Q : Beta blockers reduce blood pressure mainly through
- Reduction in Heart rate
- Reduction in cardiac output
- Negative Inotropic action
- Vascular sensitization to circulating catecholamines
- Blocks Renin secretion and reduce vascular tone.
Answer : 5 (May be 4 as well !)
Our understanding of beta blocker’s action in SHT has changed considerably over the years .The negative inotropic action on the myocardium attributed for BP reduction , is no longer considered important . Now we know , beta blockers can reduce peripheral vascular resistance significantly.(There were days , we presumed the opposite to be true , ie when beta blockers are blocked , alpha action will overshoot to cause excess vascular resistance ! ) This is more of perceived fear. This concept was never proved convincingly even in the dreaded Prinzmetal angina* where beta blockers are relatively contraindicated for fear of aggravating vasospasm.
*Note : This is may still be valid in selected few who show a tendency for Raynaud phenomenon especially in peripheral vascular system.
Additional factors influencing beta blockers in SHT
- Suppression of central adrenergic drive , modulation of brain stem vasomotor centre are aslo considered vital . This action is linearly related to the ability of beta blockers to cross the blood brain barrier which is more with lipophilic drugs like metoprolol.
- The role of beta blocker in isolated systolic hypertension in elderly is unique.Here it reduces the myocardial dp/dt (ie contractility ) and hence help them prevent systolic spikes of pressure and the resultant stroke.
- The newer vasodilating beta blockers like Nebivolol, (Nitric oxide mediated ?) and Carvidilol may have additional advantage in controlling BP.
- It needs to be appreciated , beta blockers combine well with diuretics like hydrochlorthaizide .This makes it easier to control severe forms of HT especially volume dependent ones in both young and elderly. (SHEP trial )
Final message
The modification of vascular response to catecholamines is the single most important mechanism of reduction of blood pressure.
This may be a direct consequence of 1. Blockade of vascular adrenergic receptors . 2 Indirectly through suppression of Rennin secretion.
Dr.S.Venkatesan MD 
Dear Dr.Venkatesan, I am Dr.Shajahan writing to you from Saudi Arabia. Your online cardiology service for the medical professionals is highly laudable and I pray that this service of yours should continue for a very very long time for the benefit of the medical community. Can you please simplify the understanding of the mechanisms involved in patients who are on cardiac pacemakers posted for non-cardiac surgery? I am an Anaesthetist from Saudi Arabia. Of course Textbooks deal with cardiac pacemakers but they are not so simplified and things appear to be very technical for an anaesthetist who doesn’t come across such cases frequently. Looking forward to an enlightening explanation from you, Thanks and regards, Dr.Shajahan.