Monitoring of cerebral oxygenation, cerebrovascular reactivity and circulatory function in preterm infants Brain injury in the preterm infant is associated with death and lifelong disability. Cerebral hypoxia and fluctuations in cerebral blood flow in the first two days of life have been implicated in the pathophysiology of haemorrhagic and ischaemic brain injury. Monitoring of haemodynamic changes during the early transitional circulation from in-utero to ex-utero life are currently based on standard measurements of systemic oxygenation and mean arterial blood pressure, with no reliable assessment of end-organ perfusion. In this thesis, measurements using near-infrared spectroscopy (NIRS) and functional echocardiography were made to assess cerebral perfusion and systemic blood flow in a cohort of preterm infants undergoing intensive care. This thesis is divided into four sections: i) The feasibility of continuous monitoring of cerebral oxygenation and cerebrovascular reactivity is demonstrated in a series of case reviews, and the association between cerebral oxygenation and cerebrovascular reactivity with outcome of brain injury and mortality is described. ii) Combining measurements of systemic blood flow with end organ perfusion was applied to define MABPOPT in preterm infants based on an index of cerebrovascular reactivity. Deviations below MABPOPT were associated with intraventricular haemorrhage and mortality. iii) The complexity of brain and systemic signals was studied by using multi-scale entropy analysis. Most studies using cerebral NIRS or systemic measurements of blood flow use linear analysis; however, a complex biological system, such as the human brain, includes many regulatory mechanisms that interact in a complex manner, resulting in effects that cannot be understood wholly through the analysis of its individual constituents. Lower complexity of brain signals was observed in infants who developed intraventricular hemorrhage or died. iv) Changes in systemic and cerebral oxygenation in a cohort of preterm infants in the first 48 hours of life was assessed using functional echocardiography. The patterns of changes in cardiac output and cerebral oxygenation in infants who did and did not have intraventricular haemorrhage are discussed. Furthermore, the relationship between the presence of a haemodynamically significant ductus arteriosus and brain injury is assessed.

Changes in cerebrovascular reactivity (CVR) to carbon dioxide (CO2) are reported in many neurological conditions. My aim was to validate a method for computerized prospective targeting of CO2 levels (RespiractTM) as a bedside tool for impaired CVR. I hypothesized that 1) The RespiractTM and TCD method can be used to detect impairment of CVR after SAH and that 2) CVR is impaired in SAH patients. In 18 SAH patients and 26 controls CVR index was calculated dividing the percentage change in middle cerebral artery blood flow velocity (MCAv) by the change in PETCO2. The absolute MCAv values were similar in both groups, but CVR was significantly different (hypercapnia: 0.044 ± 0.076 - controls; 0.014 ± 0.037 - SAH; p=0.0007). I showed that impaired CVR can be detected at the bedside using TCD and CO2 challenge with the RespiractTM, control of CO2 is precise and minimal changes are required.

Changes in cerebrovascular reactivity (CVR) to carbon dioxide (CO2) are reported in many neurological conditions. My aim was to validate a method for computerized prospective targeting of CO2 levels (RespiractTM) as a bedside tool for impaired CVR. I hypothesized that 1) The RespiractTM and TCD method can be used to detect impairment of CVR after SAH and that 2) CVR is impaired in SAH patients. In 18 SAH patients and 26 controls CVR index was calculated dividing the percentage change in middle cerebral artery blood flow velocity (MCAv) by the change in PETCO2. The absolute MCAv values were similar in both groups, but CVR was significantly different (hypercapnia: 0.044 ± 0.076 - controls; 0.014 ± 0.037 - SAH; p=0.0007). I showed that impaired CVR can be detected at the bedside using TCD and CO2 challenge with the RespiractTM, control of CO2 is precise and minimal changes are required.

