The AP window is displayed by 2D imaging as a direct communication between the arterial trunks. It appears as a deficiency or absence (echocardiographic “dropout”) of the wall normally interposed between the great arteries. The communication is usually large and frequently located along the anterior and leftward aspect of the aorta in the region where the great arteries are normally in apposition. The location and size of the defect, as well as its proximity to the semilunar valves are best evaluated in multiple views. Suggested TEE cross-sections to characterize an AP window are the UE PA LAX, UE Ao Arch SAX, and ME Asc Ao SAX views (Fig. 13.13 left panel, Video 13.6). The region of interest is the area of overlap between the great arteries or where they are seen in close proximity. The comprehensive evaluation of this defect may require the use of additional or non-standard TEE imaging planes, as may also be the case with other defects discussed throughout this textbook. For example, from the mid esophageal aortic valve long axis (ME AV LAX), mid esophageal long axis (ME LAX), mid esophageal ascending aortic long axis (ME Asc Ao LAX), and their respective modified views, the probe can be withdrawn slowly and sweeps performed by rotating the transducer in clockwise and counterclockwise fashions to display the great arteries and the communication (Fig. 13.14, Video 13.7). The deep transgastric long axis (DTG LAX) and sagittal (DTG Sagittal) views, as well as modified respective views that display the great arteries as they cross each other, provide complementary information. It is important to note that an AP window would represent an extremely rare new finding on TEE; therefore, when a presumptive AP window has been identified for the first time on a TEE study, it is important to exclude potential diagnostic pitfalls that can mimic this defect, such as false echocardiographic dropouts in this region. Confirmatory information should be obtained, as discussed below.

Color Doppler plays a very important role in the echocardiographic evaluation of an AP window. It verifies and characterizes the flow across the communication, providing qualitative information regarding shunt direction and magnitude. The same TEE views described in the identification of the defect can be used for this evaluation (Fig. 13.13 right panel, Videos 13.6 and 13.7). The spectral Doppler examination demonstrates antegrade continuous forward flow into the pulmonary artery near the area of the defect (Fig. 13.15). The presence of pulmonary hypertension at or near systemic levels is the norm for most large defects. In this case, low velocity, laminar, bidirectional (predominantly left-to-right) shunting across the communication reflects the negligible systolic pressure gradient between the systemic and pulmonary beds. This condition may render the identification of shunting somewhat difficult. Adjustment of the Nyquist limit for the color Doppler scale enhances the detection of low velocity flows and can be used to facilitate the detection of shunting across the communication. It is important to distinguish the low velocity color flow signal across an AP window from the “bleeding” of color flow sometimes seen across the walls of the aorta and pulmonary artery in a normal heart. Interrogation of the ascending aorta near the communication or of the descending thoracic aorta may display diastolic flow reversal in the presence of low pulmonary vascular resistance. It is similar to the nature of descending aortic flow in a moderate to large-sized PDA, and serves as indirect evidence of the presence of an AP communication, although it is not specific of the type. Smaller, restrictive defects can display a turbulent color flow Doppler pattern that corresponds to high velocity, continuous left-to-right shunting across the area.

As indicated, flow across large communications displays low velocity consistent with elevated pulmonary artery pressures. Smaller defects result in pressure restriction, displaying flow aliasing (mosaic pattern) by color Doppler and high velocity flow by spectral Doppler. The peak velocity of the continuous wave Doppler signal can be used to estimate PASP (as described for a PDA). The peak velocity of the tricuspid regurgitant jet, if present, can also be applied to estimate RVSP, as noted previously. Additional findings that may provide insight into the extent of pulmonary artery pressure elevation include: motion and configuration of the interventricular septum, right ventricular wall thickness, and degree of right ventricular dilation.

Depending on the presence of associated defects, additional aspects of the anatomy should be examined using corresponding TEE views as necessary. The hemodynamic effects of the volume load result in dilation of the pulmonary arteries and left-sided cardiac structures. The nature of the runoff associated with an AP window may result in low systemic diastolic pressures, low coronary perfusion pressures, and the potential for coronary ischemia and/or myocardial dysfunction. Thus, it is essential to assess both global and segmental left ventricular systolic function in the preoperative evaluation. This assessment is best performed in the transgastric mid short axis (TG Mid SAX), transgastric basal short axis (TG Basal SAX), transgastric two chamber (TG 2 Ch), mid esophageal four chamber (ME 4 Ch), mid esophageal two chamber (ME 2 Ch), and ME LAX views (also refer to Chap.​ 5).

In rare cases, depending upon the location of the AP window and other factors such as the size and length of the communication, consideration may be given to using a surgical approach that obliterates the communication without the need for cardiopulmonary bypass. In this setting, TEE is valuable for intraoperative monitoring and assessing the results of surgery. In addition, TEE is extremely useful for evaluating global and segmental ventricular function as well as left ventricular preload, and also for excluding the presence of intracardiac/intravascular air.

Obliteration of the abnormal AP connection in most cases requires patch closure (Fig. 13.16, Video 13.8). The post-repair TEE assessment should explore potential residual shunting and patency of the arterial roots to exclude obstruction of either the aorta or pulmonary artery. This represents the primary reason for morbidity in this lesion [48]. Semilunar valve competence also should be examined, as regurgitation can result from the repair, particularly if the AP window is proximally located near the semilunar valves.

