Control system analysis of visually evoked blood flow regulation in humans under normocapnia and hypercapnia

Among other factors, the cerebral blood flow (CBF) is regulated in accordance to the arterial CO(2) tension and the cortical activity. The CO(2) test is commonly used to measure the vascular reserve capacity. Most functional imaging studies rely on the activity-flow coupling (AFC). We aimed to combine both challenges in order to increase the insight into mechanisms of CBF regulation.

Fifteen healthy students underwent a functional transcranial Doppler test using a visual stimulation paradigm: firstly under normocapnia and secondly under conditions of hypercapnia. Hypercapnia was induced by breathing a carbogene gas mixture of 5% CO(2) and 95% O(2). The entire time course of flow velocity adaptation in the posterior cerebral artery (PCA) was analyzed mathematically using a control system approach.

Resting CBF velocities increased by nearly 26% under conditions of hypercapnia, whereas the slight increase in arterial blood pressure (ABP) and the decrease in the Pourcelot-Pulsatility index (PI) were statistically not significant. From the control system parameters which were time delay, rate time, gain, attenuation and natural frequency, only the parameter rate time, indicative for the initial steepness of flow velocity increase, showed a statistically significant decrease, consistently for the peak systolic and enddiastolic flow velocity data. As concluded from the unchanged gain parameter the absolute amount of blood flow evoked by the same visual stimulus increased also by 26%.

Evaluated by Doppler measurements hypercapnia seems to influence the AFC in two ways: It decreases the steepness of the initial increase in blood flow velocity and enhances the absolute amount of blood flow evoked by the same stimulus.