Older age is a primary risk factor for the development of cardiovascular disease in part through the stiffening of the large cardiothoracic elastic arteries (e.g., aorta, carotid arteries). Aging is also associated with reduced cognitive function, cerebrovascular reactivity and brain white matter integrity, but whether these changes in brain structure and function are associated with age-related large artery stiffness remains unclear. In contrast, older adults who have high aerobic fitness demonstrate attenuated large artery stiffness and better cognitive performance compared to their sedentary counterparts, but the effects of aerobic fitness on white matter integrity and cerebrovascular reactivity with aging are conflicting and limited. Moreover, whether high aerobic fitness-associated lower large artery stiffness in older adults is associated with, and perhaps mediates, the beneficial changes in cognitive function and white matter structure remains unknown. The purpose of this study was to investigate the extent to which high aerobic fitness is associated with preserved white matter structure, cerebrovascular reactivity, and cognitive performance in aged individuals, and if these changes in brain structure and function are associated with attenuated large artery stiffness. In young (n=19, 23.6 ± 2.5 years) and old (n=22, 64.4 ± 4.2 years) healthy adults, large elastic artery stiffness was measured by carotid-femoral pulse wave velocity (cfPWV, aortic stiffness) via non-invasive applanation tonometry of carotid and femoral pulse waveforms and carotid artery beta-stiffness index (β-stiffness index) and compliance using high-resolution ultrasound and carotid blood pressure via applanation tonometry. Aerobic fitness was measured as maximal exercise oxygen uptake (VO2max) using respiratory gas analysis on an upright cycle ergometer. Older subjects were stratified as high or low fit based on gender and age VO2max classification. Letter, pattern and N-Back cognitive tests were used to assess processing speed and working memory respectively. Fractional anisotropy (FA) from diffusion tensor images and Blood Oxygenation Level Dependent (BOLD) imaging was used to assess cerebrovascular reactivity (CVR) response to a breath hold and brain activation during a working memory task. The association between large artery stiffness and FA was then assessed using a voxel-wise general linear model approach and a region-of-interest analysis. Our results confirmed age-related increases in cfPWV, carotid β-stiffness index and central (carotid) but not brachial systolic blood pressure, and expected reductions in carotid compliance, VO2max, working memory and processing speed, and in white matter integrity in select brain regions (bilateral cingulate, frontal, occipital, temporal). In contrast, we found no age-associated differences in CVR to breath hold stimulus or change in BOLD response to the N-Back. In our cohort of health adults, we found that the age-related changes in large artery stiffness were not attenuated by high compared with low VO2max. Among older adults, large elastic artery stiffness was not associated with regional white matter integrity or cerebrovascular reactivity in any regions-of-interest. Greater carotid artery compliance and lower β-stiffness index was associated with higher processing speed, while compliance was related to higher d'Prime scores and lower reaction time on the 2-Back task among the older adults. CVR to a breath hold stimulus was not related to any measure of cognitive performance. VO2max was not associated with any measures of vascular function, brain structure, function or cognition, indicating relations between large artery stiffness and cognition were independent of aerobic fitness capacity. Taken together, these data suggest that select measures of cognitive performance, but not white matter structure or CVR, may be susceptible to age-related changes in carotid stiffness/compliance and that are unaffected by aerobic fitness. More work is needed to understand the mechanisms by which age-related declines in carotid artery compliance and increased carotid stiffness are associated with reductions in cognitive function in older adults.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第21671号 / 医博第4477号 / 新制||医||1035(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 髙橋 良輔, 教授 溝脇 尚志, 教授 黒田 知宏 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

Cerebral autoregulation is an intrinsic physiological response that maintains a constant cerebral blood flow (CBF) despite dynamic changes in the systemic blood pressure. Autoregulation is achieved through changes in the resistance of the small blood vessels in the brain through reflexive vasodilatation and vasoconstriction. Cerebrovascular reactivity (CVR) is a measure of this response. CVR is defined as a change in CBF in response to a given vasodilatory stimulus. CVR therefore potentially reflects the vasodilatory reserve capacity of the cerebral vasculature to maintain a constant cerebral blood flow. A decrease in CVR (which is interpreted as a reduction in the vasodilatory reserve capacity) in the vascular territory downstream of a larger stenosed supply artery correlates strongly with the risk of a hemodynamic stroke. As a result, the use of CVR studies to evaluate the state of the cerebral autoregulatory capacity has clinical utility. Application of CVR studies in the clinical scenario depends on a thorough understanding of the normal response. The goal of this thesis therefore was to map CVR throughout the brain in normal healthy individuals using Blood Oxygen Level Dependant functional Magnetic Resonance Imaging (BOLD MRI) as an index to CBF and precisely controlled changes in end-tidal partial pressure of carbon dioxide (PETCO2) as the global flow stimulus.

