Cardiotomy suction is known for its deleterious effects on formed and unformed blood elements. The authors investigated an "intelligent" remote controlled automatic suction system. A suction cannula with an optic sensor at its tip was connected to a special closed cardiotomy reservoir. Contact with blood immediately generated a reservoir vacuum from 0 to -100 mmHg, permitting aspiration until the blood was no longer detected (automatic shut off). Blood trauma was evaluated in a bovine model, comparing the automatic suction system vs standard continuous aspiration (control) adjusted to -100 mmHg. After full systemic heparinization, five calves (weight, 62.5 +/- 4.4 kg) for the automatic suction system group, and four (weight, 62.8 +/- 5.1 kg) for the control group, were equipped with a jugular cannula connected via a roller pump to the cardiotomy reservoir. Through a small thoracotomy, a standardized hole was created in the right atrium, allowing for a blood loss of approximately 400 ml/min. The suction cannula was placed into the chest cavity in a fixed position. Blood samples were drawn at regular intervals for cell count and chemistry. Lactate dehydrogenase values, for the automatic suction system and the control groups, respectively, expressed as percent of baseline value, were 88 +/- 14 vs 116 +/- 22 after 1 hr; 94 +/- 16 vs 123 +/- 23 after 2 hr; and 97 +/- 19 vs 140 +/- 48 after 3 hr (p < 0.05). Values for free hemoglobin in plasma (percent of baseline value), for the automatic suction system and the control groups, respectively, were 102 +/- 18 vs 200 +/- 69 after 1 hr; 98 +/- 29 vs 163 +/- 37 after 2 hr; and 94 +/- 37 vs 179 +/- 42 after 3 hr (p < 0.05). Compared with a standard continuous aspiration system, in situ regulation of suction significantly reduces blood trauma.