A blood viscosity estimation method based on pressure-flow characteristics of an oxygenator during cardiopulmonary bypass and its clinical application

Plasmapheresis for hypertriglyceridemia: The association between blood viscosity and triglyceride clearance rate

OBJECTIVES

Several factors in double filtration plasmapheresis (DFPP) were associated with triglyceride (TG) clearance rate. This study examines whether baseline whole blood viscosity was a predictor for efficient TG removal.

METHODS

Adult subjects who receiving DFPP for hyperlipidemia in Taoyuan General Hospital from January 2015 to March 2018 were classified into efficient and inefficient TG removal according to TG removal rate ≥50% vs <50%. TG removal rate was defined as following formula: (pre-apheresis TG- post-apheresis TG)/pre-apheresis TG. Whole blood viscosity (WBV) was estimated by following equation: WBV = 0.12 × hematocrit +0.17 × (total protein -2.07). Univariate linear regression was used to assess the association between TG removal rate and WBV. Odds ratios (ORs) and 95% confidence interval (95%CI) for associations between variables and efficient TG removal were evaluated by logistic regression model to including univariate and multivariate adjustment.

RESULTS

From a total of 66 subjects receiving DFPP, 37 subjects reached efficient TG removal. The difference in pre-apheresis TG levels, hematocrit, and WBV between efficient vs. inefficient TG removal groups was 4.1 vs 6.7 mmol/L; 43.1% vs 39.5%; and 6.0cP vs 5.cP (Ps <0.05). After multivariate adjustment, WBC was a significant predictor for efficient TG removal (ORs and 95% CI were 3.192 (1.300-7.838), P < 0.05). The correlation between WBV and extraction of TG was significant (r = -0.255, P = 0.039).

CONCLUSION

Hyperviscosity reduced the efficiency of TG removal in those receiving DFPP.

Blood viscosity monitoring during cardiopulmonary bypass based on pressure-flow characteristics of a Newtonian fluid

We proposed a blood viscosity estimation method based on pressure-flow characteristics of oxygenators used during cardiopulmonary bypass (CPB) in a previous study that showed the estimated viscosity to correlate well with the measured viscosity. However, the determination of the parameters included in the method required the use of blood, thereby leading to high cost of calibration. Therefore, in this study we propose a new method to monitor blood viscosity, which approximates the pressure-flow characteristics of blood considered as a non-Newtonian fluid with characteristics of a Newtonian fluid by using the parameters derived from glycerin solution to enable ease of acquisition. Because parameters used in the estimation method are based on fluid types, bovine blood parameters were used to calculate estimated viscosity (ηe), and glycerin parameters were used to estimate deemed viscosity (ηdeem). Three samples of whole bovine blood with different hematocrit levels (21.8%, 31.0%, and 39.8%) were prepared and perfused into the oxygenator. As the temperature changed from 37 °C to 27 °C, the oxygenator mean inlet pressure and outlet pressure were recorded for flows of 2 L/min and 4 L/min, and the viscosity was estimated. The value of deemed viscosity calculated with the glycerin parameters was lower than estimated viscosity calculated with bovine blood parameters by 20-33% at 21.8% hematocrit, 12-27% at 31.0% hematocrit, and 10-15% at 39.8% hematocrit. Furthermore, deemed viscosity was lower than estimated viscosity by 10-30% at 2 L/min and 30-40% at 4 L/min. Nevertheless, estimated and deemed viscosities varied with a similar slope. Therefore, this shows that deemed viscosity achieved using glycerin parameters may be capable of successfully monitoring relative viscosity changes of blood in a perfusing oxygenator.

Continuous Blood Viscosity Monitoring System for Cardiopulmonary Bypass Applications

This paper proposes an algorithm that estimates blood viscosity during cardiopulmonary bypass (CPB) and validates its application in clinical cases. The proposed algorithm involves adjustable parameters based on the oxygenator and fluid types and estimates blood viscosity based on pressure-flow characteristics of the fluid perfusing through the oxygenator. This novel nonlinear model requires four parameters that were derived by in vitro experiments. The results estimated by the proposed method were then compared with a conventional linear model to demonstrate the former's optimal curve fitting. The viscosity (η_e) estimated using the proposed algorithm and the viscosity (η) measured using a viscometer were compared for 20 patients who underwent mildly hypothermic CPB. The developed system was applied to ten patients, and η_e was recorded for comparisons with hematocrit and blood temperature. The residual sum of squares between the two curve fittings confirmed the significant difference, with p < 0.001. η_e and η showed a very strong correlation with R² = 0.9537 and p < 0.001. Regarding the mean coefficient of determination for all cases, the hematocrit and temperature showed weak correlations at 0.33 ± 0.14 and 0.22 ± 0.21, respectively. For CPB measurements of all cases, η_e was more than 98% distributed in the range from 1 to 3 mPa⋅s. This new system for estimating viscosity may be useful for detecting various viscosity-related effects that may occur during CPB.