A patient is moved from a supine to an upright position in the dental chair and reports feeling dizzy and lightheaded. What is the MOST likely explanation?

Orthostatic hypotension (postural hypotension) is a drop in systolic BP of ≥20 mmHg (or diastolic ≥10 mmHg) within 3 minutes of standing or assuming upright position. In the dental office, it occurs when rapidly moving a patient from supine (reclined) to upright. Blood pools in the lower extremities, temporarily reducing cerebral perfusion. Common in elderly patients, those on antihypertensives, diuretics, or after prolonged supine positioning. Management: lower the chair back, monitor vitals.