Part 3: Acute asthma, prognosis, and treatment

Jennifer E. Fergeson, DO, Shiven S. Patel, MD, and Richard F. Lockey, MD

Asthma affects about 300 million people globally and accounts for 1 in every 250 deaths in the world. Approximately 12 million people in the United States each year experience an acute exacerbation of their asthma, a quarter of which require hospitalization.

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PHYSIOLOGIC AND LABORATORY PARAMETERS

Serial measurements of lung function facilitate quantification of the severity of airflow obstruction and response to therapy. Measurement of peak expiratory flow (PEF) rate provides a simple, quick, and cost-effective assessment of the severity of airflow obstruction. Patients can be supplied with an inexpensive PEF meter and taught to perform measurements at home to detect deterioration of their asthma. An individual management plan will be based on the personal best PEF value. Predetermined PEF values can be set, at which time the patient is alerted to the degree of severity of symptoms and can  institute appropriate therapy, consult their physician, or both(Fig 2). In nonacute settings assessment of PEF and spirometry before and after administration of a bronchodilator can indicate the likely degree of improvement in lung function, which can be achieved by using adequate therapy. PEF values of 50% to79% of predicted or personal best value signify the need for immediate treatment with an inhaled short-acting b-agonist(SABA). Values of less than 50% indicate the need for immediate medical care. Values of less than 35% indicate a possible severe life-threatening episode. This treatment should be administered with a SABA by using a nebulizer or metered-dose inhaler (MDI). SABA dose, response, and further management are depicted in Fig 2. (2,11-14)

 

FEV1 is measured by means of spirometry to assess the volume of air exhaled over 1 second and is the most sensitive test for airflow obstruction. FEV1 is less variable than PEF and is independent of effort once a moderate effort has been made by the patient. Postbronchodilator reversibility should be assessed, and an increase in FEV1 of 12% or greater and 200 mL or greater is diagnostic of asthma.2,11

 

Most patients do not require laboratory testing for the diagnosis of acute asthma. If laboratory studies are obtained, they must not delay asthma treatment. Laboratory studies might assist in detecting other comorbid conditions that complicate asthma treatment, such as infection, cardiovascular disease, or diabetes. Measurement of brain natriuretic peptide and a 2-dimensional transthoracic echocardiogram aid in the diagnosis of congestive heart failure. For patients taking diuretics who have comorbid cardiovascular disease, serum electrolytes might be useful because frequent SABA administration can cause transient decreases in serum potassium, magnesium, and phosphate levels. A baseline electrocardiogram and monitoring of cardiac rhythm are appropriate in patients older than 50 years and those with comorbid cardiovascular disease or COPD. A complete blood cell count might be useful in patients with fever or purulent sputum; however, modest leukocytosis is common in asthmatic patients, and patients with corticosteroids might have a corticosteroid-induced neutrophil leukocytosis. Serum theophylline levels are essential for patients taking theophylline because of its narrow therapeutic window.(11)

 

Chest radiographs are not usually necessary for the diagnosis of acute asthma if examination of the chest reveals no abnormal findings other than the expected clinical signs and symptoms associated with an acute exacerbation. If a complication is suspected, such as pneumonia, pneumothorax, pneumomediastinum, congestive heart failure, or atelectasis secondary to mucous plugging, a chest radiograph should be obtained.(11)

 

Arterial blood gas analysis should be considered in patients who are critically ill and have oxygen saturations of less than 92% or an FEV1 of less than 30% who do not respond to intensive conventional treatment. Proper interpretation of the pH, arterial oxygen pressure, and partial pressure of arterial carbon dioxide (PaCO2) might help further assess the severity of an acute asthma exacerbation (Fig 1). For example, a breathless asthmatic patient presenting with a PaCO2 of 45 mm Hg or greater indicates a life-threatening attack and the need for transfer to a medical intensive care unit for further care. Less than 10% of asthmatic patients presenting to the emergency department have arterial oxygen values of less than 50 mm Hg and carbon dioxide levels of greater than 45 mm Hg.(14,15) Lactic acidosis is common in patients with severe acute asthma. However, increased lactic acid levels are also associated with high doses of inhaled b2-agonist treatment.(16)

Venous blood gases have been evaluated as a substitute for arterial measurements because venous blood is easier to obtain. However, The Expert Panel Report 3 (EPR-3) does not recommend substituting venous partial pressure of carbon dioxide (PvCO2) for arterial blood gas. Arteriovenous correlation for partial pressure of carbon dioxide is poor, and therefore PvCO2 cannot be relied on as an absolute representation of PaCO2. However, a normal PvCO2 has a good negative predictive value for a normal PaCO2. Therefore it might be used as a screening test to exclude hypercapnic respiratory distress.(11,17)