Obtaining a basic imaging protocol for characterizing a state of shock is one of the main indications for TEE in critically ill patients. The main objective is to identify disorders such as severe left or right ventricular dysfunction, severe valvular heart disease, pericardial effusion, or tamponade. Less frequent causes of shock include dynamic left ventricular outflow tract (LVOT) obstruction or extrinsic cardiac compression.21,24

In patients where the cause of shock is known, TEE can help to assess preload dependence and optimize the dosage of vasoactive and inotropic drugs. Monitoring CO requires measuring the LVOT diameter in mid-systole, in the mid-esophageal long-axis (ME-LAX) plane, and magnifying the image to improve accuracy (Fig. 3A). It also requires quantifying the velocity-time integral (VTI) of the LVOT (Fig. 3B), which is obtained in the deep transgastric 5-chamber (dTG-5C) plane.21,24 The calculation of CO requires averaging three consecutive beats when measuring the LVOT VTI. Estimating CO by TEE has the same limitations as in the case of TTE,25,26 fundamentally due to the assumption that the LVOT is a circumference (when in fact it is an oval), and from the possibility of underestimating or overestimating the measurement (due to deviation from the Doppler alignment axis or approximating the sample volume excessively towards the aortic valve, respectively). Consequently, error can be reduced by using the LVOT VTI exclusively to guide the therapeutic interventions. Additionally, the transvalvular aortic gradient can be assessed in this position. This provides information on flow dynamics and can detect dynamic LVOT obstruction when a dagger-shaped flow is observed (Fig. 3C). It also detects the presence of ascending aortic dissection as the cause of shock (Fig. 3D). Finally, diastolic function can be analyzed by studying the mitral filling pattern with pulsed Doppler in ME-4C.

Figure 3.

Study images of the patient in shock and volume response parameters.

TEE allows for the acquisition of preload parameters and the prediction of volume response. The presence of kissing walls and left ventricular (LV) obliteration identified in the transgastric short-axis plane at the level of the papillary muscles (TG-SAX PM) suggests severe hypovolemia, especially when associated with an LV end-diastolic area <10 cm2 (or <6 cm2/m2) (Fig. 3E).27

On the other hand, dynamic parameters such as a variation in peak aortic flow velocity in the LVOT > 12%, by pulsed Doppler, have been shown to be a good predictor of volume response in patients on IMV and without arrhythmias.28,29

Respirophasic variation of the superior vena cava (SVC) on its long axis, as measured in the mid-esophageal bicaval (ME-bicava) plane (Fig. 3F), is the most specific echocardiographic predictor of volume response. It surpasses the variation of the inferior vena cava (ΔIVC), the Δ LVOT VTI, and the variation of systolic volume.30–32 Vieillard-Baron et al.33 established a cut-off point of 36% (90% sensitivity [Se] and 100% specificity [Sp]) for Δ  SVC ([diametermax − diametermin]/diametermax × 100) in patients subjected to IMV. A recently published study that included ventilated postoperative patients with tidal volume < 8 ml/kg identified a superior vena cava collapsibility index (SVC-CI) cut-off point Δ of 39.4% (Se 64% and Sp 92%).34 An SVC-CI > 37% on the short axis, obtained in the mid-esophageal ascending aorta short axis plane (ME AscAo SAX), was associated with volume response in postoperative cardiac surgery patients. Measurements obtained in M-mode in the ME-bicaval and ME AscAo SAX planes are taken at different anatomical sites and are therefore not interchangeable.35 Obtaining the ME AscAo SAX view presents less technical difficulty compared to the ME-bicava plane.

The study of systemic venous congestion using ultrasound is very useful for critically ill patients.36 In the context of TEE, information can also be obtained to complete the Venous Excess Ultrasound (VExUS) protocol, although the difficulty may be greater than with the abdominal study.37

The transesophageal VExUS study begins with the evaluation of the diameter of the IVC. From the ME-4C plane, the probe is rotated clockwise to focus on the right atrium (RA), and carefully advanced to visualize the cavoatrial junction, the IVC, and the confluence with the hepatic veins. Alternatively, the IVC can be assessed from the transgastric plane by flexing and rotating the probe clockwise.

