Functional imaging of the visual cortex with bold-contrast MRI: hyperventilation decreases signal response

Dose-dependent relationship between intra-procedural hypoxaemia or hypocapnia and postoperative delirium in older patients

BACKGROUND

Previous studies indicated an association between impaired cerebral perfusion and post-procedural neurological disorders. We investigated whether intra-procedural hypoxaemia or hypocapnia are associated with delirium after surgery.

METHODS

Inpatients ≥60 yr of age undergoing anaesthesia for surgical or interventional procedures between 2009 and 2020 at an academic healthcare network in the USA (Massachusetts) were included in this hospital registry study. The primary exposure was intra-procedural hypoxaemia, defined as peripheral oxygen saturation <90% for >2 cohering min. The co-primary exposure was hypocapnia during general anaesthesia, defined as end-tidal carbon dioxide pressure ≤25 mm Hg for >5 cohering min. The primary outcome was delirium within 7 days after surgery.

RESULTS

Of 71 717 included patients, 1702 (2.4%) developed postoperative delirium, and hypoxaemia was detected in 2532 (3.5%). Of 42 894 patients undergoing general anaesthesia, 532 (1.2%) experienced hypocapnia. The occurrence of either hypoxaemia (adjusted odds ratio [OR_adj]=1.71; 95% confidence interval [CI], 1.40-2.07; P<0.001) or hypocapnia (OR_adj=1.77; 95% CI, 1.30-2.41; P<0.001) was associated with a higher risk of delirium within 7 days. Both associations were dependent on the magnitude, and increased with event duration (OR_adj=1.03; 95% CI, 1.02-1.04; P<0.001 and OR_adj=1.01; 95% CI, 1.00-1.01; P=0.005, for each minute increase in the longest continuous episode, respectively). There was no association between occurrence of hypercapnia and postoperative delirium (OR_adj=1.24; 95% CI, 0.90-1.71; P=0.181).

CONCLUSIONS

Intra-procedural hypoxaemia and hypocapnia were dose-dependently associated with a higher risk of postoperative delirium. These findings support maintaining normal gas exchange to avoid postoperative neurological disorders.

Comparison of cerebrovascular reactivity recovery following high-intensity interval training and moderate-intensity continuous training

A common inclusion criterion when assessing cerebrovascular (CVR) metrics is for individuals to abstain from exercise for 12–24 hr prior to data collections. While several studies have examined CVR during exercise, the literature describing CVR throughout post‐exercise recovery is sparse. The current investigation examined CVR measurements in nine participants (seven male) before and for 8 hr following three conditions: 45‐min moderate‐continuous exercise (at ~50% heart‐rate reserve), 25‐min high‐intensity intervals (ten, one‐minute intervals at ~85% heart‐rate reserve), and a control day (30‐min quiet rest). The hypercapnic (40–60 mmHg) and hypocapnic (25–40 mmHg) slopes were assessed via a modified rebreathing technique and controlled stepwise hyperventilation, respectively. All testing was initiated at 8:00a.m. with transcranial Doppler ultrasound measurements to index cerebral blood velocity performed prior to the condition (pre) with serial follow‐ups at zero, one, two, four, six, and eight hours within the middle and posterior cerebral artery (MCA, PCA). Absolute and relative MCA and PCA hypercapnic slopes were attenuated following high‐intensity intervals at hours zero and one (all p < .02). No alterations were observed in either hypocapnic or hypercapnic slopes following the control or moderate‐continuous exercise (all p > .13), aside from a reduced relative hypercapnic MCA slope at hours zero and one following moderate‐continuous exercise (all p < .005). The current findings indicate the common inclusion criteria of a 12–24 hr time restriction on exercise can be reduced to two hours when performing CVR measures. Furthermore, the consistent nature of the CVR indices throughout the control day indicate reproducible testing sessions can be made between 8:00a.m. and 7:00p.m.

Hyperventilation in Adult TBI Patients: How to Approach It?

Hyperventilation is a commonly used therapy to treat intracranial hypertension (ICTH) in traumatic brain injury patients (TBI). Hyperventilation promotes hypocapnia, which causes vasoconstriction in the cerebral arterioles and thus reduces cerebral blood flow and, to a lesser extent, cerebral blood volume effectively, decreasing temporarily intracranial pressure. However, hyperventilation can have serious systemic and cerebral deleterious effects, such as ventilator-induced lung injury or cerebral ischemia. The routine use of this therapy is therefore not recommended. Conversely, in specific conditions, such as refractory ICHT and imminent brain herniation, it can be an effective life-saving rescue therapy. The aim of this review is to describe the impact of hyperventilation on extra-cerebral organs and cerebral hemodynamics or metabolism, as well as to discuss the side effects and how to implement it to manage TBI patients.

Cerebrovascular Reactivity Mapping Without Gas Challenges: A Methodological Guide

Cerebrovascular reactivity (CVR) is defined as the ability of vessels to alter their caliber in response to vasoactive factors, by means of dilating or constricting, in order to increase or decrease regional cerebral blood flow (CBF). Importantly, CVR may provide a sensitive biomarker for pathologies where vasculature is compromised. Furthermore, the spatiotemporal dynamics of CVR observed in healthy subjects, reflecting regional differences in cerebral vascular tone and response, may also be important in functional MRI studies based on neurovascular coupling mechanisms. Assessment of CVR is usually based on the use of a vasoactive stimulus combined with a CBF measurement technique. Although transcranial Doppler ultrasound has been frequently used to obtain global flow velocity measurements, MRI techniques are being increasingly employed for obtaining CBF maps. For the vasoactive stimulus, vasodilatory hypercapnia is usually induced through the manipulation of respiratory gases, including the inhalation of increased concentrations of carbon dioxide. However, most of these methods require an additional apparatus and complex setups, which not only may not be well-tolerated by some populations but are also not widely available. For these reasons, strategies based on voluntary breathing fluctuations without the need for external gas challenges have been proposed. These include the task-based methodologies of breath holding and paced deep breathing, as well as a new generation of methods based on spontaneous breathing fluctuations during resting-state. Despite the multitude of alternatives to gas challenges, existing literature lacks definitive conclusions regarding the best practices for the vasoactive modulation and associated analysis protocols. In this work, we perform an extensive review of CVR mapping techniques based on MRI and CO2 variations without gas challenges, focusing on the methodological aspects of the breathing protocols and corresponding data analysis. Finally, we outline a set of practical guidelines based on generally accepted practices and available data, extending previous reports and encouraging the wider application of CVR mapping methodologies in both clinical and academic MRI settings.

Fiber-Optic Breath Sensors: A Comparison Study

The paper presents a comparative study of three fiber optic sensors based on the fiber Bragg grating (FBG). The basic monitored parameter is the respiratory rate of the human body. Fiber-optic sensors are immune to electromagnetic interference (EMI). This fact singles them out as ideal for use in magnetic resonance environments (typically in MRI - magnetic resonance imaging) as a prediction of hyperventilation states in patients. These patient conditions arise as a result of the closed tunnel environment in MR scanners. The results (10 volunteers with written consent) were compared with the results using the conventional respiratory belt (RB) in a laboratory environment and processed using the objective Bland-Altman (B-A) method.

