Beta-Agonists

Mechanism:

Beta-agonist inhalers contain a chemical that activate beta receptors. This receptor is found on many different kinds of cells, including the contracted smooth muscles cells found in the constricted airways of patients suffering from asthma attacks and COPD. The activated receptors causes the muscle cells to relax, causing the airways to open up, and allow air to flow more freely. This reduces feelings of breathlessness in patients with asthma and COPD.

Beta receptors are also found on the surface of heart muscle cells. When these receptors are activated, the heart beats harder and faster, leading it use more oxygen. If the heart muscle works harder than its oxygen supply allows, it will start to die. This effect occured in the 1960s with the first beta-agonist inhaler, which caused some asthmatics to suffer heart attacks. Fortunately, the receptors on the heart are slightly different than the ones in the airways, and research has brought us beta-agonists that specifically target the airways, with little to no effect on the heart. Most modern beta-agonists can even be used in patients with moderate levels of heart disease.

Medications:

Beta-agonists inhalers can be further divided into two groups by their duration of action, short-acting and long-acting.

The primary short-acting beta-agonist used in the US is albuterol, which starts to work within 15 minutes after administration, and continues to be active for 4 to 6 hours. It is sold in the US under a variety of names including Ventolin, ProAir, and Proventil. Due to global treaty to limit CFC pollution, the use of CFCs in inhalers was banned in America, and now all forms of albuterol in the US are sold using HFA propellants. It is often used as a rescue inhaler, as its rapid onset of activity and short duration of activity allows it to break asthma attacks effectively with few side effects.

There are several long-acting beta agonists that are sold in the US. The most commonly used ones are salmeterol and formeterol, which are sold as Severent Diskus and Foradil HFA, respectively. These medications start to work 1 to 2 hours after administration, and last for approximately 12 hours, and need to be taken twice a day. These medications are used to prevent attacks of breathlessness in patients with asthma and COPD, and cannot be used as a rescue inhaler.

While modern beta-agonist inhalers are selective for the airways, long-acting ones should not be used in COPD patients unless they are in conjunction with an inhaled corticosteroid. When long-acting beta-agonists are used alone in patients with COPD, there is a small statistical increase in the severity of COPD symptoms.