The respiratory tract may be divided into upper and lower portions.
The upper portion consists of the nose, sinuses, oropharynx, and
larynx. The lower portion comprises the trachea and lungs with their
associated airways. Disorders and drug therapy of the upper respiratory
system differ from those of the lower respiratory tract.
Disorders of the upper respiratory tract are those associated
with infections (most commonly uncomplicated viral rhinotracheitis)
and seasonal allergies (allergic rhinoconjunctivitis and rhinotracheitis).
For the most part, these dysfunctions are self-limiting, and the
drug classes used to treat them may be obtained without a prescription
(over-the-counter, OTC). Disorders of the lower respiratory tract
may be broadly classified as parenchymal infections (e.g., pneumonia)
and obstructive airway (bronchial) conditions. In general, the latter
disorders limit expiratory airflow. They are divided into bronchial
asthma, which is characterized by acute episodes, and chronic obstructive
airway disorders. Chronic obstructive airway disorders are further
subdivided into chronic bronchitis, emphysema, bronchiectasis, and
cystic fibrosis. The treatment of infections in all parts of the
respiratory tract is discussed in Chapter 27.
Manifestations of upper respiratory tract dysfunctions include
mucous and watery discharges and vasodilation, mediated in part
through histamine and other substances released from mast cells. Mast
cells are important “gate-keeper” cells that are
concentrated in the skin and other tissues near external body surfaces.
Histamine is produced from the amino acid histidine and is stored
in vesicles. The four histamine receptor subtypes characterized
to date are designated H1 to H4. H1 receptors
mediate mucous discharge and vasodilation, H2 receptors
are important in gastric acid secretion (Chapter 36), H3 receptors
are found in the central nervous system (CNS), and H4 receptors
may modulate inflammatory reactions by chemotactic effects on eosinophils
and mast cells. Secretion of histamine and other mast cell mediators
causes vasodilation of the nasal vasculature, leading to the nasal
congestion and “runny nose” commonly associated
with seasonal allergies and viral infections. Drugs used to decrease
these manifestations include H1 receptor antagonists (antihistamines)
to decrease mucus production and vasodilation, nasal decongestants
to decrease vasodilation, and mast cell stabilizers.
Bronchial congestion with cough and excessive mucus production
are also associated with viral infections. These manifestations
may be relieved with drugs that suppress coughing (antitussives) or
assist in clearance of mucus from larger airways in the lungs (expectorants).
The various drug classes used in therapy of the upper respiratory
tract are outlined in Figure 35–1. Many of the OTC drugs
are presented in Table 35–1. Mast cell stabilizers are
discussed in connection with lower respiratory tract conditions.
Drug classes used to treat upper airway dysfunction.
Initially the drug classes are divided into histamine type 1 receptor antagonists (antihistamines), decongestants, expectorants, and antitussives. Antihistamines
are divided into first and second generations. Decongestants are ...