Vital Sign Monitoring and Cardiac Triggering at 1.5 Tesla: A Practical Solution by an MR-Ballistocardiography Fiber-Optic Sensor

This article presents a solution for continuous monitoring of both respiratory rate (RR) and heart rate (HR) inside Magnetic Resonance Imaging (MRI) environments by a novel ballistocardiography (BCG) fiber-optic sensor. We designed and created a sensor based on the Fiber Bragg Grating (FBG) probe encapsulated inside fiberglass (fiberglass is a composite material made up of glass fiber, fabric, and cured synthetic resin). Due to this, the encapsulation sensor is characterized by very small dimensions (30 × 10 × 0.8 mm) and low weight (2 g). We present original results of real MRI measurements (conventionally most used 1.5 T MR scanner) involving ten volunteers (six men and four women) by performing conventional electrocardiography (ECG) to measure the HR and using a Pneumatic Respiratory Transducer (PRT) for RR monitoring. The acquired sensor data were compared against real measurements using the objective Bland⁻Altman method, and the functionality of the sensor was validated (95.36% of the sensed values were within the ±1.96 SD range for the RR determination and 95.13% of the values were within the ±1.96 SD range for the HR determination) by this means. The accuracy of this sensor was further characterized by a relative error below 5% (4.64% for RR and 4.87% for HR measurements). The tests carried out in an MRI environment demonstrated that the presence of the FBG sensor in the MRI scanner does not affect the quality of this imaging modality. The results also confirmed the possibility of using the sensor for cardiac triggering at 1.5 T (for synchronization and gating of cardiovascular magnetic resonance) and for cardiac triggering when a Diffusion Weighted Imaging (DWI) is used.

Magnetic Resonance Imaging Compatible Non-Invasive Fibre-Optic Sensors Based on the Bragg Gratings and Interferometers in the Application of Monitoring Heart and Respiration Rate of the Human Body: A Comparative Study

The publication presents a comparative study of two fibre-optic sensors in the application of heart rate (HR) and respiratory rate (RR) monitoring of the human body. After consultation with clinical practitioners, two types of non-invasive measuring and analysis systems based on fibre Bragg grating (FBG) and fibre-optic interferometer (FOI) have been designed and assembled. These systems use probes (both patent pending) that have been encapsulated in the bio-compatible polydimethylsiloxane (PMDS). The main advantage of PDMS is that it is electrically non-conductive and, as well as optical fibres, has low permeability. The initial verification measurement of the system designed was performed on four subjects in a harsh magnetic resonance (MR) environment under the supervision of a senior radiology assistant. A follow-up comparative study was conducted, upon a consent of twenty volunteers, in a laboratory environment with a minimum motion load and discussed with a head doctor of the Radiodiagnostic Institute. The goal of the laboratory study was to perform measurements that would simulate as closely as possible the environment of harsh MR or the environment of long-term health care facilities, hospitals and clinics. Conventional HR and RR measurement systems based on ECG measurements and changes in the thoracic circumference were used as references. The data acquired was compared by the objective Bland⁻Altman (B⁻A) method and discussed with practitioners. The results obtained confirmed the functionality of the designed probes, both in the case of RR and HR measurements (for both types of B⁻A, more than 95% of the values lie within the ±1.96 SD range), while demonstrating higher accuracy of the interferometric probe (in case of the RR determination, 95.66% for the FOI probe and 95.53% for the FBG probe, in case of the HR determination, 96.22% for the FOI probe and 95.23% for the FBG probe).

The absolute CBF response to activation is preserved during elevated perfusion: Implications for neurovascular coupling measures

Functional magnetic resonance imaging (fMRI) techniques in which the blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) response to a neural stimulus are measured, can be used to estimate the fractional increase in the cerebral metabolic rate of oxygen consumption (CMRO2) that accompanies evoked neural activity. A measure of neurovascular coupling is obtained from the ratio of fractional CBF and CMRO2 responses, defined as n, with the implicit assumption that relative rather than absolute changes in CBF and CMRO2 adequately characterise the flow-metabolism response to neural activity. The coupling parameter n is important in terms of its effect on the BOLD response, and as potential insight into the flow-metabolism relationship in both normal and pathological brain function. In 10 healthy human subjects, BOLD and CBF responses were measured to test the effect of baseline perfusion (modulated by a hypercapnia challenge) on the coupling parameter n during graded visual stimulation. A dual-echo pulsed arterial spin labelling (PASL) sequence provided absolute quantification of CBF in baseline and active states as well as relative BOLD signal changes, which were used to estimate CMRO2 responses to the graded visual stimulus. The absolute CBF response to the visual stimuli were constant across different baseline CBF levels, meaning the fractional CBF responses were reduced at the hyperperfused baseline state. For the graded visual stimuli, values of n were significantly reduced during hypercapnia induced hyperperfusion. Assuming the evoked neural responses to the visual stimuli are the same for both baseline CBF states, this result has implications for fMRI studies that aim to measure neurovascular coupling using relative changes in CBF. The coupling parameter n is sensitive to baseline CBF, which would confound its interpretation in fMRI studies where there may be significant differences in baseline perfusion between groups. The absolute change in CBF, as opposed to the change relative to baseline, may more closely match the underlying increase in neural activity in response to a stimulus.

Brain-state dependent uncoupling of BOLD and local field potentials in laminar olfactory bulb

The neural activities of the olfactory bulb (OB) can be modulated significantly by internal brain states. While blood oxygenation level dependent functional MRI (BOLD-fMRI) has been extensively applied to study OB in small animals, the relationship between BOLD signals and electrophysiological signals remains to be elucidated. Our recent study has revealed a complex relationship between BOLD and local field potentials (LFP) signals in different OB layers during odor stimulation. However, no study has been performed to compare these two types of signals under global brain states. Here, the changes of BOLD and LFP signals in the glomerular, mitral cell, and granular cell layers of the OB under different brain states, which were induced by different concentrations of isoflurane, were sequentially acquired using electrode array and high-resolution MRI. It was found that under deeper anesthesia, the LFP powers in all layers were decreased but the BOLD signals were unexpectedly increased. Furthermore, the decreases of LFP powers were layer-independent, but the increases of BOLD signal were layer-specific, with the order of glomerular>mitral cell>granular cell layer. The results provide new evidence that the direct neural activity levels might not be correlated well with BOLD signals in some cases, and remind us that cautions should be taken to use BOLD signals as the index of neural activities.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

End-tidal CO2: an important parameter for a correct interpretation in functional brain studies using speech tasks

