A 63-year-old male with chronic schizophrenia is brought to the emergency department after a syncopal episode at his assisted living facility. He was recently transitioned from haloperidol 10 mg daily to chlorpromazine 300 mg daily due to intolerable extrapyramidal symptoms. Staff report he stood up from a seated position and lost consciousness briefly. On examination, his supine blood pressure is 138/84 mmHg, which drops to 98/58 mmHg upon standing with a compensatory heart rate increase from 72 to 104 bpm. He denies chest pain, palpitations, or seizure-like activity. Which pharmacologic property of chlorpromazine most directly accounts for this presentation?