Edited by: Ana Ochagavía - Hospital Universitario de Bellvitge. L'Hospitalet de Llobregat. Barcelona. Spain.
More infoUltrasound is an essential diagnostic tool in critically ill patients with extracorporeal membrane oxygenation (ECMO). With it, we can make an anatomical and functional (cardiac, pulmonary and vascular) evaluation which allows us to execute an adequate configuration, guides implantation, helps clinical monitorization and detects complications, facilitates withdrawal and complete post-implant evaluation. In patients with ECMO as respiratory support (veno-venous), thoracic ultrasound allows monitoring pulmonary illness evolution and echocardiography the evaluation of biventricular function, especially right ventricle function, and cardiac output to optimize oxygen transport. In ECMO as circulatory support (veno-arterial), echocardiography is the guide of hemodynamic monitoring, allows detecting the most frequent complications and helps the weaning. In ECMO teams, for a proper management of these patients, there must be trained intensivists with advanced knowledge on this technique.
La ecografía es un instrumento diagnóstico fundamental en el paciente crítico con membrana de oxigenación extracorpórea (ECMO). Con ella podemos hacer una evaluación anatómica y funcional (cardiaca, pulmonar y vascular) para plantear una adecuada configuración; además, guía su implante, ayuda en la monitorización clínica y la detección de complicaciones, facilita su retirada y completa la evaluación postimplante. En los pacientes con ECMO como soporte respiratorio (veno-venosa), la ecografía torácica permite monitorizar la evolución de la enfermedad pulmonar y la ecocardiografía la evaluación de la función biventricular, especialmente la derecha, y el gasto cardiaco para optimizar el transporte de oxígeno. En la ECMO como soporte circulatorio (veno-arterial), la ecocardiografía supone la guía de la monitorización hemodinámica, permite detectar las principales complicaciones y ayuda al destete del dispositivo. En los equipos ECMO, para un adecuado manejo de estos pacientes, debe haber intensivistas entrenados y con conocimientos avanzados sobre esta técnica.
Extracorporeal membrane oxygenation (ECMO) is a mechanical support system used to secure total or partial cardiopulmonary stabilization in patients with circulatory and/or respiratory failure. Although the history of ECMO goes back over 50 years since it was first used for therapeutic purposes,1 it has only become widely used throughout the world in recent decades. The introduction of new devices and materials, the results obtained during the influenza A pandemic,2,3 and patient centralization in reference centers with multidisciplinary teams are a number of the factors that have caused ECMO to form part of the therapeutic measures for dealing with refractory cardiorespiratory failure. Due to the characteristics of these patients and the complexity of the processes, the available scientific evidence is limited,4,5 though many studies have evidenced the usefulness of the technique, and for this reason different scientific societies have established recommendations on its use.6–8
The use of ultrasound in critical patients is an established practice and is crucial for both the general and specific evaluation of cardiovascular and respiratory disease. Despite the complexity of the exploration and the technical requirements, it can afford information with a high diagnostic capacity at the patient’s bedside and on an immediate basis.9,10 In the case of patients with ECMO, and due to the increase in its indications and configurations, ultrasound – particularly transthoracic echocardiography (TTE) or transesophageal echocardiography (TEE) in the case of a poor exploration window – has become a key instrument.11,12 Adequately trained professionals are needed, since ultrasound allows precise anatomical and functional cardiopulmonary evaluation, guides the insertion and placement of cannulas, helps to optimize flow, allows monitoring to detect and resolve clinical changes, facilitates weaning from ECMO, and contributes to the post-implantation assessment of possible complications.13–15Table 1 summarizes the echocardiographic parameters that should be evaluated and recorded in patients with ECMO.
Echocardiographic parameters to be evaluated in patients with ECMO.
| Structure | Parameter | Reference |
|---|---|---|
| Left ventricle | ||
| • Morphology | Myocardial size and thickness | |
| Septal position | ||
| Presence of autocontrast/thrombi | ||
| • Systolic function | Ejection fraction | |
| Segmental motility | ||
| Mitral S wave (TDI) | > 6 cm/s | |
| Velocity and size of VTI of LVOT | > 1 m/s and >10 cm | |
| • Diastolic function | E/A ratio of transmitral flow | <2 |
| EDT of mitral flow | >150 ms | |
| E/e’ ratio of the mitral ring | <8 | |
| Mitral and aortic valves | Diagnosis and grading of possible insufficiency and/or stenosis | |
| Left atrium | Size and volume | <35 ml/m2 |
| Pulmonary vein flow | S wave > D | |
| Right ventricle | ||
| • Morphology | RV/LV end-diastolic area ratio | <0.6 |
| Septal position and motion | ||
| Ventricular geometry | Triangular | |
| Eccentricity index | 1 | |
| Myocardial thickness | <8 mm | |
| • Systolic function | TAPSE | >16 mm |
| Tricuspid S’ wave (TDI) | >10 cm/s | |
| Shortening fraction | >35% | |
| McConnell sign | ||
| • Diastolic function | Tricuspid flow E/A ratio | <2 |
| Tricuspid and pulmonary valves | Tricuspid insufficiency for estimation of PAPs | <3 m/s |
| Pulmonary flow (Tac) | >120 ms | |
| Right atrium | Area | <20 cm2 |
| Structures (valves, Chiari, coronary sinus) | ||
| Septal integrity (foramen ovale) | ||
| RA-LA shunt (color Doppler ± shaken saline test) | ||
| Inferior/superior vena cava | Size and respiratory variation | <20 cm |
| Position of cannulas | ||
| Thrombi | ||
| Aorta | Thrombosis/atheromatosis/dissection | |
| Pericardium/pleura | Effusion (characteristics and grading) | |
TDI: tissue Doppler; VTI: velocity-time integral; LVOT: left ventricular outflow tract; EDT: E wave deceleration time; RV: right ventricle; LV: left ventricle; TAPSE: tricuspid annular plane systolic excursion; PAPs: systolic pulmonary artery pressure; Tac: acceleration time; RA-LA: right atrium–left atrium.
