Intravenous Fluids

Prescribing IV fluids is a common task on surgery, but also important in all inpatient medical jobs. The information here is based on the NICE guidelines last updated 2017 on “intravenous fluid therapy in adults in hospital” and is designed to help you understand the concepts for your exams. When prescribing fluids follow national and local guidelines and consult seniors if you are in doubt. You should not use this as a guide to prescribing.


Fluid Compartments

Fluid within the body is distributed across different “spaces”. Water, electrolytes, glucose and proteins are constantly moving between the fluid spaces to find a balance and carry out functions. There are two categories of fluid spaces in the body:

  • Intracellular space (inside the cells) – 2/3 of the total body fluids
  • Extracellular space (outside the cells) – 1/3 of the total body fluids


The extracellular space is subdivided into three spaces:

  • Intravascular space (inside blood vessels) – 20% of the extracellular fluid
  • Interstitial space – the functional tissue space between and around cells – 80% of the extracellular fluid
  • The “third space” – the “third” extracellular space


The third space refers to areas of the body that does not normally contain fluid, and where fluid collection is not functional or desirable. This includes areas such as the:

  • Peritoneal cavity (forming ascites)
  • Pleural cavity (forming pleural effusions)
  • Pericardial cavity (forming a pericardial effusion)
  • Joints (forming joint effusions)


The third space also refers to the non-functional and excessive collection of fluid in the interstitial space, resulting in oedema.

Third-spacing refers to fluid shifting into this non-functional third space. Often this refers to the development of oedema, as excessive fluid moves into the interstitial space. It also refers to the development of ascites, effusions or other non-functional fluid collections within the body. When fluid moves into a non-functional space, this may come as the expense of the intravascular space, resulting in hypotension and reduced perfusion of tissues.


Fluid Balance

Monitoring fluid balance is common in hospital, and involves recording on a fluid balance chart all the fluid intake and all the fluid output. It is particularly helpful in acutely unwell patients (e.g. sepsis), surgical patients and those with fluid balance issues, such as diarrhoea and vomiting, heart failure or kidney disease.

Sources of fluid intake include:

  • Oral fluids
  • Nasogastric or PEG feeds
  • Intravenous fluids


Sources of fluid output include:

  • Urine output
  • Bowel or stoma output (particularly diarrhoea)
  • Vomit or stomach aspiration
  • Drain output
  • Bleeding
  • Sweating


Insensible fluid losses is a term that refers to fluid output that is difficult to measure, such as through respiration (breathed out), in stools, through burns and from sweat. This varies a lot and can only be estimated. It may account for a large volume (in excess of 800mls per day) in patients with significant diarrhoea or high fevers and significant sweating.

The fluid balance chart is helpful in assessing whether the input is matching the output to guide the prescription of IV fluids. When prescribing fluids always have a look at the fluid balance chart from that day and the previous day, to see whether they have a positive or negative fluid balance. If they have a negative fluid balance, they may require additional IV fluids, and if they have a positive fluid balance they may require less IV fluids. Sometimes a bit of skepticism needs to be applied, as the chart might not be completely accurate. The patient may be visiting the toilet or sneaking large drinks without informing the nurse, or there may be omissions from the chart, particularly when the ward is very busy.


Assessing Fluid Status

Having an abnormally low amount of extracellular fluid can be referred to as hypovolaemia or volume depletion. A negative fluid balance is when more fluid is leaving the body than coming in, and this will lead to hypovolaemia.

Having an abnormally high amount of extracellular fluid in the body can be referred to as hypervolaemia or fluid overload. A positive fluid balance is when more fluid is entering the body than being removed, and will lead to hypervolaemia.

Signs of hypovolaemia (inadequate fluid) are:

  • Hypotension (systolic < 100 mmHg)
  • Tachycardia (heart rate > 90)
  • Capillary refill time < 2 seconds
  • Cold peripheries
  • Raised respiratory rate
  • Dry mucous membranes
  • Reduced skin turgor
  • Reduced urine output
  • Sunken eyes
  • Reduce body weight from baseline
  • Feeling thirsty


Signs of fluid overload are:

  • Peripheral oedema (check the ankles and sacral area)
  • Pulmonary oedema (shortness of breath, reduced oxygen saturations, raised respiratory rate and bibasal crackles)
  • Raised JVP
  • Increased body weight from baseline


Patients with third-spacing may have a low level of fluid in the intravascular space, but excessive fluid other areas (such as the interstitial space or peritoneal cavity). This can give signs of hypovolaemia (e.g. hypotension, tachycardia and reduced capillary refill time) as well as signs of fluid overload (e.g. oedema and ascites).


