Drugs and the respiratory system
The respiratory system, extending from the nose to the pulmonary capillaries, performs the essential function of gas exchange between the body and its environment. In other words, it takes in oxygen and expels carbon dioxide.
Drugs used to improve respiratory symptoms are available in inhalation and systemic formulations. These drugs include:
- beta2-adrenergic agonists
- leukotriene modifiers
- mast cell stabilizers
- monoclonal antibodies
Beta2-adrenergic agonists are used to treat symptoms associated with asthma and chronic obstructive pulmonary disease (COPD). Drugs in this class can be either short-acting or long-acting.
Short-acting beta2-adrenergic agonists
Short-acting beta2-adrenergic agonists include:
- albuterol (systemic, inhalation)
- levalbuterol (inhalation)
- metaproterenol (inhalation)
- pirbuterol (inhalation)
- terbutaline (systemic).
Long-acting beta2-adrenergic agonists
Long-acting beta2-adrenergic agonists include:
- formoterol (inhalation)
- salmeterol (inhalation).
Pharmacokinetics (how drugs circulate)
Beta2-adrenergic agonists are minimally absorbed from the GI tract; inhaled forms exert their effects locally. After inhalation, beta2-adrenergic agonists appear to be absorbed over several hours from the respiratory tract. These drugs don’t cross the blood-brain barrier; they’re extensively metabolized in the liver to inactive compounds and rapidly excreted in urine and stool.
Pharmacodynamics (how drugs act)
Beta2-adrenergic agonists increase levels of cyclic adenosine monophosphate by stimulating the beta2-adrenergic receptors in the smooth muscle, resulting in bronchodilation. These drugs may lose their selectivity at higher doses, which can increase the risk of toxicity. Inhaled forms are preferred because they act locally in the lungs, resulting in fewer adverse reactions than systemically absorbed forms.
Pharmacotherapeutics (how drugs are used)
Short-acting inhaled beta2-adrenergic agonists are the drugs of choice for fast relief of symptoms in the patient with asthma. They’re generally used as needed for asthma (including exercise-induced asthma) and COPD. A patient with COPD may use them around-the-clock on a specified schedule. However, excessive use of a short-acting beta2-adrenergic agonist may indicate poor asthma control, requiring reassessment of the patient’s therapeutic regimen.
Safe and sound
Problems with long-acting beta2-adrenergic agonists
If a patient is taking a long-acting beta2-adrenergic agonist, make sure that he’s using it only as part of a combination therapy with other medications such as inhaled corticosteroids. Patients who use long-acting beta2-adrenergic agonists as their only means of asthma control are at serious risk for adverse effects, including death.
A good combination
Long-acting beta2-adrenergic agonists tend to be used with antiinflammatory agents, namely inhaled corticosteroids, to help control asthma.They’re especially useful for the patient with nocturnal asthmatic symptoms. These drugs must be administered on a schedule. They aren’t used to relieve acute symptoms because their onset of action isn’t fast enough. They also don’t affect the chronic inflammation associated with asthma.
Interactions are uncommon when using the inhaled forms. Beta-adrenergic blockers decrease the bronchodilating effects of beta2-adrenergic agonists. They should be used together cautiously. (See
Adverse reactions to beta2-adrenergic agonists
Adverse reactions to short-acting beta2-adrenergic agonists include:
- paradoxical bronchospasm
- dry mouth.
Adverse reactions to long-acting beta2-adrenergic agonists include: