Transesophageal echo (TEE) in COVID-19: a new outlook
TEE offers multiple advantages including better imaging windows by virtue of the TEE probe being close to the cardiac chambers and great vessels compared with conventional TTE probes. In addition, TEE is not hampered by other factors such as high PEEP positive ventilation, prone position, body habitus, and emphysematous lungs that constitute a limiting factor for obtaining good images by TTE [55,56]. Studies demonstrated that superior vena caval (SVC) collapsibility index is superior to IVC distensibility index in predicting fluid responsiveness[57,58]. As TEE is generally required to measure SVC collapsibility, it is inherently able to predict volume status much better than TTE among critically ill patients. Given the fact that ECHO is the modality of choice in investigating cardiogenic shock[59], TEE could help in providing real time information regarding LV function, trends of quantitative indices under acute therapy and whether or not cardiac dysfunction is acute and reversible (as in septic cardiomyopathy) [60]. With the challenges of volume resuscitation among COVID-19 patient with ARDS, TEE may prove to be an important tool in fluid management.
TEE is also essential in identification of acute cor pulmonale (ACP) as it provides the necessary short axis view of the heart required to identify ACP[56]. With regards to assessing RV systolic function, the RV fractional area change (FAC) measured by TEE is still considered the best parameter for measurement [61]. However, the measurement of an accurate RV FAC requires the entire endocardium to be clearly visible, which is sometimes difficult in patients who develop ACP and are on mechanical ventilation. In this subset of patients, measurement of the tricuspid longitudinal annular displacement (TMAD), a bi-dimensional strain parameter that tracks the tricuspid annular tissue motion toward the RV apex, thereby allowing an objective quantitative assessment of RV systolic function may be helpful[62]. TMAD is also angle independent and is unaffected by endocardial definition, which further adds to its ability to identify and quantify RV systolic dysfunction in patients in whom a traditional FAC measurement would be difficult. In a study by Beyls et al, a TMAD at the RV free wall (TMADlat) cutoff value of 18.5 mm was found to be statistically significant in identifying RV systolic dysfunction as compared with RV GLS. The sensitivity and specificity of TMADlat in identifying RV systolic dysfunction were 80% and 70%, respectively and the intraobserver reproducibility of TMADlat was excellent (intraclass correlation coefficient = 0.98 [0.93 to 0.99]), thereby, adding support for the use of this measurement in COVID-19 patients to identify RV systolic dysfunction[63].
Recent international guidelines recommend the use of ECMO therapy in patients with severe ARDS due to COVID-19 who have failed mechanical ventilation[64,65]. Pre-procedure TEE plays a fundamental role in ECMO initiation as it can identify unexpected and reversible findings while ruling out severe valvular abnormalities that may affect the success of venovenous(VV) or venoarterial(VA) ECMO therapy. In addition, the presence of severe LV dysfunction identified by TEE before placement of ECMO prompts consideration of VA ECMO instead of VV ECMO[66,67]. Embryologic remnants of right heart structures or other congenital abnormalities may affect the safe and appropriate placement of venous cannulas during ECMO initiation. A persistent left SVC leading to a dilated coronary sinus may be accidentally cannulated, leading to compromised oxygenation on ECMO. Similarly, a prominent Chiari network may impede cannula positioning and may increase the risk of subsequent thrombosis. TEE guidance can help confirm the course of a guidewire during insertion and help in excluding coiling of the guidewire in the right atrium, crossing of the guidewire across the interatrial septum or its entrance in the coronary sinus. It can also ensure that the return cannula is positioned clear of the interatrial septum and the tricuspid valve, thereby reducing the risk for recannulation [67–71]. TEE can also help identify the cause for worsening hypoxemia during ECMO which includes scenarios where the cannula tips are too close to each other causing recirculation, hypovolemia causing inadequate ECMO flow and thrombus formation in the cannula which may be impeding adequate flow[66].
Despite the numerous advantages TEE has over TTE, its performance is difficult as there is a decreased availability of health care providers who have adequate training and expertise in performing the procedure. There is also an increased risk of aerosol exposure to healthcare providers during a TEE when compared with a conventional TTE. Although, obtaining expertise in TEE requires a lot of hands-on training, competence in performing TEE for assessing central hemodynamics can be achieved in a short span of time after approximately 35 examinations. This number is based on the international consensus statement on training standards for advanced critical care ECHO citing evidence from a prospective, multicentric trial that validated the number of TEEs required to be performed to gain competence in monitoring central hemodynamics[72,73]. Adoption of adequate personal protective measures as per current guidelines should lead to a decreased risk of acquiring transmissible diseases while performing TEE[15].