Metered dose inhalers (MDIs) are a familiar and commonly prescribed asthma treatment. If you have asthma, the chances are almost 100% that MDIs are a familiar part of your treatment routine. MDIs are fast acting, and are meant to be used as a short-term relief from the sensations of chest tightness and breathing difficulty associated with sudden, minor asthma attacks.
The medicine in MDIs works by targeting molecules, called beta receptors, that line the walls of the respiratory passages. When stimulated by the medicine, these receptors cause the respiratory passages to expand, relieving asthma symptoms. Because of how it works, this medicine is called a beta-agonist (enhances the activity of beta receptors).
Beta receptors are also an important controller of blood vessel diameter, where their activation works to narrow the diameter of blood vessels. Beta blockers, a very common type of high blood pressure medicine, work by preventing the activation of blood vessel beta receptors, which means that the vessels stay more dilated than normal, lowering blood pressure.
It makes sense to wonder about the effect asthma medications may have on blood pressure, because of their beta-agonist activity. Namely, if asthma medications stimulate beta receptor activity, and increased beta receptor activity causes increased blood pressure, then do asthma medications increase blood pressure?
Yes and no. If you were to directly expose blood vessels to the beta agonist asthma medication, you would see some small amount of vessel constriction. However, this does not routinely occur in asthma patients using MDIs. There are several reasons for this:
- MDIs are an inhaled drug, and close to 100% of the drug remains within the lungs, and away from the beta receptors on blood vessels
- The beta agonist used in MDIs (albuterol) is selective – though not perfectly – for a subtype of beta receptor that is found in large numbers on respiratory passages, but in much smaller numbers on blood vessels
- Albuterol has a very short timeframe of activity, so even though a small amount of drug may find its way to blood vessels, and a small proportion of that drug may cause an effect on those vessels, the effect goes away quickly