Aorticopulmonary Window
This defect consists of a connection between the ascending aorta and the main pulmonary artery trunk or the right branch of the pulmonary artery. There is virtually a window between the two vessels as opposed to a vessel or duct as in PDA. In severe cases, there is complete communication between the aorta and main pulmonary artery with no development of a septum between the two vessels. This window is usually large in size and in volume of shunting blood (1,83). Associated defects include VSD, subaortic stenosis, aortic arch interruption, mitral insufficiency, double outlet right ventricle, aortic valve atresia, bicuspid aortic valve, patent ductus arteriosus, and Tetralogy of Fallot (83).
Transverse images of the aortic and pulmonary artery from the right and left sides of the thorax show the communication. Normal dropout of echoes in these imaging planes is typical. A true window has thick hyperechoic borders. Color-flow and spectral Doppler shows bi-directional shunting and flow directed toward the pulmonary valve in the main pulmonary artery segment (83).
Right to Left Shunting PDA
Right to left shunting PDA occurs when pulmonary vascular pressure exceeds systemic pressure. This may occur because of over perfusion of the pulmonary lung field resulting in vasoconstriction and structural changes that include intimal proliferation and medial hypertrophy or because of retention of the high pulmonary artery resistance and pressure found in the fetus (79,84–88). Shunting can be bidirectional or balanced when systemic and pulmonary vascular pressures are similar. The degree of pulmonary over circulation is determined by the size of the ductus, and some large diameter PDAs have been shown to result in the development of pulmonary hypertension in dogs (89–91).
The diastolic component of shunting disappears first when pulmonary hypertension develops in hearts with PDA.
The cardiac changes associated with a reverse patent ductus arteriosus are all related to pulmonary hypertension. Diastolic aortic and pulmonary pressures will equilibrate first, and the diastolic component of the shunting blood will be lost first. There will be concentric hypertrophy and possibly dilation of the right ventricular chamber and main pulmonary artery dilation (Figure 9.27). The right atrium may remain normal but when tricuspid insufficiency is present, right ventricular pressure rises significantly and the right atrium can be dilated. Paradoxical septal motion may be present as RV pressure increases (92). The left side of the heart will be small secondary to a decrease in preload.
Figure 9.27 Right to left shunting PDAs cause right ventricular hypertrophy and occasionally dilation. (A) This cat had severe right ventricular hypertrophy and no dilation associated with its right to left shunting PDA. (B) Right ventricular hypertrophy (arrow) is seen on this long-axis outflow view of the heart in this puppy with a reversed PDA. (C) This young dog had severe right ventricular dilation as a result of pulmonary hypertension and an almost reversed PDA. Plane = apical four chamber. (D) Severe right ventricular volume overload and a dilated pulmonary artery are seen in this young dog with a reverse shunting patent ductus arteriosus. RV = right ventricle, LV = left ventricle, LA = left atrium, RA = right atrium, PA = pulmonary artery, RVW = right ventricle wall, PV = pulmonary valve, AO = aorta.
Color-flow Doppler will no longer help define the ductal flow. There is a minimal pressure gradient across the defect that creates no turbulence. Additionally, the packed cell volume is high, which will also diminish flow velocity.
Flow velocities of tricuspid and pulmonary insufficiency, if present, are high and reflective of high right ventricular and pulmonary artery pressure. The pulmonary flow profile will exhibit rapid acceleration and may show systolic notching (92). See Chapters 4 and 6 for more information on calculating these pressure gradients and determining RV and PA pressure. There is no way to differentiate the cardiac changes associated with hypertension secondary to a reverse shunt from those resulting from other causes such as primary pulmonary hypertension or thromboembolic disease. A bubble study can be used to determine whether the pulmonary hypertension is secondary to a PDA (92). This procedure is described later in this chapter.
