohyponatremia due to hyperlipidemia or paraproteinemia were excluded.

Upon admission, demographic details such as age, gender, and socioeconomic status were recorded. Informed written consent was obtained from both the patient and their immediate relative or legally authorized representative after explaining the nature, purpose, and procedures of the study in their preferred language. A detailed medical history was collected, focusing on presenting complaints and their duration, past medical history including known comorbidities, detailed drug history (particularly the use of thiazide diuretics and selective serotonin reuptake inhibitors), and any recent hospitalizations, surgeries, or acute medical events. A thorough clinical examination was conducted, including assessment of the patient’s volume status. Hyponatremic patients were classified into three groups based on clinical and laboratory findings: Euvolemic, Hypovolemic, Hypervolemic. All patients underwent relevant laboratory investigations, including complete blood count, fasting blood glucose, serum electrolytes, serum urea and creatinine, liver function tests, and lipid profile. Patients with hyponatremia were further categorized according to the severity of serum sodium reduction as follows:

1.      Mild: 130–134 mEq/L

2.      Moderate: 125–129 mEq/L

3.      Severe: <125 mEq/L

Outcome parameters recorded during the ICU stay included the presence of acute renal failure, the need for mechanical ventilation and duration of ventilator support, and the final outcome in terms of discharge or mortality.

Statistical Analysis

The data were tabulated in Microsoft excel and analysed with SPSS V.24 software. The continuous variables were presented with mean and standard deviation. The categorical variables were presented with frequency and percentage. Chi square test and One way ANOVA test were used for the statistical analysis. The p value ≤0.05 was considered statistically significant.

The study included 177 patients admitted to the Medicine ICU. Most patients were aged 41–50 years (27.7%), followed by 51–60 years (26.0%), with a male predominance (52.5%). The mean age was 52.99 ± 12.60 years. The majority belonged to the upper-lower (40.1%) and lower (35.6%) socioeconomic classes. Based on BMI, overweight (33.9%) and obese (24.9%) individuals were most common, with a mean BMI of 25.71 ± 14.5 kg/m². Among comorbidities, diabetes mellitus (31.1%) and hypertension (24.9%) were most frequent, followed by chronic kidney disease (15.8%) and ischemic heart disease (9.6%), (Table 1).

Table 1. Socio-demographic profile and comorbidities of the patients

Vomiting was the most common symptom (53; 29.9%), followed by fever (52; 29.4%), breathlessness (48; 27.1%), altered sensorium (39; 22.0%), and chest pain in 21 individuals (11.9%) as depicted in figure 1.

Acute Myocardial Infarction (Acute MI) and Acute Kidney Injury (AKI) were the most prevalent diagnoses, each accounting for 17 cases (9.6%), followed by Heart Failure and Lower Respiratory Tract Infection (LRTI), both with 15 cases (8.5%). Diabetic Ketoacidosis (DKA) affects 12 cases (6.8%), while Sepsis and LRTI each represent 11 cases (6.2%), (Figure 2).

The most commonly given treatments were a combination of Fluids and Antibiotics (10.7%), Antibiotics with O₂ therapy (10.2%), and Antibiotics with Fluids (10.2%). Other frequent treatments included O₂ therapy with Antibiotics (6.8%), Insulin with Fluids (4.0%), and PAM with Atropine (4.0%). Less frequent treatments involved various combinations of inotropes, diuretics, anticoagulants, antiplatelets, and hemodialysis, each accounting for smaller percentages (0.6–1.1%).

The mean serum sodium was 134.86 ± 7.67 mEq/L, and urine sodium was 57.35 ± 34.21 mEq/L. Mean plasma glucose showed high variability (217.24 ± 331.01 mg/dL). Serum urea and creatinine levels were 82.32 ± 57.89 mg/dL and 3.04 ± 10.1 mg/dL, respectively. The mean serum albumin was 3.38 ± 0.62 g/dL. Hematological parameters showed a WBC count of 16.21 ± 4.27, hemoglobin 10.57 ± 2.04 g/dL, and platelet count 271.14 ± 72.94. Lipid profile revealed total cholesterol 138.02 ± 34.13 mg/dL, triglycerides 92.96 ± 34.71 mg/dL, LDL 82.77 ± 23.62 mg/dL, and HDL 38.87 ± 10.48 mg/d.

Among 177 patients, hyponatremia was present in 35.6% and absent in 64.4%. Regarding severity, 64.41% had normal sodium levels, while 6.21%, 11.30%, and 18.08% had mild, moderate, and severe hyponatremia, respectively. Based on volume status, 50.79% were hypervolemic, 38.10% hypovolemic, and 11.11% euvolemic, (Figure 3).

Table 2 shows that the most common causes of hyponatremia were congestive cardiac failure (25.4%), chronic kidney disease (20.6%), and gastrointestinal loss (14.3%). Other less frequent causes included translocational hyponatremia (12.7%), CLD, and cerebral salt wasting (7.9% each).

Among 177 patients, 87 (49.15%) required ventilator support, while 90 (50.85%) did not. The mean duration of ventilation was 8.59 ± 2.86 days. Of the total participants, 139 (78.5%) were discharged, and 38 (21.5%) expired.

