Expiratory tidal volume displayed on Bird 8400 STi can exceed the preset tidal volume due to cardiogenic oscillation:...

We noticed that monitored tidal volumes often exceeded preset tidal volumes in patients with large cardiogenic oscillation. To investigate whether triggering modes affect this discrepancy, we simulated cardiogenic oscillation of 90 breaths/min in a lung model, which was ventilated with a Bird 8400 STi ventilator (Bird, Palm Springs, CA, USA). The magnitude of cardiogenic oscillation was defined as peak expiratory flow fluctuation at the lung model. Two respiratory rates (5 and 10 breaths/min) and two triggering modes (flow-triggering and pressure-triggering) were applied, while tidal volume was set at 500 ml. We recorded tidal volume on a ventilator monitor and calculated the discrepancy from the set tidal volume. We also measured fluctuation in flow and airway opening pressure created by cardiogenic oscillation. During flow-triggering, larger flow fluctuation and smaller airway pressure fluctuation were observed compared with during pressure-triggering. During flow-triggering, the discrepancy between monitored tidal volume and set value ranged from 0 to +327 ml at 5 breaths/min, and from 0 to +105 ml at 10 breaths/min. There was a linear correlation between the magnitude of cardiogenic oscillation and the overestimation of tidal volume. By contrast, during pressure-triggering, the discrepancy was small. In conclusion, tidal volume is overestimated during flow-triggering but not during pressure-triggering when cardiogenic oscillation is large.