Measurement of Airway Resistance

Airway resistance can be measured during spontaneous breathing by simultaneous recording of air flow, and the pressure gradient between mouth and alveoli. In practice the alveolar pressure is difficult to obtain since it must be derived using a body plethysmograph. Intrapleural pressure can be used instead, but will include the pressure gradient required to overcome lung tissue resistance and inertial properties. Thus, this technique measures the resistance to air flow, the viscous resistance due to the lung tissue and the inertia of the lung. Under normal circumstances, these factors are negligible, but tissue viscous resistance may become more significant in pulmonary oedema and fibrosis. Inertia of the lung may be of importance during high frequency ventilation.

In clinical practice airway resistance can be assessed using, forced expiratory flow rates such as FEV1, peak expiratory flow rate (PEFR), and mid expiratory flow rate. These indices are more easily measured, but they rely upon expiratory muscle activity in addition to airway resistance, and are affected by patient technique.

Work of Breathing

Work is normally expended in inspiration, expiration being passive. The work of inspiration can be subdivided into:

• Work required to overcome the elastic forces of the lung (compliance), which is stored as elastic energy

• Work required to overcome airway resistance during the movement of air into the lung

• Work required to overcome the viscosity of the lung and chest wall tissues (tissue resistive work)

During quiet breathing, most of the work performed is elastic. This inflates the lungs but provides a store of elastic energy to be returned during expiration, and is, therefore, useful work done.

The effect of overcoming tissue resistance and airway resistance is dissipated as heat and can be viewed as wasted energy. Figure RR.13 shows an idealized loop inspiration occurring along the path AGB and expiration along BEA. The following points illustrate the relationship between the wasted work done and the useful work done.

• In an ideal elastic lung with no tissue or airway losses, the work stored in inspiration would be completely returned in expiration. Inspiration and expiration would then occur along the straight line AFB, with equal amounts of inspiratory work done and expiratory work, represented by the area AFBCD

• In the real lung with tissue and airway losses, inspiration occurs along curve AGB. The inspiratory work done (AGBCD) is thus greater than the ideal case (by the shaded area AGBF). This wasted work is only a fraction of the inspiratory work

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Figure RR.13 Work of breathing loop

• In the real lung, expiration occurs along BEA. The expiratory work returned (BEADC) is thus less than the ideal case by (hatched area BEAF)

• The total 'wasted' energy due to tissue and airway losses is thus the sum of the shaded and hatched areas, or the area of the loop

In lung diseases all three types of work are increased. Compliance work and tissue resistive work are greatly increased during restrictive diseases such as fibrosis of the lungs whereas airway resistance work is increased in pulmonary obstructive diseases.

Expiration does not normally entail active work, but may be necessary in forced breathing or when airway resistance or tissue resistance are increased. In some circumstances, expiratory work may be greater than inspiratory work, for example in asthma.

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