Protein Deficiency Scenerio

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42 march 19 :: vol 28 no 29 :: 2014 © NURSING STANDARD / RCN PUBLISHING

Abstract Assessment and careful maintenance of fluid and electrolyte balance in patients is an essential part of the nurse’s role. This article explores fluid and electrolyte balance with reference to the normal physiology of body fluids and regulation of fluids and electrolytes. It also considers some common conditions associated with fluid imbalance.

Authors Ella McLafferty Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Carolyn Johnstone Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Charles Hendry Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Alistair Farley Retired, was lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected]

Keywords Body fluids, diffusion, electrolytes, filtration, fluid balance, hormonal control, ion pump, oedema, osmosis

Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software.

Online Guidelines on writing for publication are available at www.nursing-standard.co.uk. For related articles visit the archive and search using the keywords above.

Fluid and electrolyte balance McLafferty E et al (2014) Fluid and electrolyte balance. Nursing Standard. 28, 29, 42-49. Date of submission: July 26 2011; date of acceptance: December 14 2011.

in body fat in females that accounts for the lower water content (Thibodeau and Patton 2012). The amount of fat in the body has an influence on the proportion of water – the more fat in the body, the lower the percentage of water.

Age also influences the amount of body fluids. Newborn infants’ total body mass can be up to can be up to 80% water and this can be higher in premature infants (Thibodeau and Patton 2012). Chow and Douglas (2008) stated that the percentage of water in the body gradually reduces with gestational age from around 86% at 26 weeks to 80% at 32 weeks and to about 78% at full term. This occurs as a result of the accumulation of body fat during development. In the newborn infant, body weight can be a good indicator of fluid loss and balance (Chow and Douglas 2008). As people age, there is a gradual decrease in the percentage of body water. This is a result of a gradual reduction in muscle mass and a gradual increase in body fat (Thibodeau and Patton 2012) . It is important that nurses are aware of these changes because differences in body water percentage can affect the concentration of water soluble drugs in the body (Thibodeau and Patton 2012).

Fluid compartments Body fluids exist in two main compartments: intracellular and extracellular compartments (Brooker and Nicol 2011). Fluid within the body’s cells is known as cytosol and accounts for about two thirds of all body fluids; it is separated from extracellular fluids by the cell membrane. Extracellular fluid accounts for about one third of body fluids (Tortora and Derrickson 2009a) and is further separated into two compartments, the interstitial fluid and plasma contained within the blood vessels. The cells are surrounded by interstitial fluid, which accounts for 80% of extracellular fluid. However, interstitial fluid also includes lymph, cerebrospinal fluid, synovial fluid, aqueous humor and vitreous body, and pleural, pericardial and peritoneal fluids (Tortora and Derrickson 2009b). Plasma or intravascular fluid makes up the remaining 20% of extracellular fluid; this is the fluid component of the blood and is separated from the interstitial fluid by the capillary membrane.

There is constant movement of fluids between compartments. Fluids can cross the cell membrane

FLUID AND ELECTROLYTE balance is crucial in maintaining homeostasis within the body. Nurses may play a role in regulating body fluids to ensure patient health and prevent conditions that may result from fluid and electrolyte imbalances.

Body uids Water is the most abundant compound in the body, accounting for around 55% of total body weight in a non-obese adult. Gender is associated with slight variations with water accounting for 60% of total body weight in the average male and 50% of total body weight in the female. It is the slight increase

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Channel protein

Glycolipid: carbohydrate

Lipids

Peripheral protein

Extracellular fl uid

Pore

Phospholipid bilayer

Cytosol

Integral proteins

Peripheral protein

Cholesterol

that separates intracellular fl uids from the interstitial fl uids and can readily move through the cell wall boundary between the interstitial fl uid and plasma (Brooker and Nicol 2011).

Cell membrane The cell membrane has a vital role in the maintenance of fl uid and electrolyte balance and is one of the most important components of the cell (Porth 2011). The cell membrane controls the movement of fl uids between the intracellular and extracellular compartments (Porth 2011). Structurally, the cell membrane consists mainly of lipids and proteins and is known as a phospholipid bilayer (Tortora and Derrickson 2009a) (Figure 1). The main structure is created by the back-to-back arrangement of phospholipids, which is stabilised by cholesterol and glycolipids (Thibodeau and Patton 2012). Membrane proteins – integral proteins that extend through the membrane and peripheral proteins that are attached to the outside or inside of the cell membrane – are scattered throughout this layer (Tortora and Derrickson 2009a).

