Intravenous fluid therapy

Published on 01/03/2015 by admin

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Intravenous fluid therapy

Intravenous (IV) fluid therapy is an integral part of clinical practice in hospitals. Every hospital doctor should be familiar with the principles underlying the appropriate administration of intravenous fluids. Each time fluids are prescribed, the following questions should be addressed:

Does this patient need IV fluids?

Which fluids should be given?

How much fluid should be given?

How quickly should the fluids be given?

How should the fluid therapy be monitored?

Does this patient need IV fluids?

The easiest and best way to give fluids is orally. The use of oral glucose and salt solutions may be life-saving in infective diarrhoea. However, patients may be unable to take fluids orally. Often the reason for this is self-evident, e.g. because the patient is comatose, or has undergone major surgery, or is vomiting. Sometimes the decision is taken to give fluids intravenously even if the patient is able to tolerate oral fluids. This can be because there is clinical evidence of fluid depletion, or biochemical evidence of electrolyte disturbance, that is felt to be severe enough to require rapid correction (more rapid than could easily be achieved orally).

Which IV fluids should be given?

The list of intravenous fluids that is available for prescription in many hospital formularies is long and potentially bewildering. However, with a few exceptions, many of these fluids are variations on the three basic types of fluid shown in Figure 13.1.

Fig 13.1 The three types of fluid usually used in intravenous fluid therapy are shown here with the different contributions they make to the body fluid compartments.

Plasma, whole blood, or plasma expanders. These replace deficits in the vascular compartment only. They are indicated where there is a reduction in the blood volume due to blood loss from whatever cause. Such solutions are sometimes referred to as ‘colloids’ to distinguish them from ‘crystalloids’. Colloidal particles in solution cannot pass through the (semipermeable) capillary membrane, in contrast with crystalloid particles like sodium and chloride ions, which can. This is why they are confined to the vascular compartment, whereas sodium chloride (‘saline’) solutions are distributed throughout the entire ECF.

Isotonic sodium chloride (0.9% NaCl). It is called isotonic because its effective osmolality, or tonicity, is similar to that of the ECF. Once it is administered it is confined to the ECF and is indicated where there is a reduced ECF volume, as, for example, in sodium depletion.

Water. If pure water were infused it would haemolyse blood cells as it enters the vein. Water should instead be given as 5% dextrose (glucose), which, like 0.9% saline, is isotonic with plasma initially. The dextrose is rapidly metabolized. The water that remains is distributed evenly through all body compartments and contributes to both ECF and ICF. Five per cent dextrose is, therefore, designed to replace deficits in total body water, e.g. in most hypernatraemic patients, rather than those specifically with reduced ECF volume.

How much fluid should be given?

This depends on the extent of the losses that have already occurred of both fluid and electrolytes, and on the losses/requirements anticipated over the next 24 hours. The latter depends, in turn, on both insensible losses and measured losses.

Existing losses

It may not be possible to calculate the exact deficit of water or electrolytes. This is not as critical as one might expect. Even where there is a severe deficit of water or sodium, it is important not to replace too quickly if complications of over-rapid correction are to be avoided. Unless there are severe ongoing losses it is the duration rather than the rate of fluid replacement that varies.

Anticipated losses

It is useful to know what ‘normality’ is, i.e. what the fluid and electrolyte requirements would be for a healthy subject if for some reason they were unable to eat or drink orally. Most textbooks quote a water throughput of between 2 and 3 L daily, a sodium throughput of 100 to 200 mmol/day, and a potassium throughput that varies from 20 to 200 mmol/day. These figures include the insensible losses (those that occur from skin, respiration and faeces); these are not normally measured and, for water, amount to about 800 mL/day. In artificial ventilation or excessive sweating insensible losses may increase greatly.

How quickly should the fluids be given?

The appropriate rate of fluid replacement varies enormously according to the clinical situation. For example, a patient with trauma-induced diabetes insipidus can lose as much as 15 L urine daily. The two very different clinical scenarios below illustrate the importance of the rate of IV fluid replacement.

Perioperative patient

It might be expected that intravenous fluid therapy for a patient undergoing elective surgery would be based simply on ‘normality’ (see above) and that an appropriate daily regimen should include between 2.0 and 3.0 L isotonic fluids, of which 1.0 L should be 0.9% saline (which will provide ~155 mmol sodium), with potassium supplementation. However, this approach does not take account of the metabolic response to trauma, which provides a powerful non-osmotic stimulus to AVP secretion, with resultant water retention, or of the response to physiological stress, which both reduces sodium excretion and increases potassium excretion, or of the redistribution of potassium that occurs as a result of tissue damage. In the immediate postoperative period, a daily regimen that includes 1.0 to 1.5 L IV fluid containing 30 to 50 mmol sodium and no potassium will often be adequate.

Hyponatraemia

Patients with severe hyponatraemia are vulnerable to demyelination if the serum sodium is raised acutely. The mechanism may involve osmotic shrinkage of axons, which leads to severing of the links with their myelin sheaths. Osmotic demyelination is especially likely in the pons (central pontine myelinolysis) and results in severe neurological disorders or death. For this reason, it is recommended that serum sodium should be raised by not more than 10 to 12 mmol/L per day.

How should the fluid therapy be monitored?

The best place to study monitoring of IV fluid replacement in practice is in the intensive care setting. Here, comprehensive monitoring of a patient’s fluid and electrolyte balance (Fig 13.2) allows the prescribed fluid regimen to be tailored to the patient’s individual requirement.

Fig 13.2 This patient has undergone major abdominal surgery and is now 2 days postop.

Clinical note

Assessing a patient’s fluid and electrolyte status has as much, if not more, to do with clinical skill than biochemical interpretation. Look at the patient in Figure 13.2 and think about what information is available. Your answer may include consideration of the following:

case records (details of patient history, examination)

examination of patient (JVP, CVP, pulse, BP, presence of oedema, chest sounds, skin turgor)

fluid balance and nursing charts (BP, pulse, temperature, fluid-input and output)

nasogastric and surgical wound drainage, in addition to urinary catheter bag

presence of IV fluid therapy (type, volume)

ambient temperature (wall thermometer).

Case history 8

Postoperatively, a 62-year-old woman was noted to be getting progressively weaker. There was no evidence of fever, bleeding or infection. Blood pressure was 120/80 mmHg. Before the operation her serum electrolytes were normal, as were her renal function and cardiovascular system. Three days after the operation her electrolytes were repeated.

Random urine osmolality = 920 mmol/kg

Urine [Na⁺] < 10 mmol/L

Urine [K⁺] = 15 mmol/L

• What is the pathophysiology behind these findings?

• What other information do you require in order to prescribe the appropriate fluid therapy?

Comment on page 164.

Intravenous fluid therapy

Intravenous fluid (IV) therapy is commonly used to correct fluid and electrolyte imbalance.

The simple guidelines for IV fluid therapy are:

first assess patient clinically, then biochemically, paying particular attention to cardiac and renal function

use simple solutions

in prescribing fluids, attempt to make up deficits and anticipate future losses

monitor patient closely at all times during fluid therapy.

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