Key Drugs

Historical precedent and current experience would suggest that various parenteral antihypertensive agents are theoretically/potentially suited to specific hypertensive emergencies. These are:

Sodium nitroprusside

It is rapid in onset, rapid in offset, titratable, and it works! It is a potent vasodilator causing a reduction in preload and afterload and can often result in improvement of cardiac output when there is impairment of left ventricular function. It can, however, also cause cerebral vasodilation and exacerbate raised intracranial pressure. The metabolites of nitroprusside (thiocyanate and cyanide) are excreted by the kidney and may accumulate with prolonged usage or in patients with significant renal impairment.

Nitroprusside is light sensitive and the delivery device should therefore be covered in opaque material. Because of its potency, rapid onset and offset, nitroprusside must be administered with regular monitoring in an appropriate environment (e.g. high dependency unit). Extravasation may result in local necrosis. Nitroprusside should not be used in pregnancy due to the potential for foetal poisoning with metabolites.

Learning Bite

Labetolol is the agent of choice for most hypertensive emergencies. It is predictable, titratable and it works!

Labetalol

Labetalol is a mixed alpha – and – beta blocker and lowers BP by vasodilation (vascular smooth muscle receptor blockade) and by reducing cardiac contractility (cardiac receptor blockade). Labetolol does not produce the reflex tachycardia seen with other vasodilators (e.g. nitrates and nitroprusside). It is not associated with reduced cerebral blood flow and is the agent of choice when used for hypertensive stroke syndromes. Because labetolol has a component of alpha -blockade, it can be used as in hypertensive crises that result primarily from stimulation (e.g. phaeochromocytoma) where pure beta -blockers would be contraindicated.

Nitrates

Intravenous nitrates cause vasodilation of capacitance vessels which reduces preload on the heart and left ventricular end diastolic pressure.

It is also a coronary vasodilator. It is, therefore, particularly useful in the setting of hypertension associated with myocardial ischaemia and in left ventricular failure with pulmonary oedema.

Response of the BP to nitrates is often unpredictable and tolerance occurs within 24 hours of commencing the infusion.

Hydralazine

Hydralazine has historically been used in hypertensive emergencies of pregnancy (i.e. eclampsia) because of its theoretical positive effects on uterine blood flow.

More recently, other agents (e.g. nicardipine and labetalol) have been found to be superior.

Hydralazine causes a marked reflex tachycardia and is associated with raised intracranial pressure and so should be avoided in patients with cardiac or cerebral disease.

Nicardipine

Nicardipine is a calcium channel blocker and acts predominantly as a vasodilator. It has a fairly rapid onset of action and is therefore titratable, dosing is not dependant on body weight, it appears to be safe in pregnancy, and it reduces cerebral and coronary ischaemia.

Like all calcium antagonists, it should be used with caution in patients with impaired left ventricular function because of its negative inotropic effects. It is gaining popularity in some countries as an alternative to nitroprusside but a parenteral form is not yet available in the UK.

Phentolamine

Phentolamine is an lpha blocking agent which is used exclusively for hypertensive emergencies related to excess catecholamine release (e.g. phaeochromocytoma, cocaine overdose).
It is (like hydralazine) rarely used today since other agents are effective and have a more predictable response (e.g. nitroprusside and labetalol).

Essentially, all hypertensive emergencies can be treated with one of (or a combination of) three key drugs: nitroprusside, labetalol, or nitrates.

Learning Bite

Nitroprusside, nitrates and labetalol are the three key drugs used in all hypertensive emergencies.