Cerebral autoregulation is an intrinsic physiological response that maintains a constant cerebral blood flow (CBF) despite dynamic changes in the systemic blood pressure. Autoregulation is achieved through changes in the resistance of the small blood vessels in the brain through reflexive vasodilatation and vasoconstriction. Cerebrovascular reactivity (CVR) is a measure of this response. CVR is defined as a change in CBF in response to a given vasodilatory stimulus. CVR therefore potentially reflects the vasodilatory reserve capacity of the cerebral vasculature to maintain a constant cerebral blood flow. A decrease in CVR (which is interpreted as a reduction in the vasodilatory reserve capacity) in the vascular territory downstream of a larger stenosed supply artery correlates strongly with the risk of a hemodynamic stroke. As a result, the use of CVR studies to evaluate the state of the cerebral autoregulatory capacity has clinical utility. Application of CVR studies in the clinical scenario depends on a thorough understanding of the normal response. The goal of this thesis therefore was to map CVR throughout the brain in normal healthy individuals using Blood Oxygen Level Dependant functional Magnetic Resonance Imaging (BOLD MRI) as an index to CBF and precisely controlled changes in end-tidal partial pressure of carbon dioxide (PETCO2) as the global flow stimulus.

Abstract The purpose of the present study was to gain more insight into the blood oxygen level-dependent (BOLD)-contrast functional MRI (fMRI) in the brain and its connection to EEG, both in global and local scales of their temporal and spatial relations. BOLD signal changes were studied during hyperventilation (HV) induced EEG reactivity of intermittent rhythmic delta activity (IRDA). The BOLD signal in gray matter decreased 30% more in subjects with IRDA (N = 4) than in controls (N = 4), during the first two minutes of HV. This difference disappeared during IRDA in EEG. BOLD signal changes may provide additional information about dynamic hemodynamic changes relative to HV induced EEG reactivity. BOLD signal changes were investigated during sudden deepening of thiopental anesthesia into EEG burst-suppression level in pigs (N = 5). Positive (6–8%) or negative (-3– -8%) group average BOLD signal changes correlated to the thiopental bolus injection were seen. Positive and negative responses covered 1.6% and 2.3% of the brain voxels, respectively. BOLD signal changes in brain are associated with sudden deepening of thiopental anesthesia into EEG burst-suppression level, but they are spatially inconsistent and scarce. Somatosensory BOLD response was studied in brain before and after globally induced methotrexate (MTX) exposition in pigs (N = 4). After the MTX exposure, reduced (from 2–4% to 0–1%) or negative (-2% to -3%) BOLD responses were detected. Somatosensory BOLD-contrast response shows a slight difference in brain before and after globally induced MTX exposition. An experimental epilepsy model for development of simultaneous EEG and BOLD-contrast fMRI in the localization of epilepsy was developed and tested. Dynamic penicillin induced local epilepsy was applied in deep isoflurane anesthesia in pigs (N = 6). Relatively high (10–20%) and localized BOLD signal increase was found. The dynamic penicillin induced focal epilepsy model in deep isoflurane anesthesia with simultaneous EEG and BOLD-contrast fMRI is feasible for the development of these methods for localization of epileptic focus or foci. In conclusion, with careful experimental design and analysis, BOLD-contrast fMRI with EEG provides a potential tool for monitoring and localising functional changes in the brain.

Le dioxyde de carbone (CO2) est un résidu naturel du métabolisme cellulaire, la troisième substance la plus abondante du sang, et un important agent vasoactif. À la moindre variation de la teneur en CO2 du sang, la résistance du système vasculaire cérébral et la perfusion tissulaire cérébrale subissent des changements globaux. Bien que les mécanismes exacts qui sous-tendent cet effet restent à être élucidés, le phénomène a été largement exploité dans les études de réactivité vasculaire cérébrale (RVC). Une voie prometteuse pour l’évaluation de la fonction vasculaire cérébrale est la cartographie de la RVC de manière non-invasive grâce à l’utilisation de l’Imagerie par Résonance Magnétique fonctionnelle (IRMf). Des mesures quantitatives et non-invasives de de la RVC peuvent être obtenus avec l’utilisation de différentes techniques telles que la manipu- lation du contenu artériel en CO2 (PaCO2) combinée à la technique de marquage de spin artériel (Arterial Spin Labeling, ASL), qui permet de mesurer les changements de la perfusion cérébrale provoqués par les stimuli vasculaires. Toutefois, les préoccupations liées à la sensibilité et la fiabilité des mesures de la RVC limitent de nos jours l’adoption plus large de ces méthodes modernes de IRMf. J’ai considéré qu’une analyse approfondie ainsi que l’amélioration des méthodes disponibles pourraient apporter une contribution précieuse dans le domaine du génie biomédical, de même qu’aider à faire progresser le développement de nouveaux outils d’imagerie de diagnostique. Dans cette thèse je présente une série d’études où j’examine l’impact des méthodes alternatives de stimulation/imagerie vasculaire sur les mesures de la RVC et les moyens d’améliorer la sensibilité et la fiabilité de telles méthodes. J’ai aussi inclus dans cette thèse un manuscrit théorique où j’examine la possible contribution d’un facteur méconnu dans le phénomène de la RVC : les variations de la pression osmotique du sang induites par les produits de la dissolution du CO2. Outre l’introduction générale (Chapitre 1) et les conclusions (Chapitre 6), cette thèse comporte 4 autres chapitres, au long des quels cinq différentes