All of the following drugs cause tachychardia except?
Correct Answer: Clonidine
Description: CENTRAL SYMPATHOLYTICS -Clonidine -It is an imidazoline derivative having complex actions. Clonidine is a paial agonist with high affinity and high intrinsic activity at a2 receptors, especially a2A subtype in brainstem. The major hemodynamic effects result from stimulation of a2A receptors present mainly postjunctionally in medulla (vasomotor centre) --decrease sympathetic out flow - fall in BP and bradycardia (also due to enhanced vagal tone). Plasma NA declines. Though clonidine is capable of reducing NA release from peripheral adrenergic nerve endings (release inhibitory prejunctional a2 action), this is not manifest at clinically used doses. Clonidine is a moderately potent antihypeensive. Presence of Imidazoline receptors which are distinct from a., receptors has now been confirmed both in the brain as wdl as periphery. These are activated by clonidine and related drugs but not by NA. Clonidine also appears to directly stimulate a2 receptors to produce hypotension and sedation. Rilmenidine and moxonidine are selective cerebral imidazoline receptor agonists with low a2 receptor affinity. Therefore, they have low sedative propey but equivalent antihypeensive action. Rapid i.v. injection of clonidine raises BP transiently due to activation of peripheral postsynaptic vasoconstrictor u2B receptors at the high concentrations so attained. Oral doses producing lower plasma clonidine levels cause only fall in BP, because clonidine has lower intrinsic activity on u2" receptors which predominate in vascular smooth muscle. Probably for the same reason clonidine exhibits the therapeutic window phenomenon: optimum lowering of BP occurs between blood levels of 0.2-2.0 ng/ml. At higher concentrations fall in BP is less marked. On chronic administration of clonidine decrease in c.o. contributes more to the fall in BP than decrease in t.p.r. Cardiovascular reflexes are affected little. Decreased sympathetic flow to the kidney results in reduced renin release. It does not alter plasma lipid levels. Pharmacokinetics- Clonidine is well absorbed orally; peak occurs in 2-4 hours; 1 /2 to 2/3 of an oral dose is excreted unchanged in urine, the rest as metabolites. Plasma t'h is 8-12 hours. Effect of a single dose lasts for 6-24 hours. Dose: Sta with 100 J..lg 00 or BD, max 300 J..lg TDS, oral or i v ; Side effects with clonidine are common. Sedation, mental depression, disturbed sleep; dr:- r of mouth, nose, and eyes (secretion is decreased central action), constipation (antisecretory eifc.:c the intestines). Impotence, salt and water retention, bradycardia to reduced sympathetic tone). Postural hypotension occurs but is mostly asymptomatic. Alarming rise in BP, in excess of pretreatment, with tachycardia, restlessness, anxiety, headache, nausea, and vomiting occur in patients when doses of clonidine are missed per Days. ESSENTIALS of MEDICAL PHARMACOLOGY SIXTH EDITION KD TRIPATHI Page 565.,566
Category:
Pharmacology
Get More
Subject Mock Tests
Practice with over 200,000 questions from various medical subjects and improve your knowledge.
Attempt a mock test nowMock Exam
Take an exam with 100 random questions selected from all subjects to test your knowledge.
Coming SoonGet More
Subject Mock Tests
Try practicing mock tests with over 200,000 questions from various medical subjects.
Attempt a mock test now