Drug Interactions between Ocu-Pentolate and Vazculep

MONITOR: The pressor response to phenylephrine may be potentiated by the vagolytic effect of atropine, which inhibits the reflex bradycardia that would normally accompany any increases in blood pressure induced by phenylephrine. Other antimuscarinic agents may also participate in this interaction, although clinical data are lacking. In one report, pseudo-pheochromocytoma (i.e., significant increases in blood pressure and tachycardia) occurred in seven patients who underwent eye surgery and were given phenylephrine 10% ophthalmic solution and systemic atropine, three of whom subsequently developed left ventricular failure and pulmonary edema that required intensive care monitoring. Two patients had preexisting hypertension, while others had no known history of cardiovascular disease. All had received general anesthesia with propofol, phenoperidine, and vecuronium. Since phenylephrine use alone may be associated with cardiovascular toxicities including hypertension, arrhythmia, myocardial infarction and cardiac failure, the extent of involvement by atropine is uncertain. The authors reported no further cardiovascular events following implementation of various measures that reduced phenylephrine dosage and systemic exposure, including: use of a milder strength of phenylephrine ophthalmic solution; swabbing to minimize drainage into the nasolachrymal duct to the nasal mucosa; and use of a cannula to reduce drop size. In a study of six healthy volunteers, diastolic and systolic blood pressure increased by 4 mmHg following administration of phenylephrine (0.42 mcg/kg/min), compared to 17 mmHg when phenylephrine was given after three doses of atropine (0.02, 0.01 and 0.01 mg/kg at 30 minute intervals).

MANAGEMENT: Caution is advised if phenylephrine (systemic or ophthalmic) is used in combination with atropine or other antimuscarinic agents. Cardiovascular status, including blood pressure and heart rate, should be closely monitored. When using ophthalmic formulations, measures to minimize systemic absorption should be employed, such as digital compression of the lacrimal sac or lid closure after instillation. A milder strength (< 10%) is preferable if phenylephrine ophthalmic solution is given.

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