The hepatic veins can be observed from both the distal esophageal plane, previously described, and from the transgastric plane. In this case, the probe is anteflexed and directed toward the RA starting at an angle of 0° and progressively adjusting between 0° and 60° (Fig. 4A). Once the vein is identified, the pulsed Doppler sample volume is placed at about 2−3 cm proximal to its confluence with the IVC, to evaluate hepatic blood flow (Fig. 4B).

Figure 4.

VExUS protocol by TEE. IVC: inferior vena cava; HV: suprahepatic vein; PV: portal vein.

Then, the portal vein is visualized from the deep transgastric plane by adjusting the angle between 20° and 60° (Fig. 4C). Color Doppler allows the identification of portal flow, which is directed upwards towards the transducer. The pulsed Doppler is then placed to assess the flow (Fig. 4D).

Although renal evaluation using TEE may be technically demanding in some patients, it is generally possible. Starting from the deep transgastric plane, the probe is rotated approximately 180° degrees until the descending aorta is visualized. Then, it is advanced slightly and rotated counterclockwise until the left kidney is identified (Fig. 4E). Alternatively, the kidney can be visualized in longitudinal section by maintaining a 90° angle and rotating the probe counterclockwise from the short-axis view of the descending aorta. Once the renal structure has been located, the pulsed Doppler is placed over an interlobar artery to record both the arterial and venous flow simultaneously (Fig. 4F).

In critically ill patients, acute hypoxemia may be due to various cardiopulmonary conditions, so a differential diagnosis must be established quickly and accurately. In this context, TEE has established itself as a valuable diagnostic tool. This technique allows for the simultaneous evaluation of cardiac and pulmonary causes of hypoxemia, with high spatial resolution, independent of lung parenchyma or intestinal gas interference.

In cases of refractory hypoxemia, TEE is particularly useful for ruling out the presence of intracardiac shunts, such as patent foramen ovale (PFO) with a right-to-left shunt. This condition can be observed in patients with acute respiratory distress syndrome (ARDS) who require IMV support.38 For the identification of PFO using TEE, it is advisable to use the ME-bicaval plane, centering the view in the fossa ovalis. Additionally, 9.5 ml of saline mixed with 0.5 ml of room air is injected through two syringes connected to a three-way valve to generate echocontrast (shaken saline microbubbles). The injection is considered successful if the entire RA is opacified by the contrast induced by the microbubbles. A significant PFO is defined as the passage of more than 10 bubbles within three heartbeats after RA opacification (Appendix B Video 1). This test also allows for the detection of intrapulmonary shunts, which are characterized by the late passage (>3 cycles) of bubbles through the pulmonary veins.39

In addition, TEE is useful when pulmonary thromboembolism is suspected, allowing the visualization of indirect signs such as dilatation and dysfunction of the right ventricle (RV) or thrombi located in the right chambers or pulmonary arteries.40 Right ventricular volume or pressure overload is diagnosed by assessing interventricular septal dynamics.41

The introduction of transesophageal lung ultrasound (TELU) has made it possible to expand the technique’s usefulness to analyzing the lung parenchyma, particularly in posterior zones.42 The degree of aeration has been associated with PO2/FiO2 and 28-day mortality.43 Although the technique may be more complicated, the semiology is similar to that of transthoracic ultrasound.44 The study is performed by means of a craniocaudal exploration of each hemithorax. Reference points for the apical, middle, and basal lung zones are identified by the left subclavian artery’s origin, and the left superior pulmonary vein’s origin, and the IVC’s origin, respectively (Fig. 5). From each of these reference points identified with the transducer at 0°, a rotation to 90° allows the lungs to be studied over a longitudinal axis. From the cardiac view, for the study of the left hemithorax, the probe should be rotated counterclockwise, obtaining a view that allows examination of the lung, pleura, diaphragm, spleen, and, occasionally, the left kidney. For the study of the right hemithorax, the probe is rotated clockwise, using the same technique described above, initially with the transducer at 0° and subsequently at 90°. This method explores the right hemithorax, including the pleural space, lung, diaphragm, and underlying liver.

Figure 5.

Protocol for the study of the lung parenchyma by TEE (TELU). PE: pleural effusion.

