During normal cardiac development, patency is maintained between the right and left atria by a series of fenestrations (ostium primum and ostium secundum) that eventually become the foramen ovale. This arrangement allows oxygenated blood from the maternal circulation to flow from the right to the left atrium, thereby sustaining fetal development. At later stages of intrauterine development, tissue flaps (septum primum and septum secundum) grow to occlude the foramen ovale, and in 80% of cases, the higher left-sided pressures in the heart that occur at birth permanently fuse the septa against the foramen ovale. In the remaining 20% of cases, a patent foramen ovale results; although the flap is of adequate size to cover the foramen, the unsealed septa can allow transient right-to-left blood flow, such as with a Valsalva maneuver during sneezing or straining during bowel movements. Although this typically has little significance, it occasionally manifests as paradoxical embolism, defined as venous emboli (e.g., from deep leg veins) that enter the systemic arterial circulation via a foramen ovale defect.
In contrast to patent foramen ovale, an ASD is an abnormal fixed opening in the atrial septum that allows unrestricted blood flow between the atrial chambers. A majority (90%) of ASDs are so-called “ostium secundum” defects in which growth of the septum secundum is insufficient to occlude the second ostium.
MORPHOLOGYOstium secundum ASDs(90%ofASDs)aresmooth-walleddefectsneartheforamenovale,typicallywithoutotherassociatedcardiacabnormalities.Hemodynamicallysignificantlesionsareaccompaniedbyrightatrialandventriculardilation,rightventricularhypertrophy,anddilationofthepulmonaryartery,reflectingtheeffectsofachronicallyincreasedvolumeload.
Ostium primum ASDs(accountingfor5%ofthesedefects)occuratthelowestpartoftheatrialseptumandcanbeassociatedwithmitralandtricuspidvalveabnormalities,reflectingthecloserelationshipbetweendevelopmentoftheseptumprimumandtheendocardialcushions.Inmoreseverecases,additionaldefectsmayincludeaVSDandacommon atrioventricular canal.
Sinus venosus ASDs (accountingforanother5%ofthecases)arelocatedhighintheatrialseptumandoftenareaccompaniedbyanomalousdrainageofthepulmonaryveinsintotherightatriumorsuperiorvenacava.
ASDs usually are asymptomatic until adulthood. Although VSDs are more common, many close spontaneously. Consequently, ASDs—which are less likely to spontaneously close—are the most common defects to be first diagnosed in adults. ASDs initially cause left-to-right shunts, due to lower pressures in the pulmonary circulation and the right side of the heart. In general, these defects are well tolerated, especially if they are less than 1 cm in diameter; even larger lesions do not usually produce any symptoms in childhood. Over time, however, chronic volume and pressure overloads can cause pulmonary hypertension. Surgical or intravascular ASD closure is thus performed to preempt the development of heart failure, paradoxical embolization, and irreversible pulmonary vascular disease. Mortality is low, and postoperative survival is comparable to that for an unaffected population.
Defects in the ventricular septum allow left-to-right shunting and constitute the most common congenital cardiac anomaly at birth (see
Table 11.1