The aim was to investigate the effect of different speech tasks, i.e. recitation of prose (PR), alliteration (AR) and hexameter (HR) verses and a control task (mental arithmetic (MA) with voicing of the result on end-tidal CO2 (PETCO2), cerebral hemodynamics and oxygenation. CO2 levels in the blood are known to strongly affect cerebral blood flow. Speech changes breathing pattern and may affect CO2 levels. Measurements were performed on 24 healthy adult volunteers during the performance of the 4 tasks. Tissue oxygen saturation (StO2) and absolute concentrations of oxyhemoglobin ([O2Hb]), deoxyhemoglobin ([HHb]) and total hemoglobin ([tHb]) were measured by functional near-infrared spectroscopy (fNIRS) and PETCO2 by a gas analyzer. Statistical analysis was applied to the difference between baseline before the task, 2 recitation and 5 baseline periods after the task. The 2 brain hemispheres and 4 tasks were tested separately. A significant decrease in PETCO2 was found during all 4 tasks with the smallest decrease during the MA task. During the recitation tasks (PR, AR and HR) a statistically significant (p<0.05) decrease occurred for StO2 during PR and AR in the right prefrontal cortex (PFC) and during AR and HR in the left PFC. [O2Hb] decreased significantly during PR, AR and HR in both hemispheres. [HHb] increased significantly during the AR task in the right PFC. [tHb] decreased significantly during HR in the right PFC and during PR, AR and HR in the left PFC. During the MA task, StO2 increased and [HHb] decreased significantly during the MA task. We conclude that changes in breathing (hyperventilation) during the tasks led to lower CO2 pressure in the blood (hypocapnia), predominantly responsible for the measured changes in cerebral hemodynamics and oxygenation. In conclusion, our findings demonstrate that PETCO2 should be monitored during functional brain studies investigating speech using neuroimaging modalities, such as fNIRS, fMRI to ensure a correct interpretation of changes in hemodynamics and oxygenation.

Comparison between end-tidal CO₂ and respiration volume per time for detecting BOLD signal fluctuations during paced hyperventilation

Respiratory motion and capnometry monitoring were performed during blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) of the brain while a series of paced hyperventilation tasks were performed that caused significant hypocapnia. Respiration volume per time (RVT) and end-tidal carbon dioxide (ETCO(2)) were determined and compared for their ability to explain BOLD contrast changes in the data. A 35% decrease in ETCO(2) was observed along with corresponding changes in RVT. A best-fit ETCO(2) response function, with an average initial peak delay time of 12 s, was empirically determined. ETCO(2) data convolved with this response function was more strongly and prevalently correlated to BOLD signal changes than RVT data convolved with the corresponding respiration response function. The results suggest that ETCO(2) better models BOLD signal fluctuations in fMRI experiments with significant transient hypocapnia. This is due to hysteresis in the ETCO(2) response when moving from hypocapnia to normocapnia, compared to moving from normocapnia to hypocapnia.

Changes in visual-evoked potential habituation induced by hyperventilation in migraine

Hyperventilation is often associated with stress, an established trigger factor for migraine. Between attacks, migraine is associated with a deficit in habituation to visual-evoked potentials (VEP) that worsens just before the attack. Hyperventilation slows electroencephalographic (EEG) activity and decreases the functional response in the occipital cortex during visual stimulation. The neural mechanisms underlying deficient-evoked potential habituation in migraineurs remain unclear. To find out whether hyperventilation alters VEP habituation, we recorded VEPs before and after experimentally induced hyperventilation lasting 3 min in 18 healthy subjects and 18 migraine patients between attacks. We measured VEP P100 amplitudes in six sequential blocks of 100 sweeps and habituation as the change in amplitude over the six blocks. In healthy subjects, hyperventilation decreased VEP amplitude in block 1 and abolished the normal VEP habituation. In migraine patients, hyperventilation further decreased the already low block 1 amplitude and worsened the interictal habituation deficit. Hyperventilation worsens the habituation deficit in migraineurs possibly by increasing dysrhythmia in the brainstem-thalamo-cortical network.

Prehospital hypocapnia and poor outcome after severe traumatic brain injury

BACKGROUND

The Brain Trauma Foundation (BTF) Guidelines for prehospital management of traumatic brain injury (TBI) recommend a goal end-tidal carbon dioxide of 30 mm Hg to 35 mm Hg in patients without signs of herniation.

METHODS

We examined prehospital concordance with BTF Guidelines, selected demographic and physiologic variables and outcomes for 100 consecutive admissions to a well-established Level I regional trauma center. All patients had blunt TBI with Glasgow Coma Score < or = 8 without signs of herniation. All were transported by helicopter by flight paramedics experienced with BTF Guidelines and the continuous wave form capnometer. Patients resumed spontaneous ventilation after intubation.

RESULTS

Concordance (prehospital end-tidal carbon dioxide > 29 mm Hg) was achieved in 65 of 100 cases. Mortality was 29% (19 of 65) among those in whom guideline levels were achieved prehospital and 46% (16 of 35) in those in whom guideline levels were not achieved prehospital (odds ratio, 0.49; p = 0.10). The "achieved" group was younger (p = 0.02), with higher calculated probability of survival (p = 0.01). Intracranial pressure was maintained under intensive care within acceptable limits in the hospital in both groups but was significantly higher in the "not achieved" group (p = 0.05).

CONCLUSIONS

Our data, though not statistically significant, suggest that patients who are harder to keep within the guidelines in the field are more likely to die, because of more severe TBI or complication by other factors such as age or injury severity. Whether increased awareness of this phenomenon can improve outcomes is unknown but suggests an approach to future education and research.

Anxiety, respiration, and cerebral blood flow: implications for functional brain imaging

Brain functional imaging methods, such as fMRI, are sensitive to changes in cerebral blood flow (CBF) that are normally associated with changes in regional neural activation. However, other endogenous and exogenous factors can alter CBF independently of brain neural activity, thus complicating the interpretation of functional imaging data. The presence of an anxiety disorder, as well as change in state anxiety, is often accompanied by respiratory alterations that affect arterial CO(2) tensions and produce significant changes in CBF that are independent of task-related neural activation. Therefore, the effects of trait and state anxiety need to be given close consideration in interpreting functional imaging findings. In this paper, we review the dependence of most brain functional imaging methods on localized changes in CBF and the potentially confounding effects of anxiety-related alterations of respiration on interpreting patterns of functional activation. Approaches for addressing these effects are discussed.

Optimizing the intraoperative management of carbon dioxide concentration

PURPOSE OF REVIEW

This review assesses whether there is a carbon dioxide concentration range that provides optimum benefit to the patient intraoperatively. It includes the physiological effects of carbon dioxide on various organ systems in awake and anesthetized individuals and its clinical effects in the ischemia/reperfusion setting. This review will present views on end-tidal or arterial carbon dioxide tension management in the perioperative period.

RECENT FINDINGS

Hypocapnia reduces intracranial pressure and is used by clinicians during acute traumatic brain injury, acute intracranial hemorrhage, and acutely growing brain tumors. There is mounting evidence, however, that hypercapnia improves tissue perfusion and oxygenation. Therefore, clinicians may want to induce mild-to-moderate hypercapnia during reperfusion states such as major vascular surgery, organ transplantation, tissue-graft surgery, and cases managed with low mean arterial pressures to control bleeding. As hypercapnia preserves cerebral blood flow even under relatively low perfusion pressures, it may be beneficial during global reperfusion scenarios. This hypothesis needs to be tested extensively before being considered for clinical applications. From a different perspective, current American Heart Association Guidelines recommend 12-15 breaths/min during cardiopulmonary resuscitation and stress the potential negative role of inadvertent hyperventilation on survival outcome. The importance of this concept is discussed briefly.

SUMMARY

Overall, the benefits of managing carbon dioxide concentration intraoperatively for the maintenance of cardiac output, tissue oxygenation, perfusion, intracranial pressure, and cerebrovascular reactivity are well defined.

