Abnormal baroreflex responses in patients with idiopathic orthostatic intolerance.

BACKGROUND Patients diagnosed with idiopathic orthostatic intolerance report symptoms of lightheadedness, fatigue, and nausea accompanied by an exaggerated tachycardia when assuming the upright posture. Often, these symptoms are present in the absence of any decrease in arterial pressure. We hypothesized that patients with idiopathic orthostatic intolerance would have impaired cardiac vagal and integrated baroreflex function, lower blood volume, and increased venous compliance. METHODS AND RESULTS Sixteen patients and 14 healthy control subjects underwent the modified Oxford technique to assess cardiac vagal baroreflex sensitivity. Progressive lower-body negative pressure (to -50 mm Hg; LBNP) was used to examine the integrated baroreflex response to progressive hypovolemic stimuli. Blood volume and venous compliance were also assessed. Patients with idiopathic orthostatic intolerance had lower cardiac vagal baroreflex sensitivity (12+/-1 versus 25+/-4 ms/mm Hg; P</=0.01). The integrated baroreflex response to low levels of LBNP was characterized by shorter R-R intervals and more symptoms such as lightheadedness, despite similar levels of blood pressure. There was a trend toward lower blood volume in the patient group (56+/-2 versus 63+/-3 mL/kg; P=0.054). CONCLUSIONS Patients with idiopathic orthostatic intolerance have lower cardiac vagal baroreflex sensitivity and marginally lower blood volume and respond with faster heart rates despite similar levels of arterial pressure during LBNP. These findings may contribute to the exaggerated postural tachycardia and symptoms observed in patients with this disorder.