The Science Journal of the American Association for Respiratory Care
BACKGROUND: Pressure-support (PS) and Flow-by (FB) have been advocated as methods to reduce patient work of breathing during mechanical ventilation. While patient trigger work and spontaneous work of breathing are relatively easy to determine, the actual total patient work performed during a mechanically supported breath can be difficult to ascertain. We compared simulated total patient work of breathing (TPWOB) during pressure-triggered demand flow (PTDF), flow-by (FB), pressure-triggered pressure support (PTPS) and flow-triggered pressure support (FTPS) using the Bennett 7200ae ventilator and a lung model using an indirect method for estimating TPWOB. METHOD: Spontaneous breathing was simulated at a respiratory rate of 10 bpm and a tidal volume of 500 ml with a peak flow rate of 60 L/min and a sine wave flow pattern using a two-compartment mechanical lung model (Michigan Instruments Inc., Grand Rapids, MI). For PTDF, the test ventilator was set at a sensitivity of -0.5, -1.0, -1.5 and -2.0 cmH2O, PEEP/CPAP = 0 cmH2O. For FB, settings were 6/3, 6/2, 6/1, 10/3, 10/2, 10/1 L/min. for base flow and flow sensitivity, PEEP/CPAP = 0 cmH2O. For PTPS and FTPS, pressure support was set a 5, 10, 15, and 20 cmH2O with the sensitivity and FB settings varying as described for PTDF and FB, above. TPWOB was determined using the VenTrak 1550 Respiratory Mechanics Monitoring System (Novametrix Medical Systems, Inc., Wallingford, CT) as follows:
TPWOB = WOB driver compartment linked to - WOB driver compartment alone spontaneous compartment
For each condition, TPWOB was estimated for normal compliance (CTOT = .05 L/cmH2O, Raw = 2.7 cmH2O/L/sec), decreased compliance (CTOT = .02 L/cmH2O) and increased resistance (Raw = 17.6 cmH2O/L/sec).
Results: TPWOB ranged from .71 to 1.33 J/L for PTDF and .56 to 1.13 J/L for FB. TPWOB ranged from 0 to 1.04 J/L for PTPS and 0 to 1.05 J/L for FTPS. Under all conditions for PT, as sensitivity increased, TPWOB decreased and as PS increased, TPWOB decreased. Table 1 compares the mean TPWOB for all conditions by mode.
|Total Work of Breathing (J/L) - Mean (SD)|
|* significantly less than PTDF (p = .03) ** significantly less than PTDF (p < .0001)|
There were significant differences between PTDF and FB (p < .05) prior to the addition of pressure support. There were no significant differences between PTPS and FTPS (p > .05). CONCLUSION: In the absence of pressure support, flow-by produces a small decrease in TPWOB. Flow-by combined with pressure support offered little advantage over traditional pressure-triggered pressure support ventilation in reducing simulated TPWOB.