fluid management

BASIC SCIENCE

Fluid managementAndrew Day

Tim Rockall

Lymph(1.5l)

Transcellular(1.5l)

Plasma(3.5l)

Interstitial(8.5l)

Fluid compartments

AbstractThe management of fluid administration to surgical patients before and

after an operation is an integral part of their care. Since the majority of

surgical patients, emergencies or elective, will be receiving intravenous

fluids during a proportion of their stay, it is important that members of

the surgical team have a system to guide fluid usage. This article will

discuss the normal physiology, how to determine fluid requirements for

patients and how to assess fluid balance. It will also summarize the

various types of fluids that are available and provide a framework for

the junior doctor.

Keywords Assessment; colloid; crystalloid; fluid compartments; fluid

replacement

Normal physiology and requirements

Before prescribing a fluid regime for a surgical patient an

understanding of the normal requirements is needed. A 70 kg

man will have 45 litres of total body water (which equates to

approximately 60% of their total body weight) divided between

different body compartments. Their total body water can be

divided into one-third extracellular (15 litres) and two-thirds

intracellular (30 litres). The extracellular fluid is divided further

between plasma (3.5 litres), interstitial fluid (8.5 litres), lymph

(1.5 litres) and transcellular fluid (1.5 litres) (Figure 1). Plasma is

blood minus its cell constituents, and transcellular fluid includes

cerebrospinal fluid, pleural fluid and synovial fluid.

The major extracellular fluid cation is sodium and the anion is

chloride. The major intracellular fluid cation is potassium, and to

a lesser degree magnesium, and the anion is phosphate. In each

compartment the cations and anions balance each other out to

achieve electrical neutrality.

It is important to know the normal daily requirements for an

individual in order to correctly manage their fluid balance during

the pre-/peri- and postoperative period. The normal daily

maintenance fluid requirement for an average male (Table 1) is

35 ml/kg of water, which will equate to approximately 2.5 litres

in a 70 kg man. For an average sedentary adult in temperate

conditions their intake will consist of 2 litres of water coming

from their diet and approximately 500 ml will come from their

metabolism. The average normal fluid output is divided between

Andrew Day BSc MBBS MRCS is a Laparoscopic Research Fellow at the

Minimal Access Therapy Training Unit (MATTU), Guildford, UK. Conflicts

of interest: none declared.

Tim Rockall MBBS FRCS MD is Director of the MATTU and Laparoscopic

Colorectal Surgeon at The Royal Surrey County Hospital, Guildford, UK.

Conflicts of interest: none declared.

SURGERY 28:4 151

urine (1500 ml), as the predominant source, and faeces with

sweating and respiration (1000 ml) accounting for the insensible

losses. The daily maintenance requirement for sodium is 1e1.5

mmol/kg, and for potassium is 1 mmol/kg.

Intravascular fluids, in particular those containing colloids

(large molecules that do not readily cross the capillary wall),

exert an osmotic effect. Osmosis describes the passage of water

across a semi-permeable membrane. Water will diffuse across

the membrane from a volume of low impermeant-solute

concentration to one of higher concentrations until equilibrium

is reached. The Starling equation describes the relation between

hydrostatic and osmotic pressures between the plasma and

interstitial spaces:

Jv ¼ Kf½ðPc � PiÞ � sðpc �piÞ�

Jv, net fluid flux

Kf, filtration coefficient

Pc, capillary hydrostatic pressure e normally about 25 mmHg

Pi, interstitial hydrostatic pressure e normally near zero

s, reflection coefficient e a number between 0 and 1: for capil-

laries impermeable to protein (for example cerebral, glomerular

system) e s approaches 1; for capillaries leaky to protein s is

nearer 0

pc, capillary oncotic (osmotic) pressure e normally about

25 mmHg due to plasma proteins

pi, interstitial oncotic pressure e normally near zero

Thus, the filtrative hydrostatic pressure gradient is nearly

matched by the absorptive osmotic gradient and net water flux is

small (there is a small filtration returned via the lymphatic

system). Note also that the magnitude of osmotic pressure

depends only on the concentration of the colloid and not on the

particular molecule.

Thus, addition of a colloid-containing solution (see below) to

the plasma space will help retain water in the plasma due to

the osmotic pressure it exerts. A crystalloid solution (one that

contains only small ions) has no such osmotic effect across the

Intracellular(30l)

Figure 1

� 2010 Elsevier Ltd. All rights reserved.

