HYPOCAPNIA AND CEREBRAL HYPOPERFUSION IN ORTHOSTATIC INTOLERANCE

Background and Purpose-Orthostatic and other stresses trigger tachycar dia associated with symptoms of tremulousness, shortness of breath, di zziness, blurred vision, and, often, syncope. It has been suggested th at paradoxical cerebral vasoconstriction during head-up tilt might be present in patients with orthostatic intolerance:. We chose to study m iddle cerebral artery (MCA) blood flow velocity (BFV) and cerebral vas oregulation during tilt in patients with orthostatic intolerance (OI). Methods-Beat-to-beat BFV from the MCA, heart rate, CO2, blood pressur e (BP), and respiration were measured in 30 patients with OI (25 women and 5 men; age range, 21 to 44 years; mean age, 31.3+/-1.2 years) and 17 control subjects (13 women and 4 men; age range, 20 to 41 years; m ean age, 30+/-1.6 years); ages were not statistically different. These indices were monitored during supine rest and head-up tilt (HUT). We compared spontaneous breathing and hyperventilation and evaluated the effect of CO2 rebreathing in these 2 positions. Results-The OI group h ad higher supine heart rates (P<0.001) and cardiac outputs (P<0.01) th an the control group. In response to HUT, OI patients underwent a grea ter heart rate increment (P<0.001) and greater reductions in pulse pre ssure (P<0.01) and CO2 (P<0.001), but total systemic resistance failed to show an increment. Among the cerebrovascular indices, all BFVs (sy stolic, diastolic, and mean) decreased significantly more, and cerebro vascular resistance (CVR) was increased in OI patients (P<0.01) compar ed with control subjects. In both groups, hyperventilation induced mil d tachycardia (P<0.001), a significant reduction of BFV, and a signifi cant increase of CVR associated with a fall in CO2. Hyperventilation d uring HUT reproduced hypocapnia, BFV reduction, and tachycardia and wo rsened symptoms of OI; these symptoms and indices were improved within 2 minutes of CO2 rebreathing. The relationships between CO2 and BFV a nd heart rate were well described by linear regressions, and the slope was not different between control subjects and patients with OI. Conc lusions-Cerebral vasoconstriction occurs in OI during orthostasis, whi ch is primarily due to hyperventilation, causing significant hypocapni a. Hypocapnia and symptoms of orthostatic hygertension are reversible by CO2 rebreathing.