Intravenous Fluids

Prescribing IV fluids is a common task within surgery, as well as practically all inpatient medical jobs. The information here 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 constantly move between the fluid spaces to find 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 do 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 at the expense of the intravascular space, resulting in hypotension and reduced perfusion of tissues.


Fluid Balance

Monitoring fluid balance is common in hospitals and involves recording all the fluid intake and all the fluid output on a fluid balance chart. 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. 

Some patients require a specific fluid restriction (e.g., heart failure, renal failure or hyponatraemia), meaning the total fluid intake is capped (e.g., 1.5 litres per day). Extra caution needs to be taken in these patients to calculate their input accurately. Seek advice from seniors when in doubt.

Sources of fluid intake include:

  • Oral fluids
  • Nasogastric or PEG feeds
  • Intravenous fluids (including IV medications)
  • Total parenteral nutrition


Sources of fluid output include:

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


Insensible fluid loss 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 (over 800mls per day) in patients with significant diarrhoea, high stoma output or sweating with a high fever.

TOM TIP: The fluid balance chart helps assess whether the input matches the output to guide the prescription of IV fluids. When prescribing fluids, always 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 fluid. Sometimes a bit of scepticism 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, leading 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
  • Reduced 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 saturation, raised respiratory rate and bibasal crackles)
  • Raised JVP
  • Increased body weight from baseline (regular weights are an important way of monitoring fluid balance)


Patients with third-spacing may have a low level of fluid in the intravascular space, but excessive fluid in other areas (such as the interstitial space or peritoneal cavity). This can give signs of hypovolaemia (e.g., hypotension, tachycardia and prolonged capillary refill time) and 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 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”)
  • 5% dextrose
  • 0.18% sodium chloride in 4% glucose
  • Hartmann’s solution
  • Plasma-Lyte 148


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

  • 1 litre of water
  • 154 mmol sodium (note that this is a lot of sodium, and lots of saline can result in hypernatraemia)
  • 154 mmol chloride


A 1 litre bag of 5% dextrose contains:

  • 1 litre of water
  • No electrolytes (note that lots of hypotonic fluid can result in hyponatraemia and oedema)
  • 50 g of glucose


A 1 litre bag of 0.18% sodium chloride in 4% dextrose contains:

  • 1 litre of water
  • 31 mmol sodium
  • 31 mmol chloride
  • 40 g of glucose


A 1 litre bag of Hartmann’s solution contains:

  • 1 litre of water
  • 131 mmol sodium
  • 111 mmol chloride
  • 5 mmol potassium
  • 2 mmol calcium
  • 29 mmol lactate (helps to buffer the solution – reducing the risk of acidosis)


A 1 litre bag of Plasma-Lyte 148 contains:

  • 1 litre of water
  • 140 mmol sodium
  • 98 mmol chloride
  • 5 mmol potassium
  • 1.5 mmol magnesium
  • 27 mmol acetate (helps to buffer the solution – reducing the risk of acidosis)
  • 23 mmol gluconate (helps to buffer the solution – reducing the risk of acidosis)


TOM TIP: Normal 0.9% saline has a concentration of sodium of 154 mmols/L. The normal blood concentration of sodium is 135-145 mmols/L. When you use normal saline, you are adding a lot of sodium into the system, significantly increasing the risk of hypernatraemia. Normal saline also carries a risk of causing metabolic acidosis due to adding so much chloride (hyperchloraemic acidosis). For these reasons, Hartmann’s or Plasma-Lyte are often better options. 


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 is no 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 increases 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 as a junior doctor working in hospitals, other than albumin in patients with ascitic drains, on the instructions of the gastroenterology team.



Tonicity refers to the osmotic pressure gradient between two fluids across a membrane. This determines whether water molecules will move across the membrane by osmosis, from the area of lower concentration of solutes to the area of higher concentration of solutes. The normal serum osmolality is 275 – 295 mOsmol/Kg.

Isotonic solutions (e.g., 0.9% saline, Hartmann’s and Plasma-Lyte) match the concentration of solutes (osmolality) in the plasma. 

Hypotonic solutions (e.g., 5% dextrose and 0.18% sodium chloride) 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 lower concentration of solutes to an area of higher concentration of solutes by osmosis. Theoretically, if you dilute the blood with a hypotonic solution, water will flow out of the blood into the interstitial space. This is why hypotonic solutions (e.g., 5% dextrose) are not used for fluid resuscitation. Hypotonic solutions also carry a risk of hyponatraemia (low sodium) by diluting the sodium content of the blood.


Resuscitation IV Fluids

Where your assessment suggests 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 for fluid resuscitation. This usually means a choice of either:

  • 0.9% saline 
  • Hartmann’s solution
  • Plasma-Lyte 148


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

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 cannot give a rapid infusion of a fluid with a high potassium content. 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 specific scenarios under expert supervision with cardiac monitoring and through a central line (rather than a peripheral cannula).


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 bowel obstruction. As soon as they are able to meet their nutritional needs orally, the IV fluids should be stopped.

The NICE guidelines give approximate 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
  • Approximately 70 mmol sodium, chloride and potassium
  • 50 – 100 g glucose


You can select a bag of IV fluid with additional potassium added to meet the potassium requirements of the patient. Fluids may come with an additional 20 mmol or 40 mmol of potassium in a 1 litre bag. Potassium should not be added to fluids. The mixtures should come ready-made from the manufacturer. 

The NICE guidelines suggest starting with 25-30 ml/kg/day of 0.18% sodium chloride in 4% glucose with 27 mmol/l of added potassium. This is available as a ready-made solution. They point out that more than 2.5 litres of this fluid increases the risk of hyponatraemia as it is hypotonic.


Maintenance Requirements / Day

70kg Man / Day

1 Litre 0.18% sodium chloride in 4% glucose with 27 mmol/l potassium

2 Litres


25 – 30 ml / kg

2 litres (approx)

1 litre

2 litres


1 mmol / kg

70 mmol

31 mmol

62 mmol


1 mmol / kg

70 mmol

31 mmol

62 mmol


1 mmol / kg

70 mmol

27 mmol

54 mmol


50 – 100 g

50 – 100 g

40 g

80 g


Daily monitoring (at least) is required to ensure the correct fluids are prescribed. This includes assessment of their fluid status, fluid balance chart and U&E blood tests. A maximum of 24 hours of fluids are prescribed at one time.

Too much fluid can lead to dilution of important components of the blood:

  • Sodium (with hypotonic solutions)
  • Potassium (if potassium is not included)
  • Other electrolytes, e.g., calcium or magnesium
  • Haemoglobin and haematocrit (red blood cells in the blood) causing anaemia
  • Clotting factors, platelets and fibrinogen causing coagulopathy (clotting problems)


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 or hypernatraemia)
  • Heart failure
  • Renal impairment
  • Liver impairment


TOM TIP: Surgeons and anaesthetists often prescribe Hartmann’s solution or Plasma-Lyte 148 as maintenance fluids, as they are balanced, isotonic solutions. They are less likely to cause hyponatraemia than hypotonic solutions (e.g., 5% dextrose or 0.18% sodium chloride), which is important in surgical patients. They are also less likely to cause hypernatraemia than normal saline.


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 interrupt 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. This might involve waking the patient up, disturbing important rest. If the fluids are being stopped out of hours, remember to give clear instructions to the nurses and in the notes.


Last updated August 2021