Process review

This should include infection control (knowledge of local and national guidelines, probe sterilization, cleaning, and tracking), and care of the probe (including procedures for checking integrity before each use). Nasogastric tubes are often in situ, and to ensure interruption of feed is minimized, gastric contents should be aspirated before the study, and postprobe removal protocols should be in place to ensure no displacement has occurred, and feeding is recommenced only and as soon as judged safe.^3,7 The process of consent varies between countries, and although invasive procedures are usually undertaken only after consent is obtained, exceptions are generally allowed in the case of life-threatening emergencies and/or when the patient is unable to give informed consent. In some countries, informing the patient’s documented next of kin before a nonemergency invasive procedure is normal practice. Where research is undertaken, ethical approval must be obtained and consent obtained according to the agreed research protocol and local guidelines. The study should be recorded and reported according to local, national, and international guidelines and recommendations.

Patient review

This should include the indication for TEE and the patient history in terms of the relative and absolute contraindications to TEE (Table 29-1) and to its risk-benefit to the patient. The most significant serious complication from TEE in the outpatient population is esophageal perforation, with the usually quoted rate of 1:10,000 in studies; however, this is variable and may increase with an increasingly aged patient population.⁸ TEE in the intensive care is generally regarded as safe; however, because the critically ill patient is frequently coagulopathic, knowledge of the coagulation status of the patient is vital and, where indicated, possible correction undertaken. When the patient is fully anticoagulated or coagulopathic, probe insertion with laryngoscopic guidance is recommended. This is important if the patient is receiving extracorporeal support; in this circumstance, oropharyngeal bleeding can be life threatening. Critically ill patients undergoing TEE will be intubated and ventilated, and consistent with guidelines in many ICUs, there should be an independent ICU practitioner managing the airway, ventilation, and hemodynamic status of the patient during the study.

The practitioner planning and performing the study must consider the specific question being asked and the effects of vasoactive agents, sedation, and ventilation on patient hemodynamics that might have an impact on findings. For complex studies, the practitioner must also plan any strategy required to mimic/provoke/resolve the echocardiographic findings (i.e., changing ventilator settings, volume challenge, ventricular assist device (VAD) weaning, increased afterload, pacing optimization) and ensure the appropriate skills are present to be able to apply TEE appropriately within this setting.

Image acquisition: The standard views

Guidelines for performance of a comprehensive TEE study have been published, with recommendations for a minimum dataset in a number of circumstances.^3,7 The practitioner will then focus on the specific question that requires answering. In existing guidelines and recommendations, these predominantly relate to the perioperative cardiac surgical setting; however, it is not the mode of echo (TTE vs. TEE) but, rather, the information required that drives the study. Indeed, in undertaking repeated focused studies and/or using TEE as a hemodynamic monitor, only limited views may be required.

The standard order of image acquisition depends upon a number of factors, including the urgency of the study and time frame for acquisition, the type (comprehensive vs. focused), and individual practitioner preference. Ideally, in the critically ill patient, the amount of probe manipulation should be minimized, in particular in the elderly and/or coagulopathic patient. Movement of the probe is described in terms of its position with respect to the patient: withdraw/advance, rotate left/right, anteflex/retroflex, and flex right/left. Additional movement of the ultrasound beam relates to electronic rotation (from 0-180 degrees). As with TTE, as there is anatomic variation between patients, although numbers are given regarding the degree of electronic rotation, these are for guidance only. The standard views are described in Table 29-1, together with the main structures seen (Table 29-2).

Of note, the deep transgastric view is not achievable in all patients. Again, care should be taken to avoid excessive probe manipulation. Imaging of the pulmonary veins can be achieved by standard views (as described in Table 29-1) and additionally at 0 degrees, interrogating the right and left sides of the left atrium (both superiorly and inferiorly) in turn.

Indications for transesophageal echocardiography in the critically ill

The indications for echocardiography in the ICU have been discussed in Chapters 26 and 27, including the choice of TTE versus TEE. Images of diagnostic quality are unobtainable in up to 30% of the general ICU patient population, rising to 85% in those with severe, acute respiratory failure referred for extracorporeal respiratory support (Figure 29-2).⁹ Thus, when image quality precludes obtaining adequate information from the transthoracic route, TEE may be indicated. In addition, there are specific indications for TEE as opposed to TTE (Table 29-3). Guidelines for the appropriateness of undertaking TEE have been published; however, these will inevitably change with the ongoing development of critical care technologies, for example, extracorporeal support.^10,11

Evaluation for intracardiac thrombus

Intracardiac thrombus is most commonly found in the left atrial appendage (LAA) in patients with atrial arrhythmias. Guidelines have been published regarding the use of TEE for the evaluation of the LAA in atrial fibrillation before cardioversion.¹² Critical care patients may be relatively prothrombotic, and therefore the indication for TEE in the critically ill, to exclude the presence of thrombus, may lie outside the guidelines (Figure 29-3). More research is required in this area, and when the patient is hemodynamically stable and no contraindications to TEE exist, there should be a low threshold for performing a study.

