Abstract
There are two sensing systems in ventilators, namely a flow trigger system and a pressure trigger system to synchronize the spontaneous breathing of a patient. A flow trigger system requires constant oxygen “bias flow” and therefore assumed to spend more oxygen and to be less cost effective than a pressure trigger system. In order to compare the running cost of these two types, we measured oxygen expenditure of eight ventilators.
Each ventilator was connected to a test lung and set in a synchronized intermittent positive pressure (SIMV) mode. Oxygen expenditure was measured by a digital addition flow meter (Air-Water Co. Japan) after 30 minutes of mechanical ventilation with oxygen concentration of 100%, 60%, 40%, or 21%. In some ventilators, the expenditure was calculated from the internal pressure loss of an oxygen tank connected to them.
As a result ventilators were divided into two groups according to oxygen expenditure. A “low” expenditure group includes Servo 900C, Evita 2, and Servo 300, and a “high” expenditure group includes Bennett 740, T-Bird VSO2, Bird 8400STi, and CV4000a. This result does not depend on the triggering system. In 100% oxygen, the maximum oxygen expenditure was recorded by a Bird 8400STi (16.98l·min-1). This is more than three times higher than the smallest, record by an Servo 900C (4.10l·min-1). Two of the three pressure trigger type ventilators showed higher oxygen expenditure than the flow trigger ones. This result may be attributed to the character of oxygen-air blenders or the structure of the ventilators. We calculated the reimbursement covered by the national health insurance system and the cost of oxygen used by the ventilators in condition of 100% oxygen for 200 days of operation per year. A Bird 8400STi was estimated to make about 540 thousand yen's annual loss to the hospital, while the “low” group may bring the income of 10-60 thousand yen per year.
In conclusion we call intensivists' attention to the oxygen costs of a long term ventilator therapy.
