1995 OPEN FORUM Abstracts
Direct Measurement Via An Inline Pneumotach Is Necessary To Determine Effective Tidal Volume In Children
Barbara G. Wilson, Frank H. Kern, Ira M. Cheifetz, Jon N. Meliones, Duke University, Departments of Pediatrics and Respiratory Care, Durham, NC
An accurate assessment of the effective tidal volume (VTeff) is essential to optimize the mechanical ventilation strategy and minimize ventilator induced injury in infants and children. One method of estimating the VTeff utilizes ventilator derived measurements (VTeff = VTexpired - Tubing compliance x (PIP-PEEP)). This method may be limited at different lung compliances and in ventilators which do not utilize a pneumotach at the endotracheal tube (ETT). The purpose of this study was to determine if the VTeff could be accurately predicted using parameters derived from internal ventilator measurements and known circuit compliance.
Methods: A Siemens 900C ventilator (Siemens-Elema, Solna, Sweden) was operated in volume and pressure control modes using a disposable neonatal ventilator circuit (Baxter Healthcare Corp. Deerfield, IL), a test lung (Bio-TEK Instruments Inc., Winoosky, VT) and VT appropriate ETT. Known circuit compliance was 1.34 ml/cm H20. Three VT ranges were applied in each mode: 0-50 ml, 50-100 ml, 100-150 ml for a total of 90 measurements. Inspiratory time, rate, and airway resistance were held constant. Test lung compliance was 1 and 3 ml/cm H_2O. A pneumotach was placed at the ETT and connected to a respiratory mechanics monitor (VenTrak, Novametrix Medical Systems, Wallingford, CT). Inspiratory (VTinspired) and expiratory tidal volumes (VTexpired) were recorded from the VenTrak and the 900C. VTeff was calculated using 900C data. The VenTrak VTeff was compared to VTinspired, VTexpired and the calculated VTeff of the ventilator. Measurements were compared using linear regession. Data is presented for VenTrak VTeff as compared to ventilator derived VTeff.
VTeff r Slope Intercept
0-50 ml .88 .60 4.7
50-100 ml .95 .80 4.3
100-150 ml .31 .39 5.1
0-50 ml .75 .62 8.2
50-100 ml .99 .86 3.2
100-150 ml .46 .81 4.0
Results: There was a significant difference between VenTrak and ventilator calculated VTeff (p=0.0001). Despite an adequate correlation at certain VTs, the "goodness of fit" was poor throughout as demonstrated by the deviation of the slope from 1, a lack of a consistent slope over a range of VTs and the varying intercepts. These relationships were similar for VenTrak VTeff vs VTinspired and VTexpired. Therefore, an accurate prediction of the VTeff can not be made using ventilator derived measurements. To optimize the mechanical ventilation strategy and minimize ventilator induced injury, a direct measurement of the VTeff using an inline pneumotach is required in infants and children.