Pathophysiology of Cyanotic Congenital Heart Defects

7/27/2019 Pathophysiology of Cyanotic Congenital Heart Defects

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Clinical Cyanosis

DETECTION OF CYANOSIS Cyanosis is a bluish discoloration of the skin and

mucous membranes resulting from an increasedconcentration of reduced hemoglobin to about 5 g/100mL in the cutaneous veins.

This level of reduced Hb in the cutaneous vein mayresult from either desaturation of arterial blood or increased extraction of oxygen by peripheral tissue in thepresence of normal arterial saturation (e.g., circulatory

shock, hypovolemia, vasoconstriction from cold).

Cyanosis associated with desaturation of arterial blood iscalled central cyanosis; cyanosis with normal arterialoxygen saturation is called peripheral cyanosis.


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Cyanosis is more difficult to detect in children with darkpigmentation.

Although cyanosis may be detected on many parts of thebody, the tip of the tongue is a good place to look for cyanosis; the color of the tongue is not affected by raceor ethnic background, and the circulation is not sluggishin the tongue.

In a newborn, acrocyanosis may cause confusion.

In addition, some newborns are polycythemic, which

may contribute to the appearance of cyanosis withoutarterial desaturation.

In older infants and children, chronic subclinicalcyanosis produces clubbing.


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CAUSES OF CYANOSIS

Reduced Arterial Oxygen Saturation (i.e., Central Cyanosis) INADEQUATE ALVEOLAR VENTILATION

Central nervous system depression

Inadequate ventilatory drive (e.g., obesity, pickwickian syndrome)Obstruction of the airway, congenital or acquiredStructural changes in the lungs and/or ventilation-perfusion mismatch (e.g., pneumonia, cystic

fibrosis, hyaline membrane disease, pulmonary edema, congestive heart failure)Weakness of the respiratory muscles.

Desaturated blood bypassing effective alveolar units Intracardiac right-to-left shunt (i.e., cyanotic congenital heart defect)Intrapulmonary shunt (e.g., pulmonary atrioventricular fistula, chronic hepatic disease resulting in

multiple microvascular fistulas in the lungs)Pulmonary hypertension with the resulting right-to-left shunt at the atrial, ventricular, or ductal

levels (e.g., Eisenmenger's syndrome, persistent pulmonary hypertension of the newborn)

Normal arterial oxygen saturation (i.e., Peripheral Cyanosis) Increased deoxygenation in the capillaries

Circulatory shockCongestive heart failure

Acrocyanosis of newbornsMethemoglobinemia

Congenital methemoglobinemia Toxic substances

Ingestion of water high in nitratesExposure to aniline dye teething gels


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CONSEQUENCES AND COMPLICATIONS Polycythemia

Low arterial O2 content stimulates bone marrow througherythropoietin release from the kidneys and produces an increasednumber of red blood cells.

Polycythemia, with a resulting increase in oxygen-carrying capacity,benefits cyanotic children. However, when the hematocrit reaches65% or higher, a sharp increase in the viscosity of blood occurs, andthe polycythemic response becomes disadvantageous, particularly if there is congestive heart failure (CHF).

Some cyanotic infants have a relative iron deficiency state, withnormal or lower than normal hemoglobin and hypochromia on bloodsmear. A normal hemoglobin in a cyanotic patient represents arelative anemic state. Although less cyanotic, these infants areusually more symptomatic and improve when iron therapy raises thehemoglobin.


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Clubbing

Caused by soft tissue growth under the nail bedas a consequence of central cyanosis.

The mechanism for soft tissue growth is unclear.


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Hypothesis:

Megakaryocytes present in the systemic venous blood may beresponsible for the change. In normal persons, platelets are formedfrom the cytoplasm of the megakaryocytes by fragmentation duringtheir passage through the pulmonary circulation. The cytoplasm of megakaryocytes contains growth factors (e.g., platelet-derived growth

factor and transforming growth factor ). In patients with right -to-leftshunts, megakaryocytes with their cytoplasm may enter the systemiccirculation, become trapped in the capillaries of the digits, and releasegrowth factors, which in turn cause clubbing.


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Clubbing

Clubbing usually does not occur until achild is 6 months or older, and it isseen first and is most pronounced inthe thumb.

In the early stage, it appears as shininess

and redness of the fingertips. When it is fullydeveloped, the fingers and toes become

thick and wide and have convex nail beds .

Clubbing is also seen in patients with liver

disease or subacute bacterial endocarditisand on a hereditary basis without cyanosis.


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Central Nervous System Complications

1.Brain abscess

In the past, cyanotic CHDs accountedfor 5% to 10% of all cases of brain abscesses.

R-L intracardiac shunts may bypass the normallyeffective phagocytic filtering actions of the pulmonary capillarybed. This predisposition may also result from the fact thatpolycythemia and the consequent high viscosity of blood lead totissue hypoxia and microinfarction of the brain, which are later complicated by bacterial colonization.

The triad of symptoms of brain abscesses are fever, headache, andfocal neurologic deficit.


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2.Vascular stroke

Caused by embolization arising from thrombus in the cardiac chamber or in the systemic veins may be associated with surgery or cardiaccatheterization.

Cerebral venous thrombosis may occur, often in infants younger than 2years who have cyanosis and relative iron deficiency anemia. Apossible explanation for these findings is that microcytosis further exacerbates hyperviscosity resulting from polycythemia.


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Bleeding Disorders

Disturbances of hemostasis are frequently present in children withsevere cyanosis and polycythemia.

Most frequently noted are thrombocytopenia and defective plateletaggregation.

Other abnormalities include prolonged prothrombin time and partialthromboplastin time and lower levels of fibrinogen and factors V andVIII.

Clinical manifestations: easy bruising, petechiae of the skin andmucous membranes, epistaxis, and gingival bleeding. Red cellwithdrawal and replacement with an equal volume of plasma tend tocorrect the hemorrhagic tendency and lower blood viscosity.


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Hypoxic Spells and Squatting

Although most frequently seen ininfants with tetralogy of Fallot (TOF),

hypoxic spells may occur in infants withother congenital heart defects


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Depressed Intelligent

Quotient (IQ)

Children with chronic hypoxia and cyanosis have a lower thanexpected intelligence quotient as well as poorer perceptual andgross motor functions than children with acyanotic congenital heartdefects, even after surgical repair of cyanotic heart defects.


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Scoliosis Children with chronic cyanosis, particularly girls and

patients with TOF, often have scoliosis.

Hyperuricemia and Gout Hyperuricemia and gout tend to occur in older patients

with uncorrected or inadequately repaired cyanotic heartdefects.


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Pathophysiology of Cyanotic Congenital Heart Defects

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