Keywords: Fluid balance/Input/Output/ Nursing Practice Dehydration/Overhydration Review ●This article has been double-blind Fluid balance peer reviewed Assessing hydration status and measuring fluid balance can ensure optimal hydration Measuring and managing fluid balance headaches, fatigue In this article... 5 key and dry skin. Severe dehydra- What fluid balance is and how fluid moves around the body points Fluid balance is tion can lead to Causes and signs and symptoms of dehydration and 1the balance of hypovolaemic overhydration the input and shock, organ output of fluids in failure and death How to assess fluid balance, including clinical assessment the body to allow The three How to keep an accurate fluid balance chart metabolic pro- 4elements to cesses to function assessing fluid To assess fluid balance and Author Alison Shepherd is tutor in nursing, balance, including what fluid balance is, 2balance, hydration status department of primary care and child health, and how and why it is measured. It also dis- nurses need to are: clinical Florence Nightingale School of Nursing and cusses the importance of measuring fluid know about fluid assessment, body Midwifery, King’s College London. balance accurately, and the health implica- compartments in weight and urine Abstract Shepherd A (2011) Measuring and tions of dehydration and overhydration. the body and how output; review of managing fluid balance. Nursing Times; fluid moves fluid balance 107: 28, 12-16. What is fluid balance? between these charts; and review Ensuring patients are adequately hydrated Fluid balance is a term used to describe the compartments of blood chemistry is an essential part of nursing care, yet a balance of the input and output of fluids in Dehydration is Fluid balance recent report from the Care Quality the body to allow metabolic processes to 3defined as a 5recording is Commission found “appalling” levels of care function correctly (Welch, 2010). 1% or greater loss often inadequate in some NHS hospitals, with health Around 52% of total body weight in of body mass as a or inaccurate often professionals failing to manage dehydration. women and 60% in men is fluid. This con- result of fluid loss. because of staff This article discusses the importance of sists of water and molecules containing, Symptoms include shortages, lack of hydration, and the health implications of for example, sodium, chloride and potas- impaired cognitive training or lack of dehydration and overhydration. It also sium (Mooney, 2007). These compounds function, time provides an overview of fluid balance, disassociate into particles which carry including how and why it should be an electrical charge; these particles in solu- measured, and discusses the importance tions are called electrolytes. For example, of accurate fluid balance measurements. sodium chloride (NaCl) dissolves in solu- tion to form an equal number of positively ater is essential for life, charged sodium (Na+) ions, and negatively and maintaining the cor- charged chlorine (Cl-) ions (Waugh, 2007). rect balance of fluid in the Plasma electrolytes are balanced as it is Wbody is crucial to health important to have the correct concentration (Welch, 2010). of ions in the blood, especially sodium, However, according to a recent report potassium and magnesium. Too much or from the Care Quality Commission (2011), too little of these electrolytes can cause car- some hospital patients are not being given diac arrhythmias (Docherty, 2006). enough water to drink. The report sug- To make a competent assessment of gests fluids are being left out of reach, or fluid balance, nurses need to understand are not being given at all for long periods. the fluid compartments within the body This article provides an overview of fluid and how fluid moves between these Dehydration affects brain function 12 Nursing Times 19.07.11 / Vol 107 No 28 / www.nursingtimes.net For more articles on fluid balance, go to Nursing nursingtimes.net/fluidbalance Times.net FIG 1. TYPES OF FLUIds 48% 40% Fluids comprise an solids solids average of 52% to Tissue cells 60% body weight 52% 60% 2/3 fluids fluids intracellular fluid (icf) TOTAL BODY WEIGHT FEMALE BODY TOTAL WEIGHT MALE BODY TOTAL 1/3 80% extracellular interstitial fluid (ecf) fluid 20% plasma Blood capillary compartments (Davies, 2010). Two-thirds spaces is determined by hydrostatic and disorders, such as glomerulo-nephritis, of total body fluid is intracellular, and the osmotic pressures (Day et al, 2009): nephrotic syndrome and liver failure remaining third is extracellular fluid, » Hydrostatic pressure is created by the (Schrier, 2007; Waugh, 2007). which is divided into plasma and intersti- pumping action of the heart, and the tial fluid (Docherty and McIntyre, 2002) effect of gravity on the blood within the Maintaining fluid balance (Fig 1). There is also a third space, known as blood vessels (Scales and Pilsworth, Total fluid volume fluctuates by less than “transcellular fluid”, which is contained in 2008); 1%, and fluid intake should be balanced by body cavities, such as cerebral spinal fluid » Osmotic pressure is generated by the fluid loss (Scales and Pilsworth, 2008; and synovial, peritoneal and pleural fluids molecules in a solution (Day et al, Thomas and Bishop, 2007). (Day et al, 2009). 