The cell membrane is selectively permeable, allowing some substances to move in and out of the cell, while restricting the movement of others (Tortora and Derrickson 2009a). The structure of the cell membrane accounts for some of the selectivity; the phosphate heads face outward, while the lipid tails face each other on the inside of the membrane. This infl uences how cell membranes

work because the phosphate heads can mix with water and the lipid tails cannot. Therefore, the lipid-based membrane prevents water-soluble substances from fl owing freely between intracellular and extracellular fl uids. The phospholipid bilayer forms a protective barrier around the cell and is impermeable to all but lipid-soluble substances such as fatty acids, fat-soluble vitamins, steroids, oxygen and carbon dioxide (Tortora and Derrickson 2009a, Porth 2011). Free movement through the cell membrane is also infl uenced by particle size, for example large molecules such as glucose and amino acids do not readily cross the cell membrane. Ions such as potassium and sodium cannot freely move through the cell membrane and rely on membrane proteins to assist movement.

Membrane proteins have several roles. They act as ion channels; some membrane proteins have a hole through the middle forming a channel or pore allowing certain substances such as potassium to pass through (Tortora and Derrickson 2009a). Some membrane proteins act as carriers to transport or move substances across the cell membrane. Substances are selectively regulated to be transported in and out of the cell. For example, glucose attaches to a glucose transporter protein on the outside surface of the cell membrane, the protein changes shape and the glucose passes through the membrane and is released into the cell (Tortora and Derrickson 2009a). Some membrane proteins are involved in catalysing specifi c chemical reactions inside or outside the cell. In addition, membrane proteins

FIGURE 1 Cell membrane

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can act as receptors. These proteins are involved in recognising and binding to substances such as hormones (Tortora and Derrickson 2009a). For example, if the insulin receptor did not recognise and bind to insulin, glucose would not be able to enter the cell. Some membrane proteins allow cell recognition. During tissue formation, cells with the same protein markers will recognise each other. These proteins are important in immunity and are vital to the body’s ability to recognise cells that are foreign and potentially harmful (Tortora and Derrickson 2009a).

Movement of uids and electrolytes According to Tortora and Derrickson (2009a), movement of substances across the cell membrane is essential to cell functioning. It is vital that glucose and oxygen are able to enter the cell to allow metabolism to occur, while carbon dioxide and substances produced by the cell must be removed. In the human body, the solvent is water and there are a variety of potential solutes such as oxygen, nutrients or ions. An important factor in the movement of solutes is the concentration gradient. The concentration gradient refers to the difference in concentration between two areas or substances, for example the intracellular and interstitial fluids. The movement from a high to low concentration is referred to as movement with or down the concentration gradient, while movement from an area of low to high concentration is referred to as movement against or up the concentration gradient (Tortora and Derrickson 2009a). There are two general types of movement: passive transport and active transport (Thibodeau and Patton 2012).

Passive transport The passive movement of solutes is via diffusion and of solvent (water) is via osmosis. Filtration is the movement of both solvent and solute under pressure (Thibodeau and Patton 2012). There are several types of diffusion: simple, facilitated and diffusion through ion channels.

Simple di�sion Lipid-soluble substances can move freely across the cell membrane down the concentration gradient, and this is an important method of movement for oxygen and carbon dioxide. Other substances such as fatty acids, steroids, and fat-soluble vitamins A, D, E and K also diffuse through the cell membrane in this way (Tortora and Derrickson 2009a).

Facilitated di�sion Some substances need assistance to diffuse across the cell membrane. Facilitated

diffusion involves membrane proteins without the need for energy. An example of a substance that moves by facilitated diffusion is glucose, which is too large to cross the cell membrane by simple diffusion and which requires transporter proteins that change shape to allow glucose to diffuse through the cell membrane down the concentration gradient.

Di�sion through ion channels Ion channels in membrane proteins allow the movement of specific electrolytes through the cell membrane. Some channels are open and allow specific electrolytes to leak across the cell membrane down the concentration gradient (Porth 2011). Other channels are gated, only opening when stimulated to do so, and these channels have an important role in the electrical activity of several body cell types (Tortora and Derrickson 2009a). For example, the movement of impulses along the axon of a nerve is a result of the movement of electrically charged ions across nerve cell membranes. Local anaesthetic works by blocking gated sodium channels, thereby blocking the nerve impulse (Tortora and Derrickson 2009b).

