Peak expiratory flow is the greatest flow that can be obtained during a forced expiration starting from full inflation of the lung (i.e., total lung capacity) (1). It is the most convenient of all indirect tests of ventilatory capacity. Peak expiratory flow rate (PEFR) assessment is an excellent tool for monitoring the severity of respiratory insufficiency from airway obstruction and for following the progress of children with lower airway obstruction such as asthma. PEFR assessment offers an objective and accurate measurement of lung function in asthma. The procedure is relatively simple to perform using a handheld spirometer. Recently, portable, handheld spirometers have become available for clinical use, and pager-sized monitoring systems are available for patient home recordings. PEFR assessment can be performed by the physician, nurse, or respiratory therapist in the emergency department (ED) or office setting or by a properly trained individual at home or at school. To obtain reliable results, full cooperation of the patient is required, making the test generally useful only for children older than about 5 years of age. The ability to successfully complete the procedure varies widely in the 5- to 7-year age group. Results may not reflect the severity of airway obstruction in patients who are not fully cooperative and delivering a full effort. Therefore, PEFR assessment should not be attempted in severely symptomatic patients before emergency bronchodilator therapy.

PEFR measurement has been endorsed by the National Asthma Education Program (2) for assessing the degree of airflow obstruction and severity, monitoring response to therapy, diagnosing exercise-induced asthma, and detecting asymptomatic deterioration. To help an asthma patient use home PEFR monitoring, a system of PEFR zones can be established based on the patient’s personal best PEFR or the predicted value for the patient’s height (3,4). When the zone system is adapted to a traffic light pattern for the zones, it may be easier to use and remember. The following are the common guidelines used: green (80% to 100% of personal best) signals all is clear (i.e., no asthma symptoms are present), yellow (50% to 80% of personal best) signals caution (i.e., an acute exacerbation may be present), and red (below 50% of personal best) signals a medical alert. A medical alert indicates that a bronchodilator should be administered immediately and a clinician should be notified (2).

In the ED, PEFR assessment in a cooperative, trained patient is obtained with the help of a respiratory therapist, emergency physician, or nurse. Periodic assessments may be required at baseline and post β-agonist therapy to demonstrate reversibility (minimum of 15% improvement) and reassure patients and families that the drug is effective and pulmonary function has improved. PEFR assessment is an effort-dependent procedure, which means that the best result requires a fully cooperative patient. A supervisor needs to closely monitor, coach, and give cues to the patient in order to ensure optimal performance.

The airway and lung anatomy of children differs from that of adults. The chest wall in young children is more compliant than in adults. This tends to enhance ventilation by requiring small efforts for tidal breathing in the healthy child. As children grow, the chest wall becomes more stiff (less compliant), and recoil of the lung on expiration is more effortless. The pressure necessary to expand the lungs is increased in pathophysiologic states that reduce the lung compliance or increase the “stiffness” of the lungs. Airflow resistance is an important determinant of respiration and is greatest in the upper and nasal airway during inspiration and greatest on
expiration in the intrathoracic airways. In addition, the airways of young children are relatively narrow compared with those of adults. Because flow is related to the radius of the airway to the fourth power, small reductions in the airway caliber in children due to inflammatory processes greatly reduce the flow of air per given amount of generated work. In the healthy state, these differences lead to minimal effort in chest expansion and airway ventilation. The growth of the distal airways lags behind that of the proximal airway in the first 5 years of life. These narrow distal airways account for high peripheral airway pressures necessary to optimize air flow.

In lower airway disease such as asthma, airway narrowing is caused by bronchial smooth muscle constriction, airway inflammation, and increased mucous production. This inflammation affects more distal small airways not measurable by the PEFR. Small airway measurements such as the midexpiratory flow rate (FEF_25-75) more accurately relate to the small airways. Air trapping associated with airway obstruction of the small airways places the end of inspiration and the initiation of expiration higher on the flow-volume curve, where more effort is used for a given tidal volume change. Even with the patient’s best effort, the exhalation of a breath from maximal inspiration (from total lung capacity) through the narrowed airways cannot produce the same maximal flow that occurs without this obstruction. The point of maximal flow in expiration (PEFR) is almost at the onset of expiration. It is expressed in liters per second, is effort dependent, and is a measure of large airway flow rates.

Although it is not a sophisticated method of assessing pulmonary function, PEFR assessment provides objective evidence of the severity of airway obstruction and assists in judging the response to bronchodilator therapy. It can also provide warning signs of increasing severity of asthma or resistance to bronchodilator therapy.

Children will present to the ED because of increasing severity of symptoms or more commonly in status asthmaticus that has not responded to home therapy. Cough, dyspnea, and wheezing are the major clinical features, but presentation may vary with age. In some cases, children may present with persistent cough at night or during exercise, while in others, shortness of breath may be the predominant symptom. The degree of wheezing does not correlate well with the severity of the attacks, but the relative absence of wheezing in the presence of respiratory distress, poor air entry, or hypoxia signifies severe obstruction. The use of accessory muscles of respiration and the presence of pulsus paradoxus are other indicators of marked severity. However, these signs and symptoms are subjective, and hence objective measures of the severity of airway compromise can be useful adjuncts.

Peak flow monitoring in the ED is a useful objective tool to assist in the evaluation and treatment of children. Several indications for ED use are listed in Table 77.1. PEFR measurements should be done in all patients with asthma in the ED who are able to perform this maneuver. In the ED, PEFR assessment will be useful as an adjunct in determining the need for bronchodilator therapy or admission. It is also useful when referring these patients to specialists and when discussing these patients via telephone. The importance of documenting and quantitating airway obstruction cannot be overemphasized, because patients’ reports of their symptoms and physicians’ physical findings may not correlate with the variability and severity of airflow obstruction (5,6). However, peak flow assessment should not be attempted in the severely compromised patient who is very dyspneic or in impending respiratory failure. Under these circumstances, therapy should be determined by clinical findings and other ancillary data such as transcutaneous oxygen saturation monitoring. Measurement of PEFR can then be used to follow clinical response to therapy.

A standard office peak flow meter is the simplest and easiest meter to use (7). Several peak flow meters are commercially available (Table 77.2). Specific instructions, including a step-by-step chart to assist in performing the maneuver, are contained in the literature accompanying each meter. Because different peak flow meter brands and models often yield different values when used by the same person, children should be encouraged to use the same model in the home, in the clinician’s office, and, when possible, in the ED. Although meters may have different configurations, they all usually have a disposable mouthpiece and gauge (upright, horizontal, balls, arrows, etc.) and function on the same principles.