Clonidine hypotension caused

Clonidine is a selective o -adrenergic agonist. Clonidine has expressed hypotensive action, which is associated with a reduction of general peripheral vascular resistance, reduced frequency of cardiac beats, and a reduction of cardiac output. The mechanism of action of clonidine is caused by stimulation of o -adrenoreceptors of the inhibitory structures of the brain as well as a reduction of sympathetic impulses to the blood vessels and brain. 

The fact that the modes of action of clonidine and a-methylnoradrenaline are similar to the mode of action of the physiological transmitter noradrenaline indicates the importance of the role of the latter in the central control of blood pressure. It may be mentioned that 1-dopa too, the precursor of noradrenaline, penetrates the blood-brain barrier and causes hypotension and bradycardia after systemic administration, when do-... 

Xylometazoline and oxymetazoline are direct-acting agonists. These drugs have been used as topical decongestants because of their ability to promote constriction of the nasal mucosa. When taken in large doses, oxymetazoline may cause hypotension, presumably because of a central clonidine-like effect (see Chapter 11). Oxymetazoline has significant affinity fortx receptors. 

Reduction in arterial blood pressure by clonidine is accompanied by decreased renal vascular resistance and maintenance of renal blood flow. As with methyldopa, clonidine reduces blood pressure in the supine position and only rarely causes postural hypotension. Pressor effects of clonidine are not observed after ingestion of therapeutic doses of clonidine, but severe hypertension can complicate a massive overdose. 

Caution [C, ] CrCl <30 Contra Component sensitivity, asthma, severe COPD, sinus bradycardia Disp Soln SE Irritation, bitter taste, superficial keratitis, ocular allergic Rxn EMS Drug is absorbed systemically OD May cause electrolyte disturbances (K), acidosis and bradycardia monitor ECG Doxazosin (Cardura, Cardura XL) [Antihypertensive/Alpha Blocker] Uses HTN symptomatic BPH Action < [-Adrenergic blocker relaxes bladder neck smooth muscle Dose HTN Initial 1 mg/d PO may be T to 16 mg/d PO BPH Initial 1 mg/d PO, may T to 8 mg/d XR 2-8mg qAM Caution [B, ] Use w/ PDE5 inhibitor (eg, sildenafil) can cause 1 BP Contra Component sensitivity Disp Tabs SE Dizziness, HA, drowsiness, sexual dysfxn, doses >4 mg T postural BP risk Interactions T Effects W/ nitrates, antihypertensives, EtOH i effects W/ NSAEDs, butcher s broom -t effects OF clonidine EMS Concurrent EtOH use can T drowsiness syncope may occur w/in 90 min of initial dose OD May cause profound hypotension place pt in supine position, give IV fluids, use pressors if needed... 

Oxymetazoline (Afrin, Visine, many others). This drug is used in nose drops and nasal sprays to decrease nasal congestion through alpha- 1-mediated vasoconstriction. Higher or systemic doses may also cause hypotension, presumably because central nervous system (CNS) alpha-2 receptors are stimulated in a manner similar to clonidine (see Alpha-2-Selective Agonists ). Oxymetazoline can also be administered as eye drops to decrease redness and minor eye irritation. 

Tizanidine (Zanaflex) Tizanidine is used primarily for treating spasticity.14,24 This drug is similar to clonidine, but has less vasomotor effects and is therefore less likely to cause hypotension and other cardiovascular problems. As indicated earlier, tizanidine stimulates alpha-2 receptors in the spinal cord, which results in decreased excitatory input onto the alpha motor neuron. Decreased excitation of the alpha motor neuron results in decreased spasticity of the skeletal muscle supplied by that neuron. 

Fluoxetine, paroxetine, bupropion, duloxetine, and other CYP450 2D6 inhibitors may increase TCA concentrations Cimetidine may increase plasma concentrations of TCAs and cause anticholinergic symptoms Phenothiazines or haloperidol may raise TCA blood concentrations May alter effects of antihypertensive drugs may inhibit hypotensive effects of clonidine Use of TCAs with sympathomimetic agents may increase sympathetic activity Methylphenidate may inhibit metabolism of TCAs... 

Nonopioid. The withdrawal sjmdrome is also treatable with nonopioid drugs. Lofexidine inhibits sympathetic autonomic outflow by its agonist action on central presynaptic cc -adrenoceptors and so reduces the effects of noradrenergic hyperactivity (see above). It is similar to clonidine (see p. 482) but less likely to cause hypotension. Evidence indicates that lofexidine is as effective as methadone in withdrawal supervised in residential or community settings having no street value it is not liable to be traded. 

Clonidine (Catapres) is an imidazoline which is an agonist to ctj-adrenoceptors (postsynaptic) in the brain, stimulation of which suppresses S5unpathetic outflow and reduces blood pressure. At high doses it also activates peripheral a -adrenoceptors (pre-synaptic autoreceptors) on the adrenergic nerve ending these mediate negative feedback suppression of noradrenaline release. In overdose clonidine can stimulate peripheral Oj-adrenoceptors (postsynaptic) and thus cause hypertension by vasoconstriction. Clonidine was discovered to be hypotensive, not by the pharmacologists who tested it in the laboratory but by a physician who used it on himself as nose drops for a common cold. The tl is 6 h. 

Because clonidine relies on central alpha2-adrenoceptor agonist activity for its hypotensive effects, it can cause hjrpertension in patients with autonomic dysfunction. It should therefore be used with great caution when autonomic dysfunction is suspected. 

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