The Assessment of the Proximal Left Pulmonary Artery by Transesophageal Echocardiography and Computed Tomography in Neonates and Infants: A Case Series

The use of transesophageal echocardiography (TEE) has substantially increased during the repair of congenital heart disease (CHD) ^(1–5). Determining the size and patency of the pulmonary arteries is important in planning any surgical intervention during open-heart surgery, particularly in neonates and infants. The main pulmonary artery (MPA), right pulmonary artery (RPA), and distal left pulmonary artery (LPA) have been well visualized by using TEE ⁽⁶⁾. However, imaging the proximal LPA with the use of TEE has been limited by a “blind spot” caused by the left bronchus’s interposing between the proximal LPA and the esophagus ^(7–11).

The routine assessment of the proximal LPA in neonates and infants by means of TEE has been used for many years, but the exact success rate is not known. Therefore, we assessed 256 neonates and infants undergoing open heart surgery with TEE and computed tomography (CT) of mediastinal anatomy in 40 patients per group.

Methods

We obtained approval from our hospital’s human investigation committee. With informed consent obtained from parents, from August 1997 to August 1999, single transverse plane TEE was performed in 256 newborns and infants (146 boys and 110 girls) undergoing open-heart surgery. Eighty-five newborns were 1 to 30 days (median, 12 days) old, weighing 1.7 to 4.3 kg (median, 3.1 kg), and 171 infants were 1 to 12 mo old (median, 6 mo), weighing 2.6 to 9.7 kg (me-dian, 5 kg).

All studies were performed with the use of a specially designed transesophageal neonatal probe for newborns and infants (Vingmed Ultrasound; GE, Horten, Norway). The endoscope was 6.5 mm in external diameter and 50 cm in length, with a 7.5-MHz monoplane color flow and Doppler transducer. Under general anesthesia, all infants were placed in a supine position, and the transducer tip was introduced gently into the esophagus without force. The probe was always maintained in an unlocked position, with a slight flexion of the tip. Those infants with gastroesophageal abnormalities, such as diverticuli, ulcers, or varices, were excluded from the study. In each patient, systematic transverse TEE images were obtained, with routine supracarinal and subcarinal views for imaging the ascending aorta (AAo), MPA, RPA, and all cardiac chambers. An additional left paracarinal view for imaging the pulmonary artery (PA) bifurcation and the proximal LPA was also performed.

Forty of the 249 patients had transverse chest electron beam CT (Imatron C-150; Imatron, South San Francisco, CA) images with nonionic iodinated contrast enhancement before surgery. The topical relationships of the esophagus, left main bronchus (LB), cardiac chambers, and great arteries were studied by a radiologist who was blinded to the study protocol, after serial analysis of the CT images. All of the patients had normal great arteries. The CT sections corresponding to the transverse TEE images at the carinal, subcarinal, and supracarinal levels of the trachea were determined, photographed, and analyzed.

Results

The major congenital heart lesions repaired among the 85 neonates and 171 infants are listed in Table 1. TEE was successfully performed in 249 cases, and it failed in seven newborns who developed systemic arterial hypotension and severe desaturation soon after insertion of the TEE probe: of these seven patients, four had total anomalous pulmonary venous connection and three had critical coarctation of the aorta. The TEE probe was withdrawn, and there were no further complications.

Table 1: Congenital Heart Diseases Repaired and TEE Monitored in 256 Neonates and Infants

In 234 (94%) of the 249 patients monitored by TEE, the left paracarinal TEE approach adequately demonstrated the PA bifurcation and the proximal LPA, revealing a proximal LPA lesion in 27 (10.8%) patients, as listed in Table 2 and Figure 1. Of the 12 cases (5.1%) with postoperative residual lesions associated with desaturation after termination of cardiopulmonary bypass, four, who had severe anatomic (caused by narrowing the LPA patch) or dynamic (kinking or twisting of the patch graft) stenosis (peak flow velocity >3 m/s) underwent a revised operation immediately after the primary surgery. Eight cases that had moderate proximal LPA stenosis (peak flow velocity <2 m/s) received balloon angioplasty months after surgery when followed up with echocardiography and cardiac catheterization.

Table 2: The Proximal LPA Lesions Detected by Left Paracarinal TEE Monitoring

Figure 1: (Left, top) Image from left paracarinal transesophageal echocardiography (TEE) shows the carina (C) and a narrowing (*) at the origin of the proximal left pulmonary artery (LPA) (4.6 mm) by a fibrotic shelf. (Left, bottom) Left paracarinal TEE shows multiple pulmonary stenosis (arrow) at the main pulmonary artery (MPA) and proximal LPA after tetralogy of Fallot repair requiring a revised operation. (Right, top) Left paracarinal TEE from an infant with tetralogy of Fallot shows the fibrous flap (*) at the origin of the LPA. (Right, bottom) Left paracarinal TEE shows the pulmonary artery band (PAB) (*) having slipped dorsally, encroaching at the bifurcation of pulmonary artery (PA) and resulting in proximal LPA stenosis.

