What is the risk of hypotension in a 65-year-old hypertensive patient given 12.5 mg of intramuscular prochlorperazine?

Risk of Hypotension with Intramuscular Prochlorperazine in a 65-Year-Old Hypertensive Patient

The risk of hypotension is significant when administering 12.5 mg of intramuscular prochlorperazine to a 65-year-old woman with hypertension (180/100 mmHg), and caution should be exercised due to the medication's alpha-adrenergic blocking properties. 1

Mechanism of Hypotension Risk

Prochlorperazine, as a phenothiazine derivative, produces alpha-adrenergic blockade which can lead to orthostatic hypotension, particularly in:

• Elderly patients (≥65 years) who are more sensitive to the side effects of antipsychotics 1
• Patients with pre-existing hypertension
• Those receiving the medication via intramuscular route

Risk Assessment Factors

Several factors increase this patient's risk of developing hypotension:

1. Age: At 65 years old, she falls into the geriatric population that has increased sensitivity to phenothiazines 1
2. Route of administration: Intramuscular administration can lead to faster absorption compared to oral routes
3. Dose: 12.5 mg is within the standard dosing range (12.5-25 mg) but may still pose risks 2
4. Hypertension status: Despite having elevated blood pressure (180/100 mmHg), phenothiazines can cause paradoxical hypotension through alpha-blockade

Probability of Hypotension

While exact percentages specific to this scenario are not provided in the evidence, the FDA drug label clearly identifies hypotension as an adverse effect of prochlorperazine, with geriatric patients being particularly susceptible 1. The risk is further supported by the multisociety sedation curriculum for gastrointestinal endoscopy, which lists hypotension as a known adverse effect of phenothiazines 2.

Clinical Considerations

• Phenothiazines like prochlorperazine can counteract the antihypertensive effects of certain medications like guanethidine 1
• If thiazide diuretics are being used to treat the patient's hypertension, they may accentuate the orthostatic hypotension caused by phenothiazines 1
• The hypotensive effect is more pronounced when the medication is administered intravenously compared to intramuscular routes 3, 4

Monitoring and Management

If prochlorperazine administration is necessary:

1. Ensure the patient is well-hydrated before administration
2. Monitor blood pressure before and after administration
3. Have the patient remain recumbent for at least 30 minutes after injection
4. Assist the patient to stand slowly and observe for symptoms of orthostatic hypotension
5. Consider having calcium chloride available, as it has been reported to reverse hypotensive effects of phenothiazines in some cases 5

Alternative Considerations

For a 65-year-old hypertensive patient requiring an antiemetic:

• Consider lower initial doses (e.g., 5-10 mg instead of 12.5 mg)
• Evaluate alternative antiemetics with lower hypotension risk if appropriate for the clinical situation
• If prochlorperazine is still indicated, ensure close monitoring of vital signs

The risk-benefit ratio should be carefully evaluated, especially given the patient's age and existing hypertension, as the potential for hypotension could lead to adverse outcomes including falls or inadequate organ perfusion in this population.