Osmosis Osmosis is the diffusion of water across a selectively permeable membrane, and this occurs between compartments and cells to maintain water balance (Thibodeau and Patton 2012). Water passes through membranes by moving between neighbouring phospholipid molecules via simple diffusion or through integral membrane proteins that function as water channels. Fluids in the body contain substances that cannot move across membranes, and this creates pressure on that membrane called the osmotic pressure. Osmotic pressure depends on the amount of solute in the solution, with more concentrated solutions having a higher osmotic pressure (Tortora and Derrickson 2009a).

Osmotic pressure between the cell and the interstitial fluid is balanced and constant, therefore cells do not usually swell or shrink because of water movement. Fluid in the body is said to be isotonic if it has the same electrolyte concentration as body cells, and cells bathed in it do not swell or shrink. This principle is used to determine fluid replacement therapy, for example 0.9% sodium chloride is isotonic for red blood cells so can be used as simple fluid replacement. Fluids with a lower concentration of solutes are hypotonic, while those with a higher concentration are hypertonic.

In clinical practice, these solutions are used to treat specific conditions. Hypotonic solutions are used to treat dehydration because they encourage the movement of fluid from the blood to the cells; hypotonic fluids also form the basis of sports rehydration. The administration of hypertonic fluids

PATP

increases circulating blood volume by encouraging the movement of fluid from the interstitial space to the blood. Hypertonic fluids can be useful in cases of head injury because administration will reduce cerebral swelling by encouraging the movement of fluid from the interstitial space in the brain to the blood (Tortora and Derrickson 2009a).

Active transport Active transport is required when a substance has to be transported against the concentration gradient. There are several types of active transport mechanisms, and they mainly transport ions and are known as ion pumps (Tortora and Derrickson 2009a). One example of this type of transport is the movement of sodium out of the cell and movement of potassium into the cell. Movement of this nature requires energy in the form of adenosine triphosphate (ATP), the cell energy produced as a result of cell metabolism.

The concentration of sodium is around 14 times greater outside the cell than it is inside the cell (Porth 2011), therefore sodium can naturally leak into the cell through ion channels. Since it is osmotically active, increasing levels of sodium would encourage the inflow of water and cause the cell to swell with potential to rupture. The concentration of potassium inside the cell is around 35 times greater than that outside the cell, and potassium also leaks out through ion channels. If allowed to leak out without being returned to the cell, increasing levels of interstitial and plasma potassium could have catastrophic consequences for nerve and muscle function (Porth 2011).

The sodium-potassium pump (Figure 2) consists of an integral membrane protein that is activated by the attachment of three sodium ions in the cytosol.

Following this attachment, ATP is hydrolysed forming adenosine diphosphate (ADP), attaching a phosphate group to the pump protein, and releasing energy from the phosphate bond that drives the change in shape of the pump protein. When the pump protein has changed shape, the three sodium ions are expelled from the cell. Externally, two potassium ions attach to the pump protein, causing the phosphate to be released from the protein, the protein to be returned to its original shape and the potassium ions to be released into the cell (Tortora and Derrickson 2009a). Movement against the concentration gradient is vital in maintaining sodium, potassium and water balance in the body.

This is clinically significant when physiological shock occurs because there is a deficit of energy available to drive this reaction. This failure ultimately leads to failure of the sodium-potassium pump, an increase in sodium levels inside the cell and subsequent swelling of the cell. In addition, there is an increase in potassium outside the cell, affecting nerve and muscle activity.

Movement between plasma and the interstitial space Movement of fluid, electrolytes and other dissolved solutes is relatively free between plasma and the interstitial fluid as a result of filtration and reabsorption (Tortora and Derrickson 2009a). At the arterial end, blood hydrostatic pressure inside the capillary is higher than interstitial fluid hydrostatic pressure in the interstitial fluid surrounding it, filtering fluid out of the capillary. Larger molecules such as plasma proteins and red blood cells do not normally cross the capillary membrane because of their size (Scales and Pilsworth 2008).

