Resistance to gas flow (R) occurs as a result of frictional forces within the airway and is measured as a pressure drop per unit flow rate (cmH 2O/l/s). Resistors in series are additive and a drop in pressure occurs between mouth and the branching airways, across the lung tissue itself, and across the chest wall when gas is moving at a particular flow rate. If it is possible to measure the pressure drop between all these elements during gas flow, then each component of resistance can be isolated. In practice, however, airways resistance (Raw) is commonly measured. Raw is the resistance of the branching airways, and the pressure drop is measured between the airway (Pao) and the alveolus (PA):

Although the peripheral airways are narrow, their combined cross-sectional area results in a small contribution to airways resistance. Most of the pressure drop across the airways occurs at the level of the small bronchi.

More pressure is required for a given flow rate if the flow is turbulent. Although it is possible for airflow to be laminar within the large airways, the internal surface must be smooth and the length of the airway must be more than 30 times the radius. As many of the upper airways are irregular, short, or branched, a mixture of turbulent and laminar airflow results. For the same reason airflow within endotracheal tubes is rarely laminar, particularly when the tube is kinked or coated with secretions.

Resistance changes with lung volume. As the lung deflates, so the caliber of the branching airways decreases. If the measurement of resistance is volume dependent, standardization is required before interpretation of resistance data. This problem can be solved if the hyperbolic volume:resistance relationship is converted to a straight line by using the reciprocal of resistance, i.e. conductance ( Slkei.1995).

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