The adequacy of the surgical intervention with regard to coexistent lesions should also be examined thoroughly, as well as other relevant aspects of any postbypass evaluation (e.g., ventricular systolic function, atrioventricular and semilunar valve regurgitation).

In this vascular anomaly, also referred to as a pulmonary artery sling [49], the left pulmonary artery originates abnormally from the proximal right pulmonary artery [50]. The course of the left pulmonary artery is such that it arises from the posterior aspect of the right pulmonary artery, crosses above the right mainstem bronchus, and travels posteriorly between the trachea and esophagus towards the left hilum (Fig. 13.17). Hypoplasia of the left pulmonary artery often is seen, whereas some degree of dilation of the right pulmonary artery is the norm. The boundaries of the trachea in this setting consist of the main pulmonary artery anteriorly, the left pulmonary artery to the right and posteriorly, and the ductus or ligamentum arteriosum to the left. In contrast to other vascular anomalies such as a double aortic arch or right aortic arch with a left ductus/ligamentum, a pulmonary artery sling, does not represent a true complete anatomic ring. However, this lesion is frequently considered within the context of vascular anomalies causing a ring. The pathology is attributed to a developmental malformation, although several instances have been reported in patients with a relatively recently described genetic syndrome (Mowat-Wilson) [51, 52].

A significant number of patients with this malformation (more than 50 %) have complete tracheal rings and/or other tracheobronchial abnormalities (Fig. 13.18) [53, 54]. This condition is associated with tracheal narrowing, and, if significant, the stenosis can result in airway obstruction [53, 55–57]. Tracheomalacia is present in nearly all patients. Additional cardiovascular pathology also is a frequent finding. The most commonly associated defects include intracardiac communications at either the atrial or ventricular levels, PDA, and tetralogy of Fallot [57–59]. Other extracardiac abnormalities, such as hypoplasia and agenesis of the right lung, have also been reported [53].

Although a pulmonary artery sling may present as an incidental finding, the unusual course of the anomalous vessel behind the trachea or right bronchus frequently leads to distortion of the tracheobronchial tree, airway compression, and respiratory symptoms in early infancy. The airway compromise occurs regardless of the presence of associated congenital tracheal stenosis. In some cases, the anomalous left pulmonary artery displays stenosis and/or hypoplasia. The clinical manifestation of this defect can be influenced significantly by the presence of coexistent anomalies.

The high incidence of tracheal anomalies in this lesion requires comprehensive airway assessment to formulate suitable management plans [53, 60]. The surgical approach in most cases consists of left pulmonary artery division and reimplantation into the main pulmonary artery, allowing for the vessel to be positioned anterior to the trachea. This procedure is now accomplished at most centers via a median sternotomy approach using cardiopulmonary bypass [53, 61]. Concomitant repair of tracheal stenosis and associated intracardiac anomalies is performed as indicated [62–65]. If short-segment tracheal stenosis is present, resection of the involved region is performed; while the trachea is divided, the left pulmonary artery can be translocated anterior to the trachea, obviating the need for surgical division and left pulmonary artery reimplantation.

In addition to TTE, other imaging modalities such as chest CT scanning and MRI play important roles in identifying and defining the details of the anatomy in the patient with a pulmonary artery sling prior to surgical intervention [54, 60, 66–74]. At the same time, these additional imaging studies allow for the airway to be examined. The characteristic finding is that of the abnormal origin and course of the vessel. Depending on the particular diagnostic study, an anterior indentation of the esophagus is seen, in contrast to other types of vascular rings, which produce a posterior indentation of the esophagus.

Because of concerns related to frequently associated airway pathology and potential for respiratory decompensation due to esophageal instrumentation, serious consideration of the risk-benefit ratio should be given prior to the use of TEE in affected patients, particularly considering that the anatomic abnormalities usually have been thoroughly outlined. Epicardial imaging should be considered a potentially safer alternative in this case. Another reason for hesitation relates to possible alterations in pulmonary blood flow caused by vascular compression of the anomalous vessel, which can be impinged between the trachea and the esophageal probe. All these factors have discouraged the routine use of TEE in this anomaly and likely account for the limited experience. The discussion that follows highlights known or potential applications of TEE when used in this setting.

Anomalous origin of a branch pulmonary artery branch from the aorta is a rare congenital malformation. In this anomaly, a main pulmonary segment arises normally from the right ventricle, but only one branch emerges from the pulmonary artery confluence, and the controlateral branch originates from the ascending aorta, resulting in discontinuity between the pulmonary arteries.

Several theories have been proposed to explain the developmental alterations leading to this anomaly, but no consensus has been reached [79–81]. Although this malformation has also been referred to as hemitruncus, anatomically it should be distinguished from truncus arteriosus because, in contrast to a single semilunar valve, two semilunar valves are present in this setting. This anomaly represents less than 0.1 % of all CHD.

Although the anomaly described above of an aortic origin of a branch pulmonary artery represents the most common form of this lesion [82], the anomalous vessel can originate elsewhere [83]. In some cases, the pulmonary artery originates from unusual locations, particularly in the context of complex cardiovascular malformations [84]. It has been proposed that proximal anomalous origin of a pulmonary artery from the ascending aorta and anomalous origin from the innominate artery, also referred to as absent or occult pulmonary artery [85, 86], are two distinct and separate entities from the standpoint of embryology, clinical presentation, and treatment [87–89].