The effect of global cerebral vasodilation on focal activation hemodynamics

In view of the potential of global resting blood flow level to confound the interpretation of blood oxygenation level-dependent (BOLD) fMRI studies, we investigated the effect of pronounced elevation in baseline cerebral blood flow (CBF) on BOLD and CBF responses to functional activation. Twelve healthy volunteers performed bilateral finger apposition while attending to a radial yellow/blue checkerboard. Three levels of global CBF increase were achieved by inhaling 5, 7.5 or 10% CO2. CBF and BOLD signals were simultaneously quantified using interleaved multi-slice pulsed arterial spin labeling (PASL) and T2*-weighted gradient echo sequences. Increasing basal CBF produced a significant decrease in the activation-induced BOLD response, with the slope of the optimal linear fit of activation versus basal BOLD signal changes of -0.32 +/- 0.01%/% for motor and visual cortex regions of interest (ROIs). While the modulation in basal flow level also produced a statistically significant effect on the activation-induced CBF change, the degree of relative attenuation of the flow response was slight, with a slope of -0.18 +/- 0.02%/% in the motor and -0.13 +/- 0.01%/% in the visual cortex ROI. The current findings describe a strong attenuation of the BOLD response at significantly elevated basal flow levels and call for independent quantification of resting CBF in BOLD fMRI studies that involve subjects and/or conditions with markedly elevated global perfusion.

Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Stress, acute pain and chronic pain may often result in hyperventilation (HV) which produces hypocapnia. The aim of this fMRI-study was to investigate the influence of hypocapnia on cortical activation during noxious stimulation in 14 healthy volunteers. The intensity of voluntary HV was controlled by capnometry Three tasks were performed in the fMRI sessions: (I) three 3-min HV periods with 7-min periods of recovery in between; (II) mechanically induced phasic pain stimulation--pain task (PT); (III) tapping--motor task (MT). The last two of these protocols were performed under normocapnic and hypocapnic conditions. HV decreased the fMRI signal by 3-7% in all regions of the cortex and subcortical nuclei. This decrease was most prominent in the opercular, frontal and temporal areas. When the PT was performed during hypocapnia a strong reduction in cluster sizes and lower t-values in S1 and insular cortex were found. In contrast MT was accompanied by an increase in cluster sizes and higher t-values. From this we conclude that hypocapnia significantly influences the BOLD signal in nociceptive and motor systems, indicating that either the coupling between the BOLD effect and neuronal processing changed or that the activity in the cortical network which represents the pain processing is decreased. These effects should be considered for functional brain imaging studies on the nociceptive system.

Heterogeneous oxygen extraction in the visual cortex during activation in mild hypoxic hypoxia revealed by quantitative functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) techniques were used to study haemodynamic and metabolic responses in human visual cortex during varying arterial blood oxygen saturation levels (Y(sat), determined by pulse-oximeter) and stimulation with contrast-reversing checkerboards. The visual-evoked potential amplitude remained constant at lowered Y(sat) of 0.82+/-0.03. Similarly, fMRI cerebral blood flow (CBF) responses were unchanged during reduced Y(sat). In contrast, visual cortex volume displaying blood oxygen level-dependent (BOLD) fMRI response decreased as a function of Y(sat), but the BOLD signal change of 3.6%+/-1.4% was constant. Oxygen extraction ratio (OER) during visual activation showed values of 0.26+/-0.03 for normal Y(sat). At lowered Y(sat), two OER patterns were observed. Firstly, a reduced OER of 0.14+/-0.03 in the visual cortex structures showing BOLD in hypoxia was observed. Secondly, signs of much higher OER in other parts of visual cortex were obtained. T2*-weighted magnetic resonance imaging revealed signal increases by 0.8%+/-0.4% with visual activation during lowered Y(sat) in the visual cortex structures, which showed BOLD of 3.6% in magnitude under normoxia. Because the CBF response in the visual cortex was quantitatively similar during stimulation in normoxia and hypoxia, attenuated T2*-weighted signal increase in parts of visual cortex indicated high OER during visual activation in hypoxia, which was close to that encountered in the resting brain. These spatially localised regions of tissue oxygen extraction and metabolism argue for dissociation between CBF and BOLD fMRI signals in mild hypoxia. The findings point to heterogeneity with regard to oxygen requirement and its coupling to the haemodynamic response in the brain.

Hyperventilation and amplified blood pressure response: is there a link?

Based on prior studies, the hypothesis that hyperventilation (HV) may have a pressor effect and play a causal role in hypertension has been suggested. The objective of this study was to correlate HV with blood pressure (BP)-change during a postural challenge. Consecutive subjects referred for evaluation of syncope, dizziness, chronic fatigue syndrome (CFS), fibromyalgia, or non-CFS fatigue were assessed with a 10-min supine 30-min head-up tilt test combined with capnography. We selected for analysis the records of patients aged 17-70 years, not taking vasoactive medications, having sitting systolic BP (SBP) < 140 mmHg, sitting diastolic BP (DBP) < 90 mmHg, and who completed 30 min of tilt. HV was diagnosed when end-tidal pressure of CO2 < 30 mmHg was recorded consecutively for > or = 10 min. Postural hypertension (PHT) was diagnosed when DBP on tilt > or = 90 mmHg was recorded consecutively for > or = 10 min. DBP-change was computed as (median DBP on tilt) -(median DBP supine). PHT and DBP-change were correlated with HV. A total of 320 patient charts were reviewed. PHT was present in 30 cases. The mean DBP-change in patients with PHT was +9.9 mmHg (s.d. 5.8), with three patients manifesting HV. Of the remaining 290 patients, 56 had HV, their mean DBP-change was -0.3 mmHg (s.d. 7.2). The other 234 patients without HV had a mean DBP-change +0.95 mmHg (s.d. 5.7), comparable to the DBP-change in patients with HV. In, conclusion, posturally induced HV was not associated with an increase in BP, nor was PHT associated with HV, except in a small minority of cases.

The cross-modal interaction between pain-related and saccade-related cerebral activation: a preliminary study by event-related functional magnetic resonance imaging

Pain-related cerebral activation in functional magnetic resonance imaging shows less consistent signals that decay earlier than in conventional task-related activation. This may result from pain's top-down inhibition mediated by cognitive or hemodynamic interaction that could affect activation by other modalities. Using event-related functional magnetic resonance imaging, we examined whether pain affects cerebral activation by a saccade task through such cross-modal interaction. Six right-handed volunteers underwent whole-brain echo-planar imaging on a 3.0 T magnetic resonance imaging scanner while they received thermal pain stimulus at 50 degrees C on the right forearm (P; n = 6), performed a visually guided saccade task (V; n = 6), and went through a simultaneous pain-plus-saccade paradigm (PV; n = 5). Averaged functional activation maps were synthesized and signal time courses were analyzed at activation clusters. P activated the bilateral secondary somatosensory cortex (S2). V activated the posterior, supplementary, frontal eye fields, and visual areas. PV enhanced the S2 activation and activated additional pain-related areas, including the bilateral premotor area, right insula, anterior, and posterior cingulate cortices. In contrast, V-related activation was attenuated in PV. We propose that pain caused cross-modal suppression on the oculomotor activity and that an oculomotor task enhanced pain-related activation by triggering attention toward pain.

