Ultrasound diagnosis of pulmonary sling with proximal stenosis of left pulmonary artery and patent arterial duct.

Authors discuss methods of echocardiographic diagnosis of the pulmonary sling with stenosis and hypoplasia of the left pulmonary artery and patent arterial duct with massive left-to-right shunt, based on a case of the newborn with resistant to treatment heart failure, with initial diagnosis of patent ductus arteriosus, referred to surgical treatment. The optimal echocardiographic views permitting establish diagnosis of the pulmonary sling were suggested. The special attention was paid to high parasternal and suprasternal views visualizing vessels of the upper mediastinum as well as characteristic differences between the normal and pathologic picture. The typical features of the echocardiogram suggesting pulmonary sling, like the lack of the left pulmonary artery in its expected position, and the abnormal branching pattern of the right pulmonary artery were indicated. The greatest diagnostic difficulties in visualization of the abnormal route of the left pulmonary artery were related to the presence of air-containing tissues, like lungs and central airways between the ultrasound probe and area of interest. The other was the masking influence of the large patent arterial duct, that may mimic the left pulmonary artery arising from the pulmonary trunk. The other entities requiring differentiation with sling, like aplasia of the left lung, the direct or indirect aortic origin of the left pulmonary artery, were discussed. The role of other visualization technics, like computed 3D tomography, and magnetic nuclear resonance, as well as direct visualization of central airways with bronchoscopy in establishing precise diagnosis were stressed.

The term pulmonary sling (PS) refers to the anomalous origin of the left pulmonary artery from the right pulmonary artery. It belongs to the rarest congenital defects of the cardiovascular system. The exact incidence of this defect is not known(, but it may be indirectly estimated on the basis of the number of cardiac surgeries performed in the centers which specialize in operating patients with various forms of vascular anomalies. For instance, the register of the Pediatric Cardiac Care Consortium reports 193 surgeries of vascular rings caused by the anomalies of aortic arch and 11 surgeries of slings over the period of 10 years(. In the Children's Memorial Hospital in Chicago in 1947– 2010, 502 surgeries of vascular rings were conducted and 42 operations of slings(. The above mentioned numbers confirm the rareness of this condition.

In patients with slings the left pulmonary artery (LPA) originates from the middle part of the right pulmonary artery (RPA), in close proximity of the trachea, on its right side. Initially, the LPA runs dorsally above the right main bronchus and turns at the nearly right angle to the left to surround trachea posteriorly. When heading to the left pulmonary hilum, it travels between the trachea and esophagus almost always narrowing right main bronchus and/or trachea but only rarely compressing the esophagus, which is located behind the trachea. The presence of the arterial ligament and duct placed at the left side of the trachea is also important as it additionally decreases the narrow space for the trachea. The respiratory disorders which appear usually in neonatal period may, in more than 80% of cases, be a consequence of various coexistent anomalies in the structure of the trachea and bronchi(.

The anatomical image of the pulmonary vessels is diverse. Usually, the RPA, which constitutes a direct extension of the pulmonary trunk, is better perfused and thus, wider than the left one. The disproportion of perfusion is more intense when the fragment of the LPA undergoes hypoplasia or narrowing. It is worth adding here that the morphology of the defect may encompass a wide spectrum of variants including hypoplasia and ever aplasia of the right lung with respective underdevelopment of the right pulmonary artery. The appearance of complex cardiac and cardiovascular defects is also mentioned, for instance: total anomalous pulmonary venous connection, tetralogy of Fallot, double outlet right ventricle (DORV), pulmonary atresia without ventricular septal defect (VSD), aortopulmonary window, interrupted aortic arch, Williams syndrome, patent ductus arteriosus (PDA)(.

The diagnosis of the anomalous course of the LPA on the basis of the echocardiography evaluation is almost always possible(, but requires increased alertness if other intracardiac and/or vascular disorders (LPA hypoplasia) coexist.

