oral administration and maximal effects seen within
2–4 hours. Although it is most commonly administered in a loading regimen of 0.1–0.2 mg followed by 0.1 mg hourly for several hours, evidence suggests that
comparable responses may be seen with a single 0.2 mg dose.3 The most common adverse effect seen in
the acute setting is drowsiness, affecting up to 45% of patients. Clonidine may be a poor choice when monitoring of mental status is important. Dry mouth is also a common complaint and lightheadedness is occasionally observed.
Labetalol is a combined α- and β-adrenergic blocking agent.
It can be effective given orally in a dose of 200–400 mg, which may be repeated after
2–3 hours. The onset of effect is observed within 1–2
hours.9,10 Like any β blocking agent, it has the potential to induce heart block and to worsen symptoms of bronchospasm. Therefore,
it should be avoided in patients with uncontrolled asthma or those with more than first-degree heart block, symp-
tomatic bradycardia,
or congestive heart failure.
Prazosin is an α-adrenergic blocking agent that can have limited benefit in the early management ofpheochromocytoma. Side effects include first-dose
syncope, palpitations, tachycardia, and orthostatic
hypertension.
AGENTS FOR HYPERTENSIVE
EMERGENCIES
Parenteral Agents
The following parenteral agents are effective in treating hypertensive emergencies
(see Table IV).7 Labetalol has proved particularly effective when used in bolus intravenous injections in the initial treatment of hypertensive emergencies, and can provide a controlled reduction in blood pressure to a predetermined goal.11 Once a goal pressure is achieved, injections are stopped, and the long duration of action facilitates conversion to effective oral therapy.