Table 3 shows that hyponatremia was more frequent among expired patients (44.7%) compared to discharged ones (30.9%), though this was not statistically significant (p=0.111). However, the severity of hyponatremia showed a significant association with outcome (p=0.048), with a higher mortality observed in patients having severe hyponatremia (56.3%).

The mean ventilator days for normal sodium levels was 8.08 ± 2.630, for mild hyponatremia was 9.00 ± 3.225, for moderate hyponatremia was 7.22 ± 3.232, and for severe hyponatremia was 10.53 ± 2.458. The P-value of 0.004 indicates a statistically significant association between the severity of hyponatremia and ventilator days.

In the present study, the mean age of patients was 52.99 years, with most of the patients aged 41–60 years, differing from Ghuge DM et al10, who reported hyponatremia predominantly in individuals over 70 years. Gender distribution was nearly equal, with a slight male predominance (52.5%), similar to Ghuge et al10, who observed a male predominance of 63%. The predominance of overweight and obese patients in our study correlated with a higher frequency of congestive cardiac failure and hypertension, in contrast to Ghuge et al10, where gastrointestinal losses were more common, and Kute et al11, where SIADH was a major cause. Similar to Shukla A et al12, current study found a strong association between hyponatremia and chronic comorbidities such as cardiovascular disease and diabetes.

The most common symptoms in the present study were vomiting, fever, and breathlessness which is comparable with the study done by Kute et al11, where vomiting was the most common precipitating factor (48.4%). Neurological manifestations such as altered sensorium and seizures in our study parallel the abnormal behavior and drowsiness described in their series, reinforcing the importance of recognizing central nervous system symptoms as potential indicators of severe hyponatremia.11

The biochemical profile in the present study revealed a mean serum sodium just below the normal range and relatively high urine sodium, suggestive of renal sodium losses or SIADH in a subset. In Ghuge et al10, SIADH was diagnosed in 13% of cases, and in Kute et al11, it was the leading etiology (66.4%). The disparity between our lower SIADH proportion and their higher figures could reflect differences in diagnostic approach, particularly in applying strict criteria such as those of Bartter and Schwartz, and/or the prevalence of precipitating factors like infections and strokes in their settings.

Hyponatremia prevalence in our ICU population was 35.6%, which is higher than the 20.59% reported by Parajuli S. et al13 and the 16.2% incidence of ICU-acquired hyponatremia in the study by Jae Kyeom Sim JK et al.14 The higher prevalence in present study may be attributable to the inclusion of both at-admission and ICU-acquired cases, as well as the predominance of conditions such as congestive cardiac failure and chronic kidney disease that predispose patients to sodium imbalance. Sim JK et al14 also demonstrated that increased net positive fluid balance in the first three days of ICU admission was strongly associated with the development of hyponatremia, a finding consistent with our observation that hypervolemia was the most common volume status among hyponatremic patients. Volume status analysis revealed that hypervolemic hyponatremia was the most common type, contrasting with Ghuge et al.¹⁰, who reported euvolemia as predominant and hypovolemia as second most common. This difference likely reflects the higher proportion of cardiac and renal cases in our ICU, compared to gastrointestinal and neurological causes in their study.

Outcome analysis showed a mortality rate of 21.5% in current study, comparable to Kute et al.¹¹ (20%), but higher than Ghuge et al.¹⁰ (10%) and Shukla et al.¹² (3.7%). A significant association was observed between severity of hyponatremia and mortality, unlike Kute et al11, who found no such link. This variation may relate to differences in sample size, illness severity, or onset of hyponatremia. Severe hyponatremia was also linked to longer ventilator duration, reflecting greater neurological and respiratory compromise. Overall, while the epidemiology of hyponatremia varies across ICUs depending on population and management strategies, it remains a common and clinically significant condition, where severity and underlying etiology are key determinants of outcome.

The present study has several limitations which includes- small sample size and single-center design may limit the generalizability of the findings. Referral bias due to the tertiary care setting could have resulted in a higher proportion of severe cases. Being observational in nature, the study can identify associations but cannot establish causality. Some diagnostic variability, particularly in classifying SIADH, may have introduced bias. Additionally, the study focused only on short-term ICU outcomes without long-term follow-up, and potential confounding factors such as other electrolyte imbalances or variations in fluid therapy were not fully controlled.

In this study, hyponatremia emerged as a common electrolyte disturbance in patients admitted to the Medicine ICU, affecting nearly one-third of the cohort, with a notable proportion presenting in the severe range. Hypervolemic hyponatremia was the most frequent presentation, most often linked to congestive cardiac failure and chronic kidney disease, followed by hypovolemic and euvolemic states. Although the overall presence of hyponatremia was not significantly associated with mortality, increasing severity was strongly correlated with higher mortality rates and longer durations of mechanical ventilation. These findings highlight the need for early detection, precise evaluation of volume status, and etiology-specific management strategies in critically ill patients, as the severity of hyponatremia—rather than its occurrence alone—appears to be a critical prognostic factor in the ICU setting.

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