IMPLICATIONS

Pain-related cerebral activation is enhanced by attention toward pain. It may involve top-down suppression over the unrelated neural networks of saccade.

Changes in BOLD and ADC weighted imaging in acute hypoxia during sea-level and altitude adapted states

Acute normobaric hypoxia as well as longstanding hypobaric hypoxia induce pronounced physiological changes and may eventually lead to impairment of cerebral function. The aim of the present study is to investigate the effect of hypoxia on the cerebral activation response as well as to explore possible structural changes as measured by diffusion weighted imaging. Eleven healthy sea-level residents were studied after 5 weeks of adaptation to high altitude conditions at Chacaltaya, Bolivia (5260 m). The subjects were studied immediately after return to sea-level in hypoxic and normoxic conditions, and the examinations repeated 6 months later after re-adaptation to sea-level conditions. The BOLD response, measured at 1.5 T, was severely reduced during acute hypoxia both in the altitude and sea-level adapted states (50% reduction during an average S(a)O(2) of 75%). On average, the BOLD response magnitude was 23% lower in altitude than sea-level adaptation in the normoxic condition, but in the hypoxic condition, no significant differences were found. A small but statistically significant decrease in the apparent diffusion coefficient (ADC) was seen in some brain regions during acute hypoxia, whereas ADC was slightly elevated in high altitude as compared to sea-level adaptation. It is concluded that hypoxia significantly diminishes the BOLD response, and the mechanisms underlying this finding are discussed. Furthermore, altitude adaptation may influence both the magnitude of the activation-related response, as well as micro-structural features.

The Impact of Prehospital Endotracheal Intubation on Outcome in Moderate to Severe Traumatic Brain Injury

BACKGROUND

Although early intubation to prevent the mortality that accompanies hypoxia is considered the standard of care for severe traumatic brain injury (TBI), the efficacy of this approach remains unproven.

METHODS

Patients with moderate to severe TBI (Head/Neck Abbreviated Injury Scale [AIS] score 3+) were identified from our county trauma registry. Logistic regression was used to explore the impact of prehospital intubation on outcome, controlling for age, gender, mechanism, Glasgow Coma Scale score, Head/Neck AIS score, Injury Severity Score, and hypotension. Neural network analysis was performed to identify patients predicted to benefit from prehospital intubation.

RESULTS

A total of 13,625 patients from five trauma centers were included; overall mortality was 22.9%, and 19.3% underwent prehospital intubation. Logistic regression revealed an increase in mortality with prehospital intubation (odds ratio, 0.36; 95% confidence interval, 0.32-0.42; p < 0.001). This was true for all patients, for those with severe TBI (Head/Neck AIS score 4+ and/or Glasgow Coma Scale score of 3-8), and with exclusion of patients transported by aeromedical crews. Patients intubated in the field versus the emergency department had worse outcomes. Neural network analysis identified a subgroup of patients with more significant injuries as potentially benefiting from prehospital intubation.

CONCLUSION

Prehospital intubation is associated with a decrease in survival among patients with moderate-to-severe TBI. More critically injured patients may benefit from prehospital intubation but may be difficult to identify prospectively.

The relationship between cerebral blood flow and volume in humans

The purpose of this study was to establish the relationship between regional CBF and CBV at normal, resting cerebral metabolic rates. Eleven healthy volunteers were investigated with PET during baseline conditions, and during hyper- and hypocapnia. Values for rCBF and rCBV were obtained using 15O-labelled water and carbon monoxide, respectively. The mean value of rCBF using PET was 62 +/- 18 ml 100 g(-1) min(-1) during baseline conditions, with an average increase of 46% during hypercapnia, and a decrease of 29% during hypocapnia; baseline rCBV was 7.7 ml/100 g, with 27% increase during hypercapnia and no significant decrease during hypocapnia. A regionally uniform exponential relationship was confirmed between PaCO2 and rCBF as well as rCBV. It is shown that the theoretical implication of this is that the rCBV vs. rCBF relationship should be modelled by a power function; however, due to pronounced intersubject variability, the goodness of fit for linear and nonlinear models were not significantly different. The results of the study are applied to a numerical estimation of regional brain deoxy-haemoglobin content. Independently of the choice of model for the rCBV vs. rCBF relationship, a nonlinear deoxy-haemoglobin vs. rCBF relationship was predicted, and the implications for the BOLD response are discussed.

The impact of hypoxia and hyperventilation on outcome after paramedic rapid sequence intubation of severely head-injured patients

BACKGROUND

An increase in mortality has been documented in association with paramedic rapid sequence intubation (RSI) of severely head-injured patients. This analysis explores the impact of hypoxia and hyperventilation on outcome.

METHODS

Adult severely head-injured patients (Glasgow Coma Scale score of 3-8) unable to be intubated without neuromuscular blockade underwent paramedic RSI using midazolam and succinylcholine; rocuronium was administered after confirmation of tube position. Standard ventilation parameters were used for most patients; however, one agency instituted use of digital end-tidal carbon dioxide (ETCO2) and oxygen saturation (Spo2) monitoring during the trial. Each patient undergoing digital ETCO2/Spo2 monitoring was matched to three historical nonintubated controls on the basis of age, gender, mechanism, and Abbreviated Injury Scale scores for each of six body regions. Logistic regression was used to explore the impact of oxygen desaturation during laryngoscopy and postintubation hypocapnia and hypoxia on outcome. The relationship between hypocapnia and ventilatory rate was explored using linear regression and univariate analysis. In addition, trial patients and controls were compared with regard to mortality and the incidence of "good outcomes" using an odds ratio analysis.

RESULTS

Of the 426 trial patients, a total of 59 had complete ETCO2/Spo2 monitoring data; these were matched to 177 controls. Logistic regression revealed an association between the lowest ETCO2 value and final ETCO2 value and mortality. Matched-controls analysis confirmed an association between hypocapnia and mortality. A statistically significant association between ventilatory rate and ETCO2 value was observed (r = -0.13, p < 0.0001); the median ventilatory rate associated with the lowest recorded ETCO2 value was significantly higher than for all other ETCO2 values (27 mm Hg vs. 19 mm Hg, p < 0.0001). In addition, profound desaturations during RSI and hypoxia after intubation were associated with higher mortality than matched controls. Overall mortality was 41% for trial patients versus 22% for matched controls (odds ratio, 2.51; 95% confidence interval, 1.33-4.72; p = 0.004).

CONCLUSIONS

Hyperventilation and severe hypoxia during paramedic RSI are associated with an increase in mortality.

Air medical transport of severely head-injured patients undergoing paramedic rapid sequence intubation

INTRODUCTION

The San Diego Paramedic Rapid Sequence Intubation (RSI) Trial documented an increase in mortality with paramedic RSI of patients with severe traumatic brain injury. This analysis explores the impact of air medical transport of trial patients on outcome.

