In the perioperative and critical care setting, ensuring adequate tissue oxygenation is imperative for cardiovascular management as well as routine measurement of other important variables, including stroke volume, heart rate and cardiac output . Adverse changes in the level of oxygenation can ultimately lead to organ dysfunction and poor patient outcomes . Non-invasive technologies for monitoring tissue oxygenation and providing clinicians with a prior warning on hypoxaemia include pulse oximetry and tissue and cerebral near-infrared spectroscopy (NIRS).
Pulse oximetry, for monitoring tissue oxygenation in the critical care setting, uses spectrophotometric methodology. Changes in light absorption of oxygenated and deoxygenated blood are measured at two light wavelengths, and the ratio of absorbance at these wavelengths is determined and calibrated against direct measurements of arterial oxygen saturation. Overall, studies show that the accuracy of pulse oximetry varies according to arterial oxygen saturation levels and, depending on the probe type, the response time also varies. Other limitations include inaccurate readings as a result of intravenous dyes that have been used for diagnostic purposes, as well as nail polish on fingernails of patients, which can interfere with readings. Despite these limitations, pulse oximetry can provide early warning of hypoxaemia as demonstrated in the Moller et al. study. In this study the use of an oximeter was associated with a decrease in the rate of myocardial ischaemia, however, overall, a reduction in the rate of postoperative complications was not observed [3, 4]. There is, generally, a paucity of data demonstrating improved outcomes for patients when pulse oximetry is used during surgical procedures . In part, the lack of data is a result of the impracticality of measuring the effectiveness of pulse oximetry to reduce rare events [3, 4]. However, anaesthesiologists continue to use pulse oximeters because they believe they are helpful in avoiding irreversible organ damage .
NIRS relies on optical methodology, for measuring the amount of light returning to the spectrometer after it has been directed at the muscle tissue bed. The returning light gives an indication of skeletal muscle tissue oxygen saturation of haemoglobin (StO2), and can be used to measure perfusion in a pulseless or hypothermic patient . NIRS can be used to monitor somatic regional oxygenation as well as cerebral oxygen saturation (ScO2) and act as a surrogate indicator of circulatory system functioning during cardiac surgery. However, the use of oximetry in somatic monitoring is complicated, owing to the predominance of myoglobin in muscle tissues, while oximeters only measure changes in oxygen saturation of haemoglobin. Using the brain as the index organ to monitor changes in ScO2 can potentially indicate any deterioration of global venous oxygen saturation, although this has yet to be validated [6–8]. Current evidence suggests that reductions in ScO2 during cardiac surgery may identify cardiopulmonary bypass cannula malposition. Low level evidence links low ScO2 during cardiac surgery with postoperative neurologic complications. There are insufficient data to support the notion that interventions to improve low ScO2 prevent stroke or postoperative cognitive dysfunction . Finally, the level of evidence supporting the use of NIRS in cerebral oximetry during non-cardiac surgery and the impact of its use on patient outcomes has been unclear, and whether ScO2 is a reliable surrogate of circulatory blood flow currently remains to be determined. Studies are now looking to answer this question, although a major limitation in this field is the lack of a valid methodology to quantify cerebral autoregulation. Regardless, NIRS has potential to be a valuable tool in monitoring patients undergoing non-cardiac surgery [10–13].
Whilst NIRS-based oximetry is a valuable monitoring technique, it cannot differentiate between arterial and venous blood, so provides no information on oxygen delivery. However, NIRS does have the potential to provide clinicians with an early warning of a decrease in cerebral oxygen supply. This early warning is an indication that corrective interventions are needed and when acted upon have the potential to improve postoperative outcomes. Other than tissue oxygenation, NIRS has recently been used in brain function analysis, to aid in differential diagnosis of depressive symptoms . NIRS has also been used to determine levels of biocomponents in body fluids to help in diagnosis, prognosis and management of patients in several therapeutic areas. Overall, it is clearly plausible that NIRS applications will become widespread, and the technique could become a common tool in healthcare practices. However, issues still remain, such as the lack of direct evidence of improvement in clinical outcomes with improved cerebral ScO2 as well as results being demonstrated in larger clinical settings and populations [14, 15].
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