Composition and maintenance requirements for anadult

Ion Daily maintenance

requirements

(per kg in mmol)

Composition Composition

Intracellular

(mmol/litre)

Extracellular

(mmol/litre)

Sodium 1e1.5 4e10 135e145

Potassium 1 150 3.5e5

Chloride 1 15 95e105

Phosphate 0.2 100 1.1

Water 35 ml 30 litres 15 litres

Table 1

BASIC SCIENCE

capillary wall and so water will tend to distribute within all the

extracellular spaces.

Assessment

It is important to know how to assess a surgical patient in order

to tailor appropriately their fluid requirements. First assess the

conscious level, as there will be signs of confusion when the

patient is intravascularly depleted. In the case of fluid deficiency

the patient may have dry mucus membranes, be thirsty and have

a reduced skin turgor.

The patient’s observations should be closely inspected, being

careful not to look only at an isolated reading for the morning

ward round, but also at the overall trend. When looking at the

observation chart of a fluid-depleted patient one may see an

elevated heart rate, a narrow pulse pressure (the difference

between the systolic and diastolic blood pressure), or a low blood

pressure.1

Alongside the observation chart will be the fluid balance

chart, giving you an accurate reading of the last 24 hours of fluid

in and out. The urine output of a patient should be carefully

monitored, with an indwelling urethral catheter, in the imme-

diate postoperative period and thereafter by the use of bottles.

Urine output should be approximately 0.5 ml/kg/hour for an

adult. During the initial 24-hour postoperative period, however,

this amount may be reduced secondary to the insult of the

operation. Due to the stress of the surgery there may be an

excessive release of vasopressin (ADH) from the posterior pitu-

itary gland, which will reduce the amount of water excreted from

the kidneys.2 There will also be an associated hyponatraemia.

The urine output over a 24-hour period can be compared to

the intravenous/oral fluid input during the same period thus

determining the fluid balance of the individual concerned. This

will assist you in calculating the continued fluid requirements for

the coming day.

In addition to the above observations many postoperative

patients will have a central venous catheter in situ which allows

you to measure the central venous pressures (CVP). The CVP can

be measured using either a standard water manometer or if the

ward has the monitoring facilities a more accurate transduced

reading. The actual CVP reading is useful as a baseline, the

response of the CVP to a fluid challenge is more helpful to guiding

fluid replacement.3 In response to a 200 ml bolus of colloid the

CVP will either stay the same or increase. If the CVP stays the

SURGERY 28:4 152

same, decreases or rises only transiently the patient is probably

still hypovolaemic and further fluid replacement is required. If

there is a sustained rise of greater than or equal to 3 mmHg the

intravascular compartment is probably filled appropriately.4

The patient must also have daily weights and regular

biochemical monitoring to guide both the quantity and type of

fluid replacement given. Lastly, it must be remembered that no

single indicator can be used to draw conclusions about fluid status.

Each patient must be individually assessed, taking into account

pre-morbid conditions, intra-operative blood loss, clinical

assessment and observation of fluid balance.

Fluid replacement

Preoperative

Surgical fluid management has traditionally been focused on the

postoperative period. Optimization of fluid status, however,

must begin preoperatively. Many traditional preoperative inter-

ventions have the potential to leave the patient in a hypo-

volaemic state before the operation has even begun. In the period

preceding bowel surgery a patient may be given an oral bowel

preparation (that is Picolax�), with or without covering intra-

venous fluids. They may also be kept nil by mouth for a varying

period of time. Both of these processes may cause dehydration

and there is growing evidence that they are actually unnecessary.

A recent meta-analysis has shown that there is no benefit from

using mechanical bowel preparation in colorectal surgery and it

may possibly lead to a significant increase in the anastomotic

leak rate, depending on the formulation used.5

The ‘Enhanced Recovery after Surgery’ programme, amongst

other things, aims to prepare the patient for surgery in an optimal

fashion.6 Patients are allowed to eat and drink the night before

surgery. They do not receive an oral bowel preparation, and are

given a carbohydrate loading drink the night before, and the

morning of, their operation. These procedures have the benefit of

producing a patient with less fluid and electrolyte disruptions. In

addition, the carbohydrate loading will reduce postoperative

insulin resistance, and patients are therefore in a more anabolic

state following surgery.7

Perioperative

The intra-operative fluid management of surgical patients is

extremely important in improving patient outcomes. During the

operative period it is important to maximize the oxygen delivery to

tissues. Previous work has identified that patients with a high

oxygen delivery (DO2), around the 600 ml/min/m2 mark, are more

likely to survive major surgery.8 One aspect of intra-operative

care that can significantly alter the oxygen delivery is fluid

administration. It has been shown that maintaining the cardiac

output of a patient at its maximal level can reduce morbidity and

shorten hospital stay.9 Indeed, as has been shown, it is possible to

reduce the hospital stay to 23 hours for selected colorectal resec-

tion patients if targeted fluid administration is combined within an

enhanced recovery programme.10 What is currently unknown is

the correct type of fluid to use in this situation.

