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What is hypertension?
The circulation is a closed system in which the blood is driven by the pumping of the
heart. Pressure within this system is the product of two opposing forces – the output of
the heart and the resistance to flow offered by the closed system. The pumping of the
heart is intermittent, with two distinct phases:
-
Diastole, when the chambers of the heart relax, and
Systole, when the heart muscle contracts, ejecting blood into the major arteries.
Thus blood pressure is not constant: it pulsates, with an upper (systolic) and lower
(diastolic) value.
Hypertension is a condition in which arterial blood pressure is consistently raised, with
systolic values of 140 mm Hg or higher and/or diastolic values of 90 mm Hg or higher.
Hypertension may be purely systolic, purely diastolic, or both systolic and diastolic.
Isolated systolic hypertension is more often observed in the elderly than in middle-aged
patients.
Who suffers from hypertension?
Hypertension is the most commonly diagnosed chronic condition worldwide. An
estimated 20% of the adult population in industrialised countries suffers from the
disorder. Worryingly, only half of these people are identified and treated.
The classification systems for hypertension
Hypertension can be classified by:
-
Aetiology
Severity
Extent of organ damage
Course
1. Hypertension classified by aetiology
Forms of hypertension
Causes
Proportion of cases (%)
Essential hypertension
Unknown
90
Secondary/organic hypertension
Renal
- Renal parenchymal
- Renovascular
Endocrine
Cardiovascular
Neurogenic
e.g. chronic glomerulonephritis or
pyelonephritis, nephropathy
during pregnancy
6-8
Renal artery stenosis
Cushing’s syndrome, Conn’s
syndrome, thyrotoxicosis,
phaeochromocytoma
Aortic regurgitation, hyperkinetic
heart syndrome
Encephalitis, meningitis, brain
tumours, carbon monoxide or
thallium poisoning
<1
<1
<1
2. Hypertension classified by Severity (old)
3. Hypertension classified by extent of organ damage
The most recent report (the sixth) of the Joint National Committee (JNC) on the
Detection, Evaluation and Treatment of High Blood Pressure has classified hypertension
into 3 stages.
New category (old)
Systolic BP (mm Hg)
Diastolic BP (mm Hg)
Optimal
<120
<80
Normal
<130
<85
High-Normal
<130-139
85-89
Hypertension
Stage 1 (mild)
Stage 2 (moderate)
Stage 3 (severe)
140-159
160-179
<180
90-99
100-109
<110
US JNC VI classification of blood pressure
4. Hypertension classified by course
Benign hypertension produces no organ damage.
Malignant hypertension is characterised by:
- Diastolic BP >120 mmHg over a prolonged period
- Evidence of severe organ damage, e.g. changes in retinal vessels
- Severe lesions in the heart, kidneys, and brain, renal failure, and cerebral
haemorrhage
Pathophysiology of essential hypertension
1. Psychological factors
Stress and emotional upsets may activate the adrenergic system and release adrenaline – a
hormone which cause vasoconstriction and increase cardiac output i.e. increase blood
pressure
2. Physiological factors
 Blood
 Cardiac
 Peripheral
Output
Vascular
Volume
Resistance
Hypertension
In the early stages of hypertension, cardiac output is increased and peripheral resistance is
reduced. In advanced stages of the disease, peripheral resistance is raised as a result of
increased tone in the pre-capillary resistance vessels. The resistance vessels also show
increased responsiveness to substances, which increase blood pressure such as
angiotensin and noradrenaline.
3. Genetic factors
It is possible that individuals with a familial predisposition to hypertension carry a
genetic defect responsible for decreased excretion of sodium by the kidneys, which is
known to increase blood pressure.
4. Prostaglandin’s
Renal excretion of PGE2 and kallikrein (which has a vasodilating effect) is reduced in
hypertensives. PGE2 and bradykinin have a direct vasodilator effect.
Risk factors for hypertension
Many people have a predisposition to essential hypertension, which is inherited with the
chromosomes. However, studies have shown that various external risk factors also play
an important role in the development of hypertension.
-
Obesity
High salt intake
Physical inactivity
Stress
Smoking
Alcohol consumption
Atherosclerosis
Disturbance of lipid metabolism
Insulin resistance
Complications of untreated hypertension
Long-standing untreated hypertension leads to damage to several organs and organ
systems, often resulting in serious outcomes such as heart failure, myocardial infarction
and renal failure.
Arteries
Atherosclerosis is a disease of the arteries, in which there is:
- A deposition of cholesterol, other lipids, and calcium
- Loss of elasticity
- Stenosis
- Risk of acute thrombosis and vascular occlusion
Depending on its location, atherosclerosis may lead to such diseases as angina pectoris,
myocardial infarction, stroke, and gangrene. Just as hypertension causes atherosclerosis,
so the narrowing of blood vessels in atherosclerosis cause an increase in blood pressure,
producing a “vicious circle“ between the two conditions.
