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any condition which alters the above factors could affect PEF. However, a reduction in one of the above factors may be compensated for by an increase in one of the others, meaning that PEF does not alter, therefore potentially underestimating the severity of the condition (Quanjer et al. 1997).

Related theory

The most common disorders which affect PEF are those which increase the resistance to air flow in the intrathoracic airways, such as asthma (Quanjer et al. 1997). However, PEF may also be impaired by:

disorders which limit chest movement and respiratory musculoskeletal problems

obstruction of the extrathoracic airways

impairment of nerves supplying the respiratory system.

(Quanjer et al. 1997)

Peak expiratory flow readings are subject to individual variation depending on the patient’s age, gender, ethnic origin and stature (Quanjer et al. 1997). Therefore, results need to be compared against normal results for people of the same stature and gender (Figure 12.18) and against any previous results for that individual.

Figure 12.18 Normal peak flow measurements.

Reproduced from Kumar and Clarke (2009).

Peak expiratory flow is similar to forced expired volume in 1 second (FEV1) measurements but the two are not interchangeable and both measure different aspects of lung function (Ruffin 2004). FEV1 measures the volume of air exhaled during the first second of forced vital capacity (FVC), which occurs when an individual exhales forcefully to their maximum capacity following a deep inspiration (Marieb and Hoehn 2010). FEV1 is usually 80% of FVC in healthy participants (Marieb and Hoehn 2010). FEV1 is felt to be more sensitive in detecting mild airway obstruction, and peak expiratory flow is effort dependent and so can have a greater degree of intrasubject variability (Hansen et al. 2001). However, both FEV1 and PEF were found to have similar predictive ability in relation to mortality in patients with COPD, although FEV1 was found to be a better predictor in patients with asthma (Hansen et al. 2001).

Evidencebased approaches

Rationale

Peak expiratory flow is a simple, objective procedure which can be used to measure the degree of air flow limitation and although it may not give a full representation of lung function, it can monitor efficacy of treatment and progression of the condition (Frew and Holgate 2009).

Indications

Peak expiratory flow can be used to:

monitor the severity of the condition in patients with chronic severe asthma (Reddel 2006)

monitor progression of respiratory disease (Frew and Holgate 2009)

evaluate effectiveness of treatment (Frew and Holgate 2009)

identify exacerbating factors (Reddel 2006).

Contraindications

Peak expiratory flow should be used with caution as follows.

Severe air flow obstruction, as included in the measurement, may be air coming from the collapsing airway, yielding an erroneously high result (Quanjer et al. 1997).

If the patient is unable to take a full inspiration, for example if they have a persistent cough, then the results will be inaccurate (Quanjer et al. 1997).

If the procedure itself causes an exacerbation of the air flow limitation, shown by consecutive results producing a reduction in scoring (Quanjer et al. 1997).

Young children may not understand the procedure correctly (Gorelick et al. 2004).

Methods for measuring peak flow

Treatment is often based on the PEF measurements so it is vital that they are as accurate as possible, and performed even when the patient is symptom free (Buist et al. 2006). Patients need to adhere to the peak flow monitoring regimen as trends can be more important than isolated results (unless the isolated results reflect an exacerbation) (Booker 2009). The patient needs to repeat the procedure three times with the best result of the three being documented (Quanjer et al. 1997). Unless the procedure induces an exacerbation, there should be consistency between the three results. If the top two results have a greater disparity than 40 L/min then, as long as the patient is not fatigued, a further two attempts can be made