Assessment of functional hemispheric asymmetry by bilateral simultaneous cerebral blood flow velocity monitoring.

The aim of this study was to investigate side-to-side differences of simultaneously measured middle cerebral artery (MCA) blood flow velocities during various hemisphere-specific tasks. Using a transcranial Doppler device, flow velocity changes of 24 healthy, right-handed subjects were monitored simultaneously in the left and right MCA during different hemisphere-specific tasks. Mean flow velocity (MFV) curves were averaged for each individual subject and task. Simultaneously, heart rate, blood pressure and end-tidal carbon dioxide (CO2) were measured in a subgroup of six subjects. When compared with the resting state, all stimuli produced significant (p < 0.001) bilateral MFV increases, ranging from 2.5-9.2%. A lateralization of MFV increases with a significantly (p < 0.001) more pronounced increase in MFV in the hemisphere contralateral to the performing band was observed both during simple sequential finger movements and a complex spatial task. During the complex spatial task, consistently higher MFV increases were observed in the right MCA (p < 0.001), regardless of the side of task performance. Recognition of pictorial material presented as part of a memory task, also resulted in a side-to-side difference of respective MFV increases (right > left, p < 0.001), whereas memorization did not. Whereas bilateral MFV elevations observed during stimulation with white noise were only discrete and not lateralized, exposure to overt speech produced significantly higher (p < 0.001) MFV increases in the left MCA. The time course of the MFV reaction showed a rapid increase with an initial maximum after 4-5 s. Heart rate, blood pressure, and end-tidal CO2 showed only subtle changes during the stimulation periods. In conclusion, the observed side-to-side differences of MFV reaction in the left and right MCA concur with current functional imaging data. Bilateral simultaneous repetitive transcranial Doppler monitoring is a sensitive method to detect cerebral perfusion asymmetries caused by hemisphere-specific activation, and thus may be helpful for noninvasive assessment of hemispheric dominance for language.

Relationship between cerebral blood flow velocities and cerebral electrical activity in sleep.

The dynamics of cerebral blood flow velocity during sleep were measured in the right and left middle cerebral artery of 12 and 10 healthy male volunteers, respectively. A computer-assisted pulsed (2-MHz) Doppler ultrasonography system was modified for continuous long-term and on-line recording of cerebral hemodynamics in combination with polysomnography. Mean flow velocity (MFV) decreased steadily during deepening nonrapid eye movement (NREM) sleep and increased suddenly during rapid eye movement sleep, corresponding to changes in brain function. However, spontaneous or provoked changes in sleep stage patterns as well as awakenings from NREM sleep were not regularly accompanied by corresponding changes in MFV. Differing values for MFV in subsequent sleep cycles could be shown for several sleep stages. Furthermore, MFV values in sleep stage II at the end of an NREM-sleep period were lower than in preceding slow-wave sleep. After application of short acoustic signals the electroencephalogram frequency rose, indicating an arousal, whereas MFV rapidly decreased for several seconds and then gradually returned to the prior level. These results imply an uncoupling between cerebral electrical activity and cerebral perfusion during sleep and support a dissociation in the activity of central regulatory mechanisms. In light of the proposal that cortical energy consumption can be accounted for by cerebral electrical activity, the concept that cerebral perfusion during sleep is regulated solely by the metabolic rate must be reconsidered.

Dynamics of cerebral blood flow velocities during normal human sleep.

Bilateral flow patterns of the middle cerebral artery (MCA) were recorded continuously throughout the night in 18 healthy male subjects (mean age 27.4 years) by a computer-assisted pulsed Doppler (2 MHz) system together with simultaneous polysomnography. After inception of sleep, mean flow velocity (MFV) decreased steadily during deepening sleep stages reaching -15.0 +/- 3.6% (p < 0.001) in the right MCA and -16.2 +/- 3.4% in the left MCA (p < 0.001) in stage 4 of the first sleep cycle compared to the waking state. Lowest MFV values were found in stage 2 preceding the last REM period (right MCA: -19.2 +/- 4.1%: left MCA: -19.7 +/- 5.1%). Changing from non-REM into REM sleep, a sudden rise of MFV, which varied from 8.9% (first sleep cycle, left MCA) to 18% (last sleep cycle, right MCA), could be consistently detected indicating a coupling of cerebral electrical activity and cerebral perfusion in REM sleep. During non-REM sleep this concomitant change of MFV and EEG activity was only found in the first sleep cycle, whereas no parallel changes could be observed in later sleep cycles. These results indicate a decoupling of EEG measured cerebral electrical activity and perfusion and suggest that factors other than metabolic mechanisms contribute to the regulation of cerebral perfusion during human non-REM sleep.