Patients with myocardial infarction and extensive areas of akinesia are at risk of developing ventricular thrombus. Care must be taken when using TEE to evaluate the apex of the LV because foreshortening may occur in the midesophageal views. A modified long-axis transgastric long-axis view can be useful in these circumstances.¹³ Where an associated apical ventricular septal defect (VSD) is suspected, rotation of the probe between the LV and right ventricle (RV) in this modified view can be used to demonstrate the apical portion of the interventricular septum.

In patients receiving extracorporeal respiratory and/or cardiac support, there is a significant risk of developing intracardiac thrombus despite full anticoagulation. This may be either associated with a cannula, and/or with a nonopening valve, or an akinetic area of the LV.¹⁴ Here TEE is pivotal to making the diagnosis and planning a treatment strategy (Figure 29-4).

Infective endocarditis

Guidelines for the use of TEE in the diagnosis of endocarditis have been published,¹⁵ including prosthetic valve endocarditis.¹⁵ When endocarditis is suspected, there should be a low threshold for undertaking TEE, and it is indicated in all critical care patients with suspected prosthetic valve endocarditis (Figure 29-5). Examination should not be limited to the valves and associated structures but should include lines, cannulas, pacing wires, and any intracardiac patch or device. In prosthetic valve endocarditis, large vegetations are rarely seen, the more usual echocardiographic findings being paraprosthetic regurgitation and/or abscess or fistula formation (Figure 29-6). In intensive care, the diagnosis of severe acute respiratory failure with associated bilateral pulmonary infiltrates in a patient with previous left-sided valve repair/replacement should prompt close interrogation of the relevant valve, including TEE because significant paraprosthetic regurgitation must be actively excluded (see Figure 29-2B).

Extracorporeal support

The use of echocardiography in extracorporeal support is described in detail in Chapter 28. Regular echocardiography in this patient population is highly recommended; however, repeated TEE studies should be avoided (because of the risk of hemorrhage), and TTE should generally be the first modality used.¹⁴ TEE does have specific roles in confirming the underlying diagnosis and/or estimation of left atrial pressure (patients referred for venovenous–extracorporeal membrane oxygenation [VV-ECMO]), assisting in cannula placement (either real time or postprocedurally), and detecting associated complications, including cannula displacement, worsening valvular regurgitation, tamponade, thrombus formation (cannulas and/or valves, see Figure 29-4), and inadvertent overload or excessive offloading of one or both ventricles.

Aortic disease

In the diagnosis of aortic dissection, TEE is comparable to computed tomography (CT) and aortography and superior to TTE (Video 29-1). It has the additional advantages of providing a shorter time to diagnosis and intervention, demonstration of the entry site (and coronary ostia), and characterization of the mechanism and severity of any aortic regurgitation (AR), in which holodiastolic reverse flow greater than 15 cm/sec indicates severe regurgitation (Figure 29-7). It is, however, inferior in its ability to image the descending aorta and aortic arch because of interference from the left main bronchus. In aortic trauma, TEE has been shown to have a high specificity (91%-100%) but variable sensitivity (57%-100%) compared with aortography in demonstration of aortic interruption and hematoma.¹⁶

Transesophageal echocardiography as a cardiac monitor

There are a number of publications that support TEE as a hemodynamic monitor¹⁷ (Video 29-2). Cardiac output (CO) measured using TEE has been shown to provide quantitative assessment of CO that correlates well with continuous thermodilution methods.¹⁸ However, when compared directly, it tends to underestimate CO in both ventilated and nonventilated patients, thereby precluding their use interchangeably. The use of echocardiography to measure CO can be useful, however, in patients where placement of a pulmonary artery (PA) catheter is relatively or absolutely contraindicated, or where there are multiple cardiac factors that potentially limit CO and advanced interrogation of the heart is required.