Atrial Septal Defects
Introduction
Old English sheepdogs, Samoyeds, and Boxers seem predisposed to the development of atrial septal defects, and its occurrence is reported in cats and horses (93–96). Atrial septal defect is reported as the most common congenital heart defect (56%) in Boxers (97). Three different types of atrial septal defects occur. The most common defect, ostium secundum defect, is located in the middle of the atrial septum. It is in the same area as the fossa ovalis. This defect is created when there is over absorption of the septum primum or there is flawed development of the septum secundum. Less common is the ostium primum defect located at the junction of the atrioventricular valves and the atrial septum. This ASD is a form of endocardial cushion defect and is created when the endocardial cushions do not close the ostium primum. Because the endocardial cushions are involved, there are often associated defects of the atrioventricular valves. The least common atrial septal defect is the sinus venosus type located near the junction of the pulmonary vein and left atrium. This defect occurs when there is abnormal attachment of the right pulmonary vein into the cranial or caudal vena cava. Patent foramen ovale may
exist in the same area as the ostium secundum but has less hemodynamic significance than a true atrial septal defect unless right atrial pressures become elevated and a right to left shunt develops (1).
Two-Dimensional and M-mode Evaluation
The right atrium, right ventricle, pulmonary artery, and left atrium are all involved in the shunt pathway of an atrial septal defect and will all be volume overloaded. The left ventricle remains unaffected by this shunt (1). The degree of volume overload is dependent upon the size of the defect and the pressure difference between the left and right atrium. Large defects will volume overload the chambers and artery significantly and may lead to secondary tricuspid and mitral insufficiencies. Small defects may not show any measurable or visually obvious dilation on two-dimensional images.
ASD—shunt pathway left atrium → right atrium → right ventricle →
pulmonary artery → lungs →
left atrium
Large volume overloads of the right side of the heart will elevate right ventricular diastolic pressure, and paradoxical septal motion is seen once right-sided pressures exceed left ventricular diastolic pressure. The motion is subtle when the difference is minimal but can become dramatic with larger disparities in pressure. Paradoxical septal motion is the most common echocardiographic hemodynamic change seen in man with large atrial septal defects (Figure 9.28) (98).
Figure 9.28 Paradoxical septal motion is one of the most common echocardiographic hemodynamic changes seen with large atrial septal defects. Notice the flattening of the ventricular septum on this transverse view of the left ventricle in this dog with a large atrial septal defect. RV = right ventricle, VS = ventricular septum, LV = left ventricle, LVW = left ventricular wall.
Ostium primum defects typically have abnormal atrioventricular valves since the annuluses of these
valves are located at the junction of what would normally be the atrial and ventricular septums (Figures 9.29, 9.30). Tricuspid dysplasia and atresia may be seen with this type of atrial septal defect (95,96). Cleft mitral valve leaflets are often seen as well, and the presence of a cleft mitral valve almost always implies the presence of an ostium primum atrial septal defect (1).
Figure 9.29 Ostium primum defects are located at the lower part of the atrial septum just above the mitral and tricuspid valve annuluses (arrow). The atrioventricular valves will extend across the defect. Both valves may be dysplastic and insufficient. Plane = right parasternal long-axis four-chamber views, RV = right ventricle, RA = right atrium, LV = left ventricle, LA = left atrium.
Figure 9.30 Flow through an atrial septal defect may not show much, if any, aliasing since pressure gradients are not high. Transverse images of the heart base show low velocity flow through a small ostium primum atrial septal defect. RV = right ventricle, RA = right atrium, IAS = interatrial septum, LA = left atrium, AO = aorta.
Patent foramen ovale is seen in the middle of the atrial septum just as secundum defects are. The membrane that would normally close the foramen on the left side of the atrial septum may be seen on real-time images. Without seeing the membrane of the fossa ovalis, however, it is difficult to differentiate the shunt of an ostium secundum from a patent foramen ovale (Figures 9.31, 9.32). There is often an area in the middle of the atrial septum on four-chamber views that appears to have a defect. This is usually just the thin membrane that closed the fossa ovalis, and it does not reflect sound very strongly resulting in the appearance of an orifice (Figure 4.11). Rely on color-flow Doppler to help define this as a real communication between the atria.
Drop-out of the middle portion of the atrial septum is often seen in normal hearts.
