2001 OPEN FORUM Abstracts
ASSESSMENTOF EXPIRATORY VALVE RESISTANCE IN ADULT VENTILATORS.
Jack WalshRRT,* Richard Branson BA RRT FAARC,? Robert KacmarekPhD RRT FAARC,*? Dean Hess PhD RRT FAARC.*? *MassachusettsGeneral Hospital, ?Harvard Medical School, ?Universityof Cincinnati.
In the past, the exhalation valveof mechanical ventilators has been implicated as an important contributor toexpiratory resistance. We conducted this study to assess the role of the exhalationvalve in 7 newer generation adult mechanical ventilators.
Method: The following ventilatorswere assessed ? Dräger Evita 4, Hamilton Galileo, Pulmonetic LTV-100, Puritan-Bennett7200, Puritan-Bennett 840, Servo 300, and Thermo Tbird. A standard adult circuitwas used. The ventilator was attached to the single bellows of a Michigan InstrumentsTTL set at a compliance of 20 mL/cm H2O and one of two resistances(5 and 20 cm H2O/L/s). The ventilators were set to a tidal volumeof 0.4 or 0.8 L, and PEEP of 0, 5, 10 and 20 cm H2O. An end-inspiratorypause of 1 s was set on the ventilator. Expiratory flow curves were assessedwith and without the exhalation valve. To assess the effect of no exhalationvalve, the expiratory limb tubing was disconnected from the exhalation valveduring the end-inspiratory pause. Pressure, flow, and volume were measured atthe ventilator Y-piece using a Novametrix CO2SMO and recorded toa computer for later analysis. Peak expiratory flow and SF50 were determinedas previously described (Intensive Care Med 1999; 25:799); SF50 = arctg (y50/VT50)where y50 is the flow at 50% of the exhaled tidal volume and VT50is 50% of the exhaled tidal volume. The SF50 represents the angle of the slopeof the flow-volume curve during the last 50% of the exhaled tidal volume.
PeakExpiratory Flow (L/s)
|No Valve||Valve Present||P-value||No Valve||Valve Present||P-value|
|69.7 ± 7.0||66.2 ± 7.1||0.171||1.1 ± 0.4||0.9 ± 0.3||0.125|
|Galileo||76.0 ± 5.3||73.3 ± 5.2||0.152||1.7 ± 0.6||1.3 ± 0.4||0.061|
|LTV-1000||76.6 ± 4.8||72.9 ± 4.6||0.035||1.7 ± 0.6||1.3 ± 0.4||0.009|
|PB7200||70.9 ± 7.9||60.9 ± 5.5||0.001||1.1 ± 0.4||0.6 ± 0.2||0.001|
|PB 840 1||70.7 ± 8.||63.9 ± 7.8||0.021||1.2 ± 0.4||0.9 ± 0.3||0.032|
|Servo 300||68.2 ± 7.9||62.9 ± 6.9||0.054||1.1 ± 0.4||0.8 ± 0.2||0.025|
|TBird||76.5 ± 5.7||73.2 ± 5.4||0.105||1.9 ± 0.7||1.4 ± 0.5||0.016|
Conclusions: For these ventilators,there was an increase (mean 37%) in peak expiratory flow when the exhalationvalve was removed. However, the increase in SF50 due to the exhalation valvewas modest (mean increase 7.5% for the ventilators evaluated). Consistent withthe results of others (Intensive Care Med 1999; 25:799), this suggests thatthe second 50% of exhalation may be relatively unaffected by the exhalationvalve and might, therefore, be useful to estimate patients? respiratory mechanicsduring ventilatory support. However, the initial expiratory flow may be lessuseful in this regard because it is affected to a greater extent by the resistancethrough the endotracheal tube. Our data suggests that exhalation valves in currentgeneration ventilators may contribute less to expiratory resistance than inthe past.