Cerebral Vasomotor Reserve in Hypertension during Head-up Tilt
Objective: To evaluate cerebral vasomotor reserve in hypertension during head-up tilt with hypo- and hypercapnia. Background: Cerebral vasoregulation determines degree of perfusion during increased metabolic demands such as orthostatic stress, hypo- and hypercapnia. With hypertension, silent hypoperfusion may develop if vasomotor reserve is compromised due to remodeling of the vessel wall and impaired vasodilation. Methods: We studied 15 mild hypertensive (BP 145/95 mm Hg) and 15 control men and women (aged 35–65 years) using Transcranial Doppler. Blood flow velocities (BFV) from both middle cerebral arteries (MCA), brachial artery (BRA), heart rate, blood pressure (BP), respiration and carbon dioxide (CO2) were simultaneously recorded during: hyperventilation (3 min), apnea (1 min) and CO2 rebreathing (3 min) in supine rest and head-up tilt at 80O. Vasomotor reserve (VMR) was calculated from systolic (VMRS), diastolic (VMRD) and mean BFVs (VMRM) using regression analysis as percentual increment between hypo-, normo- and hypercapnia. Results: In controls, BFVs and VMR reserve were symmetric in both MCAs and there were no significant differences between rest and head-up tilt. In hypertensive patients, VMRM and were significantly reduced during supine rest (p<0.05) and head-up tilt (p<0.01) compared to the controls by about 50% (Table 1)in both MCAs. In controls, VMRD doubled during hypercapnia, while no significant increment was found in hypertensive patients. Systolic BFVs and VMR were similar in both groups. Conclusions: Vasomotor reserve was reduced even with mild hypertension due to impaired vasodilation. Impaired vasoregulation during orthostatic stress may be associated with silent cerebral hypoperfusion.