Eyes
Blood vessels in the retina are very sensitive to changes in blood pressure, and indeed the
WHO classification of hypertension includes stages of deterioration of the fundus of the
eye to indicate the stage of hypertension. These include: haemorrhage, capillary leakage
(exudates), and papilloedema, which lead to visual disturbances, eventually progressing
to blindness.
Brain
Atherosclerosis of the vessels supplying the brain can cause:
- Inadequate blood flow (perfusion), leading to transient ischaemic attacks (short
period of dizziness and blurred vision);
- Thrombus (clot) formation in cerebral arteries, leading to an ischaemic stroke;
- Aneurysm formation (blood-filled pouches that form at weak spots in
atherosclerotic vessels) – a burst aneurysm will lead to a haemorrhage stroke.
Hypertensive encephalopathy may follow sudden, extreme rises in blood pressure. This is
characterised by cerebral oedema with headache, vomiting and visual disturbances.
Brain complications are the cause of death in about 16% of all patients with high blood
pressure.
Heart
High blood pressure causes an increase in cardiac afterload (the resistance against which
the heart has to pump), which means that the ventricles have to work harder to eject
blood into the arteries (especially into the aorta). This leads to left ventricular
hypertrophy (LVH), which progresses to the very serious complication of congestive
heart failure if blood pressure is not restored to normal.
Myocardial infarction, or “heart attack“, is the result of coronary artery disease, for which
hypertension is a major risk factor.
Cardiac complications are the main cause of death in almost 70% of all hypertensives.
Kidneys
In the kidneys, elevated blood pressure produces:
- Arteriosclerosis with medial thickening
- Blood vessel wall necrosis, and
- Glomerulosclerosis due to inadequate blood flow.
These lead to impaired renal function. In malignant hypertension, renal failure can
develop.
Clinical investigations in the patient with suspected hypertension
1. History
The following factors are important:
- Family history (obesity, diabetes, gout, hypertension)
- Dietary habits (salt/alcohol intake)
- Lifestyle (stress/sedentary)
- Medication history (analgesics, oral contraceptives).
2. Physical examination
3. Blood pressure measurement
Blood pressure should be measured in the sitting, standing and supine positions. It must
be consistently higher than normal to constitute hypertension. A single reading is not
enough, so at least two readings should be taken, several minutes apart. Blood pressure
may be measured:
- Directly (invasively), via a catheter or a cannula inserted into an artery
- Indirectly (non-invasively), using a pneumatic cuff on the upper arm
In indirect measurement, blood pressure is measured with an inflatable cuff that can
expand only inwards. The cuff is inflated by hand with a rubber bulb or – in automatic
devices – with a small electric pump. A connecting tubes leads to a measuring device
(sphygmomanometer) in which the cuff pressure is indicated by the height of a mercury
column (mercury manometer), or by a pointer on a dial (spring manometer), or digitally
in electronic instruments. Blood pressure is measured in millimetres of mercury (mm Hg)
The cuff is first inflated to a point where the brachial artery is completely compressed
and no blood can flow through it. Pressure is then slowly released. Once the pressure in
the artery is greater than that in the cuff, blood begins to flow again. A regular beat can
then be heard with a stethoscope placed over the brachial artery above the elbow crease
(auscultation)
These tapping sounds are known as Korotkoff sounds, after the Russian physician who
first described them. The start of the Korotkoff sounds indicates the systolic blood
pressure. The disappearance of these sounds, as the cuff pressure is reduced, indicates the
diastolic blood pressure. At this point, blood flow is no longer impeded, and the pressure
corresponds to that of relaxation of the heart (diastole). The Korotkoff sounds can be
auscultated with a stethoscope or, in electronic instruments, either amplified via a built-in
microphone or displayed visually as a flashing light.
Variations in values
Some variation in values is normal:
- Blood pressure varies not only from minute to minute (the systolic value by up to
4 mm Hg, the diastolic by up to 2-3 mm Hg) but also over several hours.
- Blood pressure falls in the evening, with a further 10-20% fall during sleep.
Values may also vary for the following reasons:
- Blood pressure is often higher when measured in the doctor’s surgery, particularly
in elderly patients. This phenomenon is commoner in women than in man, and is
termed “white coat hypertension”.
- Incorrect cuff size.
- Technical defects in the measuring device (the sphygmomanometer should be
calibrated every 6 months and the digital devices once yearly).
Examination for existing hypertensive disease
The heart and lungs must be osculated to discover evidence of left ventricular strain or
heart failure. Examination of the fundus of the eye provides clues to the severity and
duration of the hypertension. In addition, the body mass index (BMI) must be calculated
to assess obesity.
Laboratory tests
Biochemical investigations may indicate the presence of secondary hypertension or
additional cardiovascular risk factors e.g. LDL, cholesterol, triglycerides and glucose
tolerance. Blood tests may occasionally include the measurement of plasma renin
activity, and hormone and catecholamine levels. Urin analysis to assess kidney function
e.g. creatinine, is also performed.
Electrocardiogram
The electrocardiogram (ECG) provides information about:
- Left ventricular hypertrophy
- Episodes of atrial fibrillation, extrasystoles, and
- Other forms of arrhythmia
- Previous myocardial infarction