Continuous measurement of cerebral oxygenation by near infrared spectroscopy during induction of anesthesia.

UNLABELLED: Near infrared spectroscopy (NIRS) measures tissue oxygenation continuously at the bedside. Major disturbances of cerebral oxygenation can be detected by using NIRS, but the ability to observe smaller changes is poorly documented. Although anesthetics generally depress cerebral metabolism and enhance oxygen delivery, the administration of etomidate has been associated with cerebral desaturation. We used this difference to study the ability of NIRS to detect the small changes associated with the onset of anesthesia. Thirty-six healthy patients were randomly allocated to have anesthesia induced with either etomidate, propofol, or thiopental. We found that there was a temporal association between the onset of anesthesia and NIRS-derived indices of cerebral oxygenation. Etomidate was associated with a decrease in cerebral oxygenation, whereas propofol and thiopental were associated with an increase in cerebral oxygenation. We conclude that NIRS is capable of detecting the small changes in cerebral oxygenation associated with the induction of general anesthesia and shows promise as a bedside investigational tool for the noninvasive assessment of cerebral oxygenation. IMPLICATIONS: We conclude that near infrared spectroscopy is capable of detecting the small changes in cerebral oxygenation associated with the induction of general anesthesia and shows promise as a bedside investigational tool for the noninvasive assessment of cerebral oxygenation.

The effect of scalp ischaemia on measurement of cerebral blood volume by near-infrared spectroscopy.

Near-infrared spectroscopy (NIRS) is a noninvasive method of quantifying changes in cerebral haemodynamics from changes in the absorption of near-infrared light by oxyhaemoglobin and deoxyhaemoglobin. Measurement of neonatal cerebral blood volume (CBV) by NIRS was described in 1990 but it has been suggested that, in adults, scalp and skull blood content contribute a significant amount to the cerebral haemodynamic variables quantifiable by NIRS. To investigate this, CBV was measured in nine adult subjects, in the frontal region of the head, before and after inflating a pneumatic tourniquet proximal to the measurement site. Because a change in scalp blood content could potentially alter the pathlength of light passing through the head and hence affect the measured CBV, the optical pathlength factor was therefore also measured before and after tourniquet inflation. Blood flow occlusion was confirmed by laser Doppler velocimetry. The results showed that tourniquet inflation had no effect on the estimated value of CBV or the differential pathlength factor. We conclude that, provided the distance between light entry and exit on the surface of the scalp is sufficiently large, changes in scalp blood flow have no effect on NIRS measurement of cerebral haemodynamics.

Influence of respiration and changes in expiratory pressure on cerebral haemoglobin concentration measured by near infrared spectroscopy.

Near infrared spectroscopy (NIRS) was used to measure the changes in concentration of cerebral oxy- and deoxygenated haemoglobin ([HbO2] and [Hb]) in six healthy adult volunteers spontaneously breathing against increased expiratory pressures (IEPs) between 0 and 20 cm H2O. During expiration, an increase in [HbO2] was recorded, accompanied by a smaller decrease in [Hb], producing a small increase in total cerebral haemoglobin concentration ([Hbsum]). The mean plus/minus SD change in [Hbsum] at the maximum 1EP of 20 cm H2O was 1.2 +/- 0.7 micromol L-1 (equivalent to 1.4%). Changes in [Hbsum] correlated with IEP level (r = 0.95) and changes in MABP (r = 0.96). The results suggest that homeostatic mechanism do not maintain cerebral blood volume or flow constant over the period of a single breath in normal adults.

Use of near infrared spectroscopy to estimate cerebral blood flow in conscious and anaesthetized adult subjects.

