Principles of Transesophageal Echocardiography

Introduction

Transesophageal echocardiography (TEE) is an additional and complementary method of obtaining ultrasound images of the heart and surrounding structures. A flexible TEE probe is introduced, via the mouth, into the esophagus of the patient. The tip of the TEE probe contains a miniaturized phased array transducer capable of producing imaging planes in a full 180-degree spectrum (multiplane imaging; Fig. 4.1A ). TEE has all the imaging capabilities of transthoracic echocardiography (TTE), including two-dimensional (2D) and three-dimensional (3D) imaging, and color, spectral, and tissue Doppler. Most TEE images are obtained with the tip of the probe in the esophagus, with additional images obtained from the stomach (see Fig. 4.1B ). Given the proximity of the esophagus to the heart, TEE can use much higher transmission frequencies, resulting in better image quality and spatial resolution compared to standard TTE imaging. This is especially true for posterior structures adjacent to the esophagus such as the left atrium, left atrial appendage, pulmonary veins, atrial septum, and left-sided valves. TEE is generally used as an adjunct or follow-up test to an initial TTE exam if additional information is sought or the TTE images are inconclusive. Table 4.1 summarizes the advantages and disadvantages of TTE versus TEE.

In 1999, the American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists published guidelines for a comprehensive intraoperative TEE examination that consisted of 20 2D views. In 2013, the TEE guidelines were updated to include 28 2D views. The initial 20 views from the 1999 paper focused primarily on intraoperative imaging and decision making. The updated 2013 guidelines included additional views, such as imaging the left atrial appendage, that are obtained regularly in the nonoperative application of TEE. In addition, the guidelines also added 3D imaging and use of biplane imaging. 3D echocardiography is a valuable imaging modality that is particularly suited for TEE.

Indications

Indications for TEE fall into two basic categories: diagnostic and procedural ( Table 4.2 ). Diagnostic indications for TEE include the evaluation of cardiac and surrounding structures, where TTE is either nondiagnostic or will likely be nondiagnostic. Commonly this includes the evaluation of far field TTE structures that are in the near field for TEE imaging, given the probe location in the esophagus. Examples of this include imaging the left atrial appendage to rule out thrombus or plan suitability for percutaneous left atrial occlusion device placement. In addition, TEE is used to evaluate cardiac valvular pathophysiology because its superior image quality often facilitates medical and surgical planning and decision making. Examples of this include planning of surgical or percutaneous valve interventions, evaluation of valve masses including infectious complications such as endocarditis, and evaluation of prosthetic valve dysfunction. It is not possible to review all the diagnostic indications for TEE, but given the invasive nature of the procedure, it is important that the findings sought should alter the medical or surgical management of the patient.

Procedural indications for TEE include both surgical and interventional evaluation and guidance. The utility of TEE for the evaluation and guidance of cardiac surgical procedures is recognized by cardiologists, anesthesiologists, and surgeons. Recent practice guidelines describing the indications for perioperative imaging have been published by the American Society of Anesthesiologists and the Society of Cardiovascular Anesthesiologists. TEE should be used in all open heart and thoracic aortic surgical procedures and should be considered in coronary artery bypass procedures. The objectives of the TEE exam include confirmation and/or refinement of the preoperative diagnosis, detecting new or unsuspected pathology, and assessing the results of the surgical intervention. TEE should also be used for transcatheter intracardiac procedures. TEE is especially useful for structural heart interventions such as atrial septal defect or patent foramen ovale closure, atrial appendage occlusion, and catheter-based valve repair or replacement. The utilization of TEE is equivocal for dysrhythmia treatment. In noncardiac surgery, TEE may be indicated when the surgery or the patient’s condition might result in severe hemodynamic, pulmonary, or neurologic compromise. TEE is indicated during unexplained life-threatening circulatory instability that persists despite corrective therapy.

Appropriate use criteria for the ordering of echocardiographic examinations have been established to assist clinicians in determining when echocardiography is indicated for specific clinical scenarios. The writing taskforce developed 202 indications for echocardiographic imaging. The proposed list of indications was not meant to be exhaustive. Specifically they evaluated 15 clinical indications for TEE for appropriateness. The criteria did not include intraoperative indications for TEE. Table 4.3 lists the 15 clinical indications and their appropriateness for TEE as an initial or supplemental test.

Complications

TEE is a semiinvasive procedure that carries potential risks to the patient. Because of the invasive nature of the exam, indications for TEE should be followed and best practice guidelines adhered to for every patient. Overall, TEE is a low-risk procedure, but complications have been reported for both diagnostic and interventional/intraoperative TEE. A review of the literature by Hiberath et al. reported major TEE complications in 0.2%–0.5% of patients having nonoperative, diagnostic TEEs, with an estimated mortality less than 0.01%. TEE in patients under general anesthesia may carry some additional risk. Probe placement is not facilitated by the patient swallowing the probe; rather, the probe must be gently pushed into the esophagus. In addition, the patient cannot respond to painful stimuli, which may indicate possible injury during the exam. Despite these challenges, reported morbidity rates in the operative setting are similar to diagnostic TEEs ranging from 0.2% to 1.2%. The largest single-center intraoperative study to date was reported by Kallmeyer et al., with associated morbidity and mortality at 0.2% and 0%, respectively. The sites of potential injury during a TEE exam include the oral cavity (dental/lip trauma), oropharyngeal (laceration, perforation), esophageal (laceration, perforation, false passage), and gastric locations (laceration, perforation, bleeding; Fig. 4.2 ).

Additional more rarely reported complications include cardiac arrhythmias such as atrial fibrillation or nonsustained ventricular tachycardia, which may be related to the comorbidities of the patient population, TEE procedure, or sedation-related complications during the case. Complications in patients receiving sedation include hypoxia, unplanned need for endotracheal intubation, bronchospasm, laryngospasm, or intolerance of the exam. Malposition of the endotracheal tube during intraoperative TEE was noted in 0.03% of the cases reported by Kallmeyer et al. The absorption of ultrasound energy or heating of the TEE transducer during an exam could lead to thermal injury to the surrounding tissue. This is particularly noticeable when using ultrasound intense imaging, such as live 3D imaging, from a singular window over a prolonged period of time. Most probes have a safety sensor designed to automatically shut off the transmission of ultrasound waves when the surrounding tissue is heated to a preset temperature threshold.

To reduce the risk of injury, each patient should be evaluated for relative or absolute contraindications to TEE on a case-by-case basis ( Table 4.4 ). Depending on the specific contraindication, a risk-benefit analysis should be discussed with the patient and involved clinicians to determine the appropriateness of the procedure for the patient. Patients may require additional preprocedural workup, such as a referral to a gastroenterologist to better assess the risk of the procedure (e.g., history of dysphagia or upper gastrointestinal [GI] bleed). Potential procedural modifications in patients with relative contraindications include limiting either the length of the exam or confining the level of the exam to the midesophagus in patients with distal esophageal or gastric contraindications. The experienced echocardiographer can minimize the amount of probe manipulation during the procedure or choose a smaller probe such as a pediatric probe. If a patient has an absolute contraindication or a relative contraindication where the risks outweigh the benefits, an alternative imaging or diagnostic modality should be sought. Alternatives include cardiac computed tomography (CT) or magnetic resonance imaging (MRI) for diagnostic exams, and intracardiac ultrasound (ICE) or epiaortic ultrasound in the interventional suite or operating room, respectively. Whenever a patient has suffered a potential complication related to a TEE procedure, such as blood noted on the probe after removal, a full workup is indicated to evaluate the possibility of injury anywhere from the mouth to the abdomen.