Cardiac Drugs 3 of 5 beta-blockers

Beta Blockers (Lols)
Cardiac Drugs Series #3 of 5

In this third edition on getting to know your cardiac drugs, we cover the B-blockers, or more correctly the
Beta- Adrenoreceptor antagonists.

Examples of these drugs include :
Propranolol
Metoprolol
Atenolol

To grasp these antiarrythmic and anti hypertensive drugs, you first need to review the relationship between stress, or shock and BP.

In shock when tissue is poorly perfused, or stress (fight or flight) the Adrenal Medulla releases a cocktail of hormones that aim to elevate BP.
Two such hormones are Adrenaline (or Epinephrine by another name), and Norepinephrine.

Their effects are many and varied, but happen because they bind to specialised receptors called alpha 1 & 2, beta1 and beta2 and Beta 3 adrenergic receptors.

As the name (beta blocker) implies, this class of drugs inhibits the beta 1,2and 3 effects, but newer more beta2 selective blockers have a dramatic effect.

Beta 2 receptors increase the force of cardiac contraction and blood flow to muscle, liver and brain.  But in doing so, this places a lot of demand on the heart.  In a patient with heart failure the use of a drug to inhibit this taxing effect of adrenaline and norepinephrine can assist the heart to reduce its workload, and pump more efficiently.

Beta blockers work on vascular smooth muscle, cardiac muscle and smooth muscle in the lungs.

Where lungs bronchiole smooth muscle is normally opened by adrenaline, beta blockers can actually cause bronchoconstriction , which is a serious side effect worsening ventilation and gas exchange.  In patients with severe restrictive lung diseases (COPD or asthma), this class of drug is best avoided.

Another effect is slowing down of the heart, so in patients who are Beta blocked, they have a reduced capacity to compensate in states of shock.

When expecting tachycardia in a stressed, anxious or shocked patient, the pulse rate barely elevates in those on these medications.  It is for this effect that this class of drugs is used to
inhibit tachyarrythmias.

Other strange effects of these drugs include insomnia, vivid nightmares and dreams, and blood sugar/lipid disturbances.