In the third part of our acute care series,Nicola Cooper explains volume resuscitation Normal versus abnormal fluid needs

Doctors learn about average daily fluid needs at medical school, but often little is taught about fluid needs in illness. The average 70 kg person needs about 3 litres day to cover insensible losses and the necessary intake to maintain normal fluid balance. But the patients we deal with are physiologically stressed through illness, and this dramatically alters the volume and composition of the intracellular and extracellular spaces as well as the kinetics of fluid distribution and excretion.

JOHN GREIM/SPL

Patients may have increased fluid losses through fever, dehydration, bleeding, or breathlessness. An extra 500 ml of fluid a day is needed for every degree above 37°C. Gastrointestinal losses are often underestimated, particularly in patients with diarrhoea or bowel obstruction. Some conditions cause capillary leak syndrome, in which serious intravascular volume depletion occurs with peripheral and pulmonary oedema--severe pancreatitis and sepsis are examples. It can get complicated.

To preserve plasma volume after trauma, surgery, or in sepsis, the kidney reabsorbs water avidly when stimulated by antidiuretic hormone. Hyponatraemia is common in hospital due to this stress response and after giving hypotonic fluids (5% dextrose or 4% dextrose and 0.18% sodium chloride). Secretion of antidiuretic hormone is increased by a variety of stimuli in hospitalised patients, including pain, anxiety, opioid and anaesthetic agents, and positive pressure ventilation. Critically ill patients tend to retain sodium yet are given litres of sodium containing fluids to expand their intravascular volume. Although colloid is available in the United States suspended in 5% dextrose, all colloids in the UK are suspended in 0.9% sodium chloride. This tends to cause oedema. In general, you should aim to give fluid for a reason:

Maintenance or electrolyte losses--Hartmann's solution (avoided in renal and liver failure because of its potassium and lactate content)

• Water losses--5% dextrose or nasogastric feed
• Expanding intravascular volume--blood or other colloid

Bleeding

Acute loss of circulating volume leads to compensatory responses. Tachycardia is the first measurable sign. At the cell level, inadequately perfused cells switch to anaerobic metabolism causing lactic acidosis. Mild metabolic acidosis is common after major surgery for this reason, and a persistent or worsening acidosis could indicate inadequate fluid resuscitation. By the time blood pressure has fallen in a healthy patient because of bleeding, at least 30% of circulating blood volume has been lost. Hypotension is a late sign (fig 1).

Fig 1 Assessment of volume status

The fluid challenge

Any signs of volume depletion must be treated with a fluid challenge, which is 250 ml of colloid given over 10 minutes. Simply increasing maintenance fluid will not help--this will take hours to take effect and during this time, hypoperfusion could cause damage to an organ (fig 2).

Fig 2 Why a fluid challenge is needed

A fluid challenge should be given through a large bore cannula using a wide diameter giving set to get an adequate flow rate. Central venous catheters are less suitable for fluid resuscitation because of their length and narrow diameter. After a fluid challenge, the patient must be re-assessed. Several fluid challenges may be needed to reach euvolaemia (fig 3).

Fig 3 The dilemma

In complicated cases, in which assessing true volume status is difficult, some form of advanced monitoring is usually necessary. This monitoring is either based on pressure or flow.

Pressure based monitoring: central venous pressure

The first and most important thing to remember is that the central venous pressure is a pressure not a volume. It is being used to gauge intravascular volume, but it is actually a measure of the pressure in the right atrium, which could bear no relation to intravascular volume at all. Many factors affect the pressure in the right heart. A healthy young person who is bleeding will vasconstrict in response, and their central venous pressure will rise. After receiving a fluid challenge, his or her central venous pressure will fall. This is probably the opposite of what you have been taught (fig 4).