METHODS

Adult trauma victims with severe traumatic brain injury (Glasgow Coma Scale score of 3 to 8) were prospectively enrolled. Paramedics performed RSI using midazolam and succinylcholine; air medical crews could be called at the discretion of ground paramedics, generally for anticipated prolonged transports. Patients were matched to historical controls using the following parameters: age, gender, mechanism, injury of severity score, and abbreviated injury scale scores for each body system. Patients transported by air and ground were compared with regard to demographics, clinical parameters, vital signs, arterial blood gas data, and outcome.

RESULTS

A total of 336 patients were included (79 air medical and 257 ground transports). No significant differences arose between the groups with regard to demographic, clinical, vital sign, and arterial blood gas data. Air medical patients had decreased mortality (28% vs 31%, OR 0.9), and ground patients had increased mortality versus matched controls (33% vs 22%, OR 1.8). Discordant groups analysis revealed a statistically significant effect of transport personnel on outcome (P=.009). Neither advanced procedures nor the use of mannitol accounted for the improved outcomes; air medical crews used capnometry to guide ventilation on all study patients.

CONCLUSION

Air medical transport of severely head-injured patients undergoing paramedic RSI was associated with improved outcomes. Improved ventilation by capnometry may account for part of these improvements.

BOLD-contrast functional MRI signal changes related to intermittent rhythmic delta activity in EEG during voluntary hyperventilation-simultaneous EEG and fMRI study

Differences in the blood oxygen level dependent (BOLD) signal changes were studied during voluntary hyperventilation (HV) between young healthy volunteer groups, (1) with intermittent rhythmic delta activity (IRDA) (N = 4) and (2) controls (N = 4) with only diffuse arrhythmic slowing in EEG (normal response). Subjects hyperventilated (3 min) during an 8-min functional MRI in a 1.5-T scanner, with simultaneous recording of EEG (successful with N = 3 in both groups) and physiological parameters. IRDA power and average BOLD signal intensities (of selected brain regions) were calculated. Hypocapnia showed a tendency to be slightly lighter in the controls than in the IRDA group. IRDA power increased during the last minute of HV and ended 10-15 s after HV. The BOLD signal decreased in white and gray matter after the onset of HV and returned to the baseline within 2 min after HV. The BOLD signal in gray matter decreased approximately 30% more in subjects with IRDA than in controls, during the first 2 min of HV. This difference disappeared (in three subjects out of four) during IRDA in EEG. BOLD signal changes seem to depict changes, which precede IRDA. IRDA due to HV in healthy volunteers represent a model with a clearly defined EEG pattern and an observable BOLD signal change.

The Use of Quantitative End-Tidal Capnometry to Avoid Inadvertent Severe Hyperventilation in Patients With Head Injury After Paramedic Rapid Sequence Intubation

BACKGROUND

This study aimed to determine whether field end-tidal carbon dioxide CO2 (ETCO2) monitoring decreases inadvertent severe hyperventilation after paramedic rapid sequence intubation.

METHODS

Data were collected prospectively as part of the San Diego Paramedic Rapid Sequence Intubation Trial, which enrolled adults with severe head injuries (Glasgow Coma Score, 3-8) that could not be intubated without neuromuscular blockade. After preoxygenation, the patients underwent rapid sequence intubation using midazolam and succinylcholine. A maximum of three intubation attempts were allowed before Combitube insertion was mandated. Tube confirmation was accomplished by physical examination, qualitative capnometry, pulse oximetry, and syringe aspiration. Standard ventilation parameters (tidal volume, 800 mL; 12 breaths/minute) were taught. One agency used portable ETCO2 monitors, with ventilation modified to target ETCO2 values of 30 to 35 mm Hg. Trial patients transported by aeromedical crews also underwent ETCO2 monitoring. The primary outcome measure was the incidence of inadvertent severe hyperventilation, defined as arterial blood gas partial pressure of CO2 (pCO2) of less than 25 mm Hg at arrival, for patients with and those without ETCO2 monitoring. These groups also were compared in terms of age, gender, clinical presentation, Abbreviated Injury Score, Injury Severity Score, arrival arterial blood gas data, and survival.

RESULTS

The study enrolled 426 patients and administered neuromuscular blocking agents to 418 patients. Endotracheal intubation was successful for 355 of these patients (85.2%). Another 58 patients (13.6%) underwent Combitube insertion. For 291 successfully intubated patients, arrival pCO2 values were documented, with continuous ETCO2 monitoring performed for 144 of these patients (49.4%). Patients with ETCO2 monitoring had a lower incidence of inadvertent severe hyperventilation than those without ETCO2 monitoring (5.6% vs. 13.4%; odds ratio, 2.64; 95% confidence interval, 1.12-6.20; p = 0.035). There were no significant differences in terms of age, gender, clinical presentation, Abbreviated Injury Score, Injury Severity Score, arrival partial pressure of oxygen (PO2) and pH, or survival. The patients in both groups with severe hyperventilation had a significantly higher mortality rate than the patients without hyperventilation (56 vs. 30%; odds ratio, 2.9; 95% confidence interval, 1.3-6.6; p = 0.016), which could not be explained solely on the basis of their injuries.

CONCLUSIONS

The use of ETCO2 monitoring is associated with a decrease in inadvertent severe hyperventilation.

Effects of hydration and hyperventilation on cortical complexity

The effects of hydration and hyperventilation on cortical complexity were investigated in a sample of 19 healthy volunteers in a double-blind placebo design using magnetoencephalographic recordings. The subjects were asked to abstain from the intake of liquids 18 h before the study. Spontaneous magnetoencephalograms (MEG) were recorded before and after drinking 750 ml water (WAT group: nine subjects) or saline solution (SAL group: ten subjects) with eyes closed and open and during hyperventilation (HV) with eyes open. The MEG data were analysed using both linear (spectral power) and non-linear (pointwise dimension and largest Lyapunov exponent) algorithms. The prediction that intake of water, because of induced cell swelling, will lead to an increased synchronization and a decreased complexity of the spontaneous MEG during hyperventilation was confirmed. Hyperventilation following the drinking condition produced an increase in all power spectra with a stronger increase of delta and theta power after drinking of water. This synchronization of spontaneous MEG is accompanied by a general significant decrease of cortical complexity, especially after water drinking. Moreover, cortical complexity was inversely related to delta and theta power and partly also to alpha power. The SAL and WAT groups showed different relations between alpha power and dimensional complexity during HV: whereas in the SAL group the correlations between these measures became more negative during HV, they reversed in the WAT group to become positive. The synchronizing effect of hyperventilation, leading to a decrease of cortical complexity, is related in the SAL group to delta, theta and alpha power, whereas in the WAT group only delta and theta activity contribute to a reduction of cortical complexity.

Real-time fMRI of temporolimbic regions detects amygdala activation during single-trial self-induced sadness

Temporolimbic circuits play a crucial role in the regulation of human emotion. A highly sensitive single-shot multiecho functional magnetic resonance imaging (fMRI) technique with gradient compensation of local magnetic field inhomogeneities and real-time data analysis were used to measure increases in amygdala activation during single 60-s trials of self-induced sadness. Six healthy male and female subjects performed a validated mood induction paradigm with randomized presentation of sad or neutral faces in 10 trials per scan. Subjects reported the intensity of experienced sadness after each trial. Immediate feedback of amygdala activation was given to the subjects during the ongoing scan to reinforce mood induction. Correspondence between increased intensity of predominantly left sided amygdala activation and self-rating of sadness was found in 78% of 120 sad trials, in contrast to only 14% of neutral trials. Amygdala activation was reproducible during repeated scanning sessions and displayed the strongest correlation with self-rating among all regions. These results suggest that amygdala activation may be closely associated with self-induced sadness. This novel real-time fMRI technology is applicable to a wide range of neuroscience studies, particularly those of the limbic system, and to neuropsychiatric conditions, such as depression, in which pathology of the amygdala has been implicated.

