Contrast Echocardiography in VV-ECMO-Dependent Patients with COVID-19

To the Editor:

The use of contrast echocardiography in patients receiving venovenous extracorporeal membrane oxygenation (VV-ECMO) for severe acute respiratory failure is not widely published, and there is understandable caution surrounding its use in this population. The coronavirus disease 2019 pandemic resulted in an unprecedented burden on critical care facilities¹ due to severe acute respiratory failure. Our center is one of five nationally commissioned VV-ECMO services in the United Kingdom and has seen VV-ECMO activity quadruple during the first pandemic surge. Consequently, there has been growing emphasis on transthoracic echocardiography (TTE) to diagnose cardiac complications of coronavirus disease 2019 supported with VV-ECMO.^{2 , 3}

Mechanically ventilated patients receiving VV-ECMO are among the most challenging in whom to obtain diagnostic images on TTE. Predictably, we found that as the volume of VV-ECMO patients increased, so too did the requirement for TTE using ultrasound-enhancing agents (UEAs). Contrast echocardiography is routinely used at our institution, and this work was approved through the local governance board as a service evaluation. We carried out bedside TTE in 37 consecutive VV-ECMO patients, among whom SonoVue (Bracco International, Milan, Italy) TTE was performed in 10 (27%). SonoVue boluses (0.5–0.7 mL), reconstituted in the standard format, were administered via the postoxygenator limb of the ECMO circuit. Repeated boluses of the UEA were required in all cases, with a maximum total dose in any one patient of 2.5 mL. All patients met criteria for the use of UEAs as outlined in the recently published guidance.⁴ Very low mechanical index imaging was performed with standard commercially available tissue cancelation sequences (Philips Medical Systems, Andover, MA). Very low mechanical index imaging allows excellent tissue delineation and results in less microbubble destruction than the higher mechanical index left ventricular opacification settings. Diagnostic images were obtained in all cases.

In line with our institutional protocols for critical care echocardiography in extracorporeal support (developed in collaboration with our specialist perfusion team), during each contrast study, the VV-ECMO circuit was managed by experienced perfusionists. This included disabling the appropriate interventions on the ECMO console before UEA administration to ensure safe administration. UEAs are known to activate the protective integrated air bubble alarms, which trigger interventions to disable flow, a safety feature of the Cardiohelp ECLS system (MAQUET Medical Systems USA, Wayne, NJ,)⁵ and in all cases, the integrated detector for air bubbles was indeed triggered by the UEA. This would usually lead to a pump shutdown because of activation of additional safety interventions, and unless this alarm is cleared, a further “zero-flow mode” is engaged. This mode provides sufficient revolutions per minute to prevent backflow from the return cannula without providing forward flow, so equilibrium is maintained in the circuit. However, the resultant cessation of flow, and consequently oxygenation, can result in rapid desaturation and potentially hypoxic arrest. It is therefore of pressing importance that centers offering VV-ECMO adopt protocols and staff training to allow the safe administration of UEAs, facilitating diagnostic echocardiography in the most critical patients.

To our knowledge, this is the largest published series affirming the applicability of a UEA in VV-ECMO. Appropriate protocols should be instituted at centers offering VV-ECMO, ensuring safe management of the circuit by the perfusion team. Enhanced echocardiography may therefore be an appropriate bedside technique during the current viral surge in critical VV-ECMO supported severe acute respiratory failure, helping address diagnostic uncertainty in cases with challenging echocardiographic visualization.