Fig 4 Problems with interpreting central venous pressure

Therefore, unless you are faced with a massively oedematous patient with pulmonary oedema and flapping ears from a raised jugular venous pressure, always give a fluid challenge to assess a patient's true volume status, even in cardiogenic shock. If the central venous pressure does not budge, the patient is underfilled. If the central venous pressure rises slightly and falls again after 5-10 minutes, the patient is underfilled. If the central venous pressure rises above 10 mm Hg and stays there, the patient probably has an adequate circulating volume. This clinical rule of thumb--sometimes termed "the rule of threes"--is extrapolated from Starling's observations in that stroke volume will increase in response to fluid challenges (stretch) up to a certain point. Fluid challenges are especially important in patients who may need inotropes, because there is little point in trying to pharmacologically improve stroke volume when the heart is too empty to eject in the first place (figs 5 and 6).

Fig Fig 5 response of central venous pressure to small fluid challenges

Fig 6 Response of central venous pressure to small fluid challenges

Doctors are moving more and more to flow based monitoring in critically ill patients. Cardiac output can be measured using an oesophageal probe which is placed via the nose. This and other measurements offer a better guide to fluid and inotrope treatment and are far less invasive than catheters placed in the heart.

Different types of fluid

Crystalloids are substances which form true solutions and pass freely through semi-permeable membranes. They contain water, electrolytes, and sometimes dextrose and stay in the intravascular compartment less than an hour. Five litres of crystalloid replace one litre of lost blood because only a quarter of the volume stays in the intravascular compartment. Hypotonic crystalloids (containing dextrose) should never be used in volume resuscitation.

• 0.9% sodium chloride
• dextrose 5%
• dextrose 4% and saline 0.18%
• 0.9% sodium chloride
• dextrose 5%
• dextrose 4% and saline 0.18%
• Hartmann's solution (resembles the extracellular fluid and contains sodium, potassium, calcium, chloride, and bicarbonate)

Colloids are substances which do not form true solutions and do not pass through semi-permeable membranes. There are many different types of colloid, and all stay in the intravascular compartment for varying lengths of time, depending on molecular size.

• Gelatines (Gelofusine and Haemaccel)--stay in the intravascular compartment 1-2 hours
• Dextrans--rarely used because of allergic reactions
• Hydroxyethyl starch (various products with different molecular sizes exist)--some evidence it reduces capillary leak by an unknown mechanism and stays in the intravascular compartment several hours
• Human albumin--stays in the intravascular compartment 2-4 hours

ARTHUR BECK/SPL

Crystalloid versus colloid

Countries like the United States commonly use crystalloids in volume resuscitation whereas in the United Kingdom colloids are more commonly used. Much debate has concerned the right choice of fluid, but all the studies that have compared them have yielded conflicting results.¹ Early animal experiments showed that during trauma and surgery, the interstitial compartment contracts, and the theory is that crystalloids not only resuscitate the intravascular compartment but this compartment as well. However, large volumes of crystalloid are needed to expand intravascular volume, because it is quickly redistributed. This leads to problems with oedema which can effect gas exchange and impair wound healing. Colloids can expand intravascular volume quickly, restoring tissue perfusion earlier, with less sodium load. However, allergic reactions can occur and they are more expensive. The debate continues.

Key points

• Fluid balance in illness is complicated
• In general, give a fluid challenge first, and ask questions later
• The central venous pressure is a pressure not a volume
• Different types of fluid exist for different reasons; a mixture is often used

Case histories

1. You have just put a central venous pressure line into a patient who is unwell and has a severe biliary infection. The blood pressure is 100/50 mm Hg (the patient's normal blood pressure is 180/80 mm Hg), The central venous pressure measures 15 mm Hg. What is your next course of action?
2. A 55 year old man is on the coronary care unit when he develops a low urine output. He is alert, his pulse is 90 beats/min, his blood pressure is 110 mmHg, his respiratory rate 14/min, and his temperature 36°C. His chest is clear. His heart sounds are normal. What is your next course of action?

Answers: (1) You should assess the patient's volume status yourself, and ignore the central venous pressure reading. Then you should administer a fluid challenge and assess the response of the patient and the central venous pressure to this. If you think the patient is well filled, but there is still hypertension and oliguria (caused by profound vasodilatation in sepsis), you need to refer the patient to intensive care for inotropes. (2) Give a fluid challenge, of course.

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