Effect of basal conditions on the magnitude and dynamics of the blood oxygenation level-dependent fMRI response

The effect of the basal cerebral blood flow (CBF) on both the magnitude and dynamics of the functional hemodynamic response in humans has not been fully investigated. Thus, the hemodynamic response to visual stimulation was measured using blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in human subjects in a 7-T magnetic field under different basal conditions: hypocapnia, normocapnia, and hypercapnia. Hypercapnia was induced by inhalation of a 5% carbon dioxide gas mixture and hypocapnia was produced by hyperventilation. As the fMRI baseline signal increased linearly with expired CO2 from hypocapnic to hypercapnic levels, the magnitude of the BOLD response to visual stimulation decreased linearly. Measures of the dynamics of the visually evoked BOLD response (onset time, full-width-at-half-maximum, and time-to-peak) increased linearly with the basal fMRI signal and the end-tidal CO2 level. The basal CBF level, modulated by the arterial partial pressure of CO2, significantly affects both the magnitude and dynamics of the BOLD response induced by neural activity. These results suggest that caution should be exercised when comparing stimulus-induced fMRI responses under different physiologic or pharmacologic states.

Regional differences of fMR signal changes induced by hyperventilation: comparison between SE-EPI and GE-EPI at 3-T

PURPOSE

To evaluate whether reproducible signal change of brain tissues by hyperventilation (HV) can be seen on spin-echo (SE)-echo planar imaging (EPI) at 3-T and to examine the sensitivity of SE-EPI for measuring vascular reactivity in regions of the brain, such as the hippocampal formation, that are difficult to visualize with gradient-echo (GE)-EPI due to susceptibility artifacts.

MATERIALS AND METHODS

Six healthy human subjects performed a voluntary HV task. The task design was as follows: two minutes normal breathing (rest) followed by two minutes HV, giving a basic four-minute block that was repeated three times for a total scan time of 12 minutes for one run. Each subject performed the run both for SE-EPI and GE-EPI. Statistical analysis was performed to detect the area with significant cerebrovascular reactivity. The percentage signal change was also obtained for each cerebral region.

RESULTS

Both GE-EPI and SE-EPI showed globally significant signal decreases in the cerebral cortex. In GE-EPI, the frontal cortex showed a larger signal decrease than the other gray matter tissues (P < 0.05). In SE-EPI, the differences among gray matter tissues except for the hippocampal formation were not significant. The hippocampal formation showed the largest signal change (P < 0.05) in SE-EPI, but no significant signal change was observed in GE-EPI due to the presence of susceptibility artifacts.

CONCLUSION

HV using SE-EPI at 3-T provides robust and reproducible signal decreases and may make the evaluation of the vascular reactivity in hippocampal formation feasible.

Effect of respiratory CO(2) changes on the temporal dynamics of the hemodynamic response in functional MR imaging

Increasing end-expiratory CO(2) levels (PETCO(2)) increases the dispersion and the time of maximum of the hemodynamic response curve in human primary visual cortex. This was demonstrated using event-related multislice functional magnetic resonance imaging (fMRI) with short repetition time and 3-s flicker light stimulation. Measurements were performed at 5 different PETCO(2) levels between 20 and 60 mmHg using hyperventilation or by adding CO(2) to the inspired air. Between 30 and 60 mmHg the full-width-at-half-maximum of the hemodynamic response curve induced by visual stimulation increased nearly linearly at 130 ms per mmHg PETCO(2). Consistent with previous studies a concomitant decrease of the signal amplitude was observed at PETCO(2) values below 40 mmHg and above 50 mmHg. The relevance of these findings for the temporal resolution of fMRI and especially of event-related methods is discussed.

Effect of graded hypo- and hypercapnia on fMRI contrast in visual cortex: quantification of T(*)(2) changes by multiecho EPI

The sensitivity of functional magnetic resonance imaging (fMRI) in visual cortex to graded hypo- and hypercapnia was quantified in 10 normal subjects using single-shot multiecho echo-planar imaging (Turbo-PEPSI) with eight equidistant echo times (TEs) between 12 and 140 ms. Visual stimulation was combined with controlled hyperventilation and carbon dioxide inhalation to perform fMRI at six levels of end-expiratory pCO(2) (PETCO(2)) between 20 and 70 mm Hg. T(*)(2) in visual cortex during baseline conditions (light off) increased nonlinearly from 20 to 70 mm Hg, from 61.1 +/- 4.2 ms to 72.0 +/- 4.6 ms. Changes in T(*)(2) due to visual stimulation increased 2.1-fold, from 1.2 +/- 0.6 ms at 20 mm Hg to 2.5 +/- 0.7 ms at 50 mm Hg. An almost complete loss of functional contrast was measured at 70 mm Hg. The model of MR signal dephasing by Yablonskiy and Haacke (Mag Reson Med 1994;32:749-763) was used to predict changes in cerebral blood flow (CBF), which were found to be consistent with results from previous positron emission tomography (PET) studies. This study further emphasizes that global CBF changes (due to PETCO(2) changes even in the physiological range) strongly influence fMRI contrast and need to be controlled for.

Modulation of cerebral blood oxygenation by indomethacin: MRI at rest and functional brain activation

The modulation of blood oxygenation level-dependent (BOLD) cerebral MRI contrast by the vasoconstrictive drug indomethacin (i.v. 0.2 mg/kg b.w.) was investigated in 10 healthy young adults without and with functional challenge (repetitive and sustained visual activation). For comparison, isotonic saline (placebo, 20 mL) and acetylsalicylate (i.v. 500 mg) were investigated as well, each in separate sessions using identical protocols. After indomethacin, dynamic T2*-weighted echo-planar MRI at 2.0 T revealed a rapid decrease in MRI signal intensity by 2.1%-2.6% in different gray matter regions (P < or = 0.001 compared to placebo), which was not observed for acetylsalicylate and the placebo condition. Regional signal differences were not significant within gray matter, but all gray matter regions differed significantly from the signal decrease of only 1.2% +/- 0.7% observed in white matter (P = 0.001). For the experimental parameters used, a 1% MRI signal decrease in response to indomethacin was estimated to correlate with a decrease of the cerebral blood flow by about 12 ml/100 g/minute, and an increase of the oxygen extraction fraction by about 15%. Responses to visual activation were not affected by saline or acetylsalicylate, and yielded 5.0%-5.5% BOLD MRI signal increases both before and after drug application. In contrast, indomethacin reduced the initial response strength to 82%-85% of that obtained without the drug. The steady-state response during sustained activation reached only 47% of the corresponding pre-drug level (P < 0.01). During repetitive activation the BOLD contrast was reduced to 66% of that observed for control conditions (P < 0.001). In conclusion, indomethacin attenuates the vasodilatory force at functional brain activation, indicating different mechanisms governing neurovascular coupling.