Indications for IV Fluids

The main indications for IV fluids are:

  • Resuscitation (e.g. sepsis or hypotension)
  • Replacement (e.g. vomiting and diarrhoea)
  • Maintenance (e.g. nil by mouth due to bowel obstruction)


Generally, IV fluids should be avoided if the patient can adequately meet their fluid requirements with oral fluids.


Types of IV Fluid

There are two main groups of IV fluids:

  • Crystalloids
  • Colloids


Crystalloids are essentially water with added salts and / or glucose. The contents of crystalloid solutions will redistribute throughout the different fluid compartments of the body. Common examples are:

  • 0.9% sodium chloride (normal saline)
  • Hartmann’s solution
  • 5% dextrose


Tonicity refers to the osmotic pressure gradient between the fluid solution and the blood plasma. Isotonic solutions (e.g. 0.9% saline and Hartmann’s) match the concentration of solutes in the plasma. Hypotonic solutions (e.g. 5% dextrose) have a lower concentration of solutes than the plasma. Hypertonic solutions (e.g. 3% saline) have a higher concentration of solutes than the plasma. Water will flow from the area of low concentration of solutes to an area of higher concentration of solutes by osmosis. This means theoretically giving an intravenous hypotonic solution will result in the water flowing out of the intravascular space to the interstitial space. This is the reason hypotonic solutions (e.g. 5% dextrose) are generally used for maintenance fluids and not fluid resuscitation.

Colloids contain larger molecules that stay in the intravascular space longer. Theoretically, this helps to retain fluid in the intravascular space. However, the research suggests there isn’t really much benefit to using them in resuscitation scenarios.

One example of a colloid is human albumin solution, which may be used in patients with decompensated liver disease. Albumin is an important component of plasma, and works to increase the plasma volume. It increases the oncotic pressure of the plasma, drawing in and retaining fluid. In decompensated liver disease, the patient’s liver is not producing adequate albumin, leading to reduced oncotic pressure in the intravascular space, resulting in reduced circulating blood volume. Human albumin solution may be used to help correct this, although the effects are only temporary.

TOM TIP: During most medical and surgical jobs you will practically only prescribe crystalloid fluids. I don’t remember ever prescribing a colloid solution during my time as a junior doctor working in hospitals.


Resuscitation IV Fluids

Where your assessment suggestions the patient is hypovolaemic and needs fluid resuscitation, you can prescribe boluses of IV fluid to rapidly improve their fluid status. Seek senior input where you are unsure or there is shock (circulatory failure).

An isotonic fluid should be used (with a sodium in the range of 130-154 mmol/l). This usually means a choice of either:

  • 0.9% saline
  • Hartmann’s solution


An ABCDE assessment of the patient is used to determine their fluid status. Signs such as hypotension, tachycardia and reduced capillary refill time indicate the need for fluid resuscitation (see above for a full list of signs of hypovolaemia).

Establish the underlying cause of the hypovolaemia (e.g. sepsis).

The NICE guidelines suggest:

  • An initial 500 ml fluid bolus over 15 minutes (“stat”), followed by reassessment with an ABCDE approach
  • Repeat boluses of 250 – 500 mls of fluid if required, each time followed by a reassessment
  • Seek expert help if the patient is not responding, particularly after 2 litres of fluid


TOM TIP: You can prescribe normal 0.9% saline or Hartmann’s as a rapid infusion in a resuscitation scenario, but you cannot give a rapid infusion of a fluid containing potassium. Under normal circumstances, the rate of potassium infusion should not exceed 10 mmol/hour, as there is a risk of inducing an arrhythmia or cardiac arrest. Higher rates are only used in certain scenarios under expert supervision with cardiac monitoring.


