It is important that the site of pressure measurement and the technique used to measure resistance are stated, as differences can occur solely on the basis of the technique. The basis of measurement is that resistance is proportional to the pressure difference and inversely related to the flow rate. If the pressure drop between the airway and alveolus is known, airway resistance (Raw) can be calculated.
Airway resistance can be estimated simply from the pressure-time curves obtained from the ventilated patient. As previously described, the airway pressure tracing during constant-flow ventilation has an initial rapid upstroke. At the start of inspiration little gas has entered the chest and this pressure rise is due to airway resistance. If the inspiratory flow rate does not change (the measurements should be made during a square-wave constant-flow breath) and airway resistance is constant, there will be a constant pressure difference attributable to resistance during inspiration ( Fig..4). The pressure difference related to resistance is often calculated at the end of inspiration during an end-inspiratory pause ( Suteretal 1975).
Fig. 4 Airway resistance can be measured from airway pressure curves; resistance is equal to pressure difference divided by flow. In this diagram, pressure difference is estimated from the peak plateau pressure, and flow can be measured directly from the ventilator.
A graph of volume, transthoracic pressure, and flow demonstrates the problems of obtaining a measurement of resistance in a subject. As resistance is related to lung volume, it is clearly important to ensure that this variable, which is changing, is standardized. Furthermore, flow changes during the respiratory cycle, and if we merely divided pressure by flow a wide range of values would be obtained. Which would represent the true measurement of resistance?
Several methods have been used to calculate resistance. To avoid the problem of standardization of lung volume, this can be fixed during inspiration and expiration and the pressure and flow differences between these two states used to measure resistance. An alternative approach is to use a sinusoidal oscillator which delivers a rapidly changing small-volume waveform to the respiratory system. Yet more techniques use other ways of averaging the lung compliance and measure resistance at a particular instantaneous flow rate. When these techniques are compared, little difference is found in absolute terms. Most are also able to detect stepwise changes in resistance.
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