1 What is the best clinical indicator of airway obstruction in children with asthma?

There is no single best indicator of obstruction. It is generally true that, in younger children, observation is often more revealing than auscultation; it is hard to listen for wheezes in a crying child. Signs of airway obstruction include:

2 What is the pulmonary index?

The pulmonary index is one of many clinical scores used in the evaluation of acute asthma severity (Table 41-1). Although no single score has been shown to be superior, the pulmonary index has been used widely in asthma research. Also, there is not universal agreement on how to interpret the score. As a general guide, a score < 6 is considered mild, while a score > 10 is considered severe.

Becker AB, Nelson NA, Simons FER: The pulmonary index: Assessment of a clinical score for asthma. Am J Dis Child 138:574–576, 1984.

3 Describe the role of measuring peak expiratory flow rate during acute asthma exacerbations in children

Clinical assessment, while helpful, may underestimate the degree of airway obstruction in a child with acute asthma. The most commonly used pulmonary function test in pediatric acute care is the pulmonary expiratory flow rate (PEFR), which is the maximal rate of airflow during forced exhalation after a maximal inhalation. It is easily measured using an inexpensive, handheld metering device. Children as young as 4 or 5 years old can be taught how to perform PEFR, although it is more difficult to learn in the setting of an acute exacerbation. Advantages of PEFR include ease of performance, low cost, and ability to track changes during therapy. However, PEFR is effort-dependent and cannot be performed by very young children. In addition, PEFR measures function in medium and large airways, while much of the pathology in asthma occurs in medium and small airways; thus, even PEFR can underestimate the severity of disease. Normal values depend on the age and height of the child (a table is usually included with the meter), as well as the severity of the underlying lung disease, so results should be expressed as a percentage of predicted value, or of the patient’s usual best value if known. PEFR of at least 80% of predicted indicates mild disease, 50–80% of predicted is considered moderate, and less than 50% of predicted constitutes severe disease. Failure to achieve PEFR of at least 50% of predicted is one indication for hospital admission.

Gorelick MH, Stevens MW, Schultz TR, Scribano PV: Difficulty in obtaining peak expiratory flow measurements in children with acute asthma. Pediatr Emerg Care 20:22–26, 2004.

4 How should pulse oximetry be interpreted in acute asthma?

“Normal” values vary considerably between institutions and practitioners. An SaO₂ (saturation of O₂ measured by pulse oximetry) of 95% correlates with a PaO₂ of around 75, while an SaO₂ of 90% is close to a PaO₂ of 60, at the top of the steep portion of the oxygen desaturation curve. Most clinicians set a cut-off for normal somewhere in between. Remember that while a low saturation is worrisome, a normal oxygen saturation does not rule out severe disease.

Keahey L, Bulloch B, Becker AB, et al: Initial oxygen saturation as a predictor of admission in children presenting to the emergency department with acute asthma. Ann Emerg Med 40:300–307, 2002.

5 Why does hypoxemia sometimes increase paradoxically after treatment with inhaled bronchodilators?

Hypoxemia in asthma is due to ventilation–perfusion mismatch. The acute deleterious effects of bronchodilators on oxygenation are believed to be due to their effects on the circulation rather than on the airways. After treatment, particularly when administered with high concentrations of oxygen, there is improvement in local alveolar oxygenation. This, in turn, causes a reversal of hypoxia-induced pulmonary vasoconstriction, with the result that areas previously neither ventilated nor perfused are now perfused but not ventilated, increasing the mismatch.

6 What is Poiseuille’s law?

where n = viscosity coefficient of the gas, l = length of the tube, and r = radius of the tube. Thus, resistance to airflow increases in inverse proportion to the fourth power of the radius of the air passages. The take-home message is that a little narrowing goes a long way toward blocking air flow.

7 What are the indications for obtaining arterial blood gas (ABG) analysis in a patient with asthma?

Since oxygenation can be measured noninvasively, ABG is useful primarily for monitoring carbon dioxide concentration and acid–base status. In deciding whether to obtain an ABG, it is worth considering that hypercarbia generally only develops when the PEFR is < 20–25% of predicted. Moreover, the decision to intubate a patient with asthma is rarely made on the basis of ABG results alone. Thus, ABG is usually reserved for the following patients:

8 What is pulsus paradoxus?

Pulsus paradoxus is an exaggeration of the normal decrease in systolic blood pressure during inspiration. In acute asthma, it has been shown to correlate well with pulmonary function tests, and it can be useful in assessing asthma severity and response to treatment. A pulsus paradoxus < 10 is considered normal; 10–20 indicates moderate obstruction; and > 20 indicates severe obstruction.

To measure pulsus paradoxus, inflate the cuff of a manual sphygmomanometer, then auscultate as pressure is gradually released. Initially, Korotkoff sounds are heard only during expiration and disappear with inspiration. The difference between the pressure at which the sounds are first heard and the pressure at which they cease to disappear with expiration is the pulsus paradoxus.

