Assessment of total pulmonary airway resistance under mechanical ventilation

Abstract

We propose a procedure for assessing the pulmonary airway resistance of patients under mechanical ventilation with a volume-cycled respirator having a sine-wave flow curve and inspiration/expiration (I/E) ratio of 1/2. This simplified procedure requires only the respirator's manometer and spirometer. The method is based on Ohm's Law, dividing the pressure difference (as shown on the manometer) between the peak value and that obtained by occluding the expiratory outlet by one-tenth of the minute volume (Vm). The relationship between the Vm and flow is obtained by calculating the height of the triangle formed by the sine wave, given that the area approximates total volume and the base is derived from the frequency and I/E ratio. This method was tested in 296 measurements on 106 patients using as a control the determination of resistance with a pneumotachograph and differential manometer placed between the patient and respirator. There was a high correlation (r = 0.96) between both procedures. To further facilitate bedside use, we have prepared a graph relating common values of Vm and pressure to resistance.