Fluid and Electrolyte Disturbances : The Basics

Total Body Water (TBW)

• The total body water in a patient is calculated according to what percentage it constitutes out of the total weight of that person.

• The graph shows that age, such as birth, prenatal life, and postnatal, is mentioned on the X-axis. Whereas on Y-axis, there is a percentage of body weight because total body water is always expressed as a percentage of body weight. Before birth, in intrauterine life, the total body weight contributes to as high as 75% to 90% of the total body weight. Suppose the weight of the fetus is 1 kilogram; then 900 grams out of 1 kilogram is water, and the remaining 100 grams weight is of tissue in that baby.As the period of gestation advances, this percentage of total body water tends to decrease.
• The percentage when birth happens of a normal baby is between 75-80%, as mentioned in the graph. Over the next few months till one year of age, there is a consistent fall in total body water, but after one year of age, it relatively becomes a plateau and remains virtually the same till puberty. Once puberty begins, males have higher total body water than females.

Implications

• In a fetus, total body water contributes 75-90% of the body weight. At birth, in term child, total body water is about 70-80% of the total body weight. It further falls as age advances so that by one year, the total body water is about 60-65% of the total body weight. It remains consistent from one year till about 10 to 11 years of age.
• As puberty begins, it has been found that males tend to develop more muscle mass, and muscles have higher total body water content; therefore, males will have higher total body water, about 60%. Whereas in the case of females, the muscle mass is relatively less, but they have a higher fat content. Fats have low water content; hence females will have less total body water, about 50-55%.

Clinical Question

Q. Why do the total body water tends to fall as age advances?

• Building or growth up of muscles, cellular protein, and cells leads to falling in the total body water. The water rises, but the percentage tends to decrease as age increases.
  Fat contributes to weight, but it is poor in water content, and thus, the total body water content of an obese child will be less as compared to the average child having less fat content.

Fluid Compartments

• The place where total body water is present can broadly be divided into two parts Extracellular Fluid (ECF) and Intracellular Fluid (ICF).

• As per the graph again. In a fetus, it has been found that the amount of Extracellular Fluid (ECF) is more than that of Intracellular Fluid (ICF). As age advances, the number of cells rises, and because of this, the fluid inside the cell will also rise, so its percentage contribution will rise. However, the contribution of ECF will tend to fall.
• At birth, the ECF will still be more significant than ICF, but around 3-4 months of age, both ECF and ICF will become the same.And after that, the Intracellular Fluid will exceed the Extracellular Fluid. Extracellular Fluid is always the same in males and females, whereas Intracellular Fluid will be more in males than females.

Intracellular Fluid (ICF)

• If total body water is 60-65% of total body weight, ICF is about 30-40%, which remains the same from 1 year until adulthood.

Extracellular Fluid (ECF)

• If total body water is 60-65% of total body weight, ECF will be about 20-25%, subdivided into Interstitial Fluid consisting of about 15% and Plasma fluid of about 5%.
• In utero, ECF is greater than ICF. Between 3-4 months of postnatal life, ECF equals ICF. Beyond 4 months of age, ICF will be greater than ECF. The adult ratio of ICF: ECF is obtained from one year of age onwards.

Normal Equilibrium Between Intravascular And Interstitial Spaces

• Two forces act to maintain the balance between Intravascular and Interstitial spaces.

• Normally, osmotic and hydrostatic pressure generally balance out each other. Overall,some fluid moves into the interstitium;With this balance, fluid returns to this circulation through lymphatics.

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Pathological States

Oncotic Pressure

• Suppose a child has low Oncotic pressure (hypoalbuminemia). The hydrostatic pressure will exceed the oncotic pressure, and thus it will produce more fluid in the interstitium producing Edema in the patient.

Congestive Cardiac Failure (CCF)

• Suppose a patient has Congestive Cardiac Failure (CCF). Then the patient will have fluid retention (sodium and water retention). Also, due to compensation in CCF, the heart will try to increase the cardiac output, so there will also be minor changes in the cardiac output. So the net result of this will be, Hydrostatic Pressure will be more than Oncotic Pressure, and thus it will produce fluid in the interstitium leading to Edema. Therefore, the causes of Edema can be either low Oncotic pressure or increased Hydrostatic pressure.

