Madison

Preface
The concept of this book arose in part from a frustra-
tion with traditional textbooks, which address medicine,
surgery and pathology as separate disciplines. This sep-
aration is frequently artificial, as patients often do not
present to the doctor with an isolated medical or surgi-
cal problem. Medicine and Surgery: A concise textbook is
a new textbook in which the pathophysiology and epi-
demiology of disease is presented alongside medical and
surgical aspects to provide a truly integrated text. This
unique approach allows the book to be used as a com-
prehensive undergraduate reference book.
Another driving force behind this book was the lack of
a comprehensive text that students could turn to for the
essential knowledge required to pass their final exams.
Medicine and Surgery: A concise textbook is also a book
that can be used by final year students to enable them
to quickly and efficiently revise their knowledge. Whilst
covering the core syllabus of undergraduate medicine
and surgery we have kept the information to that which
is essential to the undergraduate.
Diseases have been arranged by system and have been
presented under consistent subheadings to aid under-
standing and revision. This is also the manner in which
students are often expected to present in exams. At the
beginning of each chapter the system is considered from
a clinical perspective with a discussion of the symp-
toms and signs relevant to that system. Investigations
and procedures that are used in multiple conditions are
also considered in the clinical sections. Specific topics
that have been excluded include diseases of childhood,
orthopaedic, ENT and ophthalmology. These have had
to be excluded to maintain the book at a manageable
size.
This book has evolved over the last few years with
the help of students and specialists who have reviewed
and revised individual chapters or sections. We hope you
find the final product useful and we encourage you as
readers to help us revise future editions by sending your
comments and suggestions for improvement.
Acknowledgements
We would like to thank all the anonymous specialists
for their review and revision of the individual chapters;
without this input the book would have not been as up
to date and comprehensive as it is. We would also like
to thank our families, friends and colleagues for their
support and encouragement, especially Dr Man Fai Shiu
and Dr Tea Johnston for their inspiration, guidance ´
and encouragement throughout the project. Finally we
would like to thank all at Blackwell Publishing, includ-
ing Fiona Goodgame, Martin Sugden and especially
Geraldine Jeffers for her tireless work and support.
Principles and practice of 1 medicine and surgery
Fluid and electrolyte balance, 1 Perioperative care, 13 Infections, 19
Fluid and electrolyte balance
Water and sodium balance
Approximately 60% of the body weight in men and 55%
in women consists of water. Most of this exists within two
physiological fluid ‘spaces’ or compartments: about two-
thirds within the intracellular compartment and one-
third in the extracellular compartment. The extracellular
compartment consists of both intravascular fluid (blood
cells and plasma) and interstitial fluid (fluid in tissues,
which surrounds the cells). Additionally a small amount
of fluid is described as in the ‘third space’, e.g. fluid in
the gastrointestinal tract, pleural space and peritoneal
cavity. Pathological third space fluid is seen with gas-
trointestinal obstruction or ileus and pleural effusion or
ascites.
Water remains in physiological balance between these
compartments because of the concentration of osmoti-
cally active solutes. Osmosis is the passage of water from
a low concentration of solute through a semipermeable
membrane to a more concentrated solution. A propor-
tion of the total osmotic pressure is due to the presence
of large protein molecules; this is known as the colloidal
osmotic pressure or oncotic pressure.
Intracellular–extracellular fluid balance:The cell mem-
brane acts as semipermeable to sodium and potas-
sium because the Na+-K+-ATPase pump keeps mov-
ing sodium out of the cell into the interstitial fluid and
moving potassium into the cell. Sodium is the main
determinant of extracellular fluid volume.
Intravascular–interstitial fluid balance: The capillary
wall is semi-impermeable to plasma proteins, whereas
sodium passes freely across the capillary wall. This
means that proteins (through oncotic pressure), rather
than sodium, exert the osmotic effect to keep fluid
in the intravascular space. The hydrostatic pressure
generated across the capillaries offsets this, driving
intravascular fluid out into the interstitial fluid. If
there is a reduction in plasma protein levels (hypoal-
buminaemia), the low oncotic pressure can lead to
oedema; this is where there is excess interstitial fluid
at the expense of intravascular fluid.
Water is continually lost from the body in urine, stool
and through insensible losses (the lungs and skin). This
water is replaced through oral fluids, food and some is de-
rived from oxidative metabolism. Sodium is remarkably
conserved by normal kidneys, which can make virtu-
ally sodium-free urine, e.g. in hypovolaemia. Obligatory
losses of sodium occur in sweat and faeces, but account
for <10 mmol. The average dietary intake of sodium in
the United Kingdom is ∼140 mmol/day, which is the
equivalent of 8 g of salt. The recommended sodium in-
take for a healthy diet is 70 mmol/day. Normal kidneys
can easily excrete this sodium load, and in a healthy per-
son the body is able to maintain normal fluid balance by
sensing the concentration of sodium and the extracel-
lular volume. This process is under the control of both
local sensing mechanisms and more distant neurohor-
monal mechanisms. These drive thirst and water intake
on the one hand and renal excretion or conservation of
sodium and water on the other. In disease states or due to

2 Chapter 1: Principles and practice of medicine and surgery

an excess or lack of salt and/or water intake, this normal

balance may be disturbed.

