Am. J. Respir. Crit. Care Med., Vol 154, No. 4, Oct 1996, 913-917.
The accuracy of pneumotachograph measurements during mechanical ventilation
JW Kreit and FC Sciurba
Division of Pulmonary, Allergy, and Critical Care Medicine, University of Pittsburgh Medical Center, Pennsylvania, USA.
We hypothesized that differences between the conditions under which a
pneumotachograph (PT) is calibrated and those during data collection might
lead to large errors in measured flow and volume during mechanical
ventilation. A Fleisch No. 1 and Fleisch No. 2 and a screen PT were
connected to "ideal" tubing configurations that optimized flow
characteristics, and to ventilator tubing with and without a Y- connector
and endotracheal (ET) tube. Each PT was also evaluated after water had
accumulated in its resistive element. Air was passed through each PT
configuration, using both a continuous and a pulsatile flow pattern, and
collected in a water-seal spirometer. "Measured" and "true" flow and volume
were determined from the PT and the spirometer, respectively. Measured flow
and volume were falsely low when the PT was adapted to ventilator tubing.
Addition of a Y-connector and ET tube caused measured flow and volume to
increase, and, in some cases the relationship between measured and true
flow became nonlinear. Water accumulation in the PT did not lead to
measurement errors. We conclude that when a PT is used during mechanical
ventilation, tubing geometry must be identical during calibration and data
collection, and that calibration should be performed over the entire range
of relevant flows.