Figure 9.31 A patent foramen ovale is located in the middle of the atrial septum. (A) The membrane that would normally close the defect is seen on the left side of the atrial septum (arrow). (B) Colorflow Doppler applied to the image in (A) shows a mildly aliased signal as blood flows through the defect from left to right (arrow). (C) A membrane (arrow) undulating on the left side of the atrial septum is seen when a foramen ovale is patent. Planes = right parasternal long axis four-chamber views, RV = right ventricle, LV = left ventricle, RA = right atrium, LA = left atrium.
Figure 9.32 A prominent aliased signal is seen flowing through a secundum atrial septal defect on this four-chamber view. RA = right atrium, IAS = interatrial septum, LA = left atrium, AO = aorta.
Spectral and Color-Flow Doppler Evaluation
Spectral and color-flow Doppler helps identify the presence of an atrial septal defect when it is too small to see or when there is doubt as to whether a perceived defect in the atrial septum is real. Color Doppler may show flow acceleration on the left side of the atrial septum, turbulence through the defect itself, and red colored slower flow as the shunted blood enters the right atrium (Figures 9.30, 9.31). Gradients are small however and depending on the transducer and the depth of interrogation, the velocity of shunting blood may not exceed the Nyquist limit. Decreasing the Nyquist limit on the color bar helps identify this low velocity flow. The more restrictive the ASD is the more likely an aliased signal will be seen.
An ASD has almost continuous positive flow. Flow is more prominent during atrial diastole (ventricular systole) coinciding with the onset of the QRS complex. This first prominent peak may be biphasic. This positive waveform should return almost to baseline, and a second lower velocity peak should be seen during atrial systole (Figure 9.33) (93–96,98,99). Vena caval flow is often recorded on right parasternal four-chamber views, and this flow needs to be differentiated from atrial septal defect flow. Both flows are positive and have two phases to flow. Vena caval flow however varies with respiration and has a sharper more defined positive flow profile with rapid acceleration and deceleration when compared to ASD flow. Vena caval flow also has a much smaller peak coinciding with the P wave (99).
Figure 9.33 Spectral Doppler of the flow across an atrial septal defect shows an initial positive peak during early during atrial diastole. This peak may be biphasic and is barely discernable in this image (arrow). The second peak coincides with atrial contraction and is of lower velocity.
The mean pressure gradient, not the peak, should be obtained by tracing the shunt flow profile, which helps differentiate between hemodynamically significant defects and small restrictive ones. Restrictive ASDs will have higher velocities because the pressures between both atria do not equilibrate as they would with large communications. A restrictive ASD will have velocities that reflect 5 to 10 mm of difference in pressure between the two chambers.
Recording tricuspid and mitral flow also helps identify the presence of an ASD. This is only valid if no mitral or tricuspid insufficiencies are present. A larger volume of blood will flow past the tricuspid valve than the mitral valve in the presence of an ASD, and flow velocity will therefore be greater across the tricuspid valve than the mitral valve when an ASD is present. These flows can be recorded from left parasternal apical and cranial transverse views of the heart.
Trans tricuspid flow velocity will be higher than transmitral flow velocity when an ASD is present. Pulmonary artery flow velocity will also be elevated.
The shunted volume of blood must flow past the pulmonary artery, and velocity of pulmonary flow is increased with large volume shunts. Although small defects may not increase flow velocities much, a shunt must be suspected when velocities are above the normal range. Pulmonary stenosis should be ruled out when pulmonary flow velocity is elevated. Comparison of flow velocity integral of the pulmonary artery and the aorta provides an estimate of the severity of the shunt. The higher the pulmonary flow integral is when compared to aortic flow integral, the greater the volume of blood that must be flowing through this vessel as compared to the aorta. Pulmonary to systemic flow ratios can be calculated as described in Chapter 4. A ratio of 2 : 1 or higher implies a hemodynamically significant shunt. Ratios greater than 2.5 are usually considered to be significant enough to warrant surgical intervention (1).
Large volume shunts may create pulmonary interstitial changes resulting in pulmonary hypertension. When this happens and right atrial pressure begins to exceed left atrial pressures, the shunt will reverse. Reverse flow through a patent foramen is often seen with congenital heart disease that elevates right sided pressure such as pulmonary stenosis and tricuspid dysplasia. Reverse flowing atrial septal defects follow a different flow pathway and the left ventricle is now involved. Changes