Cerebral and tissue oxygen monitoring as part of the MMM strategy
It has been stated that: “The proper management of brain oxygenation should be one of the principal endpoints of all anaesthesia procedures yet the brain remains one of the least monitored organs during clinical anaesthesiology” . Near infrared spectroscopy (NIRS) using a reflectance technique can be used to measure cerebral tissue oxygenation to a depth of about 2.5 cm below the measuring electrodes, which are usually placed on the forehead. Regional brain oxygen saturation (rSO2 or ScO2) also reflects overall tissue rather than arterial oxygen saturation or Spo2, as assessed with a pulse oximeter . Cerebral rSO2 is venous weighted with a characteristic value in healthy patients of about 70%. A detailed consideration of this technology is outside the scope of this article, so the reader is referred to the excellent reviews by Murkin and Arango  and Scheeren et al.  Cerebral oximeters work best as trend monitors rather than absolute monitors , emphasising the importance of obtaining a pre-induction value in
Apart from those neurological conditions where brain oxygenation is obviously suspect – for example, carotid endarterectomy  – many operations on the elderly may lead to reductions in rSO2  and poor outcome . The challenge is to define suitable interventions if rSO2 diminishes by more than about 10% from the starting value. An MMM strategy that maintains flow and DO2 almost always ensures that rSO2 will remain at or above the pre-induction level .
Consideration of the role of cerebral oximetry in cardiac surgery, where its clinical applicability is greatest, is outside the scope of this article. However, its true role remains a matter of debate , although several studies have indicated possible benefits [37, 38].
My own experience with this monitoring technology amounts to nearly 1,000 cases. These have mainly included high-risk cases with an average American Society of Anesthesiologists (ASA) class of 3 or above, of long duration, where the build-up of oxygen debt was considered potentially a problem, and in the elderly population (average age, 68 years), where oxygen debt was likely to have caused the most problems. Maintaining tissue oxygenation should reduce complications . A recent review has even suggested that cerebral and tissue oximetry should become the standard monitor of future practice .
Depth of anaesthesia monitoring as part of the MMM strategy
Figure 1 indicates why monitoring the effect of the anaesthetic on cortical suppression is important for high-risk patients but may not be as important for fit and healthy patients. Looking along the x-axis, from left to right, increasing anaesthetic concentration will inevitably lead to cardiovascular depression and decreased DO2. This obviously has implications, in that there may be a build-up of oxygen debt, complications in the postoperative period and poor outcome. Neuronal toxicity is also increasingly likely as the concentration of anaesthetic increases . Conversely, moving from right to left along the x-axis, too little anaesthetic will obviously increase the risk of a patient’s intraoperative awareness and explicit recall; the benefits of anaesthetic neuronal protection may also be lost. Consequently, there is a balance to be struck. The dashed curve indicates a low-risk patient where the target anaesthetic concentration is rather broad and the implications of a small clinical misjudgement of the depth of anaesthesia (as indicated by the shallowness of the curve) means that the patient is unlikely to be unduly harmed. However, the situation with the elderly or high-risk patient is different, and the amount of anaesthetic required will be less, with a narrower margin between too light and too deep anaesthesia. Ten years’ experience with the BIS monitor in high-risk patients – especially the elderly – has shown that there is a fourfold variation in propofol anaesthesia requirement , which cannot be predicted by clinical signs alone.
Excessive depth of anaesthesia is harmful
Monitoring with BIS is now recommended by NICE in the UK for high-risk patients and the latest Recommendations for Standards of Monitoring during Anaesthesia and Recovery 2015 from the Association of Anaesthetists of Great Britain and Ireland (AAGBI) include a requirement for depth of anaesthesia monitoring during total intravenous anaesthesia when neuromuscular blockers are used . Evidence shows that BIS-guided anaesthesia decreases postoperative delirium  and cognitive decline (POCD) , and in the US it is recommended to reduce the incidence of delirium in older patients . In a pilot trial, combining BIS and cerebral oximetry led to a reduction in POCD and significantly lower levels of S100 B, an indicator of neuronal damage, in the intervention group .
Use of an MMM strategy in high-risk patients and high-risk surgery
Recent guidelines for managing patients with a proximal femoral fracture suggest that monitoring of cardiac output, depth of anaesthesia and cerebral oxygenation should be considered alongside a basic monitoring setup in this high-risk group . An observational case series in 120 patients who were undergoing major peripheral vascular surgery at very high risk of postoperative complications suggests that there are benefits in using
the MMM strategy, in terms of a reduction in 30-day mortality (to only 0.8%) and in amputation rate (to 2% at 1 year), as well as a reduced requirement for postoperative care in the HDU/ICU (to only 8%) .
Future randomised controlled trials using flow monitoring should consider using an MMM strategy. SV maximisation and GDT to a population-based target of 600 ml/m2 body surface area alongside liberal fluids should be abandoned [6, 47, 48].
It should become routine practice to use an MMM strategy to try to maintain perioperative DO2, in order to minimise build-up of oxygen debt, as a key strategy for ensuring successful outcomes in high-risk surgical patients. With the monitoring technology currently available, this is now achievable. As visualised in Figure 2, the reduction in oxygen debt can be achieved using an MMM approach where flow (e.g. LiDCO RapidTM), depth of anaesthesia (e.g. BISTM, Medtronic, Narcotrend®, MT MonitorTechnik GmbH & Co. KG or NeuroSENSE, NeuroWave Inc) and cerebral oximetry (e.g. INVOS, Medtronic) monitors are used alongside conventional intraoperative monitoring. The recent arrival of finger-based, non-invasive, continuous blood-pressure monitoring , with provision for converting the waveform into flow, means that all high-risk patients could benefit from this strategy in the future.
MMM has advantages for the anaesthetist, the patient and the healthcare system. It pinpoints very clearly the physiological changes associated with anaesthesia and surgery, from pre-induction to post-anaesthesia care units. It allows intervention strategies to be more focused and physiologically appropriate, and enables a more rational approach to intraoperative haemodynamic management. As we gain greater insight into intraoperative physiological change in our patients we can individualise management using strategies designed to centre on minimising the build-up of oxygen debt, and this should therefore improve outcomes. The burgeoning of the elderly, high-risk patient population mandates a new perioperative anaesthetic management strategy. MMM has the potential to reduce the requirement for expensive HDU and ICU facilities, decrease hospital length of stay and therefore reduce the cost of perioperative care by reducing patient morbidity and mortality.
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