Despite its great usefulness, the use of ultrasound in patients with ECMO has important limitations. In the case of two-dimensional (2D) explorations, imaging acquisition may be adversely affected by both the patient’s position (dorsal or even prone decubitus) and the presence of invasive mechanical ventilation or devices such as vascular catheters, tubes or drains. Furthermore, the functional evaluation and interpretation of the recordings that will guide ECMO adjustment cannot be made by any operator, since advanced technical as well as clinical knowledge is needed. The level of knowledge influences decisions and adjustments that clearly may have an impact on the course of the patient.
Although there are different configurations,16 the present review focuses on the usefulness of ultrasound in patients with veno-venous (VV) and veno-arterial (VA) ECMO.
Veno-venous (VV) ECMOPre-VV ECMO evaluationPre-implant echographic evaluation of VV ECMO should be made on a routine basis, since different conditions need to be considered in order to select the type of assist and the optimum configuration.
The presence of severe left ventricular (LV) dysfunction refractory to inotropic drug treatment may require a change in configuration from VV to VA, or to veno-arterial-venous (VAV) mode.14 In addition to echocardiographic evaluation (left ventricular ejection fraction (LVEF) < 30% and/or left ventricular outflow tract (LVOT) velocity-time integral (VTI) < 10 cm), the presence of persistently elevated lactate concentrations (>5 mmol/l), central venous saturation <55%, cardiac index <2.1, the presence of arrhythmia with hemodynamic alterations, cardiac arrest and a vasoactive inotropic score >50 points during one hour or >45 points during 8 h, are factors that may prove useful for predicting claudication with VV ECMO.17
In evaluating VV ECMO implantation, another essential factor is right ventricular (RV) function. Many patients with severe acute respiratory distress syndrome (ARDS) present pulmonary hypertension and RV dysfunction in relation to hypoxia, hypercapnia, increased airway pressure and mechanical ventilation. Veno-venous ECMO, by correcting hypoxia and hypercapnia, reduces afterload and can improve RV function, thus correcting the hemodynamic instability. In the event of significant right ventricular dysfunction, echocardiographic monitoring is important, and if shock persists and the echocardiographic parameters fail to improve despite VV ECMO, we should consider changing the strategy to VA or VAV. The evaluation of RV dysfunction must consider the following: tricuspid annular plane systolic excursion (TAPSE) < 16 mm, S’ wave < 10 cm/s, shortening fraction (SF) < 35%, RV/LV end-diastolic area ratio > 0.6 (significant) and >1 (severe) or flattening/bulging (“D” form) of the interventricular septum in both systole and diastole (Supplementary material 1). This inter-dependence can be quantified using the ventricular eccentricity index, which is the ratio between the septum-inferior surface and anterior surface-inferior surface diameter in systole and diastole, with a normal value of 1.18
On the other hand, we should evaluate the existence of underlying disease conditions or anatomical alterations that may contraindicate implantation.14 In this regard, severe tricuspid valve disease (insufficiency and/or stenosis) may impair ECMO oxygenated blood flow from the right atrium (RA) to the LV. The presence of a persistent foramen ovale or interatrial communication, during weaning from ECMO, could increase the right-side pressures and generate a right-left shunt affecting oxygenation and even making removal of the device necessary. A prominent Chiari network may complicate the placement of the cannula and guide it toward the interatrial septum. The presence of a coronary sinus dilated by a left superior vena cava (SVC), if accidentally cannulated for the return, may drain the blood towards the left arm instead of to the RA.
Lastly, we must choose the best cannulation strategy according to the characteristics of the vascular accesses, emphasizing the presence of thrombi and/or anatomical variants/anomalies (Supplementary material 2).
Cannulation and start of VV supportThe systematic use of ultrasound is recommended during the different cannulation phases19:
Vascular ultrasound allows us to measure vessel diameter to select the optimum cannula size (Fig. 1). The following formula is used for this purpose: cannula caliber (French (Fr)) = 3 × vessel diameter (mm). The largest cannula size should be used for both drainage and return, in order to ensure the greatest flow possible. Ultrasound-guided vascular puncture increases the safety and success rate at the first attempt, reducing the risk of local complications (arterial cannulation, cannulation of the saphenofemoral junction or transfixation of the inguinal ligament).20,11
