Monitoring the brain in cardiac surgery – an evolving area for research

Millions of patients worldwide undergo procedures requiring anaesthesia every year, and although anaesthetic safety has increased substantially over recent decades, significant peri-operative morbidity remains. Although many of the major causes of mortality have been addressed and many of the significant morbidity issues have received considerable attention, the bar continues to be appropriately raised with respect to what we consider a good peri-operative outcome. No longer is simply emerging from anaesthesia relatively pain-free and haemodynamically stable considered sufficient when we say the patient ‘did well’. Long-term survival with freedom from significant morbidity is increasingly the focus of peri-operative outcomes studies. One of these morbidities relates to postoperative cognitive dysfunction (POCD), which is addressed by van Harten et al. in this issue of Anaesthesia [1]. Less dramatic adverse outcomes (compared with stroke), such as POCD and delirium, are clearly no less important and have themselves been associated with significant long-term morbidity [2, 3]. Multiple approaches have been utilised to add information aiding our understanding of the risk factors, pathophysiology, early detection and potential therapy for these adverse cerebral outcomes. Specific monitoring of the brain is an important aspect that has added information to each of these considerations. However, the brain, arguably our most important of organs and despite being an obvious site of anaesthetic action, ironically is inconsistently the focus of targeted monitoring during anaesthesia. There have been several notable attempts to open a window to the ‘black box’ represented by the brain, but compared with other physiologic parameters related to the respiratory and cardiovascular system, it lags far behind. van Harten et al. address several cerebral monitoring modalities, and in doing so, highlight several of the issues and limitations related to the currently available techniques [1]. Transcranial Doppler (TCD), in addition to its ability to interrogate various cerebral vessels (most commonly the middle cerebral artery) to determine blood flow velocity (an indirect measure of cerebral blood flow), has frequently also been used for detection of emboli, thus elucidating their potential relationship to POCD. Although historically emboli detected by TCD have been shown to be associated with POCD [4], this has recently been questioned [5, 6]. One of the limitations with the data linking TCD emboli to postoperative neurological outcome is the inability of current Doppler technology to differentiate reliably between particulate and gaseous emboli, despite several notable attempts [5, 7]. This poor signal-to-noise ratio is often further impaired by gaps in monitoring because of loss of signal due to patient movement. Logistical limitations, coupled with the cumbersome nature of TCD signal acquisition and probe stabilisation, have substantially limited its usefulness in the perioperative setting [8]. With a few exceptions in the postoperative period, such as for vasospasm monitoring in patients with subarachnoid haemorrhage [9], it is a cumbersome cerebral monitor whose use is difficult to recommend confidently. In contrast to TCD, electroencephalography (EEG) has made major inroads into the peri-operative arena. It has been widely available for clinical use for almost two decades, though is still inconsistently used. Although multi-channel monitoring has only select demonstrable perioperative benefit [10], single (or dual) channel EEG monitoring has had substantive demonstrable utility [11] as processed EEG depth of anaesthesia monitors, such as the bispectral index (BIS). Initially intended as a monitor to gauge hypnotic depth in order to facilitate reducing the incidence of intra-operative awareness, early BIS studies Anaesthesia 2012, 67, 213–225 Editorial