FIGURE 2 Sodium-potassium pump

Extracellular fluid

Cytosol

Na+ gradient

Na+/K+ ATPase

3 Na+

3 Na+ expelled

ADPK+ gradient

2 K+

2 K+ imported

ADP = adenosine diphosphate; ATP = adenosine triphosphate; P = phosphate; K = potassium; Na = sodium

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Plasma proteins play a vital role in the maintenance of fluid balance because they create an osmotic pressure inside the capillaries, often referred to as colloid osmotic pressure (Brooker and Nicol 2011), drawing fluid into the capillaries. As blood flows towards the venous end of the capillary, the blood hydrostatic pressure combined with the blood colloid osmotic pressure results in a net inwards pressure, thus the majority of fluid and electrolytes are reabsorbed into the capillary (Tortora and Derrickson 2009a). This process is vital in maintaining compartment fluid balance and preventing tissue swelling. There is a small deficit of fluid return to the capillaries, but this fluid does not accumulate in the interstitial space because it is returned to the circulation via the lymphatic system, ensuring fluid balance is maintained.

Fluid regulation Fluid balance in the body remains relatively stable, and is maintained mainly by the action of the kidneys. An average of 1,600mL of fluid is gained through drinking and a further 700mL is gained as part of food, with water being absorbed by the gastrointestinal (GI) tract. Around 200mL of water is produced each day as a by-product of cell metabolism (Tortora and Derrickson 2009b). About 100mL of fluid is lost via the GI tract as part of faeces, 600mL of fluid evaporates from the skin, 300mL of fluid is lost through respiration and the kidneys account for the remaining 1,500mL of fluid loss (Tortora and Derrickson 2009b). Thus, there is an average water balance of 2,500mL in and 2,500mL out daily.

It is important to remember that these levels can vary considerably, for example diarrhoea would significantly increase GI fluid loss. When fluid loss alters via other body systems changes, the action of the kidneys alters accordingly as does urine production.

Fluid gain Fluid loss and gain are carefully regulated processes. When fluid is lost from the body in increasing amounts, the simplest way to increase fluid levels is through liquid intake. Dehydration refers to total loss of body fluid associated with a decrease in circulating fluid volume and an increase in osmolarity (Welch 2010), and triggers the thirst mechanism. Fluid gain in the body is essentially a homeostatic mechanism triggered by dehydration. Signs of dehydration include weight loss, headache, rapid but shallow breathing, rapid weak thready pulse, reduced urine output, constipation, dry mouth with thicker saliva, dry skin and sunken eyes (Welch 2010).

Thirst is an important clinical sign that can be absent in older people (Welch 2010). When fluid levels fall, blood pressure falls triggering the release of renin from the kidneys and this promotes the formation of angiotensin II, which stimulates the thirst centre in the hypothalamus. The thirst centre is also stimulated by a dry mouth, an increase in blood osmolarity and lowered blood pressure (Tortora and Derrickson 2009b). When the thirst mechanism is triggered, the usual response is to drink fluids to restore fluid balance. Nurses should remember that some individuals such as young children and older adults will be unable to respond to this trigger and to drink, therefore nurses should offer these individuals fluids and assist them to drink as part of nursing care (Speakman and Weldy 2002). Without appropriate assistance, young children, older adults and those with learning difficulties are at increased risk of dehydration (Speakman and Weldy 2002, Welch 2010).

Fluid loss The regulation of fluid loss from the body occurs in the kidneys, with the volume of body fluid determined by the loss of sodium chloride in urine (Tortora and Derrickson 2009b). Daily intake of sodium chloride can vary significantly, and the kidneys alter levels of sodium chloride excreted in urine through hormonal control. The three main hormones that alter the reabsorption rates of sodium and chloride are antidiuretic hormone (ADH) angiotensin II, aldosterone and atrial natriuretic peptide (ANP) (Tortora and Derrickson 2009b). A negative feedback mechanism involving secretion of ADH maintains the blood osmotic pressure, and sodium and water concentrations, within normal limits.

Changes in sodium levels in plasma will alter blood volume, for example a significant increase in sodium will lead to an increase in circulating blood volume and therefore blood pressure, causing the blood vessel walls to stretch. This increased stretch is detected by baroreceptors in the carotid bodies, arch of the aorta and walls of the atria (Porth 2011). Once stimulated by the increase in blood volume, these receptors initiate the sympathetic nervous system response and lead to a reduction in the release of ADH from the pituitary gland. The sympathetic nervous system response increases the glomerular filtration rate along with a decrease in renin production, and less reabsorption of sodium and water. Sensors in the kidney are also stimulated by an increase in renal perfusion, which decreases the secretion of renin (Speakman and Weldy 2002).