The question is how to monitor the cardiac output effectively

during the operation. There are a number of possibilities. The

cardiac output is a product of heart rate and stroke volume. The

original method for calculating cardiac output was determined by

Fick in 1870. The Fick Principle involves a calculation based on


BASIC SCIENCE

the oxygen consumption over a given period from measurement

of the oxygen concentration of arterial blood and venous blood.

This method is not practical for the bedside and has led to the

development of other techniques. The pulmonary artery catheter

will allow the calculation of cardiac output by the thermodilution

technique. Dilution of heat in a given portion of blood will allow

calculation of the cardiac output by using the modified Stewart-

Hamilton equation. This method can also be performed using

an indicator (for example lithium) dilution technique.

An oesophageal Doppler probe is a thin plastic tube placed in

the oesophagus parallel to the descending aorta and emits an

ultrasound wave directed at the flow of blood. The cardiac output

is calculated from the amount of blood that moves past the probe

over a given time (stroke distance) and estimates the cross-

sectional area of the aorta determined from normograms. This

technique has the benefit of providing beat-to-beat analysis, is

easily reproducible and is minimally invasive.

Arterial pulse contour analysis measures the stroke volume on

a beat-to-beat basis from the arterial pulse pressure waveform.

This requires insertion of a manometer into an artery, which will

measure continuously the pulse pressure waveform. This can

either be calibrated using a standard technique such as thermo-

dilution (PiCCO�) or lithium dilution (LiDCO�) or can be used

uncalibrated (FloTrac�).11

These are some of the methods used to fairly accurately

determine the cardiac output of surgical patients during an

operation and on the intensive care unit. Their use should help to

improve the outcome of the patient.

Postoperative

The daily fluid requirements for a postoperative patient are

a combination of the maintenance fluids (35 ml/kg/day), added to

continuing losses (for example naso-gastric aspirates). Continuing

losses may not always be apparent. Trauma or pancreatitis patients

can lose litres of fluid into the transcellular compartment (some-

times exceeding 10 ml/kg/h4), which is not easily measurable. This

is termed ‘third-spacing.’ These losses can be determined by

a thorough assessment of the patient, looking at the parameters

(fluid balance, heart rate, blood pressure, etc.) explained above.

It is also important to pay close attention to electrolyte balance,

again combining maintenance and additional requirements.

Attention must be paid to the current clinical state of the patient.

For example, a patient with high naso-gastric aspirates or diar-

rhoea is likely to be losing potassium. If the patient has a raised

temperature they will increase the amount of fluid that is lost to

insensible losses via sweating and a raised respiratory rate. If the

core temperature is raised by 1 �C then an additional 1 litre of fluid

will be required over a 24-hour period. The amount of fluid

required, and particular electrolyte requirement, will determine

the type of fluid used and any additional supplements.

One should aim to discontinue intravenous fluids as soon as

a patient can manage oral fluids. This will not only remove the

risks associated with intravenous cannulas, but will also provide

a more appropriate fluid balance and reduce bacterial trans-

location in the gut.

Fluids

Intravenous fluids can be divided into two broad categories:

colloids and crystalloids.

SURGERY 28:4 153

Crystalloids

Crystalloids are solutions of salts, such as sodium chloride, and

other solutes, such as glucose, dissolved in water. The solutions

may be isotonic, hypotonic or hypertonic. There are three crys-

talloid solutions that are typically used in the surgical patient:

5% dextrose is an isotonic solution of 50 g of glucose dissolved

in each litre of water. On entering the blood, glucose is rapidly

metabolized to leave water, which will distribute evenly amongst

all body compartments. Approximately 7% of an infused aliquot

of 5% dextrose will remain in the intravascular circulation.

0.9% isotonic saline (‘normal saline’) is a solution of

approximately 0.9 g of sodium chloride in each litre of water.

Approximately 20% of an infused amount of normal saline will

remain in the intravascular compartment.

