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A method for digitized flow-volume curves and expiratory resistance measurements using standard ventilatory equipment


Pulmonary resistance may be assessed in ventilated patients by analysis of passive expiratory pressure and flow properties of the respiratory system. Such measurements are generally complex and require specialized equipment. To facilitate expiratory airway mechanics measurements, we have developed a method for automatically recording and analyzing expiratory pressure flow curves in mechanically ventilated patients using standard ventilatory equipment and a personal computer. Simultaneous digital pressure, flow, and volume recordings are obtained with this system during exhalation. These values allow continuous calculation of ventilator circuitry and total system resistance, which could be used for assessing expiratory airway resistance in intubated patients. The accuracy of these methods was tested by comparison to standard analog recorder pressure-flow methods using two lung models as well as by testing in normal volunteers ventilated through a mouthpiece. In all situations (flows, pressures, and volumes) there was excellent correlation between data generated from the automatic digital method and standard analog methods (all r values > .9, slopes 0.9 to 1.02, P < .001, flow bias -0.02 L/min, flow precision 0.08 L/min, volume bias 0.008 L, volume precision 0.027 L). Expiratory resistance (using expiratory time-constant cord methods) also correlated well between automatic digital and standard analog analyses (r = .9, slope 0.98, P < .001). We found that expiratory flow limb resistances (airway, tubing, filters, and valves) were substantial (range, 5.5 to 14.0 cm H2O/L/s) and are varied throughout the expiratory cycle. Therefore, measurement of expiratory-flow limb resistance is necessary for accurate clinical assessment of pulmonary resistance for any system analyzing expiratory flow mechanics in ventilated patients. We conclude that this simple and convenient method allows automatic and accurate construction of pulmonary expiratory flow properties and may enable measurement of passive expiratory resistance. Rapid and accurate measurements of expiratory lung mechanics may be possible at the bedside of ventilated patients using standard ventilatory equipment with these methods. © 1992.

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