Diagnostic Dilemma: A Pacemaker Lead Inside the Left Atrium or an Echocardiographic Beam Width Artifact?

Abstract

A 71-yr-old man with a permanent dual chamber pacemaker (pm), chronic atrial fibrillation with a slow ventricular response and symptomatic mitral regurgitation presented for mitral valve surgery. His intraoperative monitoring included, among others, a pulmonary artery catheter (PAC) and transesophageal echocardiography. The surgical approach was from the right atrium via an atrial septotomy, and the mitral valve was replaced. After separation from cardiopulmonary bypass, a strong reflector accompanied by distal acoustic shadowing was seen inside the left atrium (LA), in the mid-esophageal bicaval view at different degrees of rotation (see video clip at www.anesthesia-analgesia.org.). Migration of the PAC into the LA via the atrial septotomy was excluded, based on the fact that central venous and pulmonary artery pressure waveforms were transduced from the proximal and distal PAC ports, respectively. We considered that one of the permanent pm leads was apparently trapped inside the LA. However, the surgeon was adamant that this had not occurred, and the chest was closed. Further transesophageal echocardiography examination, using similar acoustic windows, failed to reproduce the artifact (Fig. 1). A postoperative portable chest radiograph verified that both permanent pm leads were anchored properly at the right atrium and right ventricular apex.Figure 1.: Modified, mid-esophageal bicaval view after chest closure. A, The left atrium (LA) is at the top of the image. Inside the right atrium (RA), a prominent Eustachian valve is seen to the left, close to the junction with the inferior vena cava (IVC), below the interatrial septum (IAS). One of the pacemaker leads (PM1) is visible. B, With further rotation to 92°, the second pacemaker lead (PM2) is visualized inside the RA.Apart from the main beam, ultrasound transducers emit secondary beams, or side lobes, propagating radially from the center of the main beam (Fig. 2). A returning echo produced by a target located in the side lobes will be displayed as if it originated inside the main beam. The ultrasound energy in the side lobes is considerably weaker than in the main beam; therefore, reflections become apparent only when they do not conflict with real, more intense echoes (1). Examples are an enlarged cardiac chamber, as in the case of a dilated aorta, when the atrial-aortic interface creates a reverberation artifact mimicking a dissection flap, (2) and/or a relatively strong reflector, such as a catheter or a wire (3). To reduce such artifacts, the gain settings should be minimized so that strong reflections from weaker lobes are decreased. If they persist, the echocardiographer may be able to differentiate an artifact from a real structure by imaging the same structure from another acoustic window. An artifact is not likely to be reproduced in multiple echocardiographic planes.Figure 2.: Main and side lobes of the ultrasound beam produced by a phased array transducer. The shadowed cylinder represents the main beam, and the surrounding truncated cone and arrows the side lobes.Shadowing is the absence of echoes behind a highly reflective target, such as mechanical devices (catheters, prostheses) or heavily calcified native structures. When shadowing occurs, an alternate acoustic window is required to view the objects or areas of interest (3). Closure of the atrial septotomy might have altered the relative position between the interatrial septum and the pm lead. As a result, part of the latter (a strong reflector), was reflected by the side lobe, off the center of the ultrasound beam. Approximation of the sternum and the influence of the ventilated lungs may have affected the size of the LA, possibly realigning the pm lead with the main beam of the ultrasound, thus “returning” the image in its true anatomic location, inside the right atrium.