Replacement IV Fluids

IV fluids can be used to replace fluids in a patient with a negative fluid balance, where the fluid losses are greater than the fluid intake. This involves calculating or estimating the losses, and prescribing additional fluids to account for these losses.


Maintenance IV Fluids

Maintenance IV fluids are used for the shortest time possible where the patient is unable to take fluid orally, for example while nil by mouth waiting for surgery, or in a bowel obstruction. As soon as they are able to meet their nutritional needs orally, the fluids should be stopped.

The NICE guidelines state the following requirements of maintenance IV fluids:

  • 25 – 30 ml / kg / day of water
  • 1 mmol / kg / day of sodium, potassium and chloride
  • 50 – 100 g / day of glucose (this is to prevent ketosis, not to meet their nutritional needs)


The weights are based on the ideal body weight, rather than their actual body mass index. This avoids excessive prescriptions for obese patients.

For example, if you had a 70kg man, they would require:

  • Approximately 2 litres of water
  • 70 mmol sodium, chloride and potassium
  • 50 – 100 g glucose


A 1 litre bag of normal 0.9% saline solution contains:

  • 1 litre of water
  • 154 mmol sodium
  • 154 mmol chloride


A 1 litre bag of 5% dextrose contains:

  • 1 litre of water
  • 50 g of glucose


You can select a bag of IV fluid that already has potassium added to meet the patients potassium requirements. Examples are fluids with 20 mmol or 40 mmol of potassium in 1 litre. Potassium should not be added to fluids, the mixtures should come ready-made from the manufacturer.

Therefore, for a 70kg man, to meet their maintenance requirements you could prescribe:

  • 1 litre normal saline with 40 mmol potassium
  • 1 litre 5% dextrose with 40 mmol potassium



Maintenance Requirements / Day

70kg Man / Day

1 Litre 0.9% Saline

1 Litre 5% Dextrose

1 Litre Saline + 1 Litre 5% dextrose


25 – 30 ml / kg

2 litres (approx)

1 litre

1 litre

2 litres


1 mmol / kg

70 mmol

154 mmol

0 mmol

154 mmol


1 mmol / kg

70 mmol

154 mmol

0 mmol

154 mmol


1 mmol / kg

70 mmol

40 mmol if added

40 mmol if added

80 mmol if added


50 – 100 g

50 – 100 g

0 g

50 g



To avoid fluid overload and problems with abnormal fluid or electrolyte distribution, take additional caution and seek senior guidance with:

  • Elderly or frail patients
  • Significant oedema
  • Sodium imbalance (hyponatraemia / hypernatraemia)
  • Heart failure
  • Renal impairment
  • Liver impairment


TOM TIP: In reality, the fluid prescriptions are usually estimated, and over time you will develop a sense of what to prescribe rather than calculating the exact requirements for each patient you see. A common prescription of maintenance fluids is to give 3 litres of fluid over 24 hours, two dextrose and one saline, two with extra added potassium (see the example below). In patients with healthy livers, kidneys and hearts, giving a bit of extra fluid will not be harmful and is better than prescribing too little fluid resulting in dehydration. The excess will simply be excreted as urine.


Practicalities of Prescribing

A fluid prescribing chart will typically require you to fill in the following sections:


When prescribing the rate, the typical options are:

  • Stat – indicates it should be given as quickly as it will run through the cannula
  • Over X hours – indicating the number of hours you want it to be infused across
  • X ml / hour – e.g. 125 ml / hour will give 1 litre over 8 hours (volume of fluid / number of hours)


TOM TIP: When prescribing maintenance fluids during a normal working day, try to ensure enough bags of fluid are prescribed to last through to the next working day (unless you want the fluids to stop). If the bag of fluid runs out at 2 AM and no further bags are prescribed, the on-call junior doctor will get a bleep to attend the ward and prescribe more fluids. Not only does this interrupts whatever that doctor is doing at the time, but they have to work out what to prescribe for a patient that they don’t know. Prescribing enough bags of fluids for your patients to last until their regular team is next in prevents any issues during the on-calls.


Last updated February 2021
WordPress Theme built by Shufflehound. Copyright 2016-2021 - Zero to Finals - All Rights Reserved