Renin usually activates the renin-angiotensin-aldosterone system, which results in conversion of angiotensin I to angiotensin

II, increasing sodium reabsorption in the renal tubules. Angiotensin II also regulates the secretion of aldosterone from the adrenal cortex, which increases sodium reabsorption while increasing the secretion of potassium (Porth 2011). The osmotic pull of sodium encourages the reabsorption of water into the capillaries, thus increasing blood volume. Therefore, a reduction in renin will decrease the reabsorption of sodium and water, ultimately lowering blood sodium levels. Intake of water in response to the thirst mechanism decreases the osmolarity of blood and interstitial fluid, and ADH secretion ceases. ANP is released from specialised cells in the atria in response to stretch caused by over filling, increasing sodium excretion in the renal tubules and reducing circulating blood volume (Porth 2011).

Electrolytes There are several electrolytes in the human body and each performs a vital role. It is beyond the scope of this article to discuss all of these, therefore the focus is on the role of sodium and potassium.

Sodium Sodium has a vital role in the maintenance of fluid balance and is responsible for plasma osmolarity. It is the most abundant extracellular cation (Hogan et al 2007). Normal plasma sodium levels are 133-148mmol/L(Blann 2006). Sodium also plays a role in nerve and muscle cell electrical activity that drive action potentials (impulses) through the neurones or muscle fibres (Tortora and Derrickson 2009b). Chloride ions are closely related to sodium and potassium, with movement of chloride through the cell membrane occurring via a shared transport protein.

Hyponatraemia can occur as a result of impaired renal function, fluid loss related to burns, impaired ADH secretion, sodium loss associated with some diuretics and hyperglycaemia in patients with diabetes (Porth 2011, Thibodeau and Patton 2012). Individuals with hyponatraemia present with muscle weakness, dizziness, headache, hypotension and tachycardia, leading potentially to coma (Tortora and Derrickson 2009b). Treatment depends on the underlying cause of and aims to maintain fluid balance between compartments. A hypertonic saline solution can be administered possibly with a loop diuretic, which will increase the serum level of sodium while encouraging the loss of excess fluid (Porth 2011). When renal impairment is diagnosed, carefully planned renal management may be required, which will include fluid and specific electrolyte restrictions.

Hypernatraemia is relatively rare, but can occur

because of excessive consumption of salt, prolonged diarrhoea or dehydration (Thibodeau and Patton 2012). Symptoms include an intense thirst, hypertension, oedema, agitation and convulsions (Tortora and Derrickson 2009b). Hypernatraemia, when associated with dehydration, is managed with oral or intravenous fluid replacement. Care must be taken if administering hypotonic fluids intravenously because there is a risk of making the blood relatively hypotonic, causing cerebral oedema (Porth 2011).

Potassium Potassium is the most abundant intracellular cation, with an intracellular concentration of 140-150mmol/L and with a normal blood plasma concentration of 3.3-5.6mmol/L (Blann 2006). Potassium has a role in maintaining normal action potentials in muscles and nerve cells, as well as assisting in cardiac muscle cell activity. Potassium also has an important role in maintaining acid-base balance, with the pH of intracellular and extracellular fluids being maintained by the movement of potassium and hydrogen between compartments (Hogan et al 2007). For example, when metabolic acidosis occurs, hydrogen moves into the cell in exchange for potassium (Porth 2011), reducing the hydrogen levels and, therefore, the acidity of the blood. Although insulin is associated with maintaining blood glucose levels, it also increases the activity of the sodium-potassium pump, thereby increasing the movement of potassium into the cell (Porth 2011).

Hypokalaemia can be caused by excess fluid loss, decreased levels of potassium intake, renal impairment, overuse of laxatives, GI losses and some diuretics (Tortora and Derrickson 2009b, Thibodeau and Patton 2012). Presenting features include thirst, muscle fatigue and cramps, increased urine output and confusion (Tortora and Derrickson 2009b, Porth 2011). Because of the role of potassium in maintaining normal cardiac cell activity, the most serious effects of hypokalaemia are electrocardiogram (ECG) changes that can lead to ventricular arrhythmias in severe cases. Management can be as simple as increasing in the dietary intake of potassium through foods such as bananas or spinach, although supplements are also available (Porth 2011). Intravenous replacement therapy must be managed with care because of the risk of hyperkalaemia.

