Focused Transesophageal Echocardiography

Introduction

Long considered to be a vital diagnostic tool utilized exclusively within the realm of cardiology, transesophageal echocardiography (TEE) is being used safely and effectively within the perioperative, critical care, and emergency room settings. Of particular importance is the ability to diagnose a vast array of diverse pathologies causing hemodynamic instability. In this capacity, a focused TEE examination has been proven to serve as an important adjunct to the history and physical examination to aid in the clinical decision-making process and to help guide treatment strategies.

Comparing TEE to TTE

The transducers utilized in transthoracic echocardiography (TTE) are typically single-plane phased array probes, requiring the sonographer to physically manipulate the probe to move from one view to the next. On the other hand, most TEE probes, and for the purposes of the discussion in this chapter, are multiplane phased array probes. Several thousand piezoelectric crystal elements are assembled in a matrix configuration ( Fig. 11.1 ) and mounted to the distal tip of an endoscope. This arrangement allows a linear scan line to be created that can be electronically rotated from 0 toward 180 degrees using two buttons on the handle of the ultrasound probe ( Fig. 11.2 ). As such, multiple views are able to be created without any physical probe manipulation. Rotating the scan line (also called the omniplane angle ) in the direction of 180 degrees is termed rotating forward , whereas rotating back to 0 degrees is termed rotating backward . Physical probe manipulation can be performed by turning the large control wheel on the handle to anteflex (anteriorly) or retroflex (posteriorly) the probe. Furthermore, the tip of the probe can be flexed left or right by rotating the smaller control wheel. The shaft of the probe can be advanced, withdrawn, turned clockwise to image right-sided structures, or turned counterclockwise to image left-sided structures ( Fig. 11.3 ).

The transducers used in TEE can generally operate at a higher frequency, as high as 8 MHz in some cases, compared with the typical 1 to 5 MHz of TTE probes. Although this leads to less depth of penetration, the overall resolution and image quality are better than TTE. Additionally, the TEE probe sits in the esophagus in close proximity to the heart, unlike in TTE, where the ultrasound waves must travel through multiple layers of tissue, ribs, and potentially aerated lung before reaching the heart.

Given the relative position of the esophagus to the heart, specific structures are better visualized with TEE compared with TTE. Anterior structures tend to be imaged better using TTE, and posterior structures tend to be better imaged using TEE. One caveat, however, is that the left atrium sits next to the TEE transducer. Because the ultrasound beam of a phased array probe starts at a narrower origin and fans out (just as it appears on the screen of the machine), the entire left atrium can generally not be visualized using TEE. However, the mitral valve (MV), aortic valve (AV), and left atrial appendage (LAA) are visualized well. TTE, on the other hand, provides better imaging of the left ventricular (LV) apex and more optimal Doppler alignment through both the aortic and tricuspid valves, allowing for more accurate calculations of cardiac output and right ventricular (RV) systolic pressures, respectively.

A standard nomenclature is used to name all of the TEE views that varies slightly from the TTE nomenclature. The first portion of the naming format refers to the transducer location (upper esophageal, midesophageal, transgastric). This is followed by the primary anatomic structure of interest and then the imaging plane (short axis versus long axis). For instance, the midesophageal aortic valve short axis (TEE) corresponds to the parasternal aortic valve short axis (TTE).

Although TTE remains a powerful diagnostic tool in the assessment of critically ill patients, it has several limitations. Unlike TEE, TTE is dependent on patient positioning and body habitus and is difficult in patients with emphysematous lung disease. Imaging can also be difficult in patients with dressings or chest tubes in place and also suboptimal in mechanically ventilated patients in the intensive care unit (ICU) compared with TEE. Conducting a TTE examination in patients undergoing cardiopulmonary resuscitation (CPR) is technically challenging. However, due to TEE’s semiinvasive nature, it is generally not tolerated as well in awake patients. TEE remains an overall safe imaging modality, with an overall complication rate of 0.2% to 1.2% and a risk of injury leading to death of less than 0.01%.

Careful consideration must be given to the associated risks of TEE ( Table 11.1 ), as well as the appropriate indications and contraindications to the placement of a TEE probe ( Table 11.2 ). A TEE examination should only be performed in patients in whom a transthoracic examination is thought to be suboptimal and in whom the benefits are deemed to outweigh the overall risks of probe placement.

The TEE Examination

Multiple medical societies encourage the use of ultrasound and have created various position papers regarding both the training requirements and educational curricula needed to achieve proficiency. Moreover, many of these societies advocate for focused or goal-directed examination protocols to help physicians make time-sensitive, potentially lifesaving clinical decisions.

Focused, or “rescue,” TEE exists for this purpose, allowing physicians to use an abbreviated examination sequence at the bedside in patients suffering from sudden, unexplained hemodynamic instability. In these situations, a comprehensive examination may waste valuable time and delay treatment. Although multiple protocols have been described, an 11-view examination sequence will be described in this chapter ( Fig. 11.4 ). Please keep in mind that not all 11 views will always be necessary or be easily obtained. Focused TEE emphasizes speed and efficiency by using a qualitative and dichotomous approach. Usually, a diagnosis can be elucidated with a select number of views. One must keep in mind that due to anatomic variations from patient to patient and the potential for pathology to distort cardiac anatomy, the described approaches for obtaining the views as they relate to both imaging depth and omniplane angle can vary considerably.

Great care must be given during both probe placement and manipulation. Before placement, the oral cavity should be inspected for signs of obvious dental damage or previous injury. A dental guard should always be used to minimize the risk of dental trauma and probe damage. The probe control lock should always be in a neutral position and never engaged during probe placement or manipulation, although slight anteflexion of the tip with the control wheel may be helpful during insertion. A generous amount of lubrication should be applied to the probe before placement in the oropharynx. The probe should never be forced if met with resistance. Gently lifting the mandible anteriorly may be helpful in placement, as is occasionally either neck flexion or extension. Direct laryngoscopy may also be used. The exam should be performed in a manner that minimizes overall probe manipulation, including minimizing the number of times the probe is advanced and withdrawn.

When performing an examination, a few basic rules optimize image quality. First, the focal zone should be set at or slightly above the anatomic structure of interest. The resolution will be the highest in this zone. Furthermore, the depth should also consider the structure of interest so as to further enhance resolution and detail. The greater the depth, the lower the overall resolution. To image deeper structures, the frequency may need to be decreased. Finally, the gain is the amplification of the ultrasound signal and should be set in a manner to improve contrast between fluid and tissues.

TEE Views