Atlas of pathophysiology, 2 Edition

Part II - Disorders

Cardiovascular disorders

Congenital Defects

The most common congenital defects of the heart are atrial septal defect (ASD), coarctation of the aorta, patent ductus arteriosus (PDA), tetralogy of Fallot, transposition of the great arteries, and ventricular septal defect (VSD). Causes of all six defects remain unknown, although some have specific clinical associations. Pathophysiology and presentation are specific to each disorder, but diagnostic tests and available treatment modalities are applicable to the entire group (see page 48).

Atrial septal defect

An opening between the left and right atria permits blood flow from the left atrium to the right atrium rather than from the left atrium to the left ventricle. ASD is present in 4 of 100,000 people and is associated with Down syndrome.

Pathophysiology

Blood shunts from the left atrium to the right atrium because left atrial pressure is normally slightly higher than right atrial pressure. This difference forces large amounts of blood through a defect that results in right heart volume overload, affecting the right atrium, right ventricle, and pulmonary arteries. Eventually, the right atrium enlarges, and the right ventricle dilates to accommodate the increased blood volume. If pulmonary artery hypertension develops, increased pulmonary vascular resistance and right ventricular hypertrophy follow. In some adults, irreversible pulmonary artery hypertension causes reversal of the shunt direction, resulting in unoxygenated blood entering the systemic circulation, causing cyanosis.

Signs and symptoms

·         Fatigue

·         Early to midsystolic murmur, low-pitched diastolic murmur

·         Fixed, widely split S2

·         Systolic click or late systolic murmur at the apex

·         Clubbing of nails and cyanosis with a right-to-left shunt

·         Palpable pulsation of the pulmonary artery

Coarctation of the aorta

Coarctation is a narrowing of the aorta, usually just below the left subclavian artery, near the site where the ligamentum arteriosum joins the pulmonary artery to the aorta. Coarctation of the aorta occurs in approximately 1 in 10,000 people and is associated with Turner's syndrome and congenital abnormalities of the aortic valve.

Pathophysiology

Coarctation of the aorta may develop as a result of spasm and constriction of the smooth muscle in the ductus arteriosus as it closes. Possibly, this contractile tissue extends into the aortic wall, causing narrowing. The obstructive process causes hypertension in the aortic branches above the constriction and diminished pressure in the vessel below the constriction.

Restricted blood flow through the narrowed aorta increases the pressure load on the left ventricle and causes dilation of the proximal aorta and ventricular hypertrophy.

As oxygenated blood leaves the left ventricle, a portion travels through the arteries that branch off the aorta proximal to the coarctation. If PDA is present, the remaining blood travels through the coarctation, mixes with deoxygenated blood from the PDA, and travels to the legs. If the ductus arteriosus is closed, the legs and lower portion of the body must rely solely on the blood that circulates through the coarctation.

Signs and symptoms

·         Heart failure

·         Claudication and hypertension

·         Headache, vertigo, and epistaxis

·         Blood pressure greater in upper than in lower extremities

·         Pink upper extremities and cyanotic lower extremities

·         Absent or diminished femoral pulses

·         Possible murmur

·         Possibly, chest and arms more developed than legs

Patent ductus arteriosus

The ductus arteriosus is a fetal blood vessel that connects the pulmonary artery to the descending aorta, just distal to the left subclavian artery. Normally, the ductus closes within days to weeks after birth. In PDA, the lumen of the ductus remains open after birth. This creates a left-to-right shunt of blood from the aorta to the pulmonary artery and results in recirculation of arterial blood through the lungs. PDA occurs in about 1 in 2,000 infants and is associated with premature birth, rubella syndrome, coarctation of the aorta, VSD, and pulmonic and aortic stenosis.

Pathophysiology

The ductus arteriosus normally closes as the neonate takes his first breath but may take as long as 3 months in some infants.

In PDA, relative resistance in pulmonary and systemic vasculature and the size of the ductus determine the quantity of blood that's shunted from left to right. Because of increased aortic pressure, oxygenated blood is shunted from the aorta through the ductus arteriosus to the pulmonary artery. The blood returns to the left side of the heart and is pumped out to the aorta once more.

Increased pulmonary venous return causes increased filling pressure and workload on the left side of the heart as well as left ventricular hypertrophy and possibly heart failure. In the final stages of untreated PDA, the left-to-right shunt leads to chronic pulmonary artery hypertension causing unoxygenated blood to enter the systemic circulation, resulting in cyanosis.

Signs and symptoms

·         Respiratory distress with signs of heart failure in infants

·         Gibson murmur

·         Thrill palpated at left sternal border

·         Prominent left ventricular impulse

·         Corrigan's pulse

·         Wide pulse pressure

·         Slow motor development and failure to thrive

Tetralogy of Fallot

Tetralogy of Fallot is a combination of four cardiac defects: VSD, right ventricular outflow tract obstruction, right ventricular hypertrophy, and an aorta positioned above the VSD (overriding aorta). This defect occurs in about 5 in 10,000 infants

P.47

P.48


and is associated with fetal alcohol syndrome, thalidomide use during pregnancy, and Down syndrome.

