Assessment of Autoregulation by Means of Periodic Changes in Blood Pressure

Background and Purpose The aim of this study was to test the hypothesis that the phase difference that occurs between an induced oscillation in blood pressure and the resultant oscillation in middle cerebral artery (MCA) flow velocity could reflect the competence of cerebral autoregulation. Methods Fourteen volunteers performed 19 cycles of 10 seconds 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. Waveforms from each cycle were added to obtain averaged waveforms of arterial blood pressure and MCA velocity. These results were processed by Fourier analysis to extract the phase difference between the fundamental components of velocity and pressure. Each volunteer performed the exercise three times: first breathing normally, secondly hyperventilating (hypocapnia), and finally while breathing air containing 5% carbon dioxide (hypercapnia). Under these conditions the volunteers were expected to have normal, enhanced, and impaired autoregulation, respectively. Results The measurements made with normal breathing showed a phase lead of velocity ahead of pressure of 46±14° (mean±SD). We noted a highly significant reduction in phase lead with hypercapnia ( P <.00015) (Wilcoxon signed rank test, two-tailed) and a highly significant increase in phase lead with hypocapnia ( P <.002). Conclusions The results support our hypothesis and may lead to a technique for assessing the competence of cerebral autoregulation.