The basic rule of this test, which is particularly important in children with respiratory disorders, is a thorough assessment of the division region of the pulmonary trunk. In PS the echocardiographic presentation is usually characteristic. Firstly, there is no division of the pulmonary trunk. In normal conditions the pulmonary trunk runs in the cranial direction (at the angle of 45° from the chest), dorsally and to the left. The left branch constitutes its extension. It travels dorsally and slightly to the left where it crosses the aorta descending from the ventral side, below the aortic opening of the ductus arteriosus. The right branch originates from the pulmonary trunk at a nearly right angle and travels to the right hilum – in the coronal plane. In pulmonary sling, the pulmonary trunk runs more to the right and the RPA constitutes its extension. The lack of the division is striking and the vessel, which is directed to the dorsal side and originates from the posterior wall of the RPA, is not visible until the wall's middle part. It is easily visualized when it is wide. However, in the case of localized stenosis or general hypoplasia, it might be unnoticed, all the more that after its short course, it “hides itself” behind the trachea which is impermeable to the ultrasound. An additional diagnostic trap is patent ductus arteriosus(, especially when it is wide. In such a situation, it may be mistaken for the LPA. In the case of large shunt, the symptoms of circulatory insufficiency may become more prominent and mask the consequences of the airway compression.

Such a case is presented in this article: pulmonary sling with the LPA stenosis and wide patent ductus arteriosus. The patient was referred for a surgery to our Department with the diagnosis of persistent ductus arteriosus with significant left to right shunt and subsequent intractable cardiopulmonary failure. According to the routine protocol, the echocardiographic examination was performed in order to make the definite diagnosis and finally qualify the patient for a surgical treatment.

Two echocardiographic views performed by means of the high left parasternal access proved to be the most useful for the diagnosis of the sling: the first – visualizing the aortic arch in the sagittal section and the second – presenting the pulmonary trunk and its right branch in the transverse section.

For the analysis of the ductus arteriosus, a standard high parasternal view was used in the sagittal section, which typically presents the pulmonary trunk and the ductus arteriosus that runs with the artery's extension and merges smoothly with the descending aorta. This view enables a precise visualization of the entire duct, the measurements of the diameter of all its fragments and accurate Doppler assessment of the shunt. In normal conditions, it shows the LPA below the ductus arteriosus running nearly parallelly to the ductus. The LPA crosses the descending aorta anteriorly in a site right below the ductus opening. A slight shift of the ultrasound beam to the left enables to trace the course and establish reciprocal relations of these vessels (fig. 1).

Fig. 1

2D visualization of the ductus arteriosus in the long axis during a Doppler examination. The high parasternal view in a nearly sagittal plane; the image performed at the moment of contraction. The ductus arteriosus (DA), with the flow coded by the red color, is nearly parallel to the aortic arch (Łuk) and the left pulmonary artery (LTP); in both arteries, the flow “away from the transducer” is coded by the blue color. The ductus arteriosus constitutes the direct extension of the pulmonary trunk (TP) and smoothly becomes continuous with the descending aorta (Ao zst.). The figure also presents the left atrium (LP). Below the site where the descending aorta crosses the left pulmonary artery, a loss in color is visible inside the aortic lumen, because it is also crucified by left main bronchus, scattering the ultrasound beam and causing strong acoustic shadow; this phenomenon creates an appearance of the interruption of aorta

In this case, the attention of the examiner was drawn to the presence of only one vessel originating from the pulmonary trunk, which connected it with the descending aorta, so it definitely was the ductus arteriosus. At the site of an anticipated course of the LPA an empty space was visualized (figs. 2 and 3).

Fig. 2

The distal part of the aortic arch giving off the left common carotid and left subclavian arteries as well as aortic isthmus and wide ductus arteriosus connecting aorta and pulmonary artery were visualized. Moreover, the descending aorta and the cavity of the left atrium are visible. In normal conditions, the left pulmonary artery runs below the ductus arteriosus. Its anticipated course is marked with ****

Fig. 3

The analogical picture with the use of color-coded flow; the image taken during systole thanks to which the blood flow in all visible arteries can be observed. One may see: the blue aortic arch and the descending aorta as well as the red ductus arteriosus entering the pulmonary trunk. Below the ductus, the empty space is marked with four asterisks (****); it is adjacent to the acoustic shadow of the left main bronchus. However, the region not covered by the shadow is also visible. It should be occupied by the left pulmonary artery. The absence of this artery suggested pulmonary sling

