Free water deficit and hypernatremia: understanding the links and the issues
The dangers of poor hydration or uncontrolled dehydration are not always known. We often talk about the lack of electrolytes or the lack of water and their consequences, but what happens when the balance between the amount of water in our body and the minerals is not good? In this article, we will discuss the topic of hypernatremia , which is the consequence of too high a concentration of sodium in the body during dehydration or poor electrolyte intake.
1. How to calculate water deficit in hypernatremia?
Calculating your free water deficit is an indicator of how much free water we need to consume to restore balance with sodium.
1.1 Formula for calculating free water deficit:
The calculation is given by the following formula:
Free water deficit = (total body water ) x { ( current serum sodium/target serum sodium ) - 1 }
Total body water depends on a person's age , gender and body fat level . To put it simply, it represents 60% of a man's weight, 50% of a woman's and 50% of an elderly person's. For example, a 70 kg man's total body water will represent 70×0.6=42 L.
The current natremia is in fact the measured concentration of sodium in the blood plasma (in mmol/liter).
The target natremia is the objective to be achieved and is set at 140 mmol/liter. Beyond this level, hypernatremia becomes increasingly serious and requires even more care.
1.2 Calculation example:
For a 70 kg man with a current natremia of 155 mmol/liter:
Total body water is equal to 42 liters.
Free water deficit = 42 × 155 140 - 1 = 42 × 0.107 = 4.5 liters.
This man therefore has a free water deficit of 4.5 liters.
2. What is hypernatremia?
2.1 Relationship between hypernatremia and free water deficit
First of all, hypernatremia is the name given to the phenomenon that occurs when the sodium concentration in the blood is too high. The problem is that hypernatremia (sodium concentration in the blood) increases our plasma osmolarity (number of active particles: sodium, potassium, sugar for example ...) and this causes intracellular dehydration .
When we tried to calculate our free water deficit, it is actually the amount of water that is needed for the sodium concentration to return to normal.
3. Causes of hypernatremia
Hypernatremia can have several forms, these are defined according to the resulting extracellular volume. There are three possibilities: either it decreases, or it remains constant, or it increases.
3.1 Hypovolemic hypernatremia
Hypovolemic hypernatremia is caused by a loss of free water greater than sodium, which increases the sodium concentration and decreases the extracellular volume. In fact, if we have a balance between water and sodium but we lose more water than sodium, then its concentration increases, this creates hypernatremia.
The main causes are: diarrhea , vomiting , excessive sweating or even diabetes.
3.2 Euvolemic hypernatremia
Secondly, we have euvolemic hypernatremia . More simply, this occurs when we lose only water and not sodium, this will once again increase its concentration, but while maintaining a stable extracellular volume. It can be the consequence of excessive sweating , hyperventilation or even renal failure.
3.3 Hypervolemic hypernatremia
Now let's look at a third case, hypervolemic hypernatremia . This is the fact of having too much sodium retention compared to water, which will then cause hypernatremia. This phenomenon, which is due to too much sodium intake in the diet, will increase the extracellular volume .
4. Symptoms of hypernatremia
The symptoms of hypernatremia are related to excessive osmolarity in the blood plasma and intracellular dehydration. They are quite numerous and depend on the different types of imbalances that we mentioned earlier:
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Additionally, neurological symptoms may also appear. So, in addition to marked thirst, you may experience fatigue, irritability, headaches , muscle weakness, and disorientation. These symptoms tend to worsen as hypernatremia progresses.
5. Diagnosis of hypernatremia
To diagnose hypernatremia, the best is clinical assessment, with measurement of plasma natremia to know your sodium concentration. To give you an idea:
A concentration between 146 and 150 mmol/L corresponds to mild hypernatremia,
Between 151 and 159 mmol/L, it becomes moderate
And if it is higher than 160 mmol/L, the case is severe.
If you go to the doctor, he will take your blood pressure, look at the condition of your mucous membranes and if there are signs of edema, to check for the different symptoms that we have seen. There may also be a urine analysis to check the sodium concentration in your body.
6. Management of hypernatremia
Hypernatremia, which is an excess of sodium in the blood, is often due to significant dehydration . To restore balance, it is essential to drink enough water, or even drinks slightly diluted in electrolytes. However, you should not rehydrate too quickly , otherwise you risk causing imbalances in the brain.
If hypernatremia is linked to an underlying pathology (such as a kidney problem or specific diabetes), medical monitoring is essential. Only a healthcare professional will be able to adapt the treatment and offer you suitable solutions.
7. Causes and risk factors of free water deficit
Free water deficit results from an imbalance between water loss and water intake, leading to hypernatremia. Several conditions can cause this. Chronic renal failure , hyperglycemia , diabetes insipidus or even taking loop diuretics are factors that will increase water elimination without adequate compensation, thus promoting intracellular dehydration.
More common causes can also be responsible for a deficit in free water such as prolonged fever, hyperventilation, excessive sweating or even repeated vomiting which will accelerate water loss, thus increasing the risk of hypernatremia.
Certain individuals are particularly vulnerable to this imbalance, including the elderly and infants who are at increased risk due to their higher fluid needs and less effective thirst regulation.
In older people, hypernatremia can develop more easily. Thus, certain factors will play in favor of this imbalance. The decrease in the sensation of thirst, medical treatments or even the reduction of renal capacity increase water loss and reduce consumption, which causes a water imbalance. In addition, the environment also has an impact. Indeed, a hot climate will cause sweating that can be excessive and therefore lead to significant water loss. This can have an impact on cardiovascular and neurological capacities and should not be neglected!
8. Water deficit: Definition and differences with free water deficit
Water deficit is a global loss of body water, which can involve both water and electrolytes , especially sodium. This type of imbalance occurs mainly in cases of vomiting, diarrhea, or excessive water loss, without sufficient compensatory intake. Unlike free water deficit, water deficit does not directly affect natremia, because water and sodium are lost proportionally, thus maintaining a relatively stable plasma osmolarity.
The management of these two situations is different. In the case of a water deficit, it is essential to provide electrolytes in addition to water to restore a stable ionic balance. Conversely, to correct a free water deficit, the intake of pure water or hypotonic solutions is preferred, depending on the symptoms and the degree of dehydration of each person.
In this article, we have seen how to calculate your free water deficit as well as the consequences and symptoms related to a water imbalance. It is important to maintain optimal hydration in order to avoid any complications. However, if any of the symptoms we have presented occur, we recommend that you consult a health professional to have a complete assessment and obtain adequate medical advice.
Bibliography
L Berwert, B Vogt, M Burnier - Swiss Medical Review, 2010 - revmed.ch
Diagnosis and treatment of hypernatremia Saif A. Muhsin MBChB a , David B. Mount MD (Associate Chief)
