Uses of α Blockers

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Chapter: Essential pharmacology : Antiadrenergic Drugs (Adrenergic Receptor Antagonists) And Drugs For Glaucoma

Pheochromocytoma :It is a tumour of adrenal medullary cells. Excess CAs are secreted which can cause intermittent or persistent hypertension. Estimation of urinary CA metabolites (VMA, normetanephrine) is diagnostic. In addition, pharmacological tests can be performed.


USES OF Α BLOCKERS

 

1. Pheochromocytoma

 

It is a tumour of adrenal medullary cells. Excess CAs are secreted which can cause intermittent or persistent hypertension. Estimation of urinary CA metabolites (VMA, normetanephrine) is diagnostic. In addition, pharmacological tests can be performed.

 

Phentolamine test Inject phentolamine 5 mg i.v. over 1 min in recumbent subject. A fall in BP > 35 Hg systolic and/or > 25 mm Hg diastolic is indicative of pheochromocytoma. However, it is not very reliable and both false positive and false negative results are obtained.

 

Provocative tests have been performed by injecting histamine, methacholine or glucagon—which provoke release of CAs and cause marked rise in BP if pheochromocytoma is present. These tests are dangerous; phentolamine must be available to counteract excessive rise in BP.

 

Therapeutic Phenoxybenzamine can be used as definitive therapy for inoperable and malignant tumours. When surgical removal of the tumour is contemplated, it is desirable to give phenoxybenzamine orally for 1–2 weeks preoperatively and infuse it i.v. during surgery because:

 

·        Due to excess circulating CAs blood volume is low (they shift fluid from vascular to extravascular compartment). Treatment with α blocker normalizes blood volume and distribution of body water.

 

·        Handling of the tumour during surgery may cause outpouring of CAs in blood marked rise in BP. This is prevented by phenoxybenzamine given pre and intraoperatively. Alternatively, phentolamine drip can be instituted during the operation.

 

·        Removal of the tumour is often attended by marked fall in BP as blood vessels dilate and the blood volume is low. This does not happen if volume has been restored before hand with the aid of an α blocker.

 

2. Hypertension

 

α blockers other than those selective for α1 like prazosin have been a failure in the management of essential hypertension, because vasodilatation is compensated by cardiac stimulation. Moreover, postural hypotension, impotence, nasal blockage and other side effects produced by nonselective α blockers are unacceptable. However, phentolamine/phenoxybenzamine are of great value in controlling episodes of rise in BP during clonidine withdrawal and cheese reaction in patients on MAO inhibitors.

 

3. Benign Hypertrophy Of Prostate (BHP)

 

The urinary obstruction caused by BHP has a static component due to increased size of prostate and a dynamic component due to increased tone of bladder neck/prostate smooth muscle. Two classes of drugs are available:

 

·        α1 adrenergic blockers (prazosin like): decrease tone of prostatic/bladder neck muscles.

 

·        5α reductase inhibitor (finasteride): arrest growth/reduce size of prostate.

 

Since activation of α1 adrenoceptors in bladder trigone, prostate and prostatic urethra increases smooth muscle tone, their blockade relaxes these structures, reducing dynamic obstruction, increasing urinary flow rate and causing more complete emptying of bladder in many patients of BHP.

 

Voiding symptoms (hesitancy, narrowing of stream, dribbling and increased residual urine) are relieved better than irritative symptoms like urgency, frequency and nocturia. The α1 blockers afford faster (within 2 weeks) and greater symptomatic relief than finasteride which primarily affects static component of obstruction and has a delayed onset taking nearly six months for clinical improvement. The α 1 blockers do not affect prostate size, but are more commonly used. However, effects last only till the drug is given. Even with continued therapy, benefit may decline after several years due to disease progression. They may be used concurrently with finasteride.

 

Terazosin, doxazosin and tamsulosin are the peferred α1 blockers because of once daily dosing. There is some evidence that terazosin and doxazosin promote apoptosis in prostate. Tamsulosin appears to cause fewer vascular side effects because of relative α1A /α1D selectivity.

 

4. Secondary Shock

 

Shock due to blood or fluid loss is accompanied by reflex vasoconstriction. If volume replacement fails to reverse this (extremities remain pale and cold, pulse pressure does not improve), therapy with an α blocker (phenoxybenzamine i.v.) can help by:

 

·        Counteracting  vasoconstriction.

·        Shifting blood from pulmonary to systemic circuit.

·  Returning fluid from extravascular to the vascular compartment so that cardiac output improves.

 

5. Peripheral Vascular Diseases

 

α blockers do increase skin and to some extent muscle blood flow in normal individuals, but these drugs are largely disappointing in peripheral vascular diseases when obstruction is organic (Buerger’s disease). However, when vasoconstriction is a prominent feature (Raynaud’s phenomenon, acrocyanosis), good symptomatic relief is afforded by prazosin or phenoxybenzamine.

 

6. Congestive Heart Failure (CHF)

 

The vasodilator action of prazosin can afford symptomatic relief in CHF in the shortterm, but longterm prognosis is not improved.

 

7. Papaverine/Phentolamine Induced Penile Erection (PIPE) Therapy For Impotence

 

In patients unable to achieve erection, injection of papaverine (3–20 mg) with or without phentolamine (0.5–1 mg) in the corpus cavernosum has been found to produce penile tumescence to permit intercourse. However, the procedure requires skill and training. Priapism occurs in 2–15% cases, which if not promptly treated leads to permanent damage. This is reversed by aspirating blood from the corpus cavernosum or by injecting phenylephrine locally. Repeated injections can cause penile fibrosis. Other complications are—local haematoma, infection, paresthesia and penile deviation. This therapy should therefore be reserved for selected situations with proper facilities.

 

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