Other conditions that may cause an elevated pulsus paradoxus include severe pneumonia, pericardial effusion, constrictive pericarditis, and conditions affecting myocardial compliance (e.g., amyloidosis, endomyocardial fibroelastosis).

9 A 6-year-old with asthma is treated for acute wheezing in the ED with only partial relief and needs to be admitted to the hospital. Should a chest x-ray be obtained?

Chest radiography is of limited value in the evaluation of the patient with acute asthma. It may be useful in identifying those patients with airleak (i.e., pneumothorax or pneumomediastinum) or a concomitant pneumonia. However, routinely obtaining radiographs in all patients with acute asthma, or even all patients with exacerbation serious enough to require hospital admission, rarely leads to an unexpected finding that changes management. In patients with known asthma, chest radiographs should be limited to cases where there is clinical suspicion of a radiographic abnormality, such as persistent rales or asymmetry of breath sounds, high fever, crepitus in the neck, or very poor response to therapy or deterioration after initial therapy.

10 What is the value of radiography in children with a first episode of wheezing?

Routine use of radiography in such children is of relatively low yield. However, most authorities recommend obtaining an x-ray in a child with a first episode of wheezing. A number of conditions other than reactive airways disease may present with wheezing, and these should be ruled out before a diagnosis of asthma is made (Table 41-2). Radiography is probably not necessary in children with clinical bronchiolitis of mild-to-moderate severity, or in older children with a family history of asthma who respond completely to inhaled bronchodilators; however, in most cases of first-time wheezing, a chest x-ray is prudent.

Table 41-2 Other Causes of Wheezing

Roback MG, Dreitlin DA: Chest radiograph in the evaluation of first time wheezing episodes: Review of current clinical practice and efficacy. Pediatr Emerg Care 14:181–184, 1998.

11 What is the role of exhaled nitric oxide (NO) measurement in the assessment of asthma?

NO is generated from the action of nitric oxide synthase on L-arginine in the lung. NO synthase activity increases during the inflammatory process, producing increased concentrations of NO in exhaled air. Measurement of exhaled NO levels has been shown to be a sensitive, noninvasive means of measuring the degree of airway inflammation in adult and pediatric patients, and it correlates well with other markers of chronic asthma severity. Available data suggest, however, that exhaled NO levels correlate poorly with acute severity in ED patients.

Gill M, Walker S, Khan A, et al: Exhaled nitric oxide levels during acute asthma exacerbation. Acad Emerg Med 12:579–586, 2005.

12 What is the preferred initial treatment for children with acute asthma?

Treatment is tailored to the severity of the exacerbation, but treatment with inhaled bronchodilators is the mainstay of initial therapy for all patients. Those with moderate to severe exacerbation (as determined by clinical status or PEFR) should receive 3–4 doses of an inhaled bronchodilator in the first hour of treatment until there is good response; those with more mild disease may require less aggressive treatment. In addition, systemic steroids should be given immediately in severely ill patients, to those who are symptomatic despite appropriate bronchodilator therapy at home, and to all patients who fail to show complete response to one or two inhalation treatments in the ED.

Expert Panel on Management of Asthma: Guidelines for the Diagnosis and Management of Asthma: Clinical Practice Guidelines. Available at: http://www.nhlbi.nih.gov/guidelines/asthma/asthgdln.htm

13 Isn’t nebulization a more effective means of delivering inhaled beta-agonists than a metered dose inhaler (MDI)?

Several studies and meta-analyses have shown that nebulizer and MDI are clinically equivalent, provided that an appropriate dose is given and that a spacer device is used with the MDI. Standard doses of nebulized beta-agonist (0.15 mg/kg per dose, maximum 5 mg) are substantially higher than for MDI. However, the particles produced by the MDI deliver medication far more effectively to the lungs. Current recommendations of ED dosages of albuterol for acute asthma are for 2–4 puffs of albuterol for young children, 4–6 puffs for older children, and 4–8 puffs for adolescents and adults. Spacer devices (e.g., Aerochamber^®) are available in a variety of sizes and configurations and can be used successfully even in young infants. In one sense the delivery systems are not equivalent: Studies have shown that use of MDI is less expensive and leads to shorter ED stays.

Cates CJ, Bara A, Crilly JA, Rowe BH. Holding chambers versus nebulisers for beta-agonist treatment of acute asthma. Cochrane Database of Systematic Reviews 2003, Issue 2. Available at: http://www.cochrane.org/cochrane/revabstr/AB000052.htm

14 What are the adverse effects of albuterol?

Note that even with high-dose therapy, while mild degrees of hypokalemia are common (one-third of children in one study), symptomatic hypokalemia requiring treatment is rare. Patients receiving high-dose continuous albuterol for more than 6–8 hours should have serum potassium monitored.