ASSESSMENT OF AUTOREGULATION BY MEANS OF PERIODIC CHANGES IN BLOOD-PRESSURE

Background and Purpose The aim of this study was to test the hypothesi s that the phase difference that occurs between an induced oscillation in blood pressure and the resultant oscillation in middle cerebral ar tery (MCA) flow velocity could reflect the competence of cerebral auto regulation. Methods Fourteen volunteers performed 19 cycles of 10 seco nds of squatting followed by 10 seconds of standing. Peak MCA velocity was measured with transcranial Doppler ultrasound, and blood pressure was measured with a servo-controlled finger plethysmograph held level with the head. Wave-forms from each cycle were added to obtain averag ed waveforms of arterial blood pressure and MCA velocity. These result s were processed by Fourier analysis to extract the phase difference b etween the fundamental components of velocity and pressure. Each volun teer performed the exercise three times: first breathing normally, sec ondly hyperventilating (hypocapnia), and finally while breathing air c ontaining 5% carbon dioxide (hypercapnia). Under these conditions the volunteers were expected to have normal, enhanced, and impaired autore gulation, respectively. Results The measurements made with normal brea thing showed a phase lead of velocity ahead of pressure of 46+/-14 deg rees (mean+/-SD). We noted a highly significant reduction in phase lea d with hypercapnia (P<.00015) (Wilcoxon signed rank test, two-tailed) and a highly significant increase in phase lead with hypocapnia (P<.00 2). Conclusions The results support our hypothesis and may lead to a t echnique for assessing the competence of cerebral autoregulation.