1 Introduction

Echocardiography has become a true bedside clinical device for full hemodynamic evaluation, monitoring and diagnosis in patients with circulatory failure. It allows real-time anatomical and functional cardiac assessment in very different situations, as in sepsis or acute respiratory distress syndrome (ARDS). Edler et al. [1] first described echocardiography in 1954, as an “ultrasonic reflectoscope for the continuous recording of the movements of heart walls”. Many advances followed, including time motion mode in 1963 [2], real time 2D in 1973 [3], spectral Doppler in 1979 [4] and tissue Doppler in 1989 [5], each time enriching the information available for physicians.

Some pioneers started to use echocardiography in the ICU in the 1980s, especially in sepsis where it was able to detect profound cardiac failure [6–8]. After many years of development and clinical studies in critically ill patients, scientific societies defined critical care echocardiography (CCE) as echocardiography performed by intensivists themselves at the bedside for diagnosis of cardiac failure and hemodynamic monitoring, with 2 different skill levels: basic and advanced [9].

2 Transthoracic (TTE) or transesophageal echocardiography (TEE)?

Transthoracic echocardiography is often used as a first-line approach as it is well tolerated by patients, safe, and widely available [10–12]. It is especially suitable for basic CCE, where intensivists have to perform a quick and focused examination to diagnose or to exclude “gross” cardiac abnormalities, as acute cor pulmonale (ACP), cardiac tamponade or major left ventricular systolic dysfunction. However, TEE is especially accurate in mechanically ventilated patients with ARDS or septic shock where advanced CCE is required. Intensivists have to repeat a full hemodynamic evaluation at various times in different situations with different ventilatory settings [10]. In these situations, TEE has a better diagnostic capability and is more reproducible than TTE. Whereas TTE is totally non-invasive, TEE is considered as semi-invasive because of esophageal intubation, even though most complications have been described in spontaneously breathing patients [13].

3 Main echocardiographic views for hemodynamic evaluation

3.1 TTE

Even though many views are available in cardiology for diagnosis and evaluation of cardiac diseases, few views are necessary in the ICU, as a first step for evaluation and hemodynamic monitoring. The apical four-chamber view allows intensivists to detect right ventricular (RV) dilatation (Fig. 1a). Using the pulsed Doppler, it also allows recording of left ventricular (LV) inflow for evaluation of LV filling pressure (Fig. 1b). From this view, by tilting the probe up, the LV outflow track may be visualized and LV stroke volume calculated (Fig. 1c). The parasternal short-axis view is perfectly suitable for evaluation of LV contraction and for calculating LV fractional area contraction, a surrogate of LV ejection fraction. It also allows detection of paradoxical septal motion (Fig. 2), leading to the diagnosis of ACP when associated with RV dilatation. Finally, a subcostal view visualizes the inferior vena cava (IVC). Whereas a small or virtual IVC (Fig. 3a) is highly suggestive of hypovolemia and the need for fluids in a context of shock, a dilated or congestive IVC is often related to cardiac failure (Fig. 3b).

Fig. 1
figure 1

Apical four-chamber view by a transthoracic approach. a RV dilatation in a patient with acute cor pulmonale. b Pulsed-Doppler at the mitral valve with the E and the A waves, demonstrating a restrictive pattern of flow, reflecting a high LV filling pressure. c Pulsed-Doppler in the LV outflow tract, allowing calculation of the LV stroke volume. RV right ventricle, LV left ventricle

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Fig. 2
figure 2

Parasternal short-axis view in a patient with acute cor pulmonale. Note the RV dilatation and D-shape of the left ventricle reflecting the paradoxical septal motion (arrow). RV right ventricle, LV left ventricle

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Fig. 3
figure 3

Subcostal view of the inferior vena cava (IVC), coupled with time-motion study. a Very small IVC with significant respiratory variations of its diameter during tidal ventilation, reflecting hypovolemia in this patient in shock. b Dilated IVC without any respiratory variations reflecting the absence of preload responsiveness and occurrence of cardiac failure

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3.2 TEE

Even though a lot of views may be obtained by TEE using a multiplane probe, from 0 to 180° in each position, 3 views are especially required to monitor hemodynamics in the mechanically ventilated patient: the mid-esophageal 4-chamber view (Fig. 4), the transgastric short-axis view (Fig. 5) and the longitudinal view of the superior vena cava (SVC) (Fig. 6). Whereas the first view gives information similar to that provided by the apical 4-chamber view, the second view is similar to the parasternal short-axis view. The longitudinal view of the SVC is more specific and interesting since SVC respiratory variations have been demonstrated to predict the response to fluids accurately [14]. An inspiratory collapse in a ventilated patient perfectly adapted to the ventilator is highly suggestive of the need for fluids (Fig. 6).