In such a case the differential diagnostics should include aplasia of the left lung, blood supply of the lung from the aorta and pulmonary sling. The aplasia was excluded because of the correct medial position of the heart, the presence of the air-containing tissues in the left half of the chest and normal return from the left pulmonary veins visualized during the Doppler examination. The source of blood supply to the LPA could not have been the ductus arteriosus, which was clearly visible between the apex of the pulmonary trunk and the descending aorta. The only remaining possibilities were: the direct origin of the LPA from the aorta, the blood supply to the left lung vessels by the collateral arteries (major aortopulmonary collateral arteries, MAPCAs) or sling. In the transverse projection which presented the site of potential division of the trunk into branches, only the right branch and the opening of the ductus arteriosus were visualized (fig. 4). Finding the LPA was more difficult due to its considerable stenosis and limited flow (figs. 5–7).

Fig. 4

Parasternal view showing the mediastinum in the transverse section. The right pulmonary artery is visible (PTP) which runs away from the pulmonary trunk (TP); in front of the right pulmonary artery, there is the ascending aorta (AoW) and the superior vena cava (ZGG). The image taken in the diastolic phase. In this phase, the flow in the pulmonary trunk and its right branch comes from the ductus arteriosus. Hence, in the pulmonary trunk, it is coded with the red color (it is directed towards the transducer) and in the branch – with blue

Fig. 6

The largest fragment of the left pulmonary artery (LTP Zw) located at the site of crossing with the trachea. The fragment beyond the trachea is covered with the acoustic shadow

Fig. 7

Further mild modification of the transducer position visualizes longer fragments of the left pulmonary artery

Slight caudal shift of the transducer visualizes the origin of an additional vessel of the LPA from about half the length of the RPA. The image in a systolic phase. The left pulmonary artery is covered by the trachea

The LPA diameter in the narrowest segment did not exceed 2 mm (fig. 6), with generation of the maximum gradient of 55 mm Hg (fig. 8). The continuous flow, typically observed in the presence of arterial stenosis, was detected.

Fig. 8

The spectrum of the flow in the narrowed fragment of anomalous left pulmonary artery. The velocity and character of the flow confirm to the significant hemodynamic stenosis

Upon the detection of the initial fragment of the LPA, it was possible to visualize its further course beyond the trachea region towards the left lung hilum and finally sling was diagnosed.

The echocardiography, however, could not provide us with certain information concerning the degree and extensiveness of the stenosis of the LPA beyond the air-containing structures. Nor could it provide information connected with the condition of airways. Therefore, the patient was directed to the Cardiology Clinic for supplementary diagnosis. The computed tomography angiogram confirmed the stenosis of the left pulmonary artery. It enabled the assessment of the airways and determined the decision concerning a surgical procedure(. The division of the ductus arteriosus was performed. After the procedure, the patient was transferred to the home cardiology clinic. The aim of this paper was to emphasize the essential role of the echocardiography in the diagnosis of this rare condition despite its significant limitations. The echocardiography is a non-invasive test which currently is conducted by increasing number physicians. What is more, it is becoming a standard in neonatal wards. According to the authors of this article, the high parasternal views (in neonates and young infants the transsternal view is also possible) allow for a relatively easy, precise imaging of the vessels located in the upper mediastinum, i.e. the aortic arch with its branches, the aortic isthmus, the pulmonary trunk and its branches, ductus arteriosus as well as pulmonary and systemic veins. Thanks to the thymus, which conducts the ultrasounds well, clear images may be obtained. Problems may be caused by lungs hyperinflation, particularly in agitated or artificially ventilated patients. Most of the congenital vascular anomalies which require urgent diagnosis are localized in this region. The systematic analysis of images obtained in high parasternal views, in most cases, enables to establish the diagnosis or, in case of doubts, to formulate questions which may be answered by means of other diagnostic methods: computer tomography, computer tomography angiogram and bronchoscopy.