A. T. Lovell

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.

Changes in cerebral blood volume with changes in position in awake and anesthetized subjects

UNLABELLED Changes in posture affect cerebral blood volume (CBV), and moderate head-up tilt is used as a therapeutic maneuver to reduce CBV and intracranial pressure. However, CBV is rarely measured in the clinical setting. Near-infrared spectroscopy allows real-time bedside monitoring of cerebral hemodynamics, and we have used this technique to measure changes in CBV with changes in posture in 10 normal subjects and 10 propofol-anesthetized patients. In the awake subjects, changes in CBV were correlated with the degree of table tilt. CBV decreased with 18 degrees head-up tilt and increased with 18 degrees head-down tilt (P < 0.0001, r = -0.924). In anesthetized patients, there were differences between head-up and head-down tilt. In the head-down position, CBV was also correlated with the degree of table tilt (P < 0.001, r = -0.782), whereas there was a clinically insignificant reduction in CBV in the head-up position. Near-infrared spectroscopy allows continuous, real time measurement of changes in CBV at the bedside. IMPLICATIONS Near-infrared spectroscopy, a bedside technique, has been used to measure changes in cerebral blood volume in normal subjects. We have used the same technique in anesthetized patients and have shown that, when a patient is placed in the head up position, the decrease in cerebral blood volume is attenuated, relative to normal subjects.

Predicting Oscillation in Arterial Saturation from Cardiorespiratory Variables

The potential of near infrared spectroscopy (NIRS) as a non invasive tissue oxygenation monitor was first outlined by J6bsis (J6bsis, 1977). Extension of the basic technique to measure tissue blood flow using a Fick technique was developed by Edwards and Reynolds (Edwards et aI., 1988; Edwards et aI., 1993). This uses a rapid change in arterial oxyhaemoglobin concentration to act as an intravascular tracer, avoiding the problem of recirculation of indicator.

Continuous Measurement of Cerebral Oxygenation by Nirs During Induction of Anaesthesia

Continuous intraoperative monitoring of cerebral oxygenation is not routine because existing techniques are either invasive, require a prolonged period of equilibration or involve the use of ionizing radiation. The potential of near infrared spectroscopy (NIRS) as a non invasive tissue oxygenation monitor was first outlined by Jobsis (Jobsis, 1977). NIRS enables the continuous measurement of oxyhaemoglobin (HbO2) and deoxyhaemoglobin (Hb). To date most of the studies that have used NIRS during anaesthesia have considered either the effects on cerebral dynamics of extreme manoeuvres, such as clamping of a carotid artery and induction of ventricular fibrillation (Kirkpatrick et al., 1995; Mason et al., 1994; Williams et al., 1995; Levy et al., 1995), or have been confined to measuring cerebral blood flow (CBF) or cerebral blood volume (CBV) (Owen-Reece et al., 1994; Owen-Reece et al., 1996).