The neurological injury in CHD surgical patients is thus multifactorial and any one single monitoring modality is unlikely to detect all pos­sible threats. Using a combination of various cur­rently available neuro-monitoring modalities may allow for early detection and possible inter­vention and thus help to improve neurological outcome in CHD surgeries.

period of cerebral perfusion and has been sug­gested as being superior to DHCA in preventing neurological damage [24, 25]. However, cerebral autoregulation (CA) dened as the maintenance of constant CBF despite changes in cerebral per­fusion pressure (CPP) is deeply affected by tem­perature. Autoregulation is preserved during normothermic CPB; it begins to alter at tempera­ture less than 25°C, and it is lost at temperature less than 20°C [26]. TCD pattern consistent with increased cerebral vascular resistance in the early post-operative period is seen in patients treated with profound hypothermia and low-ow CPB or circulatory arrest, whereas this pattern is not present in patients treated with moderately hypo­thermic CPB. This pattern of cerebrovascular changes occurred with greater frequency in patients who had a period of circulatory arrest [27]. The CBF remains decreased even after CPB rewarming whereas it returned to baseline in nor­mothermic CPB [28]. A study which used TCD as an indicator of perfusion during repair of con­genital heart defects requiring moderate or pro­found hypothermia and low-ow CPB found that there was an immediate loss of detectable CBFV in the middle cerebral artery (MCA) when CPP decreased below 9mmHg suggesting that CPP is a crucial parameter, rather than pump ow rate, in impacting brain perfusion [23]. Similarly a study by Zimmermann etal. [29] performed on 28 neonates undergoing the arterial switch opera­tion with α-stat acid-base management showed that cerebral perfusion is detected by TCD in the MCA in some neonates at bypass ow as low as 10ml/kg per minute. But a minimum bypass ow rate of 30ml/kg per minute was needed to detect cerebral perfusion in all neonates. All patients with a mean arterial pressure (MAP) of 19mmHg or greater, regardless of pump ow rate, had detectable cerebral perfusion by TCD but corre­lation between MAP and CPB pump ow rates was minimal, conrming the conclusions of Taylor etal. [26] that MAP alone is a poor indica­tor of CPP.

Regional low-ow perfusion is a technique where the brain is perfused via the right innominate artery in neonates undergoing aortic arch recon­struction surgeries. When unilateral near-infrared spectroscopy (NIRS) was used to assess the uni­lateral cerebral oxygen saturation during RLFP, the mean regional cerebral oxygen saturation (rSO2) was 6.3% lower on the left side, and about half of the patients had a difference of 10% in the absolute values of rSO2 during RLFP.When per­fusion was initiated through carotid arteries before or after RLFP/with or without bypass, bilateral rSO2 values agreed closely [32]. However, NIRS is able to detect only inadequate CBF but not excessive CBF.TCD can be a very useful monitoring tool to identify excessive cere­bral ow and aid in maintaining adequate CBF to prevent cerebral oedema and haemorrhage.

artefacts are detected in a short period of time, the number counted is meaningless. There are a number of limitations of TCD in cerebral embolus detection. The number of emboli detected in the carotid artery during paediatric congenital heart surgery does not appear to correlate with acute post-operative neurological decits [33] and the emboli are detected when already it has occurred.

CA is the intrinsic ability of cerebral vasculature to maintain adequate CBF over a wide range of cere­bral perfusion pressure despite uctuations in blood pressures. Cardiac surgery may result in alterations in CA and methods to determine CA at the baseline, during, and after the cardiac surgery may help in understanding the effects of CPB on CA as well as in understanding the pathogenesis of neurological complications in these patients [1].

TCD with “bubble test” appears as the best screening test for the detection of RLS in young and middle-aged adults. In a recent study by Palazzo et al. [39] when comparing TCD with “bubble test” to transoesophageal echocardiogra­phy (TEE), sensitivity and specicity were both 100%. In patients with cryptogenic acute cerebral ischaemic events, TCD can help to select patients potentially suitable for closure procedure after TEE conrmation.

TCD is the only clinically useful brain ultrasound modality that has been evaluated for use in adult cardiac surgery. Typically, prior to surgery a baseline TCD evaluation is performed bilaterally in all basal arteries. However, during the cardiac surgery, initial segment (M1) of MCA is moni­tored through trans-temporal acoustic window. The key elements that can be evaluated using TCD during cardiac surgery are described in detail:

Ono et al. [41] reported higher incidence of perioperative stroke in patients with impaired CA. In a study by Joshi et al. [42] all seven strokes that occurred perioperatively were in patients with impaired CBF autoregulation dur­ing CPB rewarming. Other researchers identied impaired CA during CPB as a risk factor for post­operative delirium but not for stroke. Hori etal. [43] reported fourfold higher frequency of delir­ium in patients whom MAP exceeded upper limit of autoregulation, but not with lower limit of autoregulation.

Changes in CA following DHCA have been demonstrated in adults. In a study of 67 patients undergoing aortic arch surgery, Neri and col­leagues [44] compared the inuence of intraoper­ative DHCA (n=23), selective antegrade cerebral perfusion (SACP; n=25), and retrograde cerebral perfusion (RCP; n = 19). Dynamic CA was assessed using bilateral thigh compression to pro­duce a step decrease in arterial pressure. Autoregulation was preserved only in patients managed with SACP. In the other two groups, impaired autoregulation persisted in some of the patients until the seventh postoperative day.

Bilateral or unilateral monitoring of a targeted intracranial vessel is recorded for a minimum of 30min. TCD systems equipped with automated HITS detection software that counts the number of microemboli and measures microembolic sig­nal intensity are useful. However, for a reliable diagnosis, both visual and auditory inspection and conrmation of each detected HITS are required.

Atherosclerosis of the aortic arch has also been correlated with an increased risk of stroke after cardiac surgery [53]. The greater incidence of left-hemispheric (possibly due to the tendency of arch-generated emboli to move in the direction of blood ow to the downstream carotid vessel) strokes is an indirect evidence of the importance of aortic arch atherosclerosis [54].

Despite signicant advancement in the tech­niques of CPB, it remains a source for both par­ticulate and gaseous emboli.

TCD can be extremely helpful in detecting brain complications related to cardiac surgery both in paediatric and adult patients. This technique is easy to perform during cardiac surgery and it can be considered as a good non-invasive tool for CBF monitoring and titrating perfusion ow both during cardiac surgery and when patients are under ECMO.Further research should be consid­ered in this eld given the new indications to put patients on ECMO when affected by acute lung injury.