Electrolyte Composition

• These are the major cations and significant anions:

• As per the image given by Nelson, it has been found that the Sodium ions in the plasma are 140 and Potassium ions are only 4, whereas, inside the cell, the Potassium ions are 140 and Sodium ions are 13.
• Similarly, Chloride and Bicarbonate are the predominant anions on the outside, while Phosphates and proteins are predominant inside the cell with some amount of Bicarbonate and Chloride.

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Implications Of Different Concentrations In ECF And ICF

• The serum value of various electrolytes does not always reflect actual electrolyte content (also known as Total Body Electrolyte Content), which means there may be changes between these two. This is especially true in cations like potassium because potassium is present outside the cell in minimal amounts. Sometimes there may be movement of potassium inside and outside in various pathological states. And so, the serum potassium performed is not always proper. Thus, implications mean you cannot always rely on the serum values of these electrolytes to know about the actual status. In Diabetic ketoacidosis, significant potassium ion depletion is marked by the transmembrane shift of potassium ions from the ICF to ECF.

Osmolality

• It is expressed in units of milli osmoles per kg ( mOsm/kg). Osmolality refers to the solute concentration of a fluid. Normally, it has been observed that ECF and ICF stay in osmotic equilibrium, which means ECF osmolality usually in physiological conditions equals the ICF osmolality, and the primary reason for this is although the concentration of ions is different across the two fluid spaces, the water tends to move freely across these spaces. So in normal physiological conditions, most individuals see an Osmotic equilibrium.
• If there is a change in the ions, it will alter the movement of water, and this equilibrium will be balanced. So, the normal plasma osmolality is kept within a solid range of 285-295 mOsm/kg. Formula to calculate plasma osmolality depending upon the concentration of various solutes. For all practical purposes, three solutes contribute to osmolalities Sodium, Glucose, and Blood urea nitrogen. The formula says the osmolality of plasma is given by 

Osm = 2 × (Na+) + [Glucose]/18 + [BUN]/2.8 

• It is found that the calculated osmolality is lower than the measured osmolality, but it is within 10 mOsm/kg. It is less because other solutes are available and are not a part of this formula.

Osmolal Gap

• The Osmolal gap is between measured osmolality, the actual osmolality, and calculated osmolality, which is more than 10 mOsm/kg. If the value is more than 10 mOsm/kg, then it is said to be an Osmolal gap. The substances which can lead to an Osmolal gap are excess consumption of Ethanol, Methyl alcohol, Ethylene glycol, Sorbitol, Sucrose, and Mannitol. As per Nelson, calculating the Osmolal gap indicates poisoning caused by methyl alcohol and ethylene glycol.

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Concept Of Effective Osmolality 

• Plasma has many ineffective osmoles, such as Urea. In addition,if the concentration is high  Ethanol is also considered ineffective osmoles . These solutes do contribute to the osmolality but they freely move across the plasma membrane and so they donot determine the guiding force for the movement of fluids. 
• The term devised, effective osmolality, is a better indicator of moving fluid. This is also called the Tonicity of a fluid. The formula gives effective osmolality,

Effective osmolality or Tonicity 

= (2 × [Na⁺] + [Glucose]/18)

• This Effective osmolality or Tonicity determines the osmotic force mediating the water shift between the ECF and the ICF.

Clinical Questions

Q. A patient with hyperglycemia was found to have hyponatremia despite normal total body sodium. Explain?

• Because of Hyperglycemia, there will be an increase in the effective osmolality in the ECF, which will cause the movement of water from ICF to ECF, producing Dilutional hyponatremia. In these patients, the measurement of serum sodium is faulty, so they use another index to calculate the Sodium content. It is called the calculated Sodium value. (Corrected [Na⁺] = Measured [Na⁺] + 1.6 × [glucose in mg/dl-100]/100). This is a more accurate indicator of the effective circulating plasma volume and osmolality than the simply measured serum sodium done in lab examination.

