The calculation of fluid balance involves measuring the amount of fluid entering the body and comparing it to the amount of fluid leaving the body. The purpose is to determine whether there is a deficit or an excess of fluid (Bannerman, 2018). Understanding a patient's fluid status can give an indication of overall health. Osmoregulation, the process of maintaining osmotic pressure, is how the body controls fluid and electrolyte concentrations. This is particularly important for patients who are unwell or have conditions that affect the kidneys.
Maintenance of fluid balance is an important activity and is essential for optimal health. If a patient has too much or too little fluid, this imbalance can cause health problems. There are some pathophysiological conditions that can result in fluid overload, such as kidney disease and some types of heart disease. When overload occurs, the person finds it difficult to excrete excess water from the body, leading to oedema and serious health concerns. Dehydration occurs when the body is using more fluid than has been taken in or when there has been excessive fluid loss. The presence of oedema is a sign that fluid is in the wrong places. Patients who are oedematous may have fluid overload or be hypovolaemic.
Anatomy
Fluid shifts happen when the fluid in the body moves between fluid compartments. Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressure gradients. Osmosis can be defined as the passive movement of fluid from an area of lower concentration of solutes to an area of higher concentration. This process happens as the body attempts to dilute stronger solutions to achieve an equal balance on both sides of a membrane (Peate, 2020). The two main body systems that help to maintain fluid regulation are the cardiovascular system and the renal system. The movement of fluid out of the intravascular space and into the interstitial space is dependent on opposing osmotic and hydrostatic pressures (Waugh and Grant, 2010). Hydrostatic pressure is determined by the pressure exerted at contraction of the heart and the resistance of the vessels/capillaries. This pressure forces fluid molecules that are small enough to pass through the membrane out of the vessel and into the interstitial fluid. The opposing, osmotic pressure is determined by plasma proteins that are too large to pass through the membrane even under pressure.
The opposing hydrostatic and osmotic pressures support this movement of fluid (see Table 1).
Table 1. Opposing pressures in the vascular system
| Vessel | Hydrostatic pressure | Osmotic pressure | Purpose |
|---|---|---|---|
| Arteriole | High | Low | The hydrostatic pressure exceeds the osmotic pressure, moving solutes out of the plasma and into the interstitial space |
| Vein | Low | High | Hydrostatic pressure is reduced and the osmotic pressure within the vessel is higher so water is pulled back into the vessel and circulating volume |
Source: adapted from Peate, 2021
Hormonal mechanisms and the renal system are highly influential in fluid balance. Anti-diuretic hormone (ADH) is released in response to changing osmolarity. In patients who are hypovolaemic, osmolarity rises, the ADH acts on the tubules and collecting ducts in the kidneys, inhibiting water excretion and encouraging water reabsorption. Patients who are overloaded will have a fall in plasma osmolarity, this means that the osmoreceptors are no longer stimulated. This in turn inhibits ADH release; renal tubules no longer conserve water and thirst is reduced, leading to a reduction of oral intake and restoration of balance (Peate, 2020).
Physiology of fluid
Although individual body water varies slightly depending upon age and adipose tissue, for the majority of people the body is made up of around 60% water (Marieb, 2017). Water has many essential functions in the body (see Box 1). Despite the clear importance of adequate hydration in maintaining health, it is highlighted by Pinnington et al (2016) that dehydration remains an issue for many patients both within acute care and community settings.
Box 1.Functions of water in the body
- Transportation of nutrients and gasses
- Lubrication of joints and internal organs
- Regulation of body temperature
- Moistening of tissues
- Maintaining and giving shape to cells
- Dissolving vitamins and minerals
- Maintenance of blood volume and therefore blood pressure
- Maintenance of acid–base balance.
Source: adapted from Peate, 2021
Intracellular fluid makes up the majority—approximately two thirds—of body water. The remaining third is outside of the body's cells, this is known as extracellular fluid. Extracellular fluid is divided between the interstitial compartment (approximately 80%) and the intravascular compartment (approximately 20%) (Litchfield et al, 2018).
