The selective and non-selective alpha blockers are categorised and listed in table 1 below,(see below). The principal interactions of alpha blockers are those relating to enhanced hypotensive effects. Early after introduction of selective alpha blockers it was discovered that, in some individuals, they can cause a rapid reduction in blood pressure on starting treatment (also called first-dose effect or first-dose hypotension). The risk of this may be higher in patients already taking other antihypertensive drugs. The first-dose effect has been minimised by starting with a very low dose of alpha blocker, and then escalating dose slowly over a couple of weeks. A similar hypotensive effect can occur when dose of alpha blocker is increased, or if treatment is interrupted for a few days and then re-introduced. Some manufacturers recommend giving first dose on retiring to bed, or if not, avoiding tasks that are potentially hazardous if syncope occurs (such as driving) for first 12 hours. If symptoms such as dizziness,fatigue or sweating develop, patients should be warned to lie down, and to remain lying flat until they abate completely.
It is unclear whether there are any real differences between alpha blockers in their propensity to cause this first-dose effect. With exception of indoramin, postural hypotension, syncope, and dizziness are listed as adverse effects of alpha blockers available in UK and for most it is recommended that they should be started with a low dose and titrated as required. Tamsulosin is reported to have some selectivity for alpha receptor 1A subtype, which are found mostly in prostate, and therefore have less effect on blood pressure: an initial titration of dose is therefore not considered to be necessary. Nevertheless, it would be prudent to exercise caution with all drugs in this class.
Alpha blockers are also used to increase urinary flow-rate and improve obstructive symptoms in benign prostatic hyperplasia. In this setting,their effects on blood pressure are more of an adverse effect, and their additive hypotensive effect with other antihypertensives may not be beneficial.
|Table 1 Alpha blockers|
|Selective alpha1 blockers (Alpha blockers)|
|Indoramin||BPH; Hypertension; Migraine|
|Prazosin||BPH; Heart failure; Hypertension; Raynaud’s syndrome|
|Other drugs with alpha-blocking actions|
|Moxisylate||Peripheral vascular disease; Erectile dysfunction|
|Phenoxybenzamine||Hypertensive episodes in phaeochromocytoma; Neurogenic bladder; Shock|
|Phentolamine||Erectile dysfunction; Hypertensive episodes in phaeochromocytoma|