Using the IE33 device (Philips Medical Systems, the Netherlands), two-dimensional and myocardial contrast echocardiography (TTE and MCE) are performed following the recommendations of the American Heart Association and the European Society of Cardiology (2006), and the European Association of Cardiovascular Imaging (2017). TTE and MCE are performed at the same time in the first 24 hours after ICU admission, at 48-72 hours, at 5-10 days after withdrawal of vasopressors and inotropes.

First, TTE evaluates from the apical and parasternal views:

• The Wall motion score index (WMSI) of 16 myocardial segments of the left ventricle (LV).
• The diastolic function using pulsed-wave doppler and pulsed tissue doppler at the mitral valve.
• Quantify valvular insufficiency
• Estimation of cardiac output (L/ minute).
• Evaluation of the right ventricle (RV) dimension and its the longitudinal contractility by the Tricuspid annular plane systolic excursion (TAPSE) with pulsed tissue doppler.
• Left atrial volume (ml).
• Systolic pulmonary pressure and pulmonary resistance with both continuous and pulsed-wave doppler at the tricuspid valve and the pulmonary outflow tract, respectively.

Second, MCE is performed if:

• Systolic blood pressure < 200 mmHg or > 90 mmHg,
• Heart rate < 130 or > 50 beats/minute
• Peripheral pulse oxygen saturation > 90%
• Arterial oxygen partial pressure (PaO2) 70 mmHg
• Arterial pH 7.25.

Administration of contrast agent Sonovue requires an infusion pump (Vueject, Bracco, Milan, Italy), which provides constant agitation to maintain the homogeneity distribution of Sonovue. Injection of Sonovue allows an enhancement of LV endocardial border and regional function to evaluate:

• LV end-diastolic and end-systolic volumes (ml) and ejection fraction (%) using the Simpson method.
• The WMSI of the left ventricle (LV) after Sonovue injection.

After optimization of transthoracic cardiac views, the mechanical index will settle between 0.1-0.2 and keeps unchanged during the procedure. Sonovue vial of 5 ml will dilute in in 10 ml saline solution and administrate at 0.7-1.5 ml/min. Using acquire flash-replenishment sequences during15 cardiac cycles of the apical 4-2-3 chamber views with the flash delivered after the second cardiac cycle. This technique destroys the microbubbles presents in the myocardium and allows replenishment with new microbubbles concentrations.

The volume of blood within the entire coronary circulation at rest in diastole is predominantly resided within the capillaries. The myocardial signal intensity emanating from the contrast agent reflects the concentration of microbubbles within the myocardium. It takes 5 seconds for complete replenishment of the myocardium. Any decrease in myocardial blood flow prolongs replenishment time in proportion to its reduction.

Immediately after microbubble infusion is started, all real-time MCE procedures are recorded for one minute and stored as DICOM (Digital Image Communications in Medicine) images. Offline analysis uses a specific quantification software named QLAB10 (Philips Medical Systems, the Netherlands) to convert myocardial perfusion images into time-intensity curves (TIC) corresponding to different regions of interest (ROI) of the 16 myocardial segments.

Four variables are analyzed from these TIC curves to evaluate qualitatively the myocardial

• peak intensity (PI) in decibel (dB).
• time to peak intensity in seconds (TTP).
• mean transit time in seconds (MTT).
• Area under the curve in dB/seconds (AUC).

The cardiac biomarkers including High sensivity cardiac troponin I for myocardial injury and N-terminal pro-brain natriuretic peptide (NT-proBNP) for heart failure are measured once daily in routine clinical practice.