Near infrared spectroscopy (NIRS) can be used to quantify cerebral haemodynamic states non-invasively and to estimate cerebral blood flow (CBF). In the first part of this study we have compared CBF measurements in conscious and anaesthetized subjects. In the second part we have compared paired measurements made during anaesthesia, first on the scalp and then the dura after craniotomy. Mean CBF was 17 (SD 7) ml 100 g-1 min-1 in the conscious subjects compared with 21 (8) ml 100 g-1 min-1 on the scalp during anaesthesia (P > 0.1). Mean CBF on the dura was 68 (21) ml 100 g-1 min-1 (P < 0.0001). Computer modelling suggests that the difference in magnitude between scalp and dura measurements of CBF is likely to be caused by the optical effect of extracerebral tissue which powerfully scatters light passing through it but does not contribute significantly to the measured CBF because it has only a small blood content itself. The results lend support to this method of estimating CBF although formal validation by comparison with an established technique is needed.

Brain-metabolite transverse relaxation times in magnetic resonance spectroscopy increase as adenosine triphosphate depletes during secondary energy failure following acute hypoxia-ischaemia in the newborn piglet.

The adenosine triphosphate (ATP)-dependent sodium/potassium pump extrudes intracellular sodium in exchange for extracellular potassium. Low ATP causes pump dysfunction increasing both intracellular sodium and water thereby enhancing metabolite mobility. This should be detectable by proton magnetic resonance spectroscopy (MRS) as increased metabolite transverse relaxation times (T2s). During secondary cerebral energy failure in the newborn piglet, proton and phosphorus MRS showed large increases in the T2s of choline, creatine, N-acetylaspartate, and lactate that correlated with ATP depletion. These results provide insight into factors affecting metabolite T2s and show that T2s may be useful for studying cellular oedema.

Delayed ("secondary") cerebral energy failure after acute hypoxia-ischemia in the newborn piglet: continuous 48-hour studies by phosphorus magnetic resonance spectroscopy.

Phosphorous (31P) spectra from the brains of severely birth-asphyxiated human infants are commonly normal on the first day of life. Later, cerebral energy failure develops, which carries a serious prognosis. The main purpose of this study was to test the hypothesis that this delayed ("secondary") energy failure could be reproduced in the newborn piglet after a severe acute reversed cerebral hypoxic-ischemic insult. Twelve piglets were subjected to temporary occlusion of the common carotid arteries and hypoxemia [mean arterial PO2 3.1 (SD 0.6) kPa]. Mean cerebral phosphocreatine concentration [PCr]/inorganic orthophosphate concentration [Pi] decreased from 1.40 (SD 0.29) to 0.01 (SD 0.02), and nucleotide triphosphate concentration [NTP]/exchangeable phosphate pool concentration [EPP] decreased from 0.19 (SD 0.02) to 0.06 (SD 0.04) (p < 0.001 for each decrease). On reperfusion and reoxygenation of the brain, mean [PCr]/[Pi] and [NTP]/[EPP] returned to baseline. Observations continuing for the next 48 h showed that [PCr]/[Pi] again decreased, in spite of normal arterial PO2, mean arterial blood pressure, and blood glucose, to 0.62 (SD 0.61) at 24 h (p < 0.01) and 0.49 (SD 0.37) at 48 h (p < 0.001). [NTP]/[EPP] also decreased, but to a lesser degree. Intracellular pH remained unchanged. These findings appeared identical with those seen in birth-asphyxiated human infants. No changes in cerebral metabolite concentrations took place in six control piglets. The severity of secondary energy failure, as judged by the lowest [PCr]/[Pi] recorded at 24-48 h, was directly related to the extent of acute energy depletion, obtained as the time integral of reduction in [NTP]/[EPP] (p < 0.0001). This animal model of secondary energy failure may prove useful for testing cerebroprotective strategies.

Measurement of adult cerebral haemodynamics using near infrared spectroscopy.

Near infrared spectroscopy (NIRS) is a non invasive, portable, safe technique for monitoring cerebral oxygenation and haemodynamics. Since it does not involve the use of ionising radiation it may be used repeatedly to produce serial measurements of CBF and CBV in patients, and continuously to provide trend data about cerebral circulation changes. NIRS allows measurements to be made at the bedside with minimal disturbance to other monitoring and treatment procedures. Although regional information is not yet available, good time resolution allows rapid changes in cerebral haemodynamics to be observed.