Left atrial pressure may be estimated using a combination of TEE-based techniques (combining E/E´, color Doppler propagation and pulmonary venous flow, in conjunction with ejection fraction¹⁹). Although gray zones still remain, and, as with TTE, it is not possible to measure the pressure within any cardiac chamber, extreme values may prove helpful. When a high left atrial pressure is suspected but not found on TEE in a patient who fails to wean from mechanical ventilation because of respiratory distress, stress echocardiography should be considered. The nature of stressor used will depend upon the suspected etiology of the underlying pathology (dobutamine in suspected ischemia, volume and pressor loading in shock). TEE has been proposed as a potential monitor of volume responsiveness, by analysis of the changes in stroke volume with respiration and/or inferior vena cava (IVC) diameter in response to respiration in positive-pressure ventilated patients. However, a number of technical and patient-related limitations exist.²⁰ Indeed, TEE has inherent limitations when compared with the features of an ideal monitor, requiring repeated studies and skill for performance and interpretation and providing only intermittent information. Further, many of the published studies had significant relevant exclusions to the critical care patient population. To minimize patient risk and avoid the requirement for repeated intermittent studies, a miniaturized TEE device has been devised for continuous monitoring.¹⁵ However, the greatest strength of echocardiography over all other monitors is that it provides additional insight into the underlying diagnosis and can thus be used to determine potential interventions to optimize cardiac output (Figure 29-8).

Valvular disease

There are numerous publications regarding the use of echocardiography in the evaluation of valvular heart disease.²¹ Valvular heart disease relating to or causing ICU admission is likely to be severe and affect the left-sided valves. When significant valve disease is suspected but TTE is nondiagnostic, TEE has a role in confirming or refuting the diagnosis, in evaluating possible endocarditis (and local complications), and in assessing of prosthesis malfunction (i.e., thrombosis or paraprosthetic regurgitation or valve degeneration). When critical aortic stenosis (AS) or mitral valve stenosis (MS) is present, TEE is additionally indicated to define suitability for valvuloplasty and exclusion of intracardiac thrombus.²¹ When there is severe regurgitation, TEE will aid the surgeon in planning the intraoperative strategy. Mitral regurgitation may be dynamic and reduce significantly with positive-pressure ventilation and sedation or anesthesia (Figure 29-9). Here stress echocardiography should be considered with TEE when TTE images are inadequate.²¹

Unexplained hypoxia

TTE is recommended to assist in the diagnosis of pulmonary embolism when the patient is too hemodynamically unstable to transfer to the CT scanner.²² When TEE is undertaken in critically ill patients with disproportionate/unexplained hypoxemia, on occasion, thrombus may be seen transiting the right heart, in the main pulmonary artery, or in the proximal right or left pulmonary arteries (Video 29-3); however, TEE is not the investigation of choice in patients with suspected pulmonary embolism.

TEE can be used to demonstrate presence and mechanism of intracardiac shunting. The anatomy of the interatrial septum and effects of ventilation on size and magnitude also may suggest intrapulmonary shunting, in which the timing of arrival of bubbles (and the site of their arrival, that is, from pulmonary veins) may suggest the diagnosis. In all patients with SARF, the clinician must consider the presence of an interatrial shunt because this may significantly impact management (see Figure 29-2A).

Pericardial disease

Usually evaluated in the outpatient setting with physiologic parameters of venous return, the most common pericardial disease encountered in the ICU is tamponade. When a patient has a pericardial fluid collection sufficient to cause hemodynamic instability, it is generally large, readily detected using TTE, and TEE is not indicated. An important exception is postcardiac surgery. Here the nature of the patient’s underlying cardiac disease and size and location of collections make detection difficult, with up to 50% of hemodynamically important collections missed using TTE. Although primarily a clinical diagnosis, where uncertainty exists that a patient has tamponade,TEE can be undertaken (Video 29-4). Here collections causing may be small, and/or localized (Figure 29-10), and with no respiratory variation in cardiac filling/emptying.²³

Coronary artery disease

TEE can be used to visualize the first 1 to 2 cm of the coronary arteries (Figure 29-11) and has been specifically evaluated in patients after mitral valve surgery. Its use in demonstrating significant coronary artery disruption has been validated in the cardiac perioperative setting but not in the general ICU population. However, in a ventilated patient with borderline CO or an abnormal echocardiogram, abnormal coronary Doppler flow patterns and velocities seen in association with a corresponding regional wall motion abnormality suggest significant coronary artery disease (Video 29-5).

• The indications for TEE in the ICU include suboptimal TTE image quality and specific indications for TEE.

• The experienced practitioner will be trained in both TEE and TTE and able to apply each appropriately to the clinical question being asked.

• In the critically ill patient, TEE must never be performed with an unprotected airway.

• An independent and appropriately experienced practitioner should manage the airway and hemodynamic status of the patient during the study.

• In every patient, the risk:benefit ratio of undertaking TEE must be considered before performing a study.

• Evolving techniques in the ICU are likely to result in an expansion of indications for TEE in this patient population.

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