Regulation Of Plasma Osmolality 

• The hypothalamic system majorly makes regulation of plasma osmolality. Imagine a scenario with normal plasma osmolality of about 285-295 mOsm/kg. Suppose there is an increase in plasma osmolality. Then there will be activation of two things, both happening in the hypothalamus. At first, osmoreceptors will activate in the anterior hypothalamus, which controls antidiuretic hormone synthesis. These Osmo receptors will stimulate the supraoptic and paraventricular nuclei in the hypothalamus to produce the Antidiuretic hormone (ADH), also known as Vasopressin. 
• This ADH will be released from the pituitary gland, where it further reaches the renal tubule. In the renal Collecting duct, it will act on V2 receptors and cause the insertion of water channels called Aquaporin 2. Due to the insertion of these Aquaporin 2 channels into the collecting duct, there will be increased water reabsorption which will balance out the plasma osmolality in a patient.
• In addition, separate sets of Osmoreceptors are present in the hypothalamus, which controls the thirst centre. They will cause an increase in thirst, and after drinking water, it will lead to increased water content in the plasma. Thus, with this, the plasma osmolality will get normalised. It is not All or None phenomenon , it is a Graded phenomenon , if there will be rise in plasma osmolality there will be progressive rise in ADH and if there is fall in plasma osmolality there will be  progressive fall in ADH secretion .

Effective Intravascular Volume

• Effective intravascular volume is the volume status in an individual that is sensed by the body's regulatory mechanisms.This mainly means the regulatory mechanisms occurring in kidneys.

Practical Implications

• In CCF, it is volume overload. However, the kidneys still retain sodium because although there is volume overload in CCF, there is a reduction in cardiac function, resulting in decreased effective perfusion of various organs, especially kidneys. Since these are the body's regulatory mechanisms, they perceive volume as low despite  being an overall overload. As they perceive the volume as low, Renin angiotensin aldosterone axis  (RAAS) will be activated, leading to Sodium conservation or retention in these patients.

Control Of Intravascular Volume

• Control of intravascular volume closely mirrors serum Sodium (Na+), which usually goes hand in hand. Imagine a scenario where there is a reduction in the effective intravascular volume. Then it will be sensed by the Juxtaglomerular Apparatus. Particularly, sensors are present in the afferent arterioles of the kidney, which will sense a fall in renal perfusion. Due to these senses, it sends stimulus to the Renin-Angiotensin system of the juxtaglomerular apparatus to produce more Renin.
• This increased Renin will produce Angiotensin 1 and then Angiotensin 2, which will act directly on the kidney and increase Sodium reabsorption leading to Sodium retention. Simultaneously, it will produce vasoconstriction, which will try to maintain the GFR in the patients and produce an increased amount of Aldosterone, which causes Sodium retention and Potassium excretion. Therefore, the net result will be an increase in sodium reabsorption. This increased reabsorption will cause sodium and water retention leading to improvement in intravascular volume.
• If there is an increase in effective intravascular volume, then more blood will reach the Atria, which will cause atrial distension; this atrial distension will lead to the release of Atrial Natriuretic Peptide (ANP). ANP will act on the medullary part of the Collecting Duct, and it will act to increase the Sodium loss in urine. Therefore, it will reduce the total body Sodium and Plasma Sodium/Serum Sodium, causing a relative reduction in the intravascular volume.

Side Note

• The Normal plasma osmolality is 285-295 mOsm/kg. The minimum urine osmolality is approximately 30-50 mOsm/kg. The maximum is approximately 1200 mOsm/kg. Normally, the kidney excretes less than 1% of Na⁺ filtered at the glomerulus.

Key Points 

• TBW decreases with age in infancy, stays constant till puberty, and falls slightly afterwards. Males have a higher TBW than females. Obese children have less TBW than non-obese children. Effective osmolality or Tonicity guides fluid movement between ICF and ECF.
• In hyperglycemia, the corrected sodium Na⁺ is a better indicator than the measured sodium Na⁺ . Osmolality control involves ADH release. Intravascular volume control mirrors Na⁺ levels and involves Renin-Angiotensin System (RAAS) and Atrial Natriuretic Peptide. Control of effective intravascular volume always takes precedence over control of Osmolality.

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Hope you found this blog helpful for your NEET SS Pediatrics Fluid and Electrolyte Disturbances preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs.