How should a peak expiratory flow PEF be measured?

PEAK EXPIRATORY FLOW

Peak expiratory flow (PEF) is the maximum flow achieved during a forced expiration starting from the level of maximal lung inflation.37 Primarily a measure of large airway caliber, PEF can be used to identify and assess airflow limitation in clinical practice and epidemiologic studies and can aid in the monitoring of disease progress and the effects of treatment.

In healthy subjects, PEF is determined by lung volume, airway caliber, lung elastic recoil, expiratory muscle strength, and the duration of pause at TLC before forced expiration. Traditionally, PEF was not thought to be flow limited because a plateau is not seen on isovolume pressure-flow curves, presumably because of the inability of the respiratory muscles to generate sufficient force. More recently, it has been demonstrated that PEF is determined by a wave-speed (

Flow limitation at PEF does not mean that it is independent of effort. The magnitude of PEF depends on how this maximum flow is reached. If expired volume from the TLC at which PEF is reached is small, PEF will be higher because at higher lung volume, the higher elastic recoil pressure and lower upstream resistance result in a greater wave speed and a higher PEF. In any interpretation of changes in PEF, the magnitude of effort and the volume at which PEF is reached are critical.

Miniature PEF meters are cheap and portable and can be used in the home, but there is little evidence to suggest that home PEF monitoring improves clinical outcomes. Issues with equipment accuracy, compliance, and lack of technical expertise all contribute to the unreliability of home PEF monitoring, and evidence suggests that patient education and symptom monitoring may be more useful in disease management.40

PEF increases with height during childhood; however, there is a wide range of normal values at any given height, making expression of a measured PEF as a percentage of predicted normal based on population studies unlikely to be useful. PEF may be more usefully expressed relative to each child's “personal best” determined by monitoring it for 1 to 2 weeks at a time when the child is well. This value can then be used as a basis for comparison during exacerbations of asthma.

PEAK EXPIRATORY FLOW (PEF)

Peak expiratory flow is a simple measure of airflow obstruction that can be done by the patient themselves. Monitoring peak expiratory flow (PEF) can increase patient awareness of disease control, help patients detect significant changes in symptoms and make self-management decisions, assist in evaluating the decisions made, and enhance patient-provider communications. For these reasons, PEF monitoring is frequently included in outcome evaluations, and is often examined in conjunction with symptom monitoring.

PEF monitoring is inexpensive and can be done anywhere by persons with asthma. Currently, the guidelines recommend that peak flow monitoring be done in the morning. Children as young as 5 can be taught to measure their PEF using peak flow meters (PFM). These devices are produced by a number of companies and models are available for children and adults. It should be noted that when PEF is incorporated into an evaluation of programs for children with asthma, a child's personal best PEF should be assessed at least once every six months while they are growing, thereby ensuring accurate interpretation of a PEF result. To check the accuracy of the PFM, periodic comparisons of the readings from the PFM and spirometry may be useful.

Peak Expiratory Flow Rate (PEFR)

PEFR monitoring is a simple and cost‐effective means of determining the degree of airways obstruction. PEFR is a tool for monitoring patients with asthma, but it should not be used for the diagnosis of asthma. Patients with moderate or severe asthma should be instructed in the use of PEFR and have a peak flow meter accessible for self‐monitoring.

Patients should establish a baseline “personal best” PEFR. At a time when asthma symptoms are well‐controlled, readings should be obtained at least twice daily over a 2‐ to 3‐week period, with one reading when the patient wakes up and another between noon and 2:00 pm. Readings should also be obtained pre– and post–beta‐2 agonist use.

PEFR zones are defined by the EPR2 as follows:

Green Zone: > 80% of personal best peak flow. These readings represent good asthma control, and patients should continue taking their medications without change.

Yellow Zone: > 50%, but < 80% of personal best peak flow. Patients with readings in this range should exercise caution. A short-acting inhaled beta‐2 agonist should be taken immediately, and the patient should contact the physician regarding any potential changes to the asthma regimen.

Red Zone: < 50% of personal best peak flow. Patients should be on the alert, take their short-acting inhaled beta‐2 agonist, and contact their physician or the emergency department immediately. Patients may also go to their nearest emergency room.

Aside from their use in monitoring acute exacerbations of asthma, measurement of PEFR is also useful in assessing patient response to changes in their asthma regimen.

Flow measurements

Peak expiratory flow is the maximum expiratory flow (L.s21) that a subject can produce. Of course this is dependent on the subject's motivation even with healthy lungs. The advantage of this measurement is that it can be made with simple apparatus – where the subject blows against a paddle or propeller which records flow. Although not precise this has been found to be a useful domiciliary measurement in asthma with the patient keeping a diary of his progress.

Flow-volume loops. With the subject breathing through a pneumotachograph (Fig. 4.6, p. 45) which measures flow, and by integrating that flow to provide volume, loops of inspiratory and expiratory flowvolume relationships can be recorded. (Fig. 11.3). These loops are constructed by having the patient breathe from total lung capacity down to residual volume several times. They are particularly useful in assessing chronic obstructive pulmonary disease (COPD) where the inspiratory part of the loop has a normal shape, although being of reduced volume, while the expiratory part of the loop has a characteristic ‘scooped out’ shape as flow is restricted by airway collapse.

Lung Function

Measures of PEF should be used for monitoring control. PEF should ideally be compared with the patient's best value and can be used for domiciliary monitoring. It is most useful in labile, severe asthma or when patients have difficulties in interpreting their respiratory symptoms (so-called “under-perceivers,” who may only become aware of bronchoconstriction when it is advanced). It is also helpful in documenting the effects of therapy or environmental triggers, particularly in the workplace. Such patients can benefit from an asthma action plan based on PEF in addition to or instead of symptoms. Repeated spirometry (e.g., on an annual basis) can demonstrate the development of fixed or deteriorating lung function, which may indicate an asthma-COPD overlap syndrome (ACOS) in smokers95 or the development of airway remodeling and the possible need for more intensive treatment.

LUNG FUNCTION TESTING

28.5.4.3 Small Airways

FEV1 and PEF measurements reflect changes in the caliber of the large airways. Our knowledge of anatomical and physiological changes in the small airways of patients with asthma is based on small case series of resected lung tissue from patients with asthma undergoing surgery for cancer, or on cases of fatal asthma.

These case series have demonstrated that there is significant inflammation present in the small airways (<2 mm diameter) in asthma. Fatal asthma is associated with peripheral airway inflammation and differences in the number of activated eosinophils in the distal lung. Other studies have revealed alterations in the epithelium and smooth-muscle, as well as mucous hypersecretion and distal airway plugging of the small airways. The presence of inflammation in the small airways in asthma may explain why small airways account for up to 50–90% of total airflow resistance in asthma, but only 10% of airflow resistance in normal airways.

Recently, the development of “small-particle” ICS, designed to target the peripheral lung, and the advent of new technologies—nitrogen washout, impulse oscillometry, and hyperpolarized noble gas magnetic resonance imaging, which allows assessment of peripheral lung function—have led to a resurgence of interest in the distal lung. Studies of small-particle ICS have been inconsistent; those comparing small-particle and standard-particle ICSs have failed to demonstrate improved asthma outcomes when administered in clinically comparable doses. Future asthma treatment may yet be stratified by the presence or absence of small airway inflammation.

AGING, FORCED EXPIRATORY VOLUMES, AND PEAK FLOW: IMPLICATIONS FOR COUGHING AND CLEARANCE OF AIRWAY SECRETIONS

Forced expiratory volumes and peak expiratory flow show an age-related linear decrease, probable reflecting structural changes, and chronic low-grade inflammation in peripheral airways (Enright et al., 1993). Indeed, for a male subject with a height of 180 cm, between the ages of 25 and 75, forced expiratory volume in 1 sec (FEV1) drops by 32%, forced vital capacity (FVC), by 24%, and peak expiratory flow (PEF), by 22% (Quanjer et al., 1993). In the very old, both the decrease in forced expiratory flow rates and in lung elastic recoil may compromise the efficacy of clearing airway secretions by coughing. Critical values for PEF have been reported, under which the risk of pneumonia markedly increases (Tzeng et al., 2000). In patients with neuromuscular disorders, a PEF below 270 L/min is associated with an increase in risk of pulmonary infection, and at PEF values < 160 L/min, cough is ineffective for clearing secretions from the airways (predicted PEF for an 80-year-old woman, measuring 160 cm, is 300 L/min). Coughing requires a precise coordination of laryngeal and respiratory muscle function: after a rapid inspiration, airways are submitted to a compression phase in which active glottic closure and abdominal contraction are critical, before the “explosive” expiratory phase. A decrease in expiratory muscle strength may thus compromise the efficacy of the cough reflex. The efficacy of glottic closure depends on the integrity of laryngeal muscle function and complex integrated reflexes that may be altered in the elderly by transient or permanent neurological disorders (cerebro-vascular disorders, extra-pyramidal, or cerebellar disorders). Indeed, ischemic stroke increases markedly the risk of pneumonia: in a large series of 13440 patients, pneumonia was the most frequent and serious complication, causing 31% of all deaths (Heuschmann et al., 2004). Glottal gap related to unilateral vocal cord paralysis is a potentially reversible sequel of acute cerebro-vascular events that may also increase the risk and frequency of aspiration (Fang et al., 2004).

Mucociliary clearance (progression of mucus layer lining the tracheal and bronchial epithelium) is also affected by the aging process. Even in the healthy nonsmoking aged population, mucociliary clearance rates are slowed in comparison with the young. Nasal mucociliary clearance and frequency of mucosal ciliary beat are decreased in older subjects; aging also is associated with ultrastructural changes in microtubules of ciliae of the respiratory epithelium. Indeed, both smoking and nonsmoking elderly have reduced tracheal mucus velocity compared with younger individuals (Ho et al., 2001). Dehydration, frequent in older debilitated subjects, may further compromise mucociliary clearance.

Peak Expiratory Flow Rate (PEFR)

Like FEV1, measurement of PEFR with a hand-held meter provides an objective measure of airway caliber. However, unlike spirometry, the patient can perform the maneuver at home or at work, which facilitates measurement of airflow on a daily basis. In this regard, PEFR monitoring has the capacity to provide the clinician with information not only about the severity of airflow obstruction, but also about the degree of variability of obstruction in an individual patient, which may reflect disease instability. While in asthmatic patients the am-pm variability is usually 10% or less, individuals with asthma demonstrate increased diurnal variability that further increases during periods of loss of control. For example, one study demonstrated that over 30% of patients requiring hospitalization for asthma had am-pm peak flow changes of 50%.

While these considerations make PEFR monitoring in clinical practice theoretically attractive, it must be noted that most patients are not sufficiently compliant with regular measurements to be helpful. In addition, certain PEFR meters may not provide consistently accurate results (as compared to more formal measures of PEFR by a spirometer), and further potential for error exists due to poor measurement technique, which can occur in the unmonitored home setting. In light of these limitations and lack of specific data to the contrary, it is not clear that PERF adds information regarding asthma control above that which can be obtained by soliciting patient reports of symptoms or need for rescue medications.

In sum, while the objective measurement of airflow obstruction through spirometry or PEFR measurements provides an assessment of the physiological consequences of asthma, it appears clear that neither of these tools, when used alone, is sufficiently robust to quantify overall asthma control. We recommend that these techniques be used in conjunction with other patient-derived measures to determine asthma control and adjust therapy.