Maintaining a fluid balance
The term ‘fluid balance’ indicates that the body's required amount of water is present and distributed proportionally (Litchfield et al, 2018). In a healthy person, fluid intake should be equal to output to ensure a constant electrolyte balance. Although fluid intake varies dependent upon the individual, the body regulates fluid volume within a narrow range. The renal system plays a major role in the excretion and regulation of fluids, with urine accounting for around 60% of all output. Average inputs and outputs are shown in Table 2, and it should be noted that these are approximate measurements. Additional to the input and output examples shown in Table 2, it is important to consider any additional input patients may be having such as IV fluids or enteral feeds. Patients may also have increased output from wounds or drains (see Box 2).
Table 2. Examples of average fluid intake and output
| Intake (ml) | Output (ml) | ||
|---|---|---|---|
| Drinking (60%) | 1400–1800 | Urine (60%) | 1400–1800 |
| Water from food (30%) | 700–1000 | Faeces (2%) | 100 |
| Metabolic water, produced by cells (10%) | 300–400 | Expiration from lungs (28%) | 600–800 |
| Diaphoresis (sweating) (10%) | 300–600 | ||
| Total balance (100%) | 2400–3200 | Total balance (100%) | 2400–3200 |
Source: adapted from McCance et al, 2014
Box 2.Possible fluid outputs
- Urine (catheterised/non-catheterised)
- Stools
- Stoma sites
- Wound exudate
- Gastric outlets (nasogastric tubes/gastrostomy)
- Drains
- Suction
- Blood loss
- Insensible loss
Source: adapted from Peate, 2021
In a healthy person, one of the main ways the body regulates fluid balance is via thirst receptors. Fluid balance could be disrupted by excessive fluid output, such as excessive sweating or through reduced input. Small changes to the electrolyte content or the water content of these fluids will lead to a response within the body to maintain homeostasis.
Dehydration stimulates the thirst reflex in the body in three ways: firstly, the blood osmotic pressure increases. Secondly, circulating blood volume decreases, which initiates the renin-angiotensin system. Finally, as a result of dehydration, the mucosal lining of the mouth becomes dry and the production of saliva decreases, all of which stimulates the thirst centre in the hypothalamus (Peate, 2021).
Measuring input/output
How fluid input/output is measured will depend upon the condition of the patient. It is essential that a fluid balance chart is used for accurate documentation. Nurses should be guided on the correct chart to use by their local organisation's policy.
If patients are alert it is important that they are made aware that their fluid balance is being monitored, why it is necessary and how to inform staff of any fluid intake/output. Where appropriate, patients should feel empowered to record their own fluid intake/output (Dougherty and Lister, 2015). It is important that patients are provided with the appropriate resources to measure their own output if possible. A urometer should be used for patients who are catheterised, to allow for accurate assessment of hourly urine output (Pinnington et al, 2016). Measurement of fluid output from bowels may be difficult, depending on the setting. For patients with excessive output due to loose stools, it may be appropriate to consider a bowel management system (Scales and Pilsworth, 2008). Alternatively, loose stool can be measured by volume in a bedpan or using scales to quantify fluid output. If the stool is formed it is not possible to accurately quantify, but this should still be noted on the fluid balance chart to take into account insensible fluid losses (Dougherty and Lister, 2015).
Sumnall (2007) highlights the importance of recording fluid balance hourly. The rationale for this is to obtain accurate real-time fluid balance status. Using an appropriate chart, these totals should then be added together at the end of each 24-hour period.
Professional issues
Dehydration and fluid overload can both present as challenges for patients. For this reason it is important that nurses understand how to measure and monitor fluid balance as well as the role that fluid plays in maintaining homeostasis. The Nursing and Midwifery Council (NMC) (2018) highlight the need to prioritise people and meet their individual hydration needs, while ensuring that the nurse has sufficient knowledge to assess and respond to physical needs.
The NMC (2018) also highlight the importance of accurate record-keeping, this includes the recording of a patient's fluid balance. Incorrect recording of actual or potential fluid balance disturbances could lead to incorrect management. This could result in fluid overload dehydration and/or electrolyte disturbances (Pinnington et al, 2016).
