The obstructive lung diseases are characterized by reduction of expiratory flow rates and include common disorders such as asthma and chronic obstructive pulmonary disease (COPD) and less common ones such as bronchiectasis and cystic fibers. Controversy and confusion reign over the definition of even the common ones because of the marked overlap of clinical and pathophysiological features. Some have even suggested the abandonment of the traditional names and the substitution of groupings based on signs and symptoms.
Asthma has been defined as a disease in which there is an increased responsiveness of the airways to various stimuli, causing widespread narrowing of the airways which varies over time. The stimulus may be immunological in origin, as in classic extrinsic asthma, in which mast cells, sensitized by IgE antibodies, degranulate and release Broncho active mediators following exposure to a specific antigen. The cause may also be unclear, as in adult-onset asthma, in which the patients frequently show no evidence of allergy. The airway obstruction may be due to bronchoconstriction alone or may involve mucosal inﬂammation and excessive mucus production. Symptoms may be intermittent, or they may gradually become persistent. The recognized categories of asthma are listed in Table 1. Clinical differentiation is important only in situation in which there are clear-cut, easily identiﬁable, and avoidable extrinsic factors, such as drugs or industrial substances.
Table 1. Types of asthma
The diagnosis of asthma is based on the presence of episodic dyspnea associated with wheezing. Intermittent cough, probably due to stimulation of the irritant receptors, is commonly seen and may be the sole presenting symptom in some patients. Typically, symptoms are worse at night, following exercise, after going out in the cold, while exposed to irritating gases, etc.
Laboratory studies may be required to determine the presence of specific types of asthma (Table 2). The chest x-ray demonstrates hyperinﬂation in the symptomatic patient, whereas in patients with allergic Broncho pulmonary aspergillosis serial films may show infiltrates that change location or features suggestive of central bronchiectasis. Pulmonary function studies show the findings of obstruction, which improve significantly following the acute administration of bronchodilators. During the asymptomatic phase, the diagnosis can often be made by initiating bronchospasm by the inhalation of histamine, methacholine, or cold air.
Acute severe asthma (status asthmaticus) refers to an attack of increased severity that is unresponsive to routine therapy. While the attack is sometimes prolonged, fatal episodes may occur unexpectedly with overwhelming suddenness. A history of increasing bronchodilator use with little benefit is expected, but clinical signs, including pulsus paradoxus, are extremely unreliable in judging the severity. The degree of physiological disturbance can best be appreciated by a measure of expiratory flow rates. In the emergency room management of these patients, such indices are helpful in assessing the response to therapy, as they provide immediate quantitative information and can be obtained at frequent intervals without discomfort. Complementary information can be obtained by measurement of arterial blood gases, and this may be the only measurement possible in the critically ill asthmatic. Hypoxemia is usually, but not invariably, present and does not correlate closely with airway obstruction. Paco2 is typically reduced early in an attack.
With increasing severity Pao2 falls and Paco2 returns to normal and then rises, accompanied by a mixed respiratory and metabolic acidosis, such that intubation and mechanical ventilation may become necessary. Hypercapnia at presentation is not an indication for intubation, as most patients improve with vigorous treatment, but careful monitoring is essential. In general, arterial blood gas measurements are less sensitive and specific than assessment of airway obstruction in judging the response to therapy.
Table 2. Diagnostic studies in asthma
Abbreviation: ABPA = Allergic Broncho Pulmonary Aspergillosis.
Dr. Afsaneh Jeddi