A prototype device for standardized calibration of pulse oximeters

OBJECTIVE

To develop and test a method for standardized calibration of pulse oximeters.

METHODS

A novel pulse oximeter calibration technique capable of simulating the behavior of real patients is discussed. It is based on an artificial finger with a variable spectral-resolved light attenuator in conjunction with an extensive clinical database of time-resolved optical transmission spectra of patients fingers in the wavelength range 600-1000 nm. The arterial oxygen saturation of the patients at the time of recording was derived by analyzing a corresponding blood sample with a CO-oximeter. These spectra are used to compute the modulation of the light attenuator which is attached to the artificial finger. This calibration method was tested by arbitrarily playing back recorded spectra to pulse oximeters and comparing their display to the value they displayed when the spectra were recorded.

RESULTS

We were able to demonstrate that the calibrator could generate physiological signals which are accepted by a pulse oximeter. We also present some experience of playing back recorded patient spectra. The mean difference between the original reading of the pulse oximeters and the display when attached to the calibrator is 1.2 saturation points (displayed oxygen saturation SpO2) with a standard deviation of 1.9 saturation points.

CONCLUSIONS

The tests have shown the capabilities of a spectral light modulator for use as a possible calibration standard for pulse oximeters. If some improvements of the current prototype can be achieved we conclude from the experience with the device that this novel concept for the calibration of pulse oximeters is feasible and that it could become an important tool for assessing the performance of pulse oximeters.

Accuracy of point-of-care-testing (POCT) for determining hemoglobin concentrations

BACKGROUND

While point-of-care testing (POCT) is being used increasingly as a basis for deciding on perioperative erythrocyte transfusion, no valid standards currently exist concerning the accuracy of Hb concentration measurements. For clinical employment, however, the confidence limits (+/-2 SD) of these measurements should lie close to 5 g/l. The aim of the present study was to evaluate the accuracy and precision of point-of-care testing for blood hemoglobin concentration (cHb in g/l) measurements in critically ill patients.

METHODS

Fifty blood samples from 50 postoperative patients requiring intensive care treatment were withdrawn from a cannula in the radial artery into a 2-ml heparinized syringe (containing wet sodium heparinate in the conus), in a 2-ml Monovette with 50 IE lithium heparinate, and into a 2.7-ml cuvette with 1.6 mg potassium EDTA/ml blood. The POCT battery consisted of two blood gas analyzers (ABLTM 625 and 725, Radiometer, Copenhagen), the HemoCue system (Mallinckrodt Medical, Germany), and an automated hematology analyzer (M-2000(R), Sysmex, Germany). The cyanmethemoglobin method served as the reference 'gold standard' procedure. The blood gas analyzer and HemoCue systems were tested using dry and wet heparinized blood samples.

RESULTS

Hemoglobin concentrations of the reference measurements ranged from 73.9 to 159.4 g/l. The automated hematology analyzer method did reveal a small but systematic deviation for higher cHb values. For the blood gas analyzer and HemoCue system procedures there was no systematic deviation of bias for either the first measurement or the averaged data. Bland & Altman analysis revealed a larger scattering for the wet heparinized samples.

CONCLUSIONS

The above-stated requirement for POCT systems, i.e. that the confidence limits should lie close to 5 g/l cHb, held true for the dry heparinized samples of the blood gas analyzer (1st measurement and mean of 2), the HemoCue system (mean of 3) and the automated hematology analyzer.

Detecting and quantifying absorbed irrigation fluid by measuring mannitol and sorbitol concentrations in serum samples, and by ethanol monitoring

OBJECTIVE

To describe a modified and improved technique which, in one measurement, estimates the influx of irrigation fluid during endoscopic endometrial ablation or prostate resection, and provides both rapid confirmation of the diagnosis and an estimate of the amount of fluid absorbed by detecting markers which pass from the irrigation fluid to the serum, i.e. mannitol or sorbitol.

PATIENTS AND METHODS

Control samples were taken for analysis before irrigation, and test samples were taken on four occasions during and after intervention, from each of 10 patients undergoing transurethral resection of the prostate. Irrigation fluid was also marked with ethanol (1.5% w/v) and the concentration of this agent measured in the blood and expired air of these patients. The absorbed volume was calculated according to the extracellular distribution space of mannitol.

RESULTS

Mannitol and sorbitol could be measured in 85% and 73% of the 40 test samples, respectively. The threshold for full sensitivity for breath ethanol concentration to detect absorption was 132 mL.

CONCLUSION

This method for detecting serum mannitol and sorbitol represents a valid procedure for confirming and quantifying the absorption of irrigation fluid in the clinic, which agrees closely with the already established ethanol monitoring procedure and which should now be considered as a reference procedure.

Follow-up of spongiosa plugs with contrast enhanced magnetic resonance imaging

The aim of this study was to evaluate the extent and duration of the revascularization process in spongiosa plugs when a fibrin sealant was used. 20 patients with tumour-like lesions, benign tumours and tumours with a potential for malignant transformation were studied. After intralesional tumour removal, the defect was filled with homologous spongiosa either combined with or without a fibrin sealant, according to a prospective randomization. Magnetic resonance imaging (MRI) follow-up examinations were performed within 1 week, and 1.5, 3, 4.5, 6, 8, 12, 24 and 36 months after surgery. Those patients without a fibrin sealant showed an increased revascularization zone up to the sixth week. Patients treated with fibrin, however, showed increased revascularization up to 3 months. In the 25% percentile the extent as well as the rate of revascularization is higher in those with a fibrin sealant. It is of clinical relevance that no revascularization should be expected at 3 months after surgery, which is easily demonstrated by MR follow-up.