In vitro oxyhaemoglobin saturation measurements in haemoglobin solutions using fibreoptic pulmonary artery catheters

We compared in vitro oxyhaemoglobin saturations using two pulmonaryartery catheters (catheter So₂) with oxyhaemoglobin saturations(So₂) measured by the IL282 co-oximeter and derived partialoxyhaemoglobin saturations (partial So₂) at different oxygentensions (Po₂) in six solutions: whole blood, 50:50 mixtureof whole blood and Plasmalyte A (haemodiluted blood), 50:50mixture of whole blood and 8% pyridoxylated haemoglobin-polyoxyethylene(PHP) conjugate (WB-PHP), 75:25 mixture of 8% PHP and PlasmalyteA solution (PHP66), 50:50 mixture of 8% PHP and Plasmalyte Asolution (PHP44) and stroma-free haemoglobin solution (SFH).Calculated P₅₀ values (Po₂, vs So₂) were 3.79, 3.58, 3.49, 3.15,3.04 and 2.07 kPa, respectively. However, if partial So₂ wasused the curves were shifted to the left, reducing P₅₀. CatheterSo₂ correlated well with So in whole blood (r² > 0.99 forboth catheters), haemodiluted blood (r² > 0.98 for both catheters)and WB-PHP solution (r² = 0.94 for both catheters). In PH P44(r² = 0.64 and r² = 0.57), PHP P66 (r² = 0.40 for the Oximetrixand r² = 0.25 for the Edwards catheter) and SFH solutions (r²= 0.33 for the Oximetrix and r² = 0.22 for the Edwards catheter)both catheters performed poorly. We conclude that mixed venousoxyhaemoglobin saturations measured by oximetric pulmonary arterycatheters are inaccurate in the presence of haemoglobin solutions.For accuracy a multi-wavelength co-oximeter should be used ifblood containing PHP or SFH is to be analysed. Present address: Department of Anaesthetics, Bristol Royal Infirmary,Bristol, UK