2009). When generated by the presence Water intake is obtained from fluid and It is important to remember that, of protein molecules in solution it is food in the diet, and is mostly lost through although these fluid compartments are called colloid oncotic pressure. urine output. It is also lost through the classed as separate areas, water and elec- Osmotic pressure created by dissolved skin as sweat, through the respiratory trolytes continually circulate between electrolytes in solution is called tract, and in faecal matter (Waugh 2007). them (Timby, 2008). crystalloid oncotic pressure (Scales and Fig 2 shows the normal balance of water Pilsworth, 2008). intake and output. Movement of fluids In healthy people, protein molecules Fluid intake is mainly regulated by Fluid circulates between compartments by are normally too large to pass out of the thirst, a natural response to fluid deple- diffusion. This is “the random movement of capillaries into the interstitial fluid. This tion, and is accompanied by decreased particles from regions where they are highly is because of the tight intracellular junc- secretion of saliva and dryness of the oral concentrated to areas of low concentration. tions between adjacent endothelial cells in mucosa (Waugh, 2007). Movement continues until the concentra- the capillary wall (Rassam and Counsell, As the osmotic concentration of the tion is equally distributed” (Casey, 2004). 2005). Compromising the integrity of blood increases, this draws water from the This is normally a passive process but it these tight intracellular junctions allows cells into the blood. This dehydrates spe- can be facilitated by a carrier molecule, protein molecules to pass to the interstitial cific brain cells called osmoreceptors, usually a specialist protein (Davies, 2010). spaces. The subsequent accumulation of which stimulate drinking and the release Fluid also moves by osmosis, defined by tissue fluid is known as oedema (Ganong, of antidiuretic hormone (ADH). ADH Montague et al (2005) as “the flow of water 2000). reduces water loss by lowering urine across a semipermeable membrane from a Oedema can be caused by a number of volume, producing urine that is more con- dilute solution to a more concentrated pathological mechanisms, such as venous centrated (Thornton, 2010). When water solution until stability is reached”. congestion. This increases venous hydro- intake is high, less ADH is produced, so static pressure, common in disorders such the kidneys produce large quantities of Formation of tissue fluid as cardiac failure (Paulus et al, 2008). A dilute urine (Scales and Pilsworth, 2008). Distribution and movement of water decrease in plasma oncotic pressure causes During times of fluid insufficiency, the between the intracellular and interstitial the oedema associated with common renal adrenal glands produce the hormone Fotolia www.nursingtimes.net / Vol 107 No 28 / Nursing Times 19.07.11 13 Nursing Practice Review aldosterone, which stimulates the reab- sorption of sodium from the distal renal FIG 2. FLUID IntakE AND LOss tubules and collecting ducts. This reab- Sources: McMillen and Pitcher (2010); Scales and Pilsworth (2008); Waugh (2007) sorption of sodium causes the water in the collecting ducts to be reabsorbed, main- Oral fluids and food (2,300ml) Urine output (1,500ml) taining homeostasis. Water lost through faeces, sweat and evaporation cannot be regulated in the same way by the body, and is influenced by dietary intake, illness and the environ- ML ment (Scales and Pilsworth, 2008). ML A fluctuation in fluid volume of just 2,500 5-10% can have an adverse effect on health 2,500 (Large, 2005). A deficit in fluid volume is known as a negative fluid balance and, if total total : : fluid intake is greater than output, the T U body is in positive fluid balance (Scales TP Faeces (200ml) and Pilsworth, 2008). U intake O ID ID Skin (350ml) U U Dehydration L L Dehydration is defined as a 1% or greater F F loss of body mass as a result of fluid loss, Sweat (100ml) where the body has less water than it needs Evaporation of fluid via the lungs to function properly (Madden, 2000). Metabolic water produced from (350ml) The physical symptoms of mild dehy- nutrient metabolism (200ml) dration include: » Impaired cognitive function; { { » Reduced physical performance; loss include haemorrhage, sweating, fever such as renal impairment and liver disease » Headaches, fatigue, sunken eyes and and severe burns (Mooney, 2007). (Large, 2005). dry, less elastic skin (Welch, 2010). In patients with heart failure, the If dehydration persists, the circulating Fluid overload reduced cardiac output fails to maintain volume of blood can drop. This leads to: Excessive fluid volume arises when there is adequate systemic blood pressure, causing » Hypotension; retention of both electrolytes and water in reduced renal perfusion.