Complete ganglion blockade alters dynamic cerebral autoregulation, suggesting links between systemic autonomic traffic and regulation of cerebral blood flow velocity. We tested the hypothesis that acute head-down tilt, a physiological maneuver that decreases systemic sympathetic activity, would similarly disrupt normal dynamic cerebral autoregulation. We studied 10 healthy young subjects (5 men and 5 women; age 21 Ϯ 0.88 yr, height 169 Ϯ 3.1 cm, and weight 76 Ϯ 6.1 kg). ECG, beat-by-beat arterial pressure, respiratory rate, endtidal CO2 concentration, and middle cerebral blood flow velocity were recorded continuously while subjects breathed to a metronome. We recorded data during 5-min periods and averaged responses from three Valsalva maneuvers with subjects in both the supine and Ϫ10°head-down tilt positions (randomized). Controlled-breathing data were analyzed in the frequency domain with power spectral analysis. The magnitude of input-output relations were determined with cross-spectral techniques. Head-down tilt significantly reduced Valsalva phase IV systolic pressure overshoot from 36 Ϯ 4.0 (supine position) to 25 Ϯ 4.0 mmHg (head down) (P ϭ 0.03). Systolic arterial pressure spectral power at the low frequency decreased from 5.7 Ϯ 1.6 (supine) to 4.4 Ϯ 1.6 mmHg 2 (head down) (P ϭ 0.02), and mean arterial pressure spectral power at the low frequency decreased from 3.3 Ϯ 0.79 (supine) to 2.0 Ϯ 0.38 mmHg 2 (head down) (P ϭ 0.05). Head-down tilt did not affect cerebral blood flow velocity or the transfer function magnitude and phase angle between arterial pressure and cerebral blood flow velocity. Our results show that in healthy humans, mild physiological manipulation of autonomic activity with acute head-down tilt has no effect on the ability of the cerebral vasculature to regulate flow velocity.transcranial Doppler; spectral analysis; transfer function THE CEREBRAL CIRCULATION MAINTAINS relatively constant blood flow in the face of changes in pressure and other imposed demands through normal autoregulation. Determination of factors that may modulate or disrupt normal cerebral autoregulation is important for the diagnosis and treatment of patients suffering from autonomic dysfunction. Zhang and coworkers (24) recently demonstrated that the transfer function magnitude between oscillations of mean arterial pressure and mean cerebral blood flow velocity is increased by ganglion blockade, suggesting a role for autonomic neural activity in the modulation of cerebral autoregulation. Nonpharmacological, physiological modulation of autonomic neural activity may similarly provide insight into mechanisms underlying normal cerebral autoregulation.Acute head-down tilting decreases leg volume (9) and increases central venous pressure (13). Consequent loading of baroreceptors decreases muscle sympathetic nerve activity (5, 19) and could affect regulation of cerebral blood flow velocity, although such associations have not been evaluated.The purpose of the present study was to determine the effects of acute head-down tilt...

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The cerebral CT-scan results of 72 patients with chronic atrial fibrillation (AF) were compared to those of an age- and sex-matched control group, affected by muscle-tensive headache. None of the patients in the study had any neurologic symptoms. All were normal on neurologic examination. Mean age was 68 years in both groups. Patients with atrial fibrillation had a higher prevalence of hypertension, diabetes and hyperlipidemia, although the differences were not significant. Thirty-two patients (44.4%) with AF showed hypodense lesions on cerebral CT-scan, suggestive of small infarcts, whereas this finding was present only in eight control subjects (11.1%) (p less than 0.05). These results confirm in part the observations reported in literature and suggest a more thorough examination of the problem regarding the prophylaxis of thrombo-embolic risk in patients affected by chronic AF.

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