Ross & Wilson Anatomy and Physiology in Health and Illness (57 page)

Atrial septal defect

This is commonly known as ‘hole in the heart’. Before birth, most oxygenated blood from the placenta enters the left atrium from the right atrium through the
foramen ovale
in the septum (
Fig. 5.53
). There is a valve-like structure across the opening, consisting of two partly overlapping membranes. The ‘valve’ is open when the pressure in the right atrium is higher than in the left. This diverts blood flow from the right to the left side of the heart, bypassing the pulmonary circulation, which in the unborn child is not functional because the fetus derives his oxygen supply through the placenta. After birth, when the pulmonary circulation is established and the pressure in the left atrium is the higher, the two membranes come in contact, closing the ‘valve’. Later the closure becomes permanent due to fibrosis (
Fig. 5.64
).

When the membranes do not overlap, an opening between the atria remains patent after birth. In many cases it is too small to cause symptoms in early life but they may appear later. In severe cases blood flows back to the right atrium from the left. This increases the right ventricular and pulmonary pressure, causing hypertrophy of the myocardium and eventually cardiac failure. As pressure in the right atrium rises, blood flow through the defect may be reversed, but this is not an improvement because deoxygenated blood gains access to the general circulation.

Coarctation of the aorta

The most common site of coarctation (narrowing) of the aorta is between the left subclavian artery and ductus arteriosus. This leads to hypertension in the upper body (which is supplied by arteries arising from the aorta proximal to the narrowing) because increased force of contraction of the heart is needed to push the blood through the coarctation. There may be systemic hypotension.

Fallot’s tetralogy

This is a characteristic combination of four congenital cardiac abnormalities, which causes cyanosis, growth retardation and exercise intolerance in babies and young children. The four abnormalities are:

•
stenosis of the pulmonary artery at its point of origin, which increases right ventricular workload

•
ventricular septal defect, i.e. an abnormal communicating hole between the two ventricles, just below the atrioventricular valves

•
aortic misplacement, i.e. the origin of the aorta is displaced to the right so that it is immediately above the septal defect

•
right ventricular hypertrophy to counteract the pulmonary stenosis.

Cardiac function is inadequate to meet the needs of the growing child; surgical correction carries a good prognosis.

Disorders of blood pressure

Learning outcomes

After studying this section, you should be able to:

explain the term hypertension

define essential and secondary hypertension and list the main causes of the latter

discuss the effects of prolonged hypertension on the body, including elevated blood pressure in the lungs

describe the term hypotension.

Hypertension

The term hypertension is used to describe a level of blood pressure that, taking all other cardiovascular risk factors into account, would benefit the patient if reduced. It is therefore not possible to have a definitive blood pressure value that is classified as ‘hypertension’, but measurements above 140/90 mmHg are considered higher than ‘normal’ (SIGN guidelines 2001). Blood pressure tends to rise naturally with age. Arteriosclerosis (
p. 115
) may contribute to this, but is not the only factor.

Hypertension is described as
essential
(primary, idiopathic) or
secondary
to other diseases. Irrespective of the cause, hypertension commonly affects the kidneys (
p. 345
).

Essential hypertension

Essential hypertension (hypertension of unknown cause) is a very common condition in the Western world. Treatment aims to prevent the onset of complications, primarily cardiovascular, renal and eye disease. It accounts for 95% of all cases and is subdivided according to the rate at which the disease progresses.

Benign (chronic) hypertension

The rise in blood pressure is usually slight to moderate and continues to rise slowly over many years. Sometimes complications, such as heart failure, cerebrovascular accident or myocardial infarction are the first indication of hypertension, but often the condition is symptomless and is only discovered during a routine examination.

Risk factors

Risk factors for hypertension include obesity, diabetes mellitus, family history, cigarette smoking, a sedentary lifestyle and high intakes of salt or alcohol. Stress may increase blood pressure, and there is a well-documented link between low birth weight and incidence of hypertension in later life.

Malignant (accelerated) hypertension

This is a rapid and aggressive acceleration of hypertensive disease. Diastolic pressure in excess of 120 mmHg is common. The effects are serious and quickly become apparent, e.g. haemorrhages into the retina, papilloedema (oedema around the optic disc), encephalopathy (cerebral oedema) and progressive renal disease, leading to cardiac failure.

Secondary hypertension

Hypertension resulting from other diseases accounts for 5% of all cases.

Kidney disease

Raised blood pressure is a complication of many kidney diseases. In kidney disease, there is salt and water retention, sometimes with excessive renin activity.

Endocrine disorders

Adrenal cortex

Secretion of excess aldosterone and cortisol stimulates the retention of excess sodium and water by the kidneys, raising the blood volume and pressure. Oversecretion of aldosterone (Conn’s syndrome) is due to a hormone-secreting tumour. Oversecretion of cortisol may be due to overstimulation of the gland by adrenocorticotrophic hormone secreted by the pituitary gland, or to a hormone-secreting tumour.

Adrenal medulla

Secretion of excess adrenaline (epinephrine) and noradrenaline (norepinephrine) raises blood pressure, e.g. phaeochromocytoma (
p. 227
).

Stricture of the aorta

Hypertension develops in branching arteries proximal to the site of a stricture, e.g
congenital coarctation
(
p. 124
).

Drug treatment

Hypertension may be a side-effect of some drugs, e.g. corticosteroids and oral contraceptives.

Effects and complications of hypertension

The effects of long-standing and progressively rising blood pressure are serious. Hypertension predisposes to atherosclerosis and has specific effects on particular organs.

Heart

The rate and force of cardiac contraction are increased to maintain the cardiac output against a sustained rise in arterial pressure. The left ventricle hypertrophies and begins to fail when compensation has reached its limit. This is followed by back pressure and accumulation of blood in the lungs (pulmonary congestion), hypertrophy of the right ventricle and eventually to right ventricular failure. Hypertension also predisposes to ischaemic heart disease (
p. 120
) and aneurysm formation (
p. 115
).

Brain

Stroke, caused by cerebral haemorrhage, is common, the effects depending on the position and size of the ruptured vessel. When a series of small blood vessels rupture, e.g. microaneurysms, at different times, there is progressive disability. Rupture of a large vessel causes extensive loss of function or death.

Kidneys

Essential hypertension causes kidney damage. If sustained for only a short time recovery may be complete. Otherwise the kidney damage causes further hypertension owing to activation of the renin–angiotensin–aldosterone system (
p. 338
), progressive loss of kidney function and kidney failure.