Assessment of atrial septal defects
Atrial septal defect (ASDs) is the most common congenital lesion in adults
after bicuspid aortic valve. While the defect is often asymptomatic until
adulthood, complications of an undetected lesion include irreversible
pulmonary hypertension, right ventricular failure, atrial arrhythmias,
paradoxical embolization and cerebral abscess. (See
"Pathophysiology and clinical features of atrial septal defects"
This card will discuss the methods used for assessment of ASDs and estimation
of the shunt flow volume and ratio. The management of ASDs is discussed
"Management of atrial septal defects"
！ Multiple modalities are
available for establishing the diagnosis of ASD and assessing the hemodynamic
severity. It is now also possible to obtain hemodynamic information and to
estimate the shunt flow through the defect and the pulmonary artery pressures
noninvasively in most patients. Because of certain limitations of
echocardiography and operator-dependence, previous "negative"
studies from another laboratory should not be regarded as definitive.
！ Transthoracic imaging
and an M-mode echocardiogram frequently provides the first confirmation of the
diagnosis of ASD (show
The standard view in which the area of the interatrial septum is best
seen is the apical four chamber view in which the interatrial septum is
parallel to the ultrasound beam (show
However, this can result in artifactual drop-out of the septum. Clues to
the presence of an ASD include abrupt discontinuity of the septum and slight
thickening at its termination. Hypermobility of the septum, particularly in
association with an abrupt discontinuity, is also suggestive.
The interatrial septum is perpendicular to the ultrasound beam in the
subcostal view which can afford complete visualization of the atrial septum.
This image may be suboptimal, however, particularly in obese subjects.
Off-axis apical, parasternal short axis and other non-standard views are
frequently necessary to definitively investigate the septum.
Visualization of the atrial septum may be suboptimal in a transthoracic study.
Thus, other clues to the presence of an intracardiac shunt should be assessed.
Unexplained right-sided dilatation or pulmonary hypertension is suggestive of
the presence of a shunt, as is paradoxical septal motion, pulmonary artery
flow velocity greater than 1.2 m/sec, and diastolic flattening of the
！ A right-to-left
shunt may be seen in three settings:
• With a patent foramen ovale (in which the flow
is, by definition, from right-to-left).
• With transient reversal of flow through an
uncomplicated ASD when right-sided pressure is increased (eg, with a Valsalva
maneuver) or briefly during the onset of left ventricular contraction.
• With an ASD complicated by pulmonary
hypertension in which the left-to-right shunt is reversed.
The injection of microbubbles in agitated saline can confirm the presence of a
right-to-left shunt. An easy method to produce microbubbles uses injectable
saline passed rapidly back and forth between two 10 mL syringes connected by a
3-way stopcock. Less than 0.25 cc of air is introduced into the tip of one
syringe before it is connected to the stopcock. The saline will become
slightly opaque after approximately five or six vigorous transfers. The
agitated saline is promptly injected into an intravenous line. (See
"Myocardial contrast echocardiography"
An adequate contrast injection causes opacification of the right atrium and
ventricle. If there is a right-to-left shunt, early appearance of microbubbles
in the left atrium is seen spontaneously or during the release phase of the
Valsalva maneuver. Negative contrast in the right atrium is a helpful but
insensitive sign in the diagnosis of left-to-right shunt [1
In this setting, flow from the bubble-free left atrium produces areas in the
right atrium in which bubble-produced contrast is not seen (show
In our laboratory, we routinely perform contrast injections in any patient
with an unexplained dilatation of the right atrium or ventricle. We use two
injections, one at rest and one with Valsalva. Although no serious
consequences have been reported, we do not perform these injections in
pregnant patients or those with severe pulmonary hypertension.
Doppler flow echocardiography
！ The size of
an ASD on two-dimensional echocardiography does not correlate well with shunt
flow measured at catheterization. The addition of color flow Doppler imaging
can confirm the presence of an ASD and can be used to estimate the defect size
(, , and show
The maximal width of the color flow jet across the interatrial septum in
the view from which it is best visualized correlates fairly well with the size
of the shunt at catheterization. A jet width greater than 15 mm was found to
distinguish patients with a shunt ratio of greater than 2:1 (an indication for
surgical closure of the ASD) from those with a lesser shunt [2
There are several limitations to color flow Doppler:
• It may not visualize shunt flow if the frame
rate is too slow, since flow across the ASD may occur only during a portion of
the cardiac cycle. Reducing the Nyquist limit may allow the turbulent flow to
be more easily seen.
