A 50-year-old male with treatment-resistant major depressive disorder has been on venlafaxine 75 mg daily for 8 weeks with partial response — improved sleep and appetite but persistent low energy, poor concentration, and psychomotor retardation. The PMHNP increases the dose to 225 mg daily. At his 6-week follow-up, his mood and energy have improved significantly, but his blood pressure is now 148/94 mmHg (previously 122/78 mmHg). Which of the following best explains this finding?