During cardiac surgery with cardiopulmonary bypass(CPB), blood viscosity changes as blood temperature and hematocrit values change. Because blood viscosity has a high influence on hemodynamics in surgical patients, it can be a useful parameter during CPB. To measure blood viscosity, a rotational viscometer is commonly used. When using rotational viscometer, blood sampling is required and measuring blood viscosity during CPB in real time is difficult. Thus, blood viscosity is not routinely monitored. In this study, we proposed a system for measuring blood viscosity during CPB in real time and evaluated its performance.We constructed the system by utilizing an extracorporeal ultrafiltration circuit, in which a hemoconcentrator was incorporated. We applied the Hagen-Poiseuille equation to a fluid flowing through the circuit and programmed it to calculate the viscosity of the fluid using the flow rate and the pressure difference between the two sides of the hemoconcentrator.We measured the viscosities of Newtonian glycerin solution, non-Newtonian xanthan gum solution, and bovine blood using this system and a commercially available rotational viscometer. In both measurement systems, we found that the viscosity was approximately constant in the glycerin solution, whereas it decreased with increasing shear rate in the xanthan gum solution and bovine blood. Error rates in both systems were≤±10%. The flow in the hemoconcentrator is laminar and obeys the Hagen-Poiseuille law at a flow rate of 100-500mL/min; therefore, we could efficiently calculate the viscosity using flow rate and pressure difference.The change in blood viscosity during CPB has an influence on systemic vascular resistance and CPB circuit internal pressure. This method will enable us to not only measure blood viscosity during CPB in real time but also accurately assess systemic perfusion, by considering peripheral circulation in patients and blood viscosity in a CPB circuit.
In this study, we examined the causes and coping methods of microbubble generation during cardiopulmonary bypass(CPB)using a basic experiment and by evaluation of clinical data, and discuss the prevention of microbubble generation. In the basic experiment, mock circuit was established by using CPB components of clinical grade, and filled with bovine blood. After a regulated state of the blood had been accomplished, we mixed air using a roller pump into the blood removal circuit. Microbubbles were measured using a CMD20 microbubble counter at the inlet and outlet of the CPB components. We found that microbubbles were released regardless of the venous reservoir level for all venous blood reservoirs. Moreover, the oxygenator released microbubbles regardless of the priming volume, and the removal efficiency of the microbubbles positively correlated with pressure loss. Arrangement of the arterial filter downstream of the oxygenator resulted in a reduction of microbubbles mixed into the arterial circuit. The microbubble removal rate with the oxygenator decreased with the increase of mixed air. On the basis of these results, we next evaluated our clinical data. As with the basic experiment, microbubbles mixed into the arterial circuit were found to be increased at the start of CPB during various procedures such as right atriotomy and heart retraction. Microbubbles can enter the blood flow easily owing to their low buoyancy, and the dissolution speed is extended in the blood. Consequently, the microbubbles, which are known to cause gaseous microemboli, may influence the clinical outcomes. Therefore, during surgical procedures, specific care must be taken to prevent air from getting into the blood removal circuit of CPB.
The dilution of excessive amounts of blood under alkalemic conditions causes blood to accumulate in the early stages following a cardiopulmonary bypass(CPB)or in recirculation circuits after modified ultrafiltration(MUF). We investigated changes in the shape(echinocytes)of red blood cells(RBC)in alkaline tris hydroxymethyl aminomethane(THMA)buffer at pH 7.5, 8.0, 8.5, and 9.0 using inverted microscopy. We then examined the ability of THMA buffer containing 0.5% or 5.0%(final concentrations)colloidal albumin and starch at pH 7.5 and 9.0 to protect against changes in the shape of RBC. The rates at which echinocytes appeared were significantly reduced from 13.9% in THMA buffer at pH 9.0 to 3.5% and 1.2% in THMA buffer containing 0.5% and 5.0% albumin, respectively(p<0.01 for both), and to 7.6% and 2.1% in that containing 0.5% and 5.0% starch(p<0.05 and p<0.01), respectively. These findings suggest that adding albumin and artificial colloid to CPB filling liquid might protect against blood accumulation caused by echinocytes.