Emboli generation by the Medtronic Maxima hard shell adult venous reservoir in cardiopulmonary bypass circuits: a preliminary report

Abstract

Increases in right common carotid artery Doppler ultrasound signals typical of emboli were found in cardiopulmonary bypass patients when the Medtronic Maxima hard-shell adult combined venous and cardiotomy reservoir was operated at reservoir blood volumes near the manufacturer;s recommended minimum of 300 ml. The signals were reduced by increasing the reservoir blood volume. Possible microembolus generation in the top- and bottom-entry versions of this reservoir was investigated using an in vitro circuit and a colour flow Doppler monitor that was interfaced with a microprocessor to count the signals. The reservoir blood volume was progressively lowered in 100 ml increments below 1000 ml, and signals were counted over five minutes at each new level. Signal counts downstream of the reservoir increased exponentially after the volume was decreased below 1000 ml in the bottom-entry version, and 700 ml in the top-entry version. Ultrasonic monitor ing, both upstream and downstream of the reservoir, showed that the source of these signals was the reservoir itself, and that recirculation of emboli around the circuit accounted for only a small proportion of the measured increase. Changes in circuit blood prime haematocrit within the range 0.11-0.31 did not alter the signal counts. However, counts were profoundly affected by exposure of the circuit to nitrous oxide via a membrane oxygena tor ; this showed that the emboli were bubbles. Bubble formation may occur where venous blood enters the reservoir as this results in a fountaining effect when the reservoir volume is low. This effect appears to have been serendipi tously reduced, but not eliminated, in the design of the top-entry version. It is