A blood reservoir must be incorporated in the circuit of a bypass route for extracorporeal gas exchange. The reservoir will serve as a balance indicator of the blood inflow and outflow as well as a buffering system for the volume change in the circuit. If the reservoir bag collapses during a bypass operation, it indicates that the withdrawal of blood from the venous system is not catching up with the outflow of the pump. Several kinds of detectors of the degree of reservoir inflation were devised by using pressure, force-replacement, or capacitance reactive transducers. When a large reservoir for open heart surgery had been used before, the height of the reservoir was controlled to decelerate or accelerate the speed of venous drainage by gravity according to the blood volume in the reservoir. Later the authors began to use a small reservoir of lees than 100ml, or a volume of about one stroke of a pump to prevent hypovolemic shock due to the excess blood shift from the body to the reservoir. Such a small reservoir requires continuous control of the pump to prevent an excess negative pressure in the drainage circuit. When the refill of the reservoir takes a longer time than the preset period, it means a poor venous drainage. If the reservoir is empty, the pump must be automatically stopped, and an alarm triggered. When the reservoir becomes full with blood, the pump resumes the work. Such automatic regulation of the bypass circuit has avoided the necessity of continuous and nerve racking vigilance and improved the safety of prolonged ECLA.
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