Hartmann’s or Ringer’s lactate are balanced or physiological

isotonic solutions which more closely represent plasma constit-

uents. These contain potassium, calcium and lactate or bicar-

bonate added to the sodium chloride solution.4

Colloids

A colloid is a homogeneous non-crystalline substance consisting

of large molecules dissolved in isotonic saline or isotonic balanced

electrolyte solutions. They can generally be divided into semi-

synthetic colloids (gelatins, dextrans, hydroxyethyl starches) or

naturally occurring human plasma derivatives (human albumin

solution, plasma protein fraction, fresh frozen plasma and

immunoglobulin solution).4 The colloid molecular size can vary

depending on the type of solution used. Depending on the type of

colloid used (essentially the molecular size) it will remain in the

intravascular compartment almost completely for a number of

hours.

Gelatins (Gelofusin�, Haemaccel� and Volplex�) are derived

by hydrolysis of bovine collagen to produce the appropriate

molecular size and are crosslinked with various substances.

Dextrans are produced by enzymatic cleavage of sucrose by

dextran sucrase from the bacterium Leuconostoc. Dextran colloids

are produced in two forms of dextran 40 (number averaged

molecular weights of 40,000) and dextran 70 (number averaged

molecular weights of 70,000). Hydroxyethyl starches (HES) (Vol-

uven�, Volulyte�, Pentaspan� and Hespan 6%�) are generated

from amylopectin, which is a polymer present in maize.

The normal fluid regime for a surgical patient will use crys-

talloids for day-to-day maintenance. Colloids are used for

resuscitation situations, for intra-operative fluids and for fluid

challenges. A standard maintenance regime, not including addi-

tional losses, for a 70 kg adult could be:

� 1 litre 5% dextrose þ 20 mmol potassium chloride over

8 hours

� 1 litre 5% dextrose þ 20 mmol potassium chloride over

8 hours

� 1 litre 0.9% saline þ 20 mmol potassium chloride over

8 hours

Complications

It is important to be aware of the potential complications with

intravenous fluid use. Intravenous cannulation should be dis-

continued as soon as possible to reduce the risk from phlebitis,

tissue infiltration,12 infection, haematoma or thrombosis. Close

monitoring of the fluid balance to prevent under and overfilling


BASIC SCIENCE

of patients with its subsequent associated risks is important,

particularly in patients with cardiovascular abnormalities.

Colloids have a potential risk of anaphylaxis, they can affect

haemostasis and are significantly more costly than crystalloid

solutions. If there is significant disruption to the capillary endo-

thelial cell layer they can diffuse into the interstitial space and

produce oedema secondary to their osmotic properties. A recent

Cochrane systematic review of colloids versus crystalloids for

fluid resuscitation in critically ill patients identified there was no

improvement in survival with the use of colloids over crystal-

loids13 in this cohort of patients.

When using crystalloids care must be taken with the use of

saline solutions. If too much saline is administered there is the

potential to develop hyperchloraemic metabolic acidosis due to

the high chloride load. For this reason balanced isotonic solu-

tions (Hartmann’s) should be used when larger volumes may be

required.

Special situations

Burns

Thermal injury to the skin can result in the loss of significant

amounts of fluid from exposed raw surfaces. Intravenous fluid

replacement is usually commenced when greater than or equal to

15% of total body surface is involved by burns. In order to

calculate the amount of fluid to give there are a number of

formulae available, the most commonly used is the Parkland

formula.14 This involves calculating the total body surface area

(TBSA) that is burned in percentage by using a system such as

Wallace’s rule of nines. The formula is:

TBSA burned ð%Þ �weight ðkgÞ � 4 ml

The first half of the calculated amount is given evenly over

8 hours and the rest of the volume is given over the subsequent

16 hours, from the time of the burn. The most frequent fluid used

is Hartmann’s solution.14

Trauma

Key points

C An average male adult will have 45 litres of water divided by

two-thirds to the intracellular compartment and one-third to

the extracellular compartment.

C The normal daily maintenance requirement of water for an

adult is 35 ml/kg/day.

C Assessment of a patient’s fluid status will include observations,

fluid balance, central venous pressure, weight and biochemical

tests.

C Preoperative fluid preparation is just as important as the

postoperative fluid administration.

C It is important to monitor patients closely intra-operatively

and postoperatively.

C Fluids that are commonly used are either crystalloid or colloids,

which have different properties.

Trauma management in the UK follows advanced trauma life

support (ATLS�) guidelines.15 Fluid resuscitation in the adult is

initiated with 2 litres of Hartmann’s solution given rapidly

through large peripheral cannulas. While monitoring the patient

you must then observe for a response as discussed above.

Patients will fall into three categories: responder, transient

responder and non-responder. If a patient is classified as a tran-

sient or non-responder they will likely need blood products and

of course a definitive treatment for the cause of their hypo-

volaemia to stabilize them. A

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