Where the airways are obstructed by secretions or compressible airways, as in obstructive lung disease, the cross sectional area of the airway lumen is reduced which increases airway resistance
Maximal airflow
The volume of air contained within the lungs following a full inspiration also determines the speed and volume of air expired.
In general, a larger lung volume will generate a greater PEF. If the lungs become more elastic (as in pulmonary fibrosis), this increases the energy required to take a maximal inspiration; this may also generate greater airflow on expiration.
The tube characteristics have a significant impact on airflow. The airway calibre reflects the cross-sectional properties of the airways in series, and therefore the resistance. Resistance is inversely proportional to airflow; increased resistance will impede flow.
Excess collagen deposition in the lungs in patients with pulmonary fibrosis, for instance, have increased radial traction of the airways. This maintains airway patency – even under high pressures. In these circumstances, PEF may be normal or elevated despite a restrictive pattern spirometry
Expiratory muscles generate the additional force to reach PEF. The greater the force generated by muscular contraction, the greater the difference between the pressure within the alveoli and at the mouth; so air flows out of the lungs at a greater velocity.
Expiratory muscles generate the additional force to reach PEF. The greater the force generated by muscular contraction, the greater the difference between the pressure within the alveoli and at the mouth; so air flows out of the lungs at a greater velocity.
A normal PEF essentially reflects well-preserved central and large airways, but does not provide any information on the function the peripheral airways. Whilst the PEF is not sensitive to identifying lung disease, a correctly performed measurement is useful in monitoring disease progression, particularly the variability of central and large airway obstruction in asthmatics.
Where PEF is measured in litres/ minute; remember to divide by 60 to convert from minutes to seconds, and multiply by 60 to convert from seconds to minutes
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