Cerebral haemodynamics during hypo- and hypercapnia

Summary

Aim: Transcranial Doppler sonography (TCD) is increasingly used in cerebrovascular disease for monitoring brain perfusion. It allows estimation of cerebral blood flow (CBF) by the measurement of cerebral blood flow velocity (CBFV). The CBFV as well as CBF are intimately associated with the intravascular CO₂-concentration. Thus, hyper- or hypocapnia can be used to induce a defined range of blood flows. The aim of our study was the comparison of vasomotor reactivity assessed with simultaneous TCD and quantitative regional CBFmeasurements (rCBF) by PET (serving as the reference method for in-vivo quantification of rCBF). Patients, methods: Six healthy young volunteers participated in this study. CBF was measured using ¹⁵O-butanol PET. A flow and dispersion- model was fitted to the measured time activity curves using arterial input curves. Each subject underwent five scans at five different end-tidal CO₂ levels (EtCO₂): 25, 32, 40, 48, and 55 mmHg. CBFV was assessed by continuous bilateral TCD of the middle cerebral artery (MCA). Volumes of interest for rCBF determination were placed in grey matter of the prefrontal cortex (PFC) as determined from individual MRIs. Comparisons between the rCBF, EtCO₂ and CBFV were carried out with regression and correlation analysis and paired t-tests. Results: Strong positive linear correlations of rCBF and CBFV with the CO₂-concentration and linear relationships between rCBF and CBFV were found in each individual. Normalised CO₂-reactivities measured by TCD and PET were closely correlated. Conclusions: TCD-measurements of vascular reactivity in healthy volunteers show a high correlation to those acquired with PET that serves as the reference method of quantitative rCBF-measurement. The results of the MCA insonation are a close approximation of the rCBF changes induced by variations of EtCO₂.

Zusammenfassung

Die transkranielle Doppler-Sonographie (TCD) wird zunehmend zur Überwachung der Hirnperfusion bei zerebrovaskulären Erkrankungen eingesetzt. Die TCD ermöglicht die Bestimmung des zerebralen Blutflusses (CBF) durch Messung der zerebralen Blutflussgeschwindigkeit (CBFV). CBFV und CBF sind eng an die intravaskuläre CO₂-Konzentration gekoppelt. Somit können Hypo- und Hyperkapnie zur Erzeugung definierter Blutflüsse genutzt werden. Das Ziel unserer Studie war ein Vergleich der Vasomotorreaktivitätsbestimmungen simultaner TCD-Messungen zur CBFV und quantitativer regionaler (r) CBF-PET-Messungen, wobei die PET als Referenzmethode der In-vivo-rCBF-Quantifikation fungierte. Patienten, Methoden: Untersucht wurden sechs junge Normalprobanden. Die CBF-Bestimmung erfolgte mittels ¹⁵O-Butanol-PET und Anpassung eines Flow-and-dispersion- Modells an die gemessenen Zeitaktivitätskurven mit Hilfe der arteriellen Eingangskurven. Jeder Proband wurde fünfmal während unterschiedlicher end-tidaler CO₂-Konzentrationen (EtCO₂) gemessen: 25, 32, 40, 48 und 55 mmHg. Die CBFV wurde mittels kontinuierlicher Insonation der A. cerebri media (MCA) beidseits erhoben. Zur rCBF-Bestimmung wurden Volumes of Interest in der grauen Substanz des individuell anhand des MRT abgegrenzten präfrontalen Kortex (PFC) platziert. Vergleiche zwischen rCBF, EtCO₂ und CBFV wurden mittels Regressions- und Korrelationsanalyse sowie gepaartem t-Test durchgeführt. Ergebnisse: Es zeigte sich eine hohe Korrelation von rCBF und CBF mit dem Et- CO₂ sowie eine positive lineare Beziehung zwischen rCBF und CBFV bei allen Probanden. Die normalisierten CO₂-Reaktivitäten waren ebenfalls hoch korreliert. Schlussfolgerungen: TCD-Messungen der Gefäßreaktivität zeigen eine hohe Korrelation mit Messwerten der ¹⁵O-Butanol-PET als Referenzmethode der quantitativen rCBF-Bestimmung. Die Ergebnisse der MCA-Insonation stellen eine gute Näherung an die EtCO₂-induzierten änderungen der rCBF-Werte dar.

Keywords

Cerebral blood flow velocity - cerebral vascular response to carbon dioxide - ¹⁵O-butanol PET - transcranial Doppler sonography

Schlüsselwörter

Zerebrale Blutflussgeschwindigkeit - zerebrale vaskuläre CO₂-Reaktivität - ¹⁵O-Butanol-PET - transkranielle Doppler- Sonographie

^* The authors contributed equally.

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