If patients oral/enteral intake is inadequate or they have lost a significant amount of fluid, this will need to be replaced with intravenous (IV) fluid (National Institute for Health and Care Excellence (NICE), 2017). It is the role of the nurse to administer and mange IV fluids for patients. Most commonly used replacement fluids are crystalloids and colloids, which have different effects on a range of important physiological parameters. Crystalloid solutions contain low-molecular-weight salts or sugars, which dissolve in water and freely pass between the intravascular space and the interstitial and intracellular spaces (Powell-Tuck et al, 2011). Colloids contain larger molecules that are unable to cross capillary membranes. Colloid solutions largely remain in the intravascular space (Macintosh, 2011). It is important that the nurse understands why a patient needs IV fluids and is able to regularly assess if IV fluids need to be continued. NICE (2017) recommends that decision-making is supported with daily biochemistry investigations including urea, creatinine and electrolytes.
Assessment of fluid status
Table 3 sets out the pathophysiological changes that can occur.
Table 3. Assessment of fluid status
| Assessment | Usual findings | Indications | |
|---|---|---|---|
| Fluid deficit | Fluid overload | ||
| History-taking | |||
| To understand any pre-existing conditions/medications that may place the patient at risk of fluid imbalance | Individual to each patient | For example: acute vomiting/diarrhoea. Currently taking a diuretic. Poor oral intake due to illness | For example: renal failure, reduced cardiac output, fluid retention |
| Is the patient thirsty? | Usually resolved by drinking | Unusually thirsty | Normal |
| Inspection of mucosa | Usually moist and pink | Dry and white mucosa | Appears to glisten |
| Clinical observations | |||
| Heart rate | Usually 60–90 bpm | Increased | Normal or increased |
| Peripheral pulse | Radial pulse usually easily felt with light palpation | Thready, potentially difficult to palpate | Bounding, easily palpable |
| Blood pressure | Differs for each patient—use patients normal blood pressure for a guide | Hypotensive, usually found in advanced fluid deficit due to blood volume falling beyond usual compensatory mechanisms | May become hypertensive, could also be normal |
| Central venous pressure | 3–10 mmHg | Reduced | Increased |
| Capillary refill | 2–3 seconds | Slower | Faster |
| Respiratory rate | 12–20 bpm | Increased due to compensatory mechanisms demanding more O2 | Respiratory rate could increase due to pulmonary oedema |
| Urine output | 0.5 ml/kg/hourColour: urochrome | ReducedColour: deep amber or light brown | Increased (unless renal function impaired)Colour: Clear |
| Chest auscultation | ‘Rustling’ heard mainly on inspiration | Normal | Additional sounds may be heard, increased crackles may be a sign of overload |
| Skin turgor | When pinched the skin should return to normal | Skin takes longer than normal to bounce back | Pitting oedema may be present |
| Temperature | 36.5–37.5°C | May be elevated, this could lead to increased insensible loss | Normal |
| Neurological | GCS: 15/15 | Headache, New onset of confusion, reduced GCS. Extreme dehydration may cause hyponatraemia leading to seizures | Potential for cerebral oedema, increased intercranial pressure. Can cause loss of cognitive abilities, confusion, delirium |
| Blood results | |||
| Sodium | 135–145 mmol/L | Increased | Reduced |
| Potassium | 3.5–5 mmol/L | If GI losses is driving deficit may see reduction | Normal |
| Urea | 2.5–6.4 mmol/L | Reduced | Normal |
| Creatinine | Male: 63–116 μmol/LFemale: 54–98 μmol/L | In extreme fluid deficit will increase due to poor renal perfusion | Normal |
| Serum osmolarity | 275–295 mmol/kg | Increased | Reduced |
Key: bpm=beats per minute; GCS=Glasgow Coma Scale; GI=gastroinstestinal; ml=millilitre; mmHg=millimetres of mercury; mmol/kg=millimoles per kilogram; mmol/L=millimoles per litre; μmol/L=micromoles per litre
Source: adapted from Dougherty and Lister, 2015
Conclusion
This article has provided an introduction to fluid balance, its measurement, physiology pathophysiology and importance to nursing care. It is beyond the scope of this article to give in-depth explanations of treating hypovolaemia and hypervolaemia and further reading on the subject is required.
LEARNING OUTCOMES
- Have baseline knowledge of the role of fluid in maintaining homeostasis
- Recognise the pathophysiological responses to dehydration and fluid overload
- Understand the importance of measuring fluid input and output and of accurate monitoring and record-keeping