We designed and examined venous reservoir level automatically controlling systems for an open-fashion cardiopulmonary bypass(CPB)circuit(an open circuit level controller; OLC)and a closed-fashion CPB circuit(closed circuit level controller; CLC). The OLC system consists of a CPU(central processing unit)and a slender column inserted through the top of the venous reservoir for measuring hydrostatic pressure. The CPU analyzes the hydrostatic pressure in the column as a replacement for the venous blood level and regulates the blood pump speed in proportion to the change of the hydrostatic pressure. The CLC system consists of a commercially available level sensor, which is attached in the side wall of the venous reservoir for directly measuring the venous blood level, and a CPU regulating the blood pump speed for keeping the appropriate venous blood level. Using a mock circulation model, we confirmed the both systems could maintain an appropriate blood level in the venous reservoir and regulate blood pump speed, leading to guarantee easy and safe management of the cardiopulmonary bypass.
In this cardiopulmonary bypass simulation experiment, the venous blood reservoir level was classified into high(1,000mL), medium(500mL), and low(200mL), and its influences on operators' mental workload were examined, involving 6 clinical engineering students and using physiological, behavioral, and subjective indices. No significant changes were observed in the coefficient of variation of R-R intervals, mean heart rate, or salivary amylase as physiological indices, or reaction time as a behavioral index. On the other hand, the mean weighted workload scores of the Japanese-version NASA-TLX as a subjective index significantly increased with a decrease in the reservoir level. Similarly, the mental and overall fatigue scores of the Brief Fatigue Inventory significantly increased with such a decrease. Based on these results, decreased reservoir levels may be associated with an increase in the mental workload, as well as mental and general fatigue levels, and, therefore, it may be necessary to consider operators' mental workload when setting the reservoir level.