Chronological analysis of physiological T2* signal change in the cerebrum during breath holding

The purpose of this study was to examine which physiological factors affect cerebral T2* signal intensity (SI) during breath holding (BH) (apnea after inspiration and breathing after expiration) in normal volunteers. We examined SI changes caused by anoxic gas inhalation, by respiratory movements, and by BH. High-speed echo planar images (EPI) showed changes in SI that could be divided into five phases. Reports indicate that SI changes induced by BH are due to the effects on the magnetic susceptibility of deoxygenated hemoglobin (deoxyhemoglobin (dHb)) and to hypercapnia, but these reports could not fully explain the observed five phases. In addition to deoxyhemoglobin susceptibility and hypercapnia, we found that respiratory movements play a third critical role in modifying SI by affecting blood flow into the region of interest (ROI), as judged from right carotid artery flow. Consequently, we propose that the physiological SI changes induced by BH are derived from blood oxygenation, hypercapnia, and respiratory movements.

Increasing mean airway pressure reduces functional MRI (fMRI) signal in the primary visual cortex

Changes in both blood flow and blood oxygenation determine the functional MRI (fMRI) signal. In the present study factors responsible for blood oxygenation (e.g., FiO(2)) were held constant so that changes in pixel count would above all reflect changes in regional cerebral blood flow (rCBF). Continuous positive airway pressure (CPAP) breathing at 12 cm H(2)O, which was previously shown to influence rCBF, was applied in human volunteers (n = 19) to investigate the sensitivity of fMRI for changes in rCBF caused by increased mean airway pressure. Increasing the mean airway pressure decreased the pixel count in the primary visual cortex (median (range)): baseline: 219 (58-425) pixels vs. CPAP (12 cm H(2)O): 92 (0-262) pixels). These findings indicate that fMRI is sensitive to detect a reduced rCBF-response in the primary visual cortex. The underlying mechanism is likely to be a reduced basal rCBF due to constriction and/or compression of postcapillary venoles during CPAP breathing. These findings are important for interpreting fMRI results in awake and in artificially respirated patients, in whom positive airway pressure is used to improve pulmonary function during the diagnostic procedure.

Brain imaging and the effects of caffeine and nicotine

Caffeine and nicotine are the most common psychostimulant drugs used worldwide. Structural neuroimaging findings associated with caffeine and nicotine consumption are limited and primarily reflect the putative relationship between smoking and white matter hyperintensities (WMH), a finding that warrants further appraisal of its clinical implications. The application of newer brain imaging modalities that measure subtle haemodynamic changes or tissue-based chemistry in order to better elucidate brain functional processes, including mechanisms underlying addiction to nicotine and caffeine and the brain functional consequences, provide intriguing findings. Potential influences of caffeine and nicotine on the functional contrast, or metabolic response, to neural activation also necessitates the careful appraisal of the effects that these commonly used drugs may have on the results of functional imaging.

Physiologically initiated and inhibited phantosmia: cyclic unirhinal, episodic, recurrent phantosmia revealed by brain fMRI

PURPOSE

Our goal was to use functional magnetic resonance imaging (fMRI) to demonstrate brain activation in patients with unirhinal, episodic, recurrent phantosmia who induced their phantosmia by coughing, sneezing, laughing or vigorous nasal inhalation and expiration, and inhibited it by sleep or performance of a Valsalva type maneuver.

METHODS

Three patients with unirhinal phantosmia without change in taste or smell acuity were studied by fast low angle shot (FLASH) MRI and by echo planar imaging (EPI). Brain activation was measured following memory of two tastants (salt, sweet), memory of two odorants (banana and peppermint), actual smell of three odors (amyl acetate, menthone, pyridine), memory of phantosmia (and phantageusia, where applicable), phantosmia initiated spontaneously or by vigorous nasal inhalation and exhalation, phantosmia after inhibition by Valsalva, and these stimuli before and after treatment with the neuroleptic thioridazine. Activation images were derived using correlation analysis and ratios of areas of brain activated to total brain areas were calculated. Total activated pixel cluster counts were also used to quantitative total and regional brain activation.

RESULTS

Sensory-specific brain activation was present in each section in each patient following memory of tastants and odorants, actual smell of each odor and memory, and initiation of and inhibition of phantosmia. Activation to odor memory after phantosmia initiation was very robust, whereas after phantosmia inhibition it was similar to that in normal subjects. Brain activation to unirhinal phantosmia was bihemispheric, independent of whether it was left or right sided or patient handedness. While phantosmia memory (in the absence of initiated phantosmia) produced extremely robust brain activation, after initiation and inhibition of phantosmia apparent brain activation decreased. These changes need to be related to shifting state of baseline brain activation and should be interpreted to reflect increased rather than decreased brain activation over that of phantosmia memory alone. Treatment with thioridazine inhibited brain activation to all stimuli including phantosmia and phantageusia memory, as it did previously in patients with birhinal phantosmia.

CONCLUSIONS

1) Unirhinal phantosmia can be demonstrated by brain fMRI as can birhinal phantosmia; 2) unirhinal phantosmia can be initiated and inhibited by physiological maneuvers reflected by changes in fMRI brain activation; 3) fMRI brain activation of unirhinal phantosmia is bihemispheric and independent of peripheral side of phantosmia or patient handedness; 4) anterior frontal brain region plays a significant role in both phantosmia initiation and inhibition as, to some extent, do temporal brain regions; 5) activation of brain GABAergic systems appears to play a role in inhibition of unirhinal phantosmia; and 6) unirhinal phantosmia, similar to birhinal phantosmia, may reflect a type of maladaptive brain plasticity similar to that hypothesized to be responsible for phantom limb pain.

Transforming growth factor-beta expression in mucosal biopsies in asthma and chronic bronchitis

We assessed whether transforming growth factor-beta (TGF-beta), a fibrogenic growth factor, may be involved in remodeling of asthma and chronic bronchitis; its expression was compared with that of epidermal growth factor (EGF) and granulocyte macrophage colony-stimulating factor (GM-CSF) in bronchial mucosal biopsies from 13 normal subjects, 24 asthmatics, and 19 patients with chronic bronchitis. TGF-beta immunoreactivity was highly increased in epithelium and submucosa of those with bronchitis and to a lesser extent in asthmatics. By comparison, with normal subjects, EGF immunoreactivity was significantly increased in the epithelium of bronchitic subjects and submucosa of asthmatics, and, GM-CSF immunoreactivity was increased in both epithelial and submucosal cells of asthmatics and to a lesser extent in submucosa of bronchitics. A significant correlation was found between the number of epithelial or submucosal cells expressing TGF-beta in both asthma and chronic bronchitis and basement membrane thickness and fibroblast number. No such correlation was found for EGF or GM-CSF. in situ hybridization for TGF-beta 1 mRNA confirmed the results obtained by immunohistochemistry. By combining in situ hybridization and immunohistochemistry, it was found that eosinophils and fibroblasts were synthetizing TGF-beta in asthma and bronchitis. These data suggest that TGF-beta, but not EGF or GM-CSF, is involved in airways remodeling in asthma and chronic bronchitis.