Page 6 - Medicine and Surgery

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2 Chapter 1: Principles and practice of medicine and surgery

an excess or lack of salt and/or water intake, this normal circulating volume or sodium and water balance. Exam-
balance may be disturbed. ples include patients with any history of cardiac, liver or
Thereareessentiallyfourpatternsofwaterandsodium renal failure, those with symptoms such as vomiting and
imbalance: diarrhoea, perioperative patients or any patient who has
Sodium depletion is usually due to excess sodium loss, other losses, e.g. from bleeding or drains. Clinical evalu-

e.g. due to vomiting or diarrhoea, or burns. Water is ation of fluid balance requires the observation of several
lost with the sodium, so the serum sodium usually signs that together point to whether the patient is eu-
remains normal, but hypovolaemia results. If hyper- volaemic(normalfluidbalance),fluiddepleted(reduced
tonic fluid is lost or if there has been water replace- extracellular fluid) or fluid overloaded (increased extra-
ment but insufficient sodium replacement (typically cellular fluid). In most cases when the patient is fluid
inapatientwhoisvomitingandonlydrinkingwateror depleted, there is decreased circulating volume; however
only given intravenous 5% dextrose or dextrosaline), in fluid overload, there may either be increased circulat-
hyponatraemia results, which can lead to confusion, ing volume or decreased circulating volume depending
drowsiness, convulsions and coma (see page 4). on the mechanism.
Waterdeficiency due to inadequate intake of water Fluid depletion may be suggested by an appropriate

leads to dehydration. The plasma osmolality rises and history of losses or reduced intake, but this can be un-
hypernatraemia occurs. This stimulates thirst and va- reliable.Symptomsofthirstandanyposturaldizziness
sopressin release, which increases water reabsorption should be enquired about. Signs of volume depletion
by the kidneys. Pure water depletion is rare, but many include a mild tachycardia, reduced peripheral per-
disorders mostly lead to water loss with some sodium fusion (cool dry hands and feet, increased capillary
loss. Initially water moves from the cells into the extra- refilltime >3seconds), postural hypotension and/or
cellular compartment, but then both the intracellular hypotension, and reduced skin turgor (check over the
and extracellular compartments become volume de- anterior chest wall as the limbs are unreliable, partic-
pleted, causing symptoms and signs of fluid depletion ularly in the elderly). The jugular venous pressure is
(see section Assessing Fluid Balance below). low and urine output reduced (oliguria, see later in
Sodium excess rarely occurs in isolation. It is usually this chapter).

found in combination with water excess, causing fluid Fluid overload is more likely to occur in patients with
overloadwithperipheraloedema,pulmonaryoedema cardiac, liver or renal failure, particularly if there has
and hypertension. The effect on serum sodium and been over-enthusiastic fluid replacement. Breathless-
fluid balance depends on the relative excess of sodium ness is an early symptom. Tachypnoea is common and
compared to water. Sodium excess > water excess there may be crackles heard bilaterally at the bases of
causes hypernatraemia (see page 3) whereas water ex- the chest because of pulmonary oedema. The jugu-
cess > sodium excess causes hyponatraemia. lar venous pressure is raised and sacral and/or an-
Water excess may be due to abnormal excretion e.g. kle oedema may be present (bedbound patients often

in syndrome of inappropriate antidiuretic hormone have little ankle oedema, but have sacral oedema). The
(SIADH; see page 444) or excessive intake. In normal blood pressure is usually normal (occasionally high),
circumstances the kidney excretes any excessive wa- but blood pressure and heart rate are often unreliable
ter intake, but in renal disease or in SIADH, water is because of underlying cardiac disease: in heart fail-
retained. This invariably causes hyponatraemia (see ure the blood pressure often falls with worsening fluid
page 4). Patients often remain euvolaemic, but if there overload. Pleural effusions and ascites suggest fluid
is also some degree of sodium excess there may be overload, but in some cases there may be increased
symptoms and signs of fluid overload. interstitial or third space fluid with reduced intravas-
cular fluid so that the patient has decreased circulating
volume with signs of intravascular hypovolaemia.
Assessing fluid balance
Urine output monitoring and 24-hour fluid balance
This is an important part of the clinical evaluation of charts are essential in unwell patients. Daily weights are
patients with a variety of illnesses, which may affect the useful in patients with fluid overload particularly those