Aortic arch repair is still associated with a relatively high mortality rate. In addition, inadequate cerebral perfusion during SACP can lead to unfavorable neurological outcome. The best parameters to monitor adequate cerebral perfusion are still unclear. NIRS has become widely used because it is non-invasive, easy to use, and provides a robust signal. However, the validity of the method is still disputed. It was shown that, NIRS can indicate a dislocation of the arterial cannula.(4) Since cerebral oxygenation as indicated by NIRS is the result of several factors, Denault et al developed a protocol for improvement in case of desaturation.(5) Joshi et al. even used NIRS to predict limits of cerebral autoregulation.(6) However, it seems that a constant rSO2 may not guarantee adequate perfusion. We could not see major differences in cerebral oxygenation during the procedure, except during deep hypothermic cardiac arrest, as can be seen in Fig. 1. And while MCAV showed significant decreases on the left side we saw no significant differences in NIRS measurements. This is in line with our previous findings in cardiac operations, where we found only little variation in regional oxygen saturation.(7) It can be interpreted that NIRS indicates major hemodynamic changes while more subtle changes may remain unnoticed by the method. For SACP, we therefore suggest NIRS monitoring as a minimum requirement.
TCD has long been used as a method to assess cerebral hemodynamics.(8) But whether MCAV actually reflects cerebral blood flow is subject of controversy, since basal arteries may also undergo caliber changes.(9) Nonetheless, there is mostly a consensus that TCD can detect changes in cerebral blood flow and is therefore still used in many physiological investigations.(10) The feasibility and benefit of TCD for neuro outcome has been presented by Estrera et al. but with a need of a neurosonographer to operate continuous TCD.(11) It has also been used to monitor cerebral blood flow in aortic arch surgery in a continuous setup.(2) It was our aim to develop a protocol which can improve neuromonitoring using widely available equipment. We therefore chose to use a standard B-mode ultrasound machine with TCD presets. Apart from the advantage that existing devices can be used, B-mode and color doppler facilitates localization of the MCA since the vessel can be localized visually and in difficult cases anatomical landmarks inside the skull can be used to guide the transducer angle and position. However, with this method, continuous measurements are hardly possible. In a report by Ghazy et al., the transducer was fixed with a movable arm that held the probe in place.(3) Thus, the authors could visualize cerebral circulation and decide on a unilateral brain perfusion, since the circle of Willis was intact. We intentionally accepted the disadvantage of non-continuous TCD measurements in trade-off for bilateral TCD. Denault has recently proposed a modified NIRS algorithm in which ultrasound imaging plays a role in hemodynamic evaluation and cerebral imaging in case of increased intracranial pressure.(12) We suggest that ultrasound should become even more central in cerebral monitoring to confirm NIRS findings and to evaluate cerebral hemodynamics.
Our data shows that TCD seems to be more sensitive to altered cerebral perfusion. While NIRS values react more slowly to reduced perfusion reduced MCAV can be measured promptly. Our significant reduction in left sided MCAV during SCAP is probably explained when looking at the cannulation technique. We cannulated the right subclavian artery with the standard 18 F or 20 F cannula but inserted a 10 F (?) Cannula into the left carotid artery. Blood flow is therefore obviously reduced on the left side due to the difference in cannula diameter, since the perfusionist controls overall flow during SACP, but not in the individual cannula. On the other hand anatomical differences in the arterial vessels may also influence or reduce flow and cause substantial hypoperfusion which needs to be prevented.
That NIRS showed no difference on the left side might be due to compensation via the circle of Willis or show that even if flow is reduced cerebral oxygenation might still be sufficient.
Unfortunately, it remains unknown how to determine adequate cerebral blood flow with TCD. Adding to that, we cannot present data which might have hinted towards TCD values that could have been used as landmarks to determine sufficient perfusion. As much of the decision making depends on trends and the baseline MCAV this is a crucial measurement but also difficult to evaluate, since flow in the cerebral arteries may already be compromised by the dissection. We noticed that pre-CPB MCAV was significantly lower than in the following measurements during CPB. This may be a sign of malperfusion before CPB or could conversely represent hyperperfusion in the following measurements, which may occur during CPB and which we described previously.(13) Wang et al. provide some suggestions on sufficient cerebral blood flow, but also report a close correlation between NIRS and TCD measurements, which we cannot confirm in our patients nor did we observe this preciously.(7) Thus, a wider use of TCD and further investigations will be needed to find definite answers on how to measure adequate cerebral blood flow with TCD of the MCA.
TCD measurements led to adjustment of CPB flow or repositioning of the cannulae in two patients where a flow deficit was not indicated by NIRS. This may be an argument for bilateral continuous TCD monitoring. But apart from the lack in availability, a fixation of the probe may also be impractical, since small movements of the head during surgery may impair the doppler signal and may necessitate regular repositioning of the transducer. For continuous measurement, a headframe with doppler-only probes could also be used and may represent ideal monitoring for SACP. However, attaching the headframe and searching the MCA flow signal can be time consuming and requires special training.(11) Despite some disadvantages B-mode TCD should be used as a standard tool to guide SACP, as Ghazy et al. have also suggested.(3)
While BIS monitoring has also been used and has become part of standard monitoring, its utility to assess cerebral perfusion during hypothermia with Thiopental boluses is reduced. Naturally, BIS values during SACP are significantly lower than in other phases of the operation. Therefore, although BIS can be an indicator of inadequate brain perfusion, in SACP this is not the case. However, BIS can conversely be used as an indicator of the suppressive effect of Thiopental and hypothermia.
Determination of adequate cerebral perfusion is difficult and may not be reflected by one monitoring modality alone. A multimodal monitoring approach has also been suggested by Zanatta et al. with TCD, NIRS and somatosensory evoked potentials (SEPs).(14) SEP measurements are well validated, but the method requires training, may be time consuming and may be considered impractical for Stanford type A dissections.
In a previous study, we reported that in surgery with CPB, a combination of BIS, NIRS and TCD may increase patient safety.(7) We prpopse that this is also the case for patients undergoing SACP. While BIS is of lesser importance in SACP, NIRS can be used to reflect cerebral metabolism and TCD to reflect cerebral hemodynamics. We show that TCD gives us another tool to make sure that the patients brain is well perfused. However, the relevance for outcome has yet to be established.
There are some limitations inherent in this study. We can draw no conclusion about the outcome relevance of the reported method due to the small number of cases. Although monitoring had some impact on management, its effect on outcome yet remains to be shown. Also, by choosing a non-continuous TCD measurement approach, there is missing information between measurements which was a trade-off for bilateral monitoring with an existing device. We think that this is acceptable since with NIRS there is still one continuous monitoring modality in place which can at least indicate major changes.
In summary we presented an easy and widely available method of monitoring cerebral perfusion with continuous NIRS and intermittent TCD monitoring in patients undergoing aortic arch surgery for Stanford type A dissection even if more substantial research is necessary to learn more about cerebral perfusion monitoring during SACP.