Patients in the Intensive Care Unit often present with low levels of plasma sodium and are therefore often administered high amounts of sodium, both as an additive to intravenous glucose solutions and as a constituent of various drugs and infusion fluids. Recent findings question the benefit of these large quantities of sodium and raise the question whether the individual physician takes the total sodium administration into account when sodium additives are prescribed. It can also be suspected that sodium prescription differs significantly between physicians.

Treatment with intravenous fluids (fluid therapy) is a cornerstone in intensive care. The primary aim of fluid therapy is to optimize the amount of fluid in the various spaces of the body: intravascularly (plasma), intracellularly and interstitially. Secondarily, one tries to optimize the constitution of fluid in plasma, since this is the only fluid space that is available for analysis. Fluid constitution is referring to the physical properties such as pH-value, osmolality, and partly the concentration of different elements solved in plasma (i.e. sodium, potassium, chloride, magnesium, and proteins like albumin). For this reason, there are a lot of different fluids that the physician in the intensive care can choose from. In addition, the physician can also choose to prescribe different additives to adapt these fluids to fulfil the individual patient's needs. Therefore, to prescribe both the amount and constituency of fluid therapy is a key task for ICU-physicians.

One of the most important additives to be prescribed is sodium. Sodium is the molecule (except for water) that has the highest concentration in plasma (normally around 140 mmol/L) and is therefore of utmost importance for plasma osmolality. Sodium concentration in plasma is often decreased in critically ill patients (hyponatremia) which, in turn, is associated with a worse outcome, and thus a normal task for an ICU-physician to attempt to regulate through administering amounts of sodium that are substantially higher than the physiological needs.

Recent findings have revealed that there is a large variation between different physicians when it comes to the amount of fluid the patient receives, a variation that has no scientific basis.

Fluids used in the operating room and in the intensive care often contain large quantities of sodium to create a physiological osmolality. It is thus hypothesized that it is not only the large quantities of fluid that causes a worse outcome in fluid overload, but also the un-physiologically large amount of administered sodium.

The aim of the study is partly to gain knowledge about how sodium prescription is performed in clinical practice in the intensive care, as well as how this administration relates to actual changes in plasma sodium.

Statistical method: Normal distribution will be tested via the Shapiro-Wilks test. All tests of significance are performed two-sided. P-value lower than 0.05 is considered significant. No corrections for multiple comparisons are performed if nothing else is stated.

Missing values: Single missing values are principally not replaced. In exceptional cases, if missing data leads to a whole series of data being misleading, then missing data can be imputed by calculating the mean of two adjacent values, under the condition that this results in a reasonable value.

Inclusion Criteria:

• Adult
• Documented length of stay at the Central Intensive Care Unit (CIVA) at the University Hospital of Linköping of seven (7) days minimum.
• The patient has been discharged from the ICU.
• Body Mass Index (BMI) between 18.0 and 48.0 kg/m2 at admission.

Exclusion Criteria:

• The patient has previously been included in the study in connection with another ICU-admission.
• The patient has a severe hyponatremia (<129 mmol/L) upon arrival to the ICU.
• The patient has a severe hypernatremia (>149 mmol/L) upon arrival to the ICU.
• The patient is severely hyperosmolar (>330 mmol/L) upon arrival to the ICU.
• The patient is severely hyperglycemic (>30 mmol/L) upon arrival to the ICU.
• The patient has an elevated serum urea (>30 mmol/L) upon arrival to the ICU.
• The patient receives any dialysis treatment during the observation time.
• The patient is suspected to have an elevated intracranial pressure during the observation time or is suspected to have other intracranial pathology that is likely to affect sodium homeostasis
• The patient has an intoxication with toxic alcohols as a primary diagnosis.

Critically ill patients with a documented length of stay at the Intensive Care Unit of seven days minimum