Clinical techniques

The simplest way to measure SvO2 is by sampling blood through the distal port of a pulmonary artery catheter. Unfortunately, central venous blood sampling does not correlate well with true mixed venous samples (M.┬žd.!D,,eLa/ 1992). This is due to the large differences in the supply-to-demand ratios of the organs draining into the major venous system (e.g. resting skin venous saturation exceeds 80 per cent; heart venous saturation is less than 30 per cent); thus passage through the right heart is required to ensure mixing. The measurement of hemoglobin saturation using a modern multiwavelength co-oximeter is extremely accurate, but the drawing of a mixed venous sample is error prone. Blood needs to be aspirated very slowly as the resistance to flow leads to the generation of high negative pressures which can lower gas tensions (and hence saturation) and predisposes to the introduction of air into the sample. A rapid sampling rate can also lead to drawback of blood which has been exposed to gas-exchanging pulmonary vasculature, giving a falsely high oxygen saturation, as does drawing samples in the wedged position.

There are now a number of commercially available systems which allow continuous monitoring of SvO? via an optical fiber built into the pulmonary artery catheter. Two- and three-wavelength systems are available and, although there is some evidence that the three-wavelength system is more accurate and neither are as accurate as a co-oximeter, both are sufficiently accurate for clinical purposes ( Scuderi,ef a/ 1992). As with all optical methods for measuring hemoglobin saturation, there is a possibility of interference from other optically active compounds (e.g. carboxyhemoglobin, methylene blue, very high bilirubin levels). This is relatively unusual and in practice the major problem with these systems is drift, which may be significant within relatively short periods and is often significant between the preinsertion calibration and the check calibration immediately after insertion. The systems should be recalibrated against a co-oximeter every 4 to 6 h, and should be rechecked before clinical decisions are made primarily on the basis of SvO2. Migration of the catheter to a permanently wedged position (or failure to deflate the balloon) will lead to a sustained rise in the SvO? reading to arterial levels or above.

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