Cardiopulmonary resuscitation (CPR) guidelines recommend using TTE as a diagnostic tool during cardiorespiratory arrest (CA) to identify potentially reversible causes such as pneumothorax, cardiac tamponade, pulmonary thromboembolism, ischemic heart disease, or severe hypovolemia.45,46

TEE offers significant advantages during CPR because it provides real-time imaging without interrupting chest compressions. This facilitates identifying cardiac rhythm recovery and allows for optimized ECMO cannulation when indicated.47,48 Recent studies have shown that the area of maximum cardiac compression is not always located at the center of the thorax. The use of TEE allows for the precise targeting of compression maneuvers, thus increasing their efficacy.49

The use of echocardiography during CA has been shown to improve the prognosis, as it allows for continuous cardiac rhythm monitoring, detection of fine ventricular fibrillation, identification of intracardiac thrombi or pericardial effusion, and assessment of right and left ventricular function.50 Additionally, echocardiography is useful for differentiating between pulseless electrical activity (PEA), which is characterized by the absence of myocardial contraction, and pseudo-PEA, which is characterized by visible myocardial contraction despite the absence of a palpable pulse or effective arterial pressure. The recommended imaging planes during CPR are described in Table 4.

TEE is an essential diagnostic tool during cardiac surgery51,52 and also in the immediate postoperative period.53 Its intraoperative use can contribute to improved clinical outcomes,54 and the technique can effectively complement postoperative hemodynamic assessment when used in conjunction with other monitoring systems.55

In the postoperative context of cardiac surgery, TEE’s main value lies in its ability to provide an immediate, accurate, and often conclusive hemodynamic and etiological assessment for clinical decision-making.

During hemodynamic evaluation, TEE allows visualization of ventricular function and interdependence (septal position, ejection fraction, and/or shortening fraction in ME-4C and TG-SAX PM), the estimation of stroke volume and volemia status, as described above, and detection of LVOT obstructions or dynamic gradients (ME-LAX or dTG-5C).

Regarding etiological evaluation or the assessment of possible complications, TEE can detect the presence of right ventricular dysfunction56 and segmental contractility alterations (ME-4C, ME-MC, ME-LAX, and TG-SAX PM), particularly in coronary revascularization surgery. TEE can also detect the presence of pericardial effusion (Appendix B Video 2) with or without evidence of tamponade (ME-4C, ME-LAX and TG-SAX PM), and signs of prosthetic dysfunction or dehiscence (Appendix B Videos 3 and 4) (Table 5).53

ECMO and ventricular assist devices (VADs) have become established tools in cardiac intensive care medicine.57 They are essential in cases of severe heart and/or respiratory failure, in which TEE is a critical diagnostic technique.25,58,59

In the case of veno-arterial (VA) ECMO in particular, where many conventional hemodynamic monitoring systems are not validated, TEE has become consolidated as the cornerstone for patient evaluation and follow-up.60–63 TEE enables immediate assessment at the patient’s bedside, with a high diagnostic capacity. However, its correct use requires the intervention of adequately trained professionals with specific experience in this clinical context.64,65

Using a systematic exploration protocol allows TEE to assess the anatomy and function of the cardiopulmonary system before implantation. It also guides the insertion and proper positioning of the cannulas, optimizes circulatory flow according to the clinical conditions, performs dynamic monitoring to detect complications or hemodynamic changes, supervises support removal, and evaluates the heart’s structural and functional state after removal (Fig. 6) (Table 5).62,63,66,67

Figure 6.

Role of TEE in the critically ill patient with ECMO. gr6.

TEE is useful for a wide range of diagnoses, including patient-related aspects and the technique employed.68 It has proven effective in multiple scenarios, including implantation in patients with SARS-CoV-2 infection,69 situations of refractory CA oriented to the use of ECMO,70 or as a guide for specific ECMO configurations, the insertion of other devices, and the performance of invasive procedures.68,69,71,72

In patients with stroke, it is essential to investigate a possible cardiac cause as the source of cerebral embolism.73 The presence of intracardiac thrombi in the left atrium and appendage should be excluded in the ME-2C view, especially in patients with atrial fibrillation or atrial dysfunction, and in the LV in patients with ischemic heart disease or dilated cardiomyopathy. However, the LV apex is best analyzed with TTE. As previously described, the presence of PFO should be evaluated, especially in patients under 60 years of age with cryptogenic stroke.74 Similarly, the heart valves must be carefully assessed in all planes to rule out signs of endocarditis (Appendix B Video 5) and the presence of intracardiac tumors. Finally, it is advisable to complete the study by exploring the ascending aorta and aortic arch to detect ulcerated or mobile plaques measuring more than 3 mm in size.75,76