Reflection spectrophotometry (OxySat-2®) for monitoring oxygen saturation and hematocrit during extracorporeal circulation

Abstract

Continuous measurement of sO₂ and hematocrit during extracorporeal circulation has become feasible using reflection spectrophotometry. We have compared simultaneous measurements with in-line reflection spectrophotometry (OxySat-2® American Bent-ley), and in-vitro transmission spectrophotometry (OSM3 Hemoximeter® Radiometer) on discrete samples.

In 8 patients undergoing open heart surgery during hypothermic extracorporeal circulation with hemodilution arterial and venous sO₂ and hematocrit were measured continuous in-line in the extracorporeal circuit. Simultaneous and anaerobically collected blood samples were analyzed within one minute in the OSM3. The relationship between hemoglobin concentration (cHb) measured in the OSM3 and hematocrit (Hct) measured by the OxySat-2 was estimated from a mean cell hemoglobin concentration of 20 mmol·l^-1, i.e. Hct (%) = 100 cHb/20.

There was a systematic difference between the OxySat-2 and the reference method for sO 2</sb and cHb, the mean difference being: −2.4 s.d.2.8 and-2.1 s.d. 1.7. Linear regression analysis shows the following equations : OxySat-2 = Y, OSM3 = X:

sO 2</sb <%): Y= 1.06 · X − 7.6 r = 0.97 n = 122

Hct (%): Y = 0.68 · X + 5.5 r= 0.77 n = 122

The results indicates that continuous in-line measurements of sO₂ and cHb with reflection spectrophotometry are reliable and with clinical acceptable precission and accuracy, provided that the concentration of dyshemoglobins, i.e. carboxy- and methemoglo-bin is negligible.

Key-words:

• cardiopulmonary bypass
• continuous monitoring
• hemoglobin
• oxyhemoglobin
• oxyhemoglobin fraction
• volumen fraction of erythrocytes