What is asthma, and how is it diagnosed?

Asthma is a common disease characterized by reversible airway obstruction with components of hyperresponsiveness and inflammation. Asthma “attacks” are characterized by episodes of shortness of breath, wheezing, and often cough. These episodes usually follow exposure to known triggers (e.g., pollen, dust, animal dander, smoke, change in weather, viral illness). The diagnosis is most commonly made by history and examination alone. Measurements of peak expiratory flow (PEF) rate and forced expiratory volumes and their response to bronchodilator administration may sometimes be necessary.

What are the characteristic pulmonary function test findings seen in obstructive and restrictive lung disease, asthma, and chronic obstructive pulmonary disease?

Pulmonary function tests (PFTs) comprise spirometry and flow-volume loops. An arterial blood gas and diffusion of carbon monoxide can also be included. Typical values in obstructive and restrictive lung disease are shown in Table 78-1 .

TABLE 78-1

Results of Pulmonary Function Tests in Obstructive and Restrictive Lung Disease

Value Obstructive Restrictive
FVC Normal or decreased Decreased
FEV 1 /FVC Decreased Normal or increased
MMEFR (FEF 25%-75% ) Decreased Normal
MBC Decreased Normal
TLC Normal or increased Decreased
RV Increased Decreased
DLCO Decreased in COPD Decreased
Normal in asthma

COPD, Chronic obstructive pulmonary disease; DLCO, diffusion capacity of the lung for carbon monoxide; FEF 25%-75% , forced expiratory flow between 25% and 75% of forced vital capacity; FEV 1 , forced expired volume in the first second; FVC, forced vital capacity; MBC, maximum breathing capacity; MMEFR, maximal midexpiratory flow rate; RV, residual volume; TLC, total lung capacity.

Asthmatics usually have normal PFTs between exacerbations. Narrowing limited to small airways may yield a normal ratio of forced expired volume in the first second (FEV 1 ) to forced vital capacity (FVC), but the more sensitive forced expiratory flow between 25% and 75% of FVC (FEF 25%-75% ) would be decreased. Also, the FEV 1 /FVC ratio is effort dependent and requires patient cooperation for accurate measurement, whereas FEF 25%-75% does not depend on patient effort. FEF 25%-75% is obtained by dividing the volume expired between 25% and 75% of the FVC by the time elapsed between these two points. Occasionally, bronchospasm may be intentionally triggered during PFT evaluation with methacholine or histamine to assess airway reactivity in patients with normal baseline PFTs who are suspected to have asthma.

For patients with chronic obstructive pulmonary disease (COPD), measurements are repeated after inhaled bronchodilators to evaluate the degree of reversibility. The fixed component of COPD is due to inflammation and airway destruction.

Flow-volume curves may also be helpful. A normal curve and typical curves from patients with obstructive and restrictive disease are shown in Figure 78-1 . By convention, inspiration is below the x axis, and expiration is above the x axis. In restrictive disease, airway resistance is normal with no flow limitation, whereas lung volumes are reduced. In obstructive disease, the expiratory flow curve shows a characteristic flattening secondary to increased airway resistance.

FIGURE 78-1 ■

Flow-volume curves in a normal patient (continuous curve) , in a patient with obstructive lung disease (dashed curve) , and in a patient with restrictive lung disease (dotted curve) .

Curves from patients with fixed airway obstruction and variable extrathoracic and intrathoracic obstructions are shown in Figure 78-2 . Fixed obstruction, exemplified by tracheal stricture or compression by a tumor or goiter, causes decreases in inspiratory and expiratory flows. Variable extrathoracic obstruction, such as vocal cord paralysis or marked pharyngeal muscle weakness, causes airway collapse during inspiration because negative inspiratory pressure is transmitted to the extrathoracic airway. Mobile surrounding tissues are drawn into the airway, obstructing the flow of gas. With variable intrathoracic obstruction (e.g., caused by tracheal or endobronchial tumor), airway narrowing increases during forced expiration.

Jul 14, 2019 | Posted by in ANESTHESIA | Comments Off on Asthma
Premium Wordpress Themes by UFO Themes