• Ghosting of color across the interatrial septum
sometimes gives the false impression of shunt flow.
• Sinus venosus defects, which are high up in the
septum, may not be seen with transthoracic or transesophageal echocardiography
A sinus venosus defect is one of the disorders that should be suspected
in a patient with a left-to-right shunt who has no evidence for an ASD or a
VSD. Cardiac catheterization is usually performed in such patients.
• Estimation of jet width size does not take into
account shunt flow due to associated anomalous pulmonary venous connections.
transesophageal approach is superior to transthoracic echocardiography in its
ability to image the interatrial septum and is extremely accurate in the
diagnosis of all three types of septal defects.. Estimation of defect size
using the diameter of the Doppler color flow jet correlates with surgical
findings. However, while transesophageal imaging can be used to verify the
absence of normal pulmonary venous connections, the level of anomalous venous
connection to the superior or inferior vena cava is often out of the imaging
planes and cannot be seen [3
In this setting, MRI can be used to define the anatomy of the anomalous
connections. Phase contrast imaging can estimate the size of all three types
of atrial septal defects and is superior to spin echo MRI [4
ESTIMATION OF SHUNT FLOW RATIO AND VOLUME
Operative closure of an atrial septal defect has traditionally been
recommended when the ratio of pulmonary flow to aortic flow is greater than
1.5:1 or 2:1. Noninvasive imaging can identify ASDs with significant shunt
flow. A number of parameters have been studied which can be used to separate
significant from insignificant shunts.
Measurement of the pulmonary flow to systemic flow ratio (Qp/Qs) can be
undertaken with Doppler echocardiography using the transthoracic approach. The
stroke volume through each valve is measured using the formula:
Stroke volume = cross sectional area x velocity
time integral (VTI) of the Doppler flow signal
Left-sided stroke volume is measured from the left ventricular outflow tract (LVOT)
by assuming a circular geometry. The diameter of the LVOT is always measured
in the parasternal long axis view. The maximum Doppler flow velocity apical to
the aortic valve (VTI-LVOT) is taken in the apical four chamber view. The
right-sided velocity time integral (VTI-PA) is measured in the pulmonary
artery well before the bifurcation. The diameter of the pulmonary artery is
then measured at the same level. Substitution into the stroke volume formula
results in the following formula:
Qp/Qs = LVOT diameter(2) x VTI-LVOT ‖ PA
diameter(2) x VTI-PA
The number two in parentheses (2) refers to the square of the LVOT or PA
The equation requires that the diameters of the left ventricular outflow tract
and the pulmonary artery formula be squared, making exact measurements of
prime importance in the calculation of the shunt ratio. Measurement of the
pulmonary artery diameter, taken in the parasternal short axis view at the
base of the heart can be problematic in some patients. Thus, this is the term
in the equation that is most often responsible for inaccurate estimates of the
shunt ratio using this method. Nevertheless, this method correlates well with
catheterization in patients with adequate transthoracic views [5
Shunt flow volume can be estimated using the area of the defect (again
assuming a circular geometry) and the velocity time integral of the shunt flow
measured with the Doppler beam parallel to shunt direction [6
Once the existence of an acceptable correlation between echocardiographic and
catheterization laboratory estimates of shunt flow has been established for an
individual laboratory, it is not unreasonable to forgo cardiac catheterization
in young patients when possible associated anomalies have been ruled out by
transesophageal echocardiography. Catheterization is still necessary for
evaluation of coronary lesions in patients at risk for coronary
Determination of the presence of an ASD by catheterization involves sampling
of the oxygen content in the inferior vena cava, superior vena cava, right
atrium, and right ventricle may reveal a "step-up" in oxygen
saturation which is suggestive of a shunt. A formal shunt run measures the
oxygen content in the blood at multiple sites and the Fick equation is then
used to calculate the ratio of pulmonary to systemic flow. Oxygen consumption
must be measured directly, rather than assumed from available nomograms. Some
authors believe that the partial pressure of oxygen must be measured in the
pulmonary veins if a right to left shunt is present, because dissolved oxygen
may be a substantial proportion of the oxygen content [7
In addition to the invasive nature of catheterization to determine shunt flow
ratio and volume, there are a number of potential pitfalls in the measurement
of shunt flow by catheterization. Accurate catheter position is essential and
measurement of outputs by the Fick method requires substantial patient