| Susan E Wiegers, MD Martin G St John Sutton, MBBS |
Apr 22, 1998 |
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 separately. (See "Management of atrial septal defects").
EVALUATION ！ 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.
Echocardiography ！ Transthoracic imaging and an M-mode echocardiogram frequently provides the first confirmation of the diagnosis of ASD (show echocardiogram 1).
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 echocardiogram 2).
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 interventricular septum.
Contrast echocardiography ！ 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 echocardiogram 3).
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 echocardiogram 6).
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 (show figure 1).
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 echocardiography ！ The 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 diameter.
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 atherosclerosis.
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 cooperation.