In simple uncomplicated congenital heart disease including atrial septal defect, restrictive ventricular septal defects, and patent ductus arteriosus, there is usually a net shunting of blood from the higher pressure left side to the lower pressure right side of the heart, with little, if any, right-to-left shunting. Upon short-term exposure to hypobaric hypoxia, right-sided pressures may increase, resulting in an increase in the right-to-left shunting with resultant arterial oxygen desaturation as more of the deoxygenated right-sided blood is bypassing oxygenation in the lungs.
This has been confirmed via cardiac catheterization. One group has shown that in a patient with an atrial septal defect and a resting left-to-right shunt, there was a significant increase in pulmonary artery pressure on hypoxic exposure resulting in reversal of the shunt. Interestingly, most studies of individuals who were born at and lived at high altitude reveal an increase in prevalence of patent foramen ovale and patent ductus arteriosus, likely due to a persistence of the fetal pattern of pulmonary vasculature, that is, thick smooth muscle cells, narrow lumen, small pulmonary vessels, and increased pulmonary and right ventricular pressures.
Because of the potential catastrophic effects of an acute increase in right-to-left shunting and/or pulmonary artery pressures in patients with congenital heart disease with shunts, exposure to high altitude should be avoided. However, exposure to moderate altitude has been reported to be safe even in adults with cyanotic congenital heart disease.
John P. Higgins, MD, MPhil, Troy Tuttle, MS, Johanna A. Higgins, MD