The Journal of ExtraCorporeal Technology Issue 55-4 | Page 22

172 T . Matsumoto et al .: J Extra Corpor Technol 2023 , 55 , 167 – 174

Table 3 . Simultaneous reproducibility with test liquids 8 – 10 in Experiment 2 .

Measurements were performed 10 times for each test liquid and each measurement method , and CVs (%) were calculated . With COP measurement by the Test Method , all CVs were in the range of 1.67 ± 0.51 %. With measurement using the colloid osmometer , all CVs were in the range of 0.90 % ± 0.04 %. Abbreviations : COP , colloid osmometer ; SD , standard deviation ; CV , coefficient of variation ; Hct , hematocrit value .

of 0.99 to be reasonable . Hence , only the measurement error is affecting the correlation . The difference in measurements increases as COP increases ; therefore , systematic error may be involved . However , we have not investigated the cause of this error .

Our test method measures the hydrostatic pressure difference at the equilibrium point . The inner diameter of the tube used to measure differences in the fluid level does not affect the hydrostatic pressure . However , if air is mixed into the measurement circuit , there is a possibility that the error will increase . To ensure that the mixed air can be easily removed , a moderately-sized circuit diameter is preferable .

Figure 5 . Correlation between COP measurements from the colloid osmometer and by the test method in Experiment 2 . n = 30 ; Pearson ’ s correlation : p < 0.01 ; R 2 = 0.997 . COP , colloid osmotic pressure . y = 0.5351251 + 0.9813299 x .

However , the test method was not affected by this performance degradation . This may have been due to the fact that the COP measurement was performed after circulating blood for 5 min without filtration in preparation for taking a measurement . This setting was originally devised to avoid measuring COP with concentrated blood immediately after filtration ; however , this may have had a cleansing effect by washing out concentration polarization , etc . [ 20 ]. Nevertheless , the extent of this effect is unknown . In Experiment 1 , once the test liquid was prepared , COP was measured continuously 70 times with a single ultrafiltration membrane . Consequently , the ultrafiltration time with this ultrafiltration membrane lasted over 4 h . A standard cardiopulmonary bypass takes roughly 3 h ; longer use of the ultrafiltration membrane did not result in any degradation of the membrane or any other disadvantages that could affect COP measurement . This result implies that measurement remains accurate over long-term use ; however , further research is necessary to prove this assumption .

Correlation with colloid osmometer

The correlation between COP values as measured by the test method and the colloid osmometer was quite high ( R 2 = 0.99 , Figures 4 and 5 ). Since the test method and the colloid osmometer operate according to a similar principle and the measurement error is small , we consider this correlation coefficient

Measurement of time

Measurements should be easy to conduct and obtain during cardiopulmonary bypass procedures . Our test method does not require zero-point calibration for the colloid osmometer . Complex calculations , such as albumin concentration , are also unnecessary and no sampling is required to conduct the measurement . Hence , considerable time and effort can be saved .

In Experiments 1 and 2 , the time required to complete the solvent transfer and reach equilibrium increased with increasing COP . With the colloid osmometer , the time until the results were displayed also increased . This likely results from an increase in the amount of solvent transfer accompanying an increase in COP .

One limitation of our test method is that the amount of time required for solvent transfer to reach equilibrium is unknown . With no apparent signs to confirm the equilibrium point , care must be taken not to measure during solvent transfer . For that reason , we recommend setting a standby time of approximately three minutes . The maximum amount of time required for measurements using a colloid osmometer is four minutes , which does not differ much from the time required for our method .

The effects of measurement time in real-world clinical practice should be considered . Before measurement , blood must be circulated with ultrafiltration stopped , and the concentrated blood must be washed out of the circuit . This exchange of blood takes several minutes ; combined with 3 min of standby time , a roughly 5-min period occurs during which ultrafiltration cannot be performed . An inability to perform ultrafiltration for 5 min in cardiopulmonary bypass procedures is unlikely to yield any disadvantages .