Question

Mr. Green is a 60-year-old man with atherosclerosis and poorly controlled hypertension of 170/110 mm.Hh. He has been taking several antihypertensive drugs including minoxidil. Minoxidil is a powerful arteriolar vasodilator that does not act on autonomic receptors. Which of the following effects will be observed if no other drugs are used? PLEASE EXPLAIN IN 2-3 SENTENCES.

a. Tachycardia and increased cardiac contractility

b. Tachycardia and decreased cardiac output

c. Decreased mean arterial pressure and decreased cardiac contractility

d. Decreased mean arterial pressure and increased salt and water excretion by the kidney

e. Bradycardia and decreased cardiac contractility

Answer 1

If no other drugs are not used ,the following effects will be observed ....

Tachycardia and decreased cardiac output,Decreased mean arterial pressure and increased salt and water excretion by the kidney and Bradycardia and decreased cardiac contractility.

Tachycardia and decreased cardiac output

The main reason is minoxidil will nor act on autonomic receptors. Detail explanation follows ...Adrenergic receptors (adrenoceptors) are receptors that bind adrenergic agonists such as the sympathetic neurotransmitter NE and the circulating hormone epinephrine (EPI). The most important adrenoceptor in the heart (not including coronary vascular adrenoceptors) is the β1-adrenoceptor. When activated by a β1-agonist such as NE or EPI, heart rate is increased (positive chronotropy), conduction velocity is increased (positive dromotropy), contractility is increased (positive inotropy), and the rate of myocyte relaxation is increased (positive lusitropy).

There are also β2-adrenoceptors in the heart and stimulation by β2-agonists has similar cardiac effects as β1-adrenoceptor stimulation. The β2-adrenoceptors become functionally more important in heart failure because β1-adrenoceptors become down regulated.

NE can also bind to α1-adrenoceptors found on myocytes to produce small increases in inotropy. Circulating catecholamines (epinephrine) released by the adrenal medulla also bind to these same alpha and beta adrenoceptors in the heart.

In addition to sympathetic adrenergic nerves, the heart is innervated by parasympathetic cholinergic nerves derived from the vagus nerves. Acetylcholine (ACh) released by these fibers binds to muscarinic receptors in the cardiac muscle, especially at the SA and AV nodes that have a large amount of vagal innervation. ACh released by vagus nerve binds to M2 muscarinic receptors, a subclass of cholinergic receptors. This produces negative chronotropy and dromotropy in the heart, as well as negative inotropy and lusitropy in the atria (the negative inotropic and lusitropic effects of vagal stimulation are relatively weak in the ventricles).

Decreased mean arterial pressure and increased salt and water excretion by the kidney and Bradycardia and decreased cardiac contractility.

Beta-blocker or other sympathetic nervous system suppressants

When therapy with minoxidil ( tablets) is begun, the dosage of a beta-adrenergic receptor blocking drug should be the equivalent of 80 to 160 mg of propranolol per day in divided doses.

If beta-blockers are contraindicated, methyldopa (250 to 750 mg, b.i.d.) may be used instead. Methyldopa must be given for at least 24 hours before starting therapy with minoxidil ( tablets) because of the delay in the onset of methyldopa's action. Limited clinical experience indicates that clonidine may also be used to prevent tachycardia induced by minoxidil (tablets) ; the usual dosage is 0.1 to 0.2 mg twice daily.

Sympathetic nervous system suppressants may not completely prevent an increase in heart rate due to minoxidil ( tablets) but usually do prevent tachycardia. Typically, patients receiving a beta-blocker prior to initiation of therapy with minoxidil ( tablets) have a bradycardia and can be expected to have an increase in heart rate toward normal when minoxidil is added. When treatment with minoxidil( tablets) and beta-blocker or other sympathetic nervous system suppressant are begun simultaneously, their opposing cardiac effects usually nullify each other, leading to little change in heart rate

Minoxidil can cause salt and water retention. This can lead to congestive heart failure. To avoid this, the patient has to take diuretic with minoxidil.