Hyperkalaemia is related to high potassium intake levels, tissue trauma caused by burns or crush injuries, or renal failure where the kidneys cannot excrete potassium (Tortora and Derrickson 2009b, Thibodeau and Patton 2012). Presenting features include skeletal muscle weakness, possibly leading to

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paralysis, irritability, nausea and vomiting (Tortora and Derrickson 2009b, Thibodeau and Patton 2012). The most serious effects of hyperkalaemia relate to cardiac muscle function bringing about significant changes to normal cardiac muscle electrical activity and ECG changes. These changes can be so severe that ventricular fibrillation and cardiac arrest can occur (Porth 2011).

Managing hyperkalaemia is crucial, but the speed of the increase in the serum level of potassium will be linked to the cause, which will dictate treatment. Dietary intake of potassium can be reduced and is an essential part of therapy when renal failure is the underlying cause of chronic hyperkalaemia. Many salt substitutes are rich in potassium and care must be taken when using these (Hogan et al 2007). As renal failure progresses, renal replacement therapy (renal dialysis) may be needed. The administration of calcium counteracts the effects of potassium on the myocardium by exchanging calcium for potassium ions, although this effect is short lived and must be supported by other therapies such as administration of glucose and insulin (Porth 2011).

Conditions associated with uid imbalance Imbalances in fluid volume are more likely to occur in infants, young children and older people. Kidney function is not yet mature in the young, while kidney function and the sensation of thirst decreases in older people (Speakman and Weldy 2002).

Fluid de‡cit Dehydration refers to an insufficiency in body water as a result of excessive fluid loss or inadequate fluid gain (Gould 2006). Porth (2011) referred to an isotonic deficit as being a fluid deficit where there is a proportional loss of fluid and sodium, leading to a decrease in extracellular fluid. When there is a reduction in the circulating blood volume, the balance of electrolytes remains relatively unchanged (hypovolaemia). Fluid can be lost from the body in several ways, including GI losses related to vomiting and diarrhoea, excessive sweating related to strenuous exercise and diuresis associated with diabetic ketoacidosis, for example (Porth 2011). Conditions such as peritonitis can lead to a shift of fluid from the circulation to the interstitial spaces, creating a relative hypovolaemia (Gould 2006).

The signs and symptoms of fluid deficit include thirst; reduced urine output and increasing concentration of the urine as the body attempts to conserve water; drier mucous membranes, particularly visible in the mouth where the saliva will be more viscous; and sunken eyes. Other less visible signs include headache and confusion,

a weak thready pulse as well as increased capillary refill time, reduced blood pressure and rapid shallow breathing (Gould 2006, Porth 2011). Blood test results will indicate an increase in red blood cell and urea concentrations. Porth (2011) classified fluid deficit in terms of severity, with a mild deficit relating to a 2% loss of body weight, a moderate deficit relating to a 5% loss and a severe deficit relating to more than 8% loss of body weight.

Managing hypovolaemia is relatively simple and involves appropriate fluid replacement along with managing the underlying condition. Fluid replacement can be oral if tolerated or intravenous if required. An accurate record of fluid balance must be maintained to ensure prevention of fluid overload.

Fluid excess Isotonic fluid excess refers to an increase in overall sodium levels and the related retention of water. It is possible for this to occur as a result of an increase in sodium intake, but it is more likely to be related to a reduction in sodium elimination because of an underlying condition (Porth 2011). Several conditions such as renal failure, heart failure and liver failure can lead to decreased levels of sodium elimination (Porth 2011). Cardiac failure occurs when the heart’s pumping ability is impaired, leading to activation of the renin-angiotensin-aldosterone system and ultimately to retention of sodium and water (Nicholas 2004). Renal failure leads to a reduced ability to filter the blood because of a reduced glomerular filtration rate and reduced ability to reabsorb electrolytes leading to increasing levels of sodium in the blood. Aldosterone is metabolised by the liver and in liver failure this is impaired, resulting in an increase in sodium and water retention (Porth 2011). Increasing levels of sodium and water lead to an increase in extracellular fluids.

Fluid excess presents as oedema and associated weight gain. Increased fluid volume in the vascular compartment can lead to a bounding pulse and venous distension. As the condition worsens, pulmonary oedema can occur leading to symptoms of breathlessness (Porth 2011). Treatment will often centre on managing the underlying condition although sodium and fluid restriction can generally reduce the blood volume. Diuretics are useful to encourage sodium loss (Porth 2011).

