with indication for PP and lack of invasive neuro­monitoring, both before institution of PP to iden­tify patients at higher risk of developing critical reduction in cerebral perfusion and during PP to detect changes in ICP and cerebral perfusion pressure, which are concerning for secondary brain injury.

ECMO and as a tool for optimization of cerebral perfusion, especially in patients undergoing V-A ECMO but also for V-V ECMO [23, 34, 35]. In pediatric ECMO population, an increase in TCD velocities (especially if localized and asymmet­ric) has been associated with neurologic injury, and it seems to represent an early sign, anticipat­ing by approximately 1–6days of the clinical rec­ognition [23, 35]. Reactive hyperemia secondary to uncoupling of cerebral metabolism and cere­bral ow in the context of ischemic event may explain this nding, and it is associated with increased risk of intracranial hemorrhage (espe­cially in the context of anticoagulation) [35]. Changes in pulsatility (PI) and/or resistivity (RI) indices have also been associated with increased risk of cerebrovascular complications in neonatal and pediatric ECMO patients [35, 36]. When analyzing TCD velocities, the type of ECMO, the site of cannulation, and the circuit ow rate are important parameters to take into consideration [35].

Patent foramen ovale (PFO) prevalence in gen­eral adult population is approximately 25% [39] but shunting through PFO is rare in healthy indi­viduals as the foramen is usually kept closed by a ap when left atrial pressure is higher than right atrial pressure. However, in conditions associated with increased right atrial pressure (e.g., pulmo­nary hypertension, pulmonary embolism, ARDS), right-to-left shunting may occur, with possible worsening hypoxemia. For example, shunting through PFO has been observed in almost 20% of patients with ARDS and it is asso­ciated with poorer response to PEEP, greater use of adjunctive interventions, and longer ICU stay [40]. Intrapulmonary shunt is also recognized as a cause of worsening hypoxemia during ARDS, and it is observed in approximately 25% of patients affected by this condition [41]. Contrast transesophageal echocardiography (TEE) is con­sidered the gold standard test for identication of right-to-left shunt. Contrast TCD represents an alternative method for the detection of right-to­left shunt. Identication of microembolic events [high-intensity transient signals (HITS)] in the cerebral circulation has been shown to be as accurate as cTEE (or even more) for detection of right-to-left shunting, especially at rest [42, 43]. Moreover, cTCD can be used as a semiquantita­tive method to estimate the volume of shunting. In the context of hypoxemia due to right-to-left shunting, different supportive treatment options aimed at reducing right ventricular load condi­tions and intrapulmonary shunt may be consid­ered, as nitric oxide, alveolar recruitment, and prone positioning. Contrast TCD may also have a signicant role in both detecting right-to-left shunting and assessing the response to these interventions [44].

ous intraparenchymal hemorrhage and aneurys­mal subarachnoid hemorrhage, and it represents a signicant risk factor for unfavorable outcome and death. An appropriate uid management strategy is of great importance in neurocritically ill patients, due to the risks associated with both uid overload and intravascular depletion. Fluid responsiveness can be estimated by echocar­diography in both spontaneously breathing and mechanically ventilated patients, by the use of passive leg raising [61] and by the respiratory variations of maximal Doppler velocity in the left ventricular outow tract [62] representing the most accurate indices, respectively.