Oscillatory cerebral hemodynamics – the macro- versus microvascular level

The phase shift between oscillations of blood pressure (BP) and Doppler middle cerebral artery flow velocity (MCAFV) reflects continuous cerebral autoregulatory action. It is not known whether such a phase shift can also be observed for cortical hemodynamics ('microvascular level') assessed by near infrared spectroscopy (NIRS) and whether it is then also altered in pathophysiological conditions. This study investigates the phase relations between oscillations of BP, MCAFV and NIRS parameters in 38 healthy older adults and 28 patients with unilateral severe obstructive carotid disease. BP was recorded noninvasively by fingerplethysmography. Stable 0.1Hz oscillations of all hemodynamic parameters were induced by regular breathing at a rate of 6/minute. Basic results were that: (1) BP-induced cerebral microvascular oscillations (NIRS) follow those of macrovascular oscillations (MCAFV) with a certain phase of 80–90° (corresponding to 2–2.5s at 0.1Hz), most likely reflecting a transit time phenomenon; (2) oxy- and deoxyhemoglobin thereby oscillate with a counterphase behaviour; (3) hemodynamic compromise in carotid obstruction leads to (a) delayed NIRS oscillations in comparison to BP which are highly correlated to a shorter phase lead of MCAFV against BP and (b) an uncoupling of the oxy-/deoxyhemoglobin counterphase to 240°. We conclude that also microvascular hemodynamic responses to BP oscillations follow specific phase relationships due to cerebral autoregulatory action and circulatory transit times. With hemodynamic impairment, as in unilateral carotid obstruction, these phases are significantly changed reflecting disturbed autoregulation.