Hypocapnia induced by involuntary hyperventilation during mental arithmetic reduces cerebral blood flow velocity.

Functional neuroimaging studies have shown that cognitive processes increase regional cerebral blood flow in relation with enhanced neuronal activity. However, cognition induces elevation of blood pressure, heart rate and respiratory rate, each of which also affects cerebral circulation. For proper interpretation of functional neuroimaging data, it is necessary to dissociate the effects of systemic and local metabolic reactions on regional cerebral circulation. To elucidate this interaction, we examined the changes in cerebral blood flow velocity, which were caused by voluntary hyperventilation-induced hypocapnia without cognitive effort and hypocapnia evolving during mental arithmetic task. The cerebral blood flow velocity was recorded in the middle cerebral arteries, using transcranial Doppler sonography. Respiratory rate, end-tidal partial pressure of CO(2), heart rate and arterial blood pressure were simultaneously monitored. Data were statistically evaluated. Hypocapnia induced by voluntary hyperventilation without cognition decreased the cerebral blood flow velocity. During mental arithmetic, the cerebral blood flow velocity first increased, but the hypocapnia, which was induced by involuntary hyperventilation related to cognitive effort, reduced it. This implies temporary vasoconstriction of cerebral microvessels, and the increase in cerebral vascular resistance index supports this finding. These results suggest that hypocapnia, which develops during cognition, may decrease blood flow velocity in the middle cerebral arteries, which interferes with the neuronal activity-driven regulation of cerebral circulation. In conclusion, when interpreting the results of functional neuroimaging studies on cognitive mechanisms, the tight coupling of the effects of mental processes and autonomic/metabolic reactions should be considered.

[Investigation of cerebral autoregulation in Parkinson's disease--a transcranial Doppler study]. / Az agyi autoreguláció vizsgálata Parkinson-betegségben.

BACKGROUND AND PURPOSE: The frequent orthostatic intolerance in Parkinson's disease could be the consequence of cardiovascular autonomic failure and/or a damaged cerebral autoregulation (AR). To clarify this question the regulation of cerebral circulation was investigated by polygraphic method. METHODS: On a tilt table simultaneous and continuous registrations were made of MCA velocity (V(MCA)) by transcranial Doppler, arterial blood pressure by non-invasive method, and end-tidal CO2, in supine and in tilted positions of 10 degrees, 30 degrees, 70 degrees grades. The cerebral autoregulation was characterized by the slope of the curve of the arterial blood pressure at the level of the Willis-circle (BP(W) as MCA perfusion pressure) plotted against the MCA velocity, achieved by linear regression (y = ax + b function, a=AR, or index of autoregulation). PATIENTS: The data of 17 parkinsonian patients (PP) and eight age-matched controls (C) were analyzed. RESULTS: The decrease of blood pressure in parkinsonian patients was significantly lower than in the controls when supine position was restored from 70 degrees (deltaABP 70 degrees - 0 degree pp = -3.1 +/- 7.5 Hgmm; deltaABP 70 degrees - 0(C) degrees = -11.1 +/- 7.3 Hgmm; p < 0.05), which suggests a damage to the sympathetic cardiovascular system. A disturbance of the cerebral autoregulation in patients was suggested by a 'progressively decreasing MCA average velocity (V(MCA)) during graded tilt, which was significant at 70 degrees (deltaV(ACM) = 9.8 +/- 8.82% cms(-1); p(C-PP) P 0.5), and by a higher slope of pressure-velocity curve (AR(C) = 0.143 +/- .125% cms(-1)/Hgmm; AR(PP) = 0.38 +/- 0.25% cms(-1)/Hgmm; p(C-PP) < 0.05). CONCLUSIONS: The results show that the cerebral blood flow of patients is more dependent on perfusion pressure compared to healthy controls. The disturbance of the sympathetic cardiovascular system and of cerebral autoregulation could be the consequence of a damage to the postganglionic structures in Parkinson's disease. These results could explain the frequent orthostatic intolerance of patients even with normal blood pressure.

Correlation between blood flow velocity in the middle cerebral artery and EEG during cognitive effort.

Cognitive effort modifies blood flow velocity (BFV) in the middle cerebral artery (MCA) which can be recorded by transcranial Doppler sonography (TCD). EEG parameters can be used as indicators of cortical activation. To find temporal and spatial relation between circulatory and bioelectric phenomena, we used combined EEG and TCD measurements during cognitive experiments. Bilateral BFV in the MCAs and 16-channel scalp EEG were recorded during mental arithmetic (MA) and verbal fluency (VF) tests in 12 healthy volunteers. Temporal profile of BFV, heart rate (HR), EEG central frequency (CF), relative alpha power (ralphap), and laterality index (Li) for BFV and CF were statistically analysed. During mental effort, BFV changes showed a reproducible pattern, which was different in MA and VF tests. The Li(BFV) correlated with handedness in 9/12 subjects (75%) in the VF, and in 6/12 subjects (50%) in the MA test. Significant correlation was found between Li(BFV) and Li(CF) during VF (r(2) = 0.69). Li was more indicative for the hemispheric dominance in the VF than in the MA test. During VF test, correlation between HR and BFV was significant in 7/12 subjects. CF and ralphap provide real time assessment of the functional state of the brain tissue during cognition. The correlation between CF and BFV during mental activity suggests a short latency neurogenic and a long latency, supposedly chemical regulation of regional blood flow. Parallel analysis of EEG and flow parameters increases the confidence of determining hemispheric dominance and provides an alternative to study physiological consequences of cognitive processes.

[Effect of mental arithmetic and verbal fluency on blood flow velocity in the middle cerebral arteries]. / A fejszámolás és a szóalkotás hatása az arteria cerebri mediákban mért véráramlás-sebességre.

INTRODUCTION: Using transcranial Doppler sonography (TCD), changes in blood flow velocity (BFV) can be measured in the Medial Cerebral Artery (MCA) during cognitive effort. Our goal was to define the time-course and laterality of BFV in healthy volunteers during arithmetic and verbal fluency tasks according to handedness. METHODS: Twelve subjects (8 right-handed, 4 left-handed) were assessed. The TCD registered BFV in both MCA simultaneously. Heart rate was also recorded using TCD. Finally we included a 16-channel EEG. BFV laterality index (LI) was calculated. Participants were asked to count silently and generate words beginning with a specified letter. To estimate hemispheric differences in BFV, two-tailed Wilcoxon tests were utilized along with correlational analyses. RESULTS: During cognitive effort the BFV changed in a tri-phasic manner in all participants. A 6-8% elevation of BFV was observed in MCAs without latency at the time of the evoking signal. Laterality of BFV developed after 5-13 seconds during cognitive effort in right-, and several seconds later in left-handed subjects. During tasks the BFV increased in the dominant hemisphere up to 2.6-4.7% compared to the subdominant one. We also calculated the LI. During the verbal task the LI agreed with the handedness in 9 out of 12 subjects. During the mental arithmetic task, agreement was found in 6 out of 12 subjects. According to LI results we found a discrepancy between verbal and arithmetic tests in 3 out of 12 subjects. CONCLUSION: Cognitive effort elicites significant bilateral BFV increases in the MCAs, which suggests fast neurogenic regulation. The course of BFV during mental arithmetic proved to be different from course BFV assessed during the word fluency task. Based on the laterality of the BFV, the word-generation task was more sensitive in determining the dominant hemisphere when compared to the mental arithmetic task. The use of LI may help to estimate hemispheric functions even in pathologic circumstances.