be continued. Some people need long-
term treatment with anticoagulant drugs
to prevent the formation of blood clots
around the new valve.
heat crampsPainful contractions in
muscles that are caused by excessive
salt loss as a result of profuse sweating.
Heat cramps are usually brought on by
strenuous activity in extreme heat. The
condition may occur independently, or
is sometimes a symptom of heat ex-
haustionor heatstroke. Prevention and
treatment consist of taking salt tablets
or drinking weak salt solution.
heat disordersThe body functions
most efficiently around 37°C, and any
major temperature deviation disrupts
body processes. The malfunctioning or
overloading of the body’s mechanisms
for keeping internal temperature con-
stant may cause a heat disorder.
The mechanisms by which the body
loses unwanted heat are controlled by
the hypothalamusin the brain. When
blood temperature rises, the hypothal-
amus sends out nerve impulses to
stimulate the sweat glandsand dilate
blood vessels in the skin, which cools the
body down. However, excessive sweat-
ing may result in an imbalance of salts
and fluids in the body, which may lead
to heat crampsor heat exhaustion. When
the hypothalamus is disrupted (for ex-
ample, by a fever), the body may overheat,
leading to heatstroke. Excessive exter-
nal heat may cause prickly heat.
Most heat disorders can be prevented
by gradual acclimatization to hot condi-
tions and taking salt tablets or solution.
A light diet and frequent cool baths or
showers may also help. Alcohol and
strenuous exercise should be avoided.
heat exhaustionFatigue, culminating in
collapse, caused by overexposure to heat.
There are 3 main causes of heat exhaus-
tion: insufficient water intake, insufficient
salt intake, and a deficiency in sweat
production. In addition to fatigue, symp-
toms may include faintness, dizziness,
nausea and vomiting, headache, and,
when salt loss is heavy, heat cramps. The
skin is usually pale and clammy, breath-
ing is fast and shallow, and the pulse is
rapid and weak. Unless it is treated, heat
exhaustion may develop into heatstroke.heart valveA structure at the exit of a
heartchamber that allows blood to flow
out of the chamber, but prevents back-
wash. There are 4 heart valves: aortic,
pulmonary, mitral, and tricuspid. Their
opening and closing during each heart
cycle produces heart sounds.
Any of the 4 heart valves may be
affected by stenosis(narrowing), which
causes the heart to work harder to force
blood through the valve, or by incompe-
tence or insufficiency (leakiness), which
makes the valve unable to prevent back-
wash of blood. These defects cause
characteristic heart murmurs.
Heart-valve defects may be present at
birth (see heart disease, congenital), or
they may be acquired later in life. The
most common congenital valve defects
are aortic stenosisand pulmonary stenosis.
Acquired heart-valve disease is usually
the result of degenerative changes or
ischaemiaaffecting part of the heart and
leading to aortic stenosis or mitral in-
competence. Rheumatic fevercan cause
mitral stenosis, mitral incompetence,
aortic valve defects, tricuspid stenosis
and tricuspid incompetence. The heart
valves may also be damaged by bacteri-
al endocarditis.
Heart-valve disorders commonly lead
to heart failure, arrhythmias, or symp-
toms resulting from reduced blood
supply to body tissues.
Heart-valve defects may be diagnosed
by auscultation, chest X-ray, ECG,or
echocardiography and may be corrected
by heart-valve surgery.
heart-valve surgeryAn operation to
correct a heart valvedefect or to remove
a diseased or damaged valve. A heart
valve may have to be repaired, widened,
or replaced because it is either incom-
petent (leaky) or stenotic (narrowed).
Widening of a valve may involve valv-
otomyor valvuloplasty. A damaged valve
can be replaced by a mechanical one
(fashioned from metal and plastic), a
valve constructed from human tissue,
a pig valve, or a valve taken from a
human donor after death. A heart–lung
machineis used during replacement.
After heart-valve surgery, symptoms
such as breathlessness may take weeks
to improve and require medication toHEART VALVE HEAT EXHAUSTIONH