There are essentially four patterns of water and sodium

imbalance:

Sodium depletion is usually due to excess sodium loss,

e.g. due to vomiting or diarrhoea, or burns. Water is

lost with the sodium, so the serum sodium usually

remains normal, but hypovolaemia results. If hyper-

tonic fluid is lost or if there has been water replace-

ment but insufficient sodium replacement (typically

in a patient who is vomiting and only drinking water or

only given intravenous 5% dextrose or dextrosaline),

hyponatraemia results, which can lead to confusion,

drowsiness, convulsions and coma (see page 4).

Water deficiency due to inadequate intake of water

leads to dehydration. The plasma osmolality rises and

hypernatraemia occurs. This stimulates thirst and va-

sopressin release, which increases water reabsorption

by the kidneys. Pure water depletion is rare, but many

disorders mostly lead to water loss with some sodium

loss. Initially water moves from the cells into the extra-

cellular compartment, but then both the intracellular

and extracellular compartments become volume de-

pleted, causing symptoms and signs of fluid depletion

(see section Assessing Fluid Balance below).

Sodium excess rarely occurs in isolation. It is usually

found in combination with water excess, causing fluid

overload with peripheral oedema, pulmonary oedema

and hypertension. The effect on serum sodium and

fluid balance depends on the relative excess of sodium

compared to water. Sodium excess > water excess

causes hypernatraemia (see page 3) whereas water ex-

cess > sodium excess causes hyponatraemia. Water excess may be due to abnormal excretion e.g.

in syndrome of inappropriate antidiuretic hormone

(SIADH; see page 444) or excessive intake. In normal

circumstances the kidney excretes any excessive wa-

ter intake, but in renal disease or in SIADH, water is

retained. This invariably causes hyponatraemia (see

page 4). Patients often remain euvolaemic, but if there

is also some degree of sodium excess there may be

symptoms and signs of fluid overload.

Assessing fluid balance

This is an important part of the clinical evaluation of

patients with a variety of illnesses, which may affect the

circulating volume or sodium and water balance. Exam-

ples include patients with any history of cardiac, liver or

renal failure, those with symptoms such as vomiting and

diarrhoea, perioperative patients or any patient who has

other losses, e.g. from bleeding or drains. Clinical evalu-

ation of fluid balance requires the observation of several

signs that together point to whether the patient is eu-

volaemic (normal fluid balance), fluid depleted (reduced

extracellular fluid) or fluid overloaded (increased extra-

cellular fluid). In most cases when the patient is fluid

depleted, there is decreased circulating volume; however

in fluid overload, there may either be increased circulat-

ing volume or decreased circulating volume depending

on the mechanism.

Fluid depletion may be suggested by an appropriate

history of losses or reduced intake, but this can be un-

reliable. Symptoms of thirst and any postural dizziness

should be enquired about. Signs of volume depletion

include a mild tachycardia, reduced peripheral per-

fusion (cool dry hands and feet, increased capillary

refill time >3 seconds), postural hypotension and/or

hypotension, and reduced skin turgor (check over the

anterior chest wall as the limbs are unreliable, partic-

ularly in the elderly). The jugular venous pressure is

low and urine output reduced (oliguria, see later in

this chapter).

Fluid overload is more likely to occur in patients with

cardiac, liver or renal failure, particularly if there has

been over-enthusiastic fluid replacement. Breathless-

ness is an early symptom. Tachypnoea is common and

there may be crackles heard bilaterally at the bases of

the chest because of pulmonary oedema. The jugu-

lar venous pressure is raised and sacral and/or an-

kle oedema may be present (bedbound patients often

have little ankle oedema, but have sacral oedema). The

blood pressure is usually normal (occasionally high),

but blood pressure and heart rate are often unreliable

because of underlying cardiac disease: in heart fail-

ure the blood pressure often falls with worsening fluid

overload. Pleural effusions and ascites suggest fluid

overload, but in some cases there may be increased

interstitial or third space fluid with reduced intravas-

cular fluid so that the patient has decreased circulating

volume with signs of intravascular hypovolaemia.

Urine output monitoring and 24-hour fluid balance

charts are essential in unwell patients. Daily weights are

useful in patients with fluid overload particularly those