The Peak Expiratory Flow (PEF)

The greatest velocity of airflow during expiration is represented by the sharp peak of the flow volume loop. This is the peak expiratory flow rate (PEF).

In order to achieve an acceptable upstroke (Figure 3.14), the preceding inspiration should of course be full. The forceful expiration should follow without hesitation as this reduces central airway elasticity. Performed correctly, the PEF should be reached within 0.1 seconds (100 milliseconds) of initiating the forceful expiration.

PEF on Flow-volume loop

To reach maximum velocity, which reflects the physiological and anatomical properties of the airways, maximum effort is required. For maximum effort, you should instruct the patient clearly about the urgency of the manoeuvre.

A sub-maximal effort will reduce the steepness and increase time to PEF – remember that it is effort dependent! A maximal expiratory effort should be achievable for most patients, although certain conditions may result in a less steep upstroke, thus increasing the time to peak flow.

Let’s break down the components that determine PEF.

The energy required to inflate the lung to full inflation is stored as elastic potential energy. During expiration, the elastic recoil of the lung facilitates lung emptying and is a passive process.

The damage caused by smoking destroys elastic tissue in the lungs; the lungs become more compliant and the passive process of elastic recoil declines

Conversely, a process where the lungs become more elastic (e.g. with excess collagen deposition in pulmonary fibrosis) will facilitate the passive elastic recoil of the lungs

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