études sont présentées sous forme d’articles scientifiques qui ont été acceptés à des fins de publication dans différentes revues scientifiques. Chaque chapitre débute par sa propre introduction, qui consiste en une description plus détaillée du contexte motivant le(s) manuscrit(s) associé(s) et un bref résumé des résultats transmis. Un compte rendu détaillé des méthodes et des résultats peut être trouvé dans le(s) dit(s) manuscrit(s). Dans l’étude qui compose le Chapitre 2, je compare la sensibilité des deux techniques ASL de pointe et je démontre que la dernière implémentation de l’ASL continue, la pCASL, offre des mesures plus robustes de la RVC en comparaison à d’autres méthodes pulsés plus âgées. Dans le Chapitre 3, je compare les mesures de la RVC obtenues par pCASL avec l’utilisation de quatre méthodes respiratoires différentes pour manipuler le CO2 artérielle (PaCO2) et je démontre que les résultats peuvent varier de manière significative lorsque les manipulations ne sont pas conçues pour fonctionner dans l’intervalle linéaire de la courbe dose-réponse du CO2. Le Chapitre 4 comprend deux études complémentaires visant à déterminer le niveau de reproductibilité qui peut être obtenu en utilisant des méthodes plus récentes pour la mesure de la RVC. La première étude a abouti à la mise au point technique d’un appareil qui permet des manipulations respiratoires du CO2 de manière simple, sécuritaire et robuste. La méthode respiratoire améliorée a été utilisée dans la seconde étude – de neuro-imagerie – où la sensibilité et la reproductibilité de la RVC, mesurée par pCASL, ont été examinées. La technique d’imagerie pCASL a pu détecter des réponses de perfusion induites par la variation du CO2 dans environ 90% du cortex cérébral humain et la reproductibilité de ces mesures était comparable à celle d’autres mesures hémodynamiques déjà adoptées dans la pratique clinique. Enfin, dans le Chapitre 5, je présente un modèle mathématique qui décrit la RVC en termes de changements du PaCO2 liés à l’osmolarité du sang. Les réponses prédites par ce modèle correspondent étroitement aux changements hémodynamiques mesurés avec pCASL ; suggérant une contribution supplémentaire à la réactivité du système vasculaire cérébral en lien avec le CO2. / Carbon dioxide (CO2) is a natural byproduct of cellular metabolism, the third most abundant substance of blood, and a potent vasoactive agent. The resistance of cerebral vasculature and perfusion of the brain tissue respond to the slightest change in blood CO2 content. The physiology of such an effect remains elusive, yet the phenomenon has been widely exploited in studies of the cerebral vascular function. A promising avenue for the assessment of brain’s vascular function is to measure the cerebrovascular reactivity to CO2 (CVR) non-invasively using functional MRI. Quantitative and non-invasive mapping of CVR can be obtained using respiratory manipulations in arterial CO2 and Arterial Spin Labeling (ASL) to measure the perfusion changes associated with the vascular stimulus. However, concerns related to the sensitivity and reliability of CVR mea- sures by ASL still limit their broader adoption. I considered that a thorough analysis and amelioration of available methods could bring a valuable contribution in the domain of biomedical engineering, helping to advance new diagnostic imaging tools. In this thesis I present a series of studies where I exam the impact of alternative manipulation/ASL methods on CVR measures, and ways to improve the sensitivity and reliability of these measures. I have also included in this thesis a theoretical paper, where I exam the possible contribution of an unappreciated factor in the CVR phenomenon: the changes in blood osmotic pressure induced by the products of CO2 dissolution. Apart from a general introduction (Chapter 1) and conclusion (Chapter 6), this thesis comprises 4 other chapters, in which five different research studies are presented in the form of articles accepted for publication in scientific journals. Each of these chapters begins with its own specific introduction, which consists of a description of the background motivating the study and a brief summary of conveyed findings. A detailed account of methods and results can be found in the accompanying manuscript(s). The study composing Chapter 2 compares the sensitivity of two state-of-the-art ASL techniques and show that a recent implementation of continuous ASL, pCASL, affords more robust measures of CVR than older pulsed methods. The study described in Chapter 3 compares pCASL CVR measures obtained using 4 different respiratory methods to manipulate arterial CO2 (PaCO2) and shows that results can differ significantly when manipulations are not designed to operate at the linear range of the CO2 dose-response curve. Chapter 4 encompasses two complementary studies seeking to determine the degree of reproducibility that can be attained measuring CVR using the most recent methods. The first study resulted in the technical development of a breathing apparatus allowing simple, safe and robust respiratory CO2 manipulations. The improved respiratory method was used in the second – neuroimaging – study, in which I and co-authors investigate the sensitivity and reproducibility of pCASL measuring CVR. The pCASL imaging technique was able to detect CO2-induced perfusion responses in about 90% of the human brain cortex and the reproducibility of its measures was comparable to other hemodynamic measures already adopted in the clinical practice. Finally, in Chapter 5 I present a mathematical model that describes CVR in terms of PaCO2-related changes in blood osmolarity. The responses predicted by this model correspond closely to the hemodynamic changes measured with pCASL, suggesting an additional contribution to the reactivity of cerebral vasculature to CO2.