CONGENITAL HEART DEFECTS

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Pathophysiology

Unoxygenated venous blood entering the right side of the heart may pass through the VSD to the left ventricle, bypassing the lungs, or it may enter the pulmonary artery, depending on the extent of the pulmonic stenosis. The VSD usually lies in the outflow tract of the right ventricle and is generally large enough to permit equalization of right and left ventricular pressures. However, the ratio of systemic vascular resistance to pulmonic stenosis affects the direction and magnitude of shunt flow across the VSD.

Signs and symptoms

·         Cyanosis

·         Cyanotic or “blue” spells (Tet spells)

·         Clubbing of digits, diminished exercise tolerance, dyspnea on exertion, growth retardation, and eating difficulties

·         Squatting to reduce shortness of breath

·         Loud systolic murmur, continuous murmur of the ductus

·         Thrill at left sternal border

·         Right ventricular impulse and prominent inferior sternum

Transposition of great arteries

The aorta rises from the right ventricle and the pulmonary artery from the left ventricle, producing two noncommunicating circulatory systems. This defect occurs in 40 of 100,000 infants and is associated with VSD, VSD with pulmonic stenosis, ASD, and PDA.

Pathophysiology

The transposed pulmonary artery carries oxygenated blood back to the lungs, rather than to the left side of the heart. The transposed aorta returns unoxygenated blood to the systemic circulation rather than to the lungs. Communication between the pulmonary and systemic circulations is necessary for survival. In infants with isolated transposition, blood mixes only at the patent foramen ovale and at the PDA, resulting in slight mixing of unoxygenated systemic blood and oxygenated pulmonary blood. In infants with concurrent cardiac defects, greater mixing of blood occurs.

Signs and symptoms

·         Hypoxemia, cyanosis, and tachypnea

·         Gallop rhythm, tachycardia, dyspnea, hepatomegaly, and cardiomegaly

·         Murmurs of ASD, VSD, or PDA; loud S2

·         Diminished exercise tolerance, fatigue, and clubbing

Ventricular septal defect

VSD is an opening in the septum between the ventricles that allows blood to shunt between the left and right ventricles. It's estimated that up to 1% of neonates are born with a VSD. However, in the majority, the defect is small and will close spontaneously. VSD is associated with Down syndrome and other autosomal trisomies, renal anomalies, prematurity, fetal alcohol syndrome, PDA, and coarctation of the aorta.

Pathophysiology

In neonates with a VSD, the ventricular septum fails to close completely by 8 weeks' gestation. VSDs are located in the membranous or muscular portion of the ventricular septum and vary in size. Some defects close spontaneously; in other defects, the septum is entirely absent, creating a single ventricle.

A VSD isn't readily apparent at birth because right and left pressures are approximately equal and pulmonary artery resistance is elevated. Alveoli aren't yet completely opened, so blood doesn't shunt through the defect. As the pulmonary vasculature gradually relaxes, between 4 and 8 weeks after birth, right ventricular pressure decreases, allowing blood to shunt from the left to the right ventricle. Initially, large VSD shunts cause left atrial and left ventricular hypertrophy. Later, an uncorrected VSD causes right ventricular hypertrophy due to increasing pulmonary resistance. Eventually, right- and left-sided heart failure and cyanosis (from reversal of the shunt direction) occur. Fixed pulmonary hypertension also may occur much later in life with right-to-left shunting, causing cyanosis and clubbing of the nail beds.

Signs and symptoms

·         Failure to thrive

·         Loud, harsh systolic murmur (along the left sternal border at the third or fourth intercostal space), palpable thrill

·         Loud, widely split pulmonic component of S2

·         Displacement of point of maximal impulse to left or down

·         Prominent anterior chest

·         Liver, heart, and spleen enlargement

·         Diaphoresis, tachycardia, and rapid, grunting respirations

·         Cyanosis and clubbing

Diagnostic test results

·         Chest X-ray reveals cardiomegaly and ventricular and aortic enlargement.

·         Electrocardiography may be normal or may reveal ventricular hypertrophy or axis deviation.

·         Echocardiography detects the presence and size of a defect.

·         Fetal echocardiogram can reveal a defect before birth.

·         Cardiac catheterization confirms the diagnosis and damage.

·         Arterial blood gas analysis reveals hypoxemia and acid-base disturbances.

·         Atrial balloon sepostomy (for transposition of the great arteries)

·         Treatment of complications

Treatment

·         Surgery

·         Medications, such as diuretics, angiotensin-converting enzyme inhibitors, indomethacin (for PDA), and prostaglandin

·         Oxygen therapy

·         Antibiotic prophylaxis

·         Atrial balloon septostomy (for transposition of the great arteries)

·         Treatment of complications

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