Friday, August 13, 2010

Congenital heart disease: Introduction


Congenital Heart Disease (Cong. HD.) is defined as: All cardiovascular anomalies which are present at birth.


The incidence of Cong. HD. is estimated at about 8/1000 live births.

Frequency of individual anomalies at birth:

The 3 famous anomalies with left-to-right shunts constitute about 50% of all cases:
  1. Ventricular Septal Defect (VSD): 30%.
  2. Atrial Septal Defect (ASD): 10%.
  3. Patent Ductus Arteriosus (PDA): 10%.

Four more anomalies constitute about 26% of cases, each about 6-7%:
  1. Pulmonary Stenosis (PS).
  2. Tetralogy of Fallot (TOF).
  3. Aortic Stenosis (AS).
  4. Coarctation of the aorta.

Transposition of the great arteries (TGA) and other mal-position complexes constitute about 4%, Persistent Truncus Arteriosus (PTA) and tricuspid atresia each about 2%, and all other anomalies constitute the remaining 16%.

Etiology of Cong. HD:

Cong. HD. may result from environmental or genetic factors:

Environmental factors include:
  • Maternal Rubella.
  • Maternal Systemic Lupus Erythematosus (SLE).
  • Maternal use of drugs or alcohol. 
  • Irradiation.
  •  Hypoxia. 
  • Vitamin deficiency.

Genetic factors include:
  • Single gene mutations
  • And/or chromosomal abnormalities.


The following measures may help prevent Cong. HD:
  • Immunization against Rubella.
  • Avoidance of teratogenic drugs during pregnancy.
  • Avoidance of exposure to radiation during pregnancy.
  • Amniotic fluid or villous biopsy examination for abnormal chromosomes or enzymes.

    Fetal circulation:

    The fetal circulation is essentially different from the circulation after birth. Another important feature is that the placenta, and not the lungs, is the organ of fetal respiration.

    Three cardiovascular communications make the difference:
    • Foramen ovale (FO): shunting the blood from the Right Atrium (RA) to the Left Atrium (LA).
    • Ductus Arteriosus (DA): shunting the blood from the pulmonary arterial system to the aorta (AO).
    • Ductus venosus: carrying the blood from the portal venous system to the Inferior Vena Cava (IVC).

    About 75 % of the fetal circulation is carried by the IVC: the non-oxygenated blood is carried by the 2 Umbilical Arteries (UA) to the placenta for oxygenation. Oxygenated blood comes out of the placenta via the single Umbilical Vein (UV) to the portal vein, where it is mixed with the venous return from the lower half of the body, then to the ductus venosus, the IVC, and finally the RA. Most of this blood is then shunted by the FO to the LA, the Left Ventricle (LV) and finally to the AO, to be distributed to the systemic circulation.

    The remaining 25 % of the fetal circulation is carried by the Superior Vena Cava (SVC) which drains non-oxygenated blood from the upper half of the body into the RA, Right Ventricle (RV) and finally the pulmonary trunk. Most of this blood is then shunted by the DA to the AO, and does not go to the lungs.

    As a result of blood mixing, the partial pressure of oxygen (PO2) in the umbilical vein is about 35 mmHg and the oxygen saturation (SAT) is about 80%, while in the ascending AO the PO2 is only 28 mmHg and the SAT is only 60%.

    Fetal circulation

    Circulatory changes:

    At birth and forwards, three main changes occur:
    • Drop of pulmonary vascular resistance (Rp).
    • Closure of FO.
    • Closure of DA.

    Drop of Rp:

    At birth Rp is about 8-10 units, which normally drops to the adult level of about 1-3 units within 6-8 weeks. Droop of Rp after birth is essential to allow effective blood flow to the lungs, now the organ of respiration.

    Droop of Rp is thought to be facilitated by:
    • Increase in oxygen SAT, which is thought to induce direct vasodilatation, and probably leads to the release of vasodilator mediators such as Bradykinin & Prostaglandin I2 (PG-I2).
    • Suspension of alveoli in air, instead of fluid.
    • Reflex autonomic vasodilatation.
    • Mechanical expansion of the lungs.

    Closure of FO:

    Functional closure of FO occurs immediately after birth, due to:
    • Drop of RA pressure due to cessation of placental flow coming to RA.
    • Elevation of LA pressure due to increase in blood flow to the lungs and pulmonary venous return to LA.

    The increased LA pressure, against the decreased RA pressure, will push the septum primum to the right, to come in close contact with the septum secondum; thus immediately sealing the shunt through the foramen ovale.

    Permanent anatomical closure of FO needs several months.

    Closure of DA:

    Functional closure occurs about 72 hours after birth, due to:
    • Vasoconstriction of the ductus.
    • Decreased vasodilator prostaglandins (PG); such as PG-E2 & PG-I2.

    Permanent anatomical closure needs several weeks.

    Drop of the Rp, closure of the FO & closure of the DA will shift the circulation from the fetal to the adult pattern; now all systemic venous return collected in the RA is delivered to the pulmonary circulation for oxygenation, and the oxygenated pulmonary venous return collected in the LA is delivered to the systemic circulation, i.e. the two circulations work in series, with no more mixing between them.

    Post-natal circulation: the systemic and pulmonary circulations work in series with no more mixing between them

    Persistence of Fetal Circulation (PFC):

    Persistence of fetal circulation (PFC) is defined as the persistence of high pulmonary vascular resistance after birth, with right-to-left shunt usually via patent foramen ovale.

    Right-to-left shunt may also occur via the ductus arteriosus if patent, but this needs a pulmonary arterial pressure above the systemic pressure.

    PFC is thought to be due to:
    • Failure of the pulmonary vasculature to dilate after birth,
    • Or recurrence of its constriction.

    Clinical picture of PFC:
    • Severe tachypnea and cyanosis in the first few hours of life.
    • Manifestations of pulmonary hypertension (PHT).
    • Manifestations of RV hypertrophy (RVH).

    Differential diagnosis of PFC:

    PFC should be differentiated from many conditions including:
    • Cardiac conditions:
      • Acute left ventricular failure (LVF).
      • Cyanotic congenital heart disease.
    • Blood diseases:
      • Polycythemia.
    • Lung diseases:
      • Acute respiratory distress syndrome (RDS).
      • Parenchymal lung disease.

    Investigations for PFC:
    • Arterial blood gases (ABG) analysis will usually show:
      • Severe hypoxia.
      • Metabolic Acidosis.
    • Chest x-ray (CXR) usually reveals:
      • Lung oligemia.
    • Echocardiography is an important tool which:
      • Demonstrates the right-to-left shunt via patent foramen ovale (PFO) or patent ductus arteriosus (PDA).
      • Calculates the pulmonary artery (PA) pressure.

    Treatment of PFC:
    • O2 therapy, as needed.
    • Sodium bicarbonate (Na HCO3), for metabolic acidosis, as needed.
    • Mechanical ventilation may be required.
    • Tolazoline (pulmonary vasodilator) infusion may help. It may, however, cause Hypotension, and GI-bleeding.


    prozac and atrial septal defects said...

    Where could I find more informations about heart birth defects? said...

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