INTRODUÇÃO: A autorregulação cerebral representa um dos mecanismos fisiopatológicos incertos na hemorragia intracerebral espontânea, cujo comprometimento pode influenciar no resultado prognóstico e terapêutico. O objetivo deste trabalho é avaliar a autorregulação cerebral dinâmica em modelo suíno de hemorragia intracerebral espontânea através do índice de reatividade pressórica cerebrovascular e determinar a eficácia das intervenções clínicas e cirúrgicas. MÉTODOS: Foram estudados 21 suínos híbridos machos com idade de 3 meses. O modelo experimental simulou o efeito expansivo de uma hemorragia intracerebral espontânea de grande volume quando comparado ao cérebro humano. Foram avaliados volumes de expansão diferentes, distribuídos em três grupos com sete suínos cada. O protocolo anestésico incluiu uma monitoração hemodinâmica invasiva associada a preservação da autorregulação cerebral. Os experimentos foram submetidos a monitoração neurológica multimodal e divididos em 5 fases. O índice de reatividade pressórica cerebrovascular estimou a autorregulacão cerebral durante todas as fases, sendo as três primeiras sem intervenções terapêuticas e as duas últimas para avaliar a eficácia das intervenções salina hipertônica e cirurgia. RESULTADOS: Os grupos avaliados foram homogêneos e sem diferença estatística quanto ao comprometimento da autorregulação cerebral comparando os diferentes volumes e tempos de compressão durante as duas primeiras horas da expansão do volume intracraniano. O comprometimento do índice de reatividade pressórica cerebrovascular ocorreu em alguns experimentos influenciando nas fases de tratamento subsequentes, salina hipertônica e cirurgia. CONCLUSÕES: Volumes expansivos elevados podem comprometer a autorregulação cerebral dinâmica e apresentar desfecho terapêutico desfavorável. A intervenção clínica e cirúrgica tem benefício nos experimentos com preservação do índice de reatividade pressórica cerebrovascular / INTRODUCTION: Cerebral autoregulation represents one of the uncertain pathophysiological mechanisms in spontaneous intracerebral hemorrhage, whose impairment may influence prognostic and therapeutic outcome. The aim of this study was to evaluate the dynamic cerebral autoregulation in the swine model of spontaneous intracerebral hemorrhage through the cerebrovascular reactivity index and to determine the efficacy of clinical and surgical interventions. METHODS: Twenty-one male hybrid pigs aged 3 months were studied. The experimental model simulated the expansive effect of a large intracerebral hemorrhage when compared to the human brain. Different volumes were evaluated, distributed in three groups with seven pigs each. Each experiment was divided in five phases. The anesthetic protocol included invasive hemodynamic monitoring associated with the preservation of cerebral autoregulation. Multimodallity monitoring was realised in all experiments. The cerebrovascular reactivity index estimated the cerebral autoregulation during all phases. The first three phases were without therapeutic interventions, and the last two phases were with therapeutic intervention of hypertonic saline solution and neurosurgery respectively. RESULTS: The evaluated groups were homogeneous and without statistical difference regarding the impairment of the cerebral autoregulation comparing different volumes and compression times during the first two hours of the intracranial volume expansion. CONCLUSIONS: Elevated expansive volumes may compromise dynamic cerebral autoregulation and have unfavorable therapeutic outcome. Clinical and surgical intervention had benefit in the experiments with preservation of cerebrovascular reactivity index