8/12/2019 Review Autonomics

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8/12/2019 Review Autonomics

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-  Alpha 2 agonists: reduce IOP (reduce synthesis of aqueous

humor) 

Effects on Vascular system

Alpha 1 agonist

(ex.

Phenylephrine)

Constrict skin and splanchnic blood vessels

Inc PVR and BP

Evoke compensatory reflex bradycardia

Alpha 2 agonists

(ex. Clonidine)

IV/ topical: vasoconstriction

PO: acculumate in CNS, reduce sympathetic

outflow and BP

Beta 1 agonists Little effect on blood vessel

Beta 2 agonists Reduce PVR and BP

Dopamine Vasodilation in splachnic and renal vascular

beds (D1)

High doses – activate B receptors

Higher doses- activate alpha

-  Sympathomimetics with both alpha and beta1 effects (ex.

NE) have a reflex increase in vagal outflow (evoke

baroreceptor reflex)

o  Inc BP with bradycardia

o  If reflex is blocked (ganglion blocker), inc BP with

tachycardia

- -  Pure alpha agonist (phenylephrine): bradycardia due to

baroreceptor reflex

-  Pure B agonist (isoproterenol): tachycardia

-  Beta 1 agonists: increase renin secretion

-  Beta 2 agonists: increase insulin secretion and liver

glycogenolysis

Clinical Uses

Phenylephrine Mydriasis

No effect on accommodation/cycloplegia

Reduce conjunctival itching and congestion

glaucoma

Terbutaline Tocolysis

Relax uterus

Epinephrine Immediate treatment of anaphylactic shock

glaucoma

B2 selective

antagonists

Acute asthmatic bronchoconstriction

Salmeterol Prophylaxis of bronchoconstriction

Alpha 1 agonists Spinal shock (not for septic shock or MI)

Often mixed with local anesthetics : to

reduce loss of anesthetic into circulation

Ephedrine or

Midodrine (a1)

Chronic orthostatic hypotension

B2 agonists

(Terbutaline,

Ritodrine)

Suppress premature labor

Ephedrine Urinary incontinence in children (enuresis)

Adrenoceptor Blockers

Alpha blocking drugs

Irreversible long acting Phenoxybenzamine

Reversible, shorter acting Phentolamine

Reversible Alpha 1 selective Prazosin

Alpha 2 selective Yohimbine and Rauwolscine

-  Phentolamine – rarely given orally 

-  Phenoxybenzamine – short elimination t ½ but long

duration of action (48h) 

MOA: Phenoxybenzamine binds covalently to alpha receptor

Effects of Alpha blocking drugs

Nonselective Blockers

-  Reduction in vascular tone

-  Reduction of both arterial and venous pressures

-  No significant direct cardiac effects

-  Baroreceptor reflex tachycardia

Epinephrine reversal

-  Large doses of epinephrine with alpha blocker

-  Pressor response (alpha) to depressor response (B2)

Clinical Uses

Phenoxybenzamine Pheochromocytoma- preparatory

Phentolamine Pheochromocytoma- during sx

Phentolamine Prevent tissue damage

Phentolamine Rebound hypertension

Phentolamine/

Yohimbine

Erectile dysfunction

Prazosin

Doxazosin

Terazosin

Hypertension

Prazosin

Doxazosin

Terazosin

Tamsulosin

Urinary hesitancy and prevention of

urinary retention in patients with

BPH

-  Sudden cessation of clonidine therapy leads to rebound

hypertension

Toxicity

-  Orthostatic hypotension

-  Nonselective alpha: marked reflex tachycardia

Beta Blockers

-  Competitive pharmacologic antagonists

-  Prototype: propranolol

Nonselective B1 Selective Combined a/B

Nadolol

Propranolol

Timolol

Acebutolol

Atenolol

Esmolol

Metoprolol

Labetalol

Carvedilol

Partial Agonist Activity (ISA) Full Antagonist

Can cause bronchodilation Cause severe bronchospasm

Pindolol

Acebutolol

Propranolol (not for patients

with airway disease)

Esmolol: short acting, only used parenterally

Nadolol: longest acting B blocker

Toxicity

-  Bradycardia, AV block, heart failure

-  Severe asthma attacks in patient with airway disease