Increase of posterior cerebral artery blood flow velocity during threshold repetitive magnetic stimulation of the human visual cortex: hints for neuronal activation without cortical phosphenes.

To analyze the effects of low intensity repetitive transcranial magnetic stimulation over the occipital cortex on regional cerebral perfusion, bilateral simultaneous monitoring of posterior cerebral artery blood flow velocity was performed using transcranial Doppler ultrasonography in 14 healthy subjects. During 20 s of unilateral magnetic stimulation with 3 Hz and 6 Hz a significant increase of ipsilateral flow velocity was observed during both stimulus conditions (3 Hz, 10.2 +/- 3.7%; 6 Hz, 12.8 +/- 4.7%). A significantly smaller flow velocity increase occurred also in the contralateral posterior cerebral artery (3 Hz, 8.6 +/- 4.0%; 6 Hz, 10.6 +/- 4.1%). Flow velocity increases were similar to values reported in the literature for physiological activation paradigms so that excessive excitation of the visual cortex does not seem to occur during repetitive magnetic stimulation. The largest increase of flow velocity was observed in the ipsilateral posterior cerebral artery of 5 subjects who experienced phosphenes in the contralateral visual half-field during stimulation (14.3 +/- 4.1%: mean of 3 and 6 Hz stimulation). In the other 9 subjects significant velocity increases indicated cortex activation in the absence of cortical phosphenes. The occurrence of maximum velocity responses within 2-3 heart beats following the first cortex stimulus points to a fast adjustment of cerebral perfusion in response to transcranial brain stimulation.

[Intracranial blood flow parameters in cerebral functional changes and cognitive cerebral performance]. / Intrakranielle Strömungsparameter bei zerebralen Funktionsänderungen und kognitiven Hirnleistungen.

Extensive studies have revealed a close relationship between neuronal activity and regional cerebral blood flow. However, SPECT and PET, the technologies most commonly used in these studies, are of limited value for assessment of the dynamics of cerebral blood flow changes at different states of functional brain activity. The introduction of transcranial Doppler sonography and the extended application of stimuli presentation and perception have now been added to the investigator's armamentarium. Simple sensory stimulation (visual, acoustic and tactile) and complex mental tasks (viewing of complex pictures, tactile differentiation of objects) changed the blood flow velocity in the basal intracranial arteries. These changes corresponded to the current concepts of functional cortical organization. The magnitude of the flow velocity increases upon visual stimulation was dependent on the complexity of the stimuli used, and was up to 38% in our studies. The introduction of continuous and bilateral simultaneous Doppler recordings, the calculation of mean flow velocity from cardiac cycle to cardiac cycle and a specially designed averaging method for data analysis allowed effective elimination of non-specific influences and made it possible to demonstrate rapid changes of perfusion in both middle cerebral artery territories in direct response to hemisphere-specific tasks. These changes were correlated with known functional cerebral asymmetries. A language task, for instance, was associated with a significantly larger flow velocity increase in the middle cerebral artery of the dominant hemisphere than in the corresponding artery of the non-dominant hemisphere (5.2 +/- 1,8% vs 3.0 +/- 1.8%, p < 0.001). The excellent time resolution of this technology made it possible to record hemodynamic changes taking place in response to modifications of neuronal activity within less than 1 s. The shortest time interval between stimulus presentation and the first significant increase in flow velocity was on average 717 +/- 191 ms. The latency of less than 1 s suggest that the coupling between alterations of neuronal activity and the regional cerebral blood flow response is mediated by an remarkably rapid mechanism.