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 )
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.