Drugs affecting the respiratory system with particular reference to asthma.

In 70-80% of asthmatic patients, an attack of asthma will follow 6-10 min of strenuous exercise. The increase in airway resistance is accompanied by hyperinflation and arterial hypoxemia and makes it difficult to continue or resume exercise. The past 10 yr have seen major advances in the prevention and treatment of exercise-induced asthma (EIA). The beta-sympathomimetic drugs, when administered as aerosols, have been shown to be most effective in reversing the airway obstruction, hyperinflation, and exercise-induced hypoxemia in patients with asthma. When administered as aerosols prior to exercise, the increase in airways resistance, hyperinflation, and hypoxemia are blocked in about 90% of patients. In addition to the sympathomimetic agents, sodium cromoglycate will also ameliorate or prevent EIA in 60-70% of patients. Oral administration of beta-sympathomimetics or methylxanthines is less efficacious in the prevention or treatment of EIA. Delay in absorption from the gastrointestinal tract requires at least 1-2 h before a significant response is observed. Prevention of EIA requires an adequate blood level, possibly because a high concentration of the drug does not reach the airway when the drug is given by the oral route. More importantly, aerosol administration prevents EIA at a fraction of the dose required orally. The side effects, such as nausea, tachycardia, and skeletal muscle tremor commonly observed following oral bronchodilator administration are rare with aerosol therapy.