The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

A COMPARISON OF TOTAL PATIENT WORK OF BREATHING (TPWOB) BETWEEN ENDOTRACHEAL (ETT) AND TRACHEOSTOMY (TT) TUBES DURING PRESSURE SUPPORT VENTILATION (PSV) IN A SPONTANEOUSLY BREATHING LUNG MODEL. 


Brandi A. Blackburn, BS CRT; David L. Vines, MHS, RRT; Jennifer L. Boenisch, BS, CRT; David C. Shelledy, PhD, RRT; and Jay I. Peters, MD. The University of Texas Health Science Center at San Antonio, San Antonio, Texas.

RATIONALE:
It is known that as the level of PSV increases, the TPWOB usually decreases. We assessed the effect of increasing PSV on various size airways to determine at what point differences in TPWOB would be eliminated. We also compared peak inspiratory flow (PIF), inspired and expired tidal volume (VT), peak inspiratory pressure (PIP), and mean airway pressure (MAP) between ETTs and TTs. 

Methods:
Spontaneous breathing was simulated using a two-compartment mechanical lung model (Michigan Instruments, Inc., Grand Rapids, MI). The driver lung B was attached to a Bennett 7200 ae ventilator (Puritan Bennett, Pleasanton, CA). Work of breathing (WOB) was first measured on lung B alone at VT of 300, 400, 500, and 600mL as the PIF varied from 40, 60, 80, and 100 L/min with a sine wave flow pattern. WOB was measured by the Ventrak 1550 Respiratory Mechanics Monitoring System (Novametrix Medical Systems, Inc., Wallingford, CT). WOB was then measured at these volumes for lung B to drive lung A at normal compliance (0.05 L/cm H2O) and resistance (2.7 cm H2O/L/sec) while the following airways were attached: 8.0, 7.0, 6.0 and 5.0 mm ID ETT (Mallinckrodt Critical Care, Glens Falls, NY), 8.0, 7.0, 6.0 mm ID TT (Sims Portex Inc., Keene, NH), and 5.0 mm ID #4 Shiley TT(Mallinckrodt, St. Louis, MO). A consistent curvature of all ETTs was assured by conforming them to a stylet simulating the natural curvature of the upper airway. WOB was next measured for lung B to driver lung A attached to the various airways while receiving assistance from PSV on the Puritan Bennett 840 ventilator (Puritan Bennett, Pleasanton, CA). The PSV levels were varied from 5, 10, 15, and 20 cm H2O. If airtrapping occurred in lung A, driver rate was lowered. The TPWOB was calculated using the following formula: [TPWOB= WOB(B+A) - WOBB]. PIF, inspired and expired VT, PIP, and MAP were measured using the CO2SMO Plus (Novametrix Medical Systems, Inc., Wallington, CT). 

RESULTS: 
The table below contains mean values and standard deviations for TPWOB. All significant differences are p < 0.05. There were no significant differences in PIP between the various airways across all PSV levels. Airtrapping was noted during PSV of 15 and 20 cm H2O for 5.0 mm ID airways and 6.0 mm ID ETT.

Airway Spont PSV-5 PSV-10 PSV-15 PSV-20
  TPWOB TPWOB TPWOB TPWOB TPWOB
8.0 ETT 1.040(.251)a,b,c .862 (.317)a,b .688 (.392)a,b .522 (.392)a,b .415 (.360) a
8.0 TT 0.923(.218)a,b,c .755 (.299)a,b,c .562 (.359)a,b,c .413 (.336)a,b,c .322 (.293) a,b
7.0 ETT 1.143(.285)a .993 (.353)a .805 (.414)a .709 (.458)a .558 (.437) a
7.0 TT 1.007(.240)a,b,c .832 (.321)a,b,c .620 (.379)a,b,c .466 (.365)a,b .363 (.322) a,b
6.0 ETT 1.326(.333) 1.186 (.389) 1.033 (.442) .874 (.494) .754 (.503)
6.0 TT 1.056(.257)a,b,c .925 (.332)a .742 (.397)a .601 (420)a .496 (.395) a
5.0 ETT 1.532(.363) 1.458 (.424) 1.316 (.448) 1.213 (.489) 1.070 (.515)
5.0 TT 1.33(.342) 1.219 (.419) 1.110 (434) .970 (.493) .810 (.515)


a) significantly less than 5 mm ID ETT b) significantly less than 5 mm ID TT c) significantly less than 6 mm ID ETT 

CONCLUSION: 
The mean TPWOB for the various airways decreased as PSV increased from 5 to 20 cm H20, so that the TPWOB of an 8 ETT during spontaneous breathing was approximately the same as a 7 ETT at a PSV of 5 cm H2O, a 6 ETT at a PSV of 10 cm H2O, and a 5 ETT at a PSV of 20 cm H2O. The mean differences in TPWOB between the individual airways at a set level of PSV were not eliminated as PSV increased. Airtrapping may occur as airway size decreases on a PSV level of 15 or 20 cm H2O.

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