Prevalence of congenital cardiac anomalies at high altitude.

The effect of high altitude on the prevalence of congenital heart disease was tested by examining 1,116 school children at four study sites in the People's Republic of China. Sites ranged in altitude from sea level to 4,500 m above sea level. Children were screened by physical examination, and an echocardiogram and electrocardiogram were performed on each child suspected of having a cardiac anomaly. A high prevalence of patent ductus arteriosus and atrial septal defect was found at the three high altitude sites and the effect of altitude was progressive. Both anomalies were postulated to be the result of the lower atmospheric oxygen tension present at high altitude. Failure of lower oxygen tension to constrict the ductus is thought to be the mechanism in patent ductus arteriosus. It is theorized that the persistence of high pulmonary vascular resistance and high right heart pressures at high altitude inhibits early closure of the foramen ovale. Subsequent growth may result in stretching of the fossa ovalis and incompetence of the flap and may produce an atrial septal defect. The high prevalence of atrial septal defect in tetralogy of Fallot is cited as a possible analogy because right ventricular pressure is high and right ventricular compliance is low from birth.