Fig. 4
figure 4

Transesophageal 4-chamber view in a ventilated patient. The probe is introduced into the esophagus 30 cm from the dental arch. Note that the right ventricle appears normally triangular. RA right atrium, RV right ventricle, LA left atrium, LV left ventricle

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Fig. 5
figure 5

Short-axis view of the left ventricle by a transgastric approach. LV left ventricle, RV right ventricle, PW posterior wall, AW anterior wall

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Fig. 6
figure 6

Long-axis view of the superior vena cava by a transesophageal approach, coupled with the time-motion study. a Before volume expansion, note the collapse of the vessel during each tidal ventilation. b After volume expansion, blood pressure was normalized and the collapse disappeared. SVC superior vena cava

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4 Functional hemodynamic monitoring: the concept

Over the last few years, a new approach to hemodynamic evaluation called functional hemodynamic monitoring [15] has been developed. It is less based on numbers and invasive tools, and more on qualitative and “functional” assessment, and on prediction of treatment effect. Echocardiography is perfectly adapted to this new approach. Vieillard-Baron et al. [16] have validated such a functional approach in patients with septic shock using TEE. The 3 previously described views were fully able to detect LV systolic dysfunction, preload responsiveness and RV failure [16]. More recently, Bouferrache et al. [17] reported how this functional approach is weakly related to the Surviving Sepsis Campaign guidelines for fluids and inotropic drug management.

5 Some usual indications of echocardiography in the ICU

5.1 Assessment of fluid responsiveness

Hypovolemia is a crucial problem for intensivists. It leads to organ ischemia and potentially organ failure. However, fluid overload also has marked deleterious effects, as pulmonary edema and finally increased mortality [18, 19]. This explains why intensivists progressively move from a “fluid challenge” approach to a prediction of fluid responsiveness. In this field, new echocardiographic parameters have been validated, based on heart–lung interactions, i.e. the IVC distensibility index (Fig. 3a) [20, 21], the SVC collapsibility index [14] (Fig. 6) and respiratory variations of the velocity–time integral of aortic flow [22]. The latter requires that patients be in sinus rhythm.

Static parameters, as cardiac filling pressure evaluation by pulsed Doppler at the mitral valve, are not accurate in detecting the need for fluids, but may be used to evaluate tolerance of volume expansion and to exclude (or diagnose) a cardiogenic factor in a patient with acute respiratory failure (Fig. 1b).

5.2 Adaptation of respiratory settings to RV function

Acute cor pulmonale occurs when the RV is overloaded [23, 24]. Common causes in the ICU are ARDS, “aggressive” mechanical ventilation and massive pulmonary embolism. Because ACP has been reported to have a significant impact on prognosis in ARDS patients [24], its demonstration may lead to adaptation of the ventilatory settings to RV function, by decreasing the plateau pressure, and by limiting the PEEP and hypercapnia [25, 26]. In this setting in ARDS patients, it is now recommended to perform at least one echocardiographic examination per day during the first 3 days to evaluate RV function and to adapt the ventilator.

5.3 Hemodynamic management in septic shock

Septic shock is a complex process where different mechanisms, alone or associated, may be responsible for circulatory failure throughout disease progression: hypovolemia, vasoplegia and septic cardiomyopathy. As discussed above, echocardiography is perfectly adapted to optimize volemia. But echocardiography has also proven valuable in accurately detecting LV systolic dysfunction in patients with septic shock [27]. In our usual practice, we use serial echocardiographic examinations, in association with invasive blood pressure monitoring and lactate assay, to adapt and to optimize hemodynamic resuscitation.

6 Acquisition of the skills required for CCE

Echocardiography is as an indispensable tool in the ICU for diagnosis and also for hemodynamic monitoring, and the question then is how can intensivists acquire the necessary skills? Recently, a round table of experts representing most of the international societies of intensive care medicine in the world proposed guidelines [28]. Briefly, basic CCE requires intensivists to participate in a 10-hour training course and to perform at least 30 fully supervised TTE examinations. Competence-based training in basic CCE should be included in the curriculum of all intensivists. For advanced CCE, intensivists must complete 40 h of course work and perform 150 fully supervised TTE and 25–50 TEE examinations. A process of certification, accreditation, or delivery of a diploma validating the acquisition of the required skills is essential.

In France, two types of certifications have been developed for intensivists. The first was initially intended for cardiologists, but was enlarged to include intensivists through a collaboration between the societies of cardiology and of intensive care. It is a 2-year training program that teaches the skills needed by future trainers. The second certification is specifically intended for intensivists, and is awarded after a one-year course.

7 Conclusion

Echocardiography should be widely used first line in the ICU. It safely provides hemodynamic information about acutely ill patients, as well as functional information about the condition of the heart. It can also be used as a diagnostic tool for circulatory or respiratory failure. Every intensivist should receive training at least to the basic level, and acquire an appropriate diploma approved by an intensive care society.