Oedema Oedema is swelling caused by an increase in the interstitial fluid volume (Porth 2011). There are several possible causes such as an increase in capillary filtration pressure, a decrease in colloid osmotic pressure, increase in capillary membrane permeability and obstruction to lymph flow (Porth 2011).

Capillary filtration pressure (pressure inside the capillary that forces the movement of fluid across the capillary membrane to the interstitial space) can be increased as a result of an increase in blood (hydrostatic) pressure (Casey 2004). This increase in pressure can be caused by an increase in circulating blood volume. Colloid osmotic pressure is created by proteins in the blood that exert an osmotic pull, drawing fluids back into the capillaries; this pressure will be reduced if blood levels of protein fall. Because the liver manufactures plasma proteins, the level of plasma proteins will fall in liver failure. Starvation can reduce the manufacture of plasma protein as a result of the reduced intake of amino acids, while renal failure leads to loss of plasma proteins in the urine.

Localised oedema is often a result of damage to capillary membranes because of burns, inflammation or the immune response, and this increases membrane permeability, allowing proteins to move across cells taking fluid with them. The amount of fluid reabsorbed following filtration through the capillary wall is not equal and there is a net deficit of fluid return. This excess fluid in the interstitial space is reabsorbed by the lymphatic capillaries and returned to the circulation. If lymphatic vessels become blocked or damaged, this return of fluid can be impaired, leading to oedema (Porth 2011). Obstruction to lymphatic flow often occurs following mastectomy and the removal of axillary lymph glands, leading to localised lymphoedema in the affected arm (Porth 2011).

Conclusion Alterations in fluid and electrolyte balance can have serious consequences. Fluid and electrolyte balance is vital to life and it is clear that many conditions can affect this balance. Maintaining

and monitoring fluid balance is usually the responsibility of the nurse, therefore it is essential that nurses understand the importance of this when providing patient care NS

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Brooker C, Nicol M (2011) Alexander’s Nursing Practice. Fourth edition. Churchill Livingstone Elsevier, Edinburgh.

Casey G (2004) Oedema: causes, physiology and nursing management. Nursing Standard. 18, 51, 45-51.

Chow JM, Douglas D (2008) Fluid and electrolyte management in the premature infant. Neonatal Network. 27, 6, 379-386.

Gould B (2006) Pathophysiology for the Health Professions. Third edition. Saunders Elsevier, Philadelphia PA.

Hogan MA, Gingrich MM, Overby P, Ricci MJ (2007) Fluids, Electrolytes, & Acid-Base Balance. Second edition. Pearson Prentice Hall, Upper Saddle River NJ.

Nicholas M (2004) Heart failure: pathophysiology, treatment and nursing care. Nursing Standard. 19, 11, 46-51.

Porth CM (2011) Essentials of Pathophysiology. Third edition.

Wolter Kluwer Health/Lippincott Williams & Wilkins, Philadelphia PA.

Scales K, Pilsworth J (2008) The importance of fluid balance in clinical practice. Nursing Standard. 22, 47, 50-57.

Speakman E, Weldy NJ (2002) Body Fluids & Electrolytes. A Programmed Presentation. Eighth edition. Mosby, St. Louis MO.

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Welch K (2010) Fluid balance. Learning Disability Practice. 13, 6, 33-38.

POINTS FOR PRACTICE Nurses can be encouraged to develop strategies to ensure that fluid management is a priority if the following questions are asked regularly: Are any patients in your care at risk of fluid and electrolyte imbalance? Are patients’ fluid requirements considered as part of regular nursing care?  Is fluid balance monitored appropriately?  Is managing fluid balance considered an important part of nursing care in your clinical area? Does the importance of managing fluid balance need to be re-established in your clinical area?

GLOSSARY Diffusion Passive movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. Filtration The movement of a liquid and some substances dissolved in it through a barrier. The barrier prevents some larger molecules from passing through. Hypertonic A hypertonic solution will have a high concentration of electrolytes (a higher osmotic pressure) compared with body cells and can cause cells to shrink as a result of osmosis. Hypotonic A hypotonic solution will have a low concentration of electrolytes (a lower osmotic pressure) compared with body cells and can cause cells to swell as a result of osmosis. Isotonic An isotonic solution has the same electrolyte concentration (same osmotic pressure) as body cells. Osmosis Movement of water through a semi-permeable membrane. Movement of the water occurs because of a difference in concentration on each side of the membrane and continues until equilibrium is reached. The water moves because the molecules dissolved in the fluid are too large to cross the membrane.

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