Continuous in vivo venous oxygen saturation is achieved by reflectance oximetry from a fiber-optic catheter in the jugular venous bulb, and improves detection of transient global cerebral ischemic insults which may be missed by periodic in vitro analysis (An.d.r.ew.s.et,.§l, 1991).
Red and near-infrared light is transmitted at 1-ms intervals down a transmitting fiber and reflected along a receiving fiber to a photoelectric sensor. Reflected signals are averaged over 5 s and updated every second. The Oximetrix 3 system displays trended and current oxygen saturation.
Reflected light intensity is also displayed and when normal appears as a bar between two dotted lines. The reflected light intensity should always be examined before interpreting data. High reflected light intensity indicates that the catheter tip is abutting too closely against the vessel wall, while low intensity suggests catheter-tip obstruction which should be rectified by aspiration and flushing ( Fig, 1). Inability to display a light intensity signal larger than a dot between the upper and lower dotted lines may signify damage to the fiber-optics.
Fig. 1 Displays of SjO2 using the Oximetrix system. (a) Acceptable SjO2 recording: upper trace, SjO2 between 75 and 80 per cent showing characteristic fluctuations; lower trace, vertical bars between two dotted lines indicate adequate light intensity. (b) Unacceptable SjO2 recording: the upper trace is reasonable but the lower trace shows the light intensity below the lower dotted lines, indicating catheter obstruction which requires either flushing or catheter replacement. (c) A period of acceptable recordings is followed by a sudden increase in the SjO2 trace accompanied by a rise in the light intensity display to above the upper dotted lines. This was due to the patient being turned, resulting in a vessel wall artifact. Subsequent repositioning of the patient restored accuracy as confirmed by an in vivo calibration (I, C).
A technique for inserting the Oximetrix 3 jugular venous catheter is described in Table 1. The system requires calibration before insertion to confirm its integrity and compares the reference signal with a standard optical reference. A.n.d..r.®W..S,et.a.l (1.9.91) found that use of the preinsertion calibration for jugular oximetry led to over-reading by 7 per cent. Therefore it was necessary to perfom an in vivo calibration against a laboratory calibrated oximeter and to repeat this every 8 to 12 h to ensure accurate measurements.
Table 1 Technique for inserting the Oximetrix SjO2 catheter
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