Vasodilators

Hydralizine

Arterial vasodilator given parentally in emergency. It is given in combination with other drugs.

Hydralizine is a Phenelzine derivative. It is a direct arterial dilator having negligible effects on the veins. More effects are produced on the blood vessels of the heart, kidney, brain and less effects are produced on blood vessels of skin and muscles. On prolonged treatment, increase in blood pressure occurs due to:

Reflex increase in sympathetic activity
Increase in noradrenaline
Increase in renin

Pharmacokinetics

Hydralizine is well absorbed after oral route of administration. Peak plasma concentration occurs in 30-120 minutes. It is given I/V in hypertensive emergency. It is metabolized in the liver by the process of acetylation, with some genetic variations. In fast acetylators, half life is shorter (3-4 hours) while in slow acetylators half life is about 5 hours. It has prolonged duration of action due the active metabolite (12 hours).

Adverse Effects

Due to vasodilatation there is nasal congestion, flushing.
When given for prolonged periods, arthralgia and myalgia have been reported.
In high doses there is SLE like syndrome, serum sickness vasculitis, rarely hemolytic anemias are observed, glomerulonephritis is seen with it and there is polyneuropathy (responsive to pyridoxine).

Dose

20-40 mg I/V in hypertensive emergencies
25 mg tablet given orally twice or thrice daily depending on the blood pressure of the patient.

Diazoxide

Arteriolar vasodilator

Chemistry

Chemically related to thiazide diuretics but has no diuretic activity, rather causes salt and water retention.

Mechanism of Action

Exerts vasodilator effects, by causing increase in efflux of K+ and decrease in the influx of Ca++.

Adverse Effects

Excessive hypotension may occur, leading to MI and stroke.
In patients with ischemic heart disease, aggravates the angina due to reflex tachycardia.
Inhibits the release of insulin from pancreas, can be used to treat hypoglycaemia.
Can cause salt and water retention.

Therapeutic Uses

Hypertensive emergencies –main use
Treatment of hypoglycemias due to insulinomas (tumour secreting insulin)

Minoxidil

Arterial dilator. Prodrug, which is activated in the liver by sulphotransferrase enzyme in to minoxidil sulphate.

Mechanism of Action

It produces vasodilatation by causing increase in cGMP. In addition, causes increased efflux of K+ leading to hyperpolarization and interferes with mobilization of Ca++. It has more effect of the blood vessels of the heart, kidney, GIT, skin and has less effect on blood vessels of brain. When given for prolonged period of time, it causes increase in blood pressure, due to:

Reflex increase in sympathetic activity
Increase in noradrenaline
Increase in renin

This is why diuretics are added.

Pharmacokinetics

Well absorbed after oral administration. Peak plasma levels are observed in about 1 hour. 20% of the drug is eliminated as such in urine. Rest is metabolized by the liver. Half life is 3-4 hours.

Toxicity

Salt and water retention
Tachycardia due to reflex sympathetic activity
Hypertrichiosis (increase in growth of fine hair over scalp, abdomen and legs)
Allergic manifestations
Rarely thrombocytopenia

Dose

40 mg tablet.
For I/V use, 10-20 mg
For topical preparations 2% lotions are used for treatment of baldness, but have only temporary effect.

Sodium Nitroprusside

Arteriolar and venous dilator used in hypertensive emergencies and post operation hypertension. It is given through continuous I/V infusions. It is actually a complex of cyanide, iron and nitroso group. It is metabolized in the RBCs and here cyanide is liberated. This cyanide interacts with sulphur groups in mitochondria, producing thiocyanate.

Toxicity

2 types of toxicity are seen

Cyanide toxicity

Toxicity of Thiocyanate

In cyanide toxicity, there can be metabolic acidosis, arrhythmias, hypotension and even death. It can be treated by

Giving sodium thiosulphate (increases metabolism of cyanide)
Hydroxycobalamine (cyanocobalamine formed)

Thiocyanate toxicity is seen in the elderly and patients with renal insufficiency, as it is slowly eliminated through the kidneys. Muscle weakness, muscular cramps are seen and patients can manifest in form of psychosis, hallucination disorders, etc.