Analysis of the chest CT images revealed that at the supracarinal level, the esophagus usually lies left of and behind the trachea. A TEE window was found imaging the AAo and aortic arch. At the subcarinal level, the esophagus is dorsal to the RPA in this tomographic image. It lies between the right main bronchus and LB (Fig. 2 A, left). A TEE window was found when we visualized the MPA, AAo, and part of the RPA (Fig. 2 A, right). If the transducer tip of the endoscope was manipulated clockwise in rightward rotation, AAo, RPA, and the superior vena cava could be visualized. There was no window for the visualization of the proximal LPA because of an air-block produced by the LB, which lies left and ventral to the esophagus. The CT image at the carinal level showed that the esophagus lies dorsally to the left edge of the carina, between the carina and descending aorta (Fig. 2 B, left). The left paracarinal transverse plane TEE view well visualized the MPA and proximal LPA, as shown in Figure 2 B (right). Further leftward rotation of the tip of the endoscope provided a good left paracarinal window for the visualization of the LPA arising from the MPA. The descending aorta and LPA could be comprehensively imaged with leftward rotation of the tip of the endoscope.

Figure 2: (A) Typical standard transverse images from chest computed tomography (CT) (left) and corresponding transesophageal echocardiography (TEE) (right) at the level of the subcarina in an infant, showing the main pulmonary artery (MPA) and the right pulmonary artery (RPA). The esophagus (*) or the TEE transducer (T) is dorsal to the left main bronchus (LB). There is no window for imaging the proximal left pulmonary artery (LPA). (B) Typical chest CT (left) and TEE (right) images from the left paracarinal window showing the MPA and the proximal LPA in the same infant. The esophagus (*) or the TEE transducer (T) is left dorsal to the carina. Pulmonary bifurcation (#) and proximal LPA are visualized in this view. DAo = descending aorta; AAo = ascending aorta; RB = right main bronchus; SVC = superior vena cava; C = carina.

In 7.5% (3 of 40) of the patients, by using CT analysis, the esophagus was found to be lying right lateral to the vertebra, which hampered the imaging of the proximal LPA by the paracarinal window of TEE

Discussion

Early neonatal repair of CHD has become possible in recent years ⁽¹²⁾. Intraoperative TEE is routinely used during cardiac surgery to improve the quality of surgical repair and to potentially reduce the morbidity and mortality ⁽¹³⁾. Because of the mediastinal anatomy, however, visualization of the proximal LPA has been poor or limited because of the air-filled LB interposing between the esophagus and proximal LPA.

Our case series shows that the proximal LPA could be visualized from the left paracarinal window by TEE in 94% and by CT in 92.5% of patients, with good correlation between both imaging techniques in neonates and infants. If the patient’s esophagus lies right lateral to the vertebra, the paracarinal view may not visualize the proximal LPA. The incidence of the esophagus’s lying right lateral to the vertebra was found in our series of 40 neonates and infants with CHD to be 7.5%. This anatomic variation may hamper the left paracarinal TEE approach for imaging the proximal LPA. However, the insertion of a rigid endoscope transducer, 6.5 mm in diameter, into the esophagus in neonates and infants will change the shape of the collapsed esophagus and also shift the position of the esophagus to the middle or left. This procedure may also push the LB more anteriorly, thus providing an enlarged left paracarinal view, as demonstrated in the chest CT images of neonates and infants who had a large-bore nasogastric tube in place. Our clinical experience showed that if the tip of TEE probe were tilted to the left, the left paracarinal TEE window would also be better visualized.

Monitoring the size and patency of the proximal LPA is important in planning any surgical intervention. The proximal LPA pathology occurs frequently in cases of tetralogy of Fallot, pulmonary atresia, persistent truncus arteriosus, right atrial isomerism, and other complex anomalies. Identifying proximal LPA can avoid the confusion in the diagnosis of LPA or large patent ductus arteriosus in neonates and infants. The proximal LPA stenosis may occur as a discrete membrane, as previous B-T shunting, as fibrotic tissues from patent ductus arteriosus, or as mechanical obstruction by a migrating PA band and inadequate surgical repair.

Our case series shows that the left paracarinal TEE approach may suffice during surgery to provide the important images of proximal LPA and PA bifurcation. The “blind areas,” representing the PA bifurcation and proximal LPA caused by the left bronchus, are thus eliminated in most patients. Because of the safety and limitation in imaging with the monoplane TEE transducer in neonates and infants, transgastric TEE images with color Doppler imaging have recently added a new dimension to intraoperative echocardiography. An approach from the fundus of the stomach has been tried; this approach, however, can image only the outflow tract of the right ventricle and the proximal pulmonary trunk. It also has the adverse effects of hypotension and increased peak inspiratory pressures because of the anteflexion of the probe ^(14–15). Currently, the standard care of pediatric TEE is biplane examination. However, a longitudinal view at the orthogonal planes can visualize the distal LPA clearly, but it visualizes the proximal LPA and PA bifurcation only with much difficulty.

In summary, our case series shows that through serial analysis of the chest CT and the corresponding transverse plane TEE images in neonates and infants, a left paracarinal TEE provided information of the proximal LPA in most cases, except in a few (7.5%) cases with anatomic variations of the esophagus in the right lateral to the vertebrae. This approach seems to provide a useful tool in visualizing this “blind” area, which echocardiographers have deemed to be a very difficult area to assess.