Prospective targeting and control of end-tidal CO2 and O2 concentrations

The role of physiological noise in resting-state functional connectivity

Functional connectivity between different brain regions can be estimated from MRI data by computing the temporal correlation of low frequency (<0.1Hz) fluctuations in the MRI signal. These correlated fluctuations occur even when the subject is "at rest" (not asked to perform any particular task) and result from spontaneous neuronal activity synchronized within multiple distinct networks of brain regions. This estimate of connectivity, however, can be influenced by physiological noise, such as cardiac and respiratory fluctuations. This brief review looks at the effect of physiological noise on estimates of resting-state functional connectivity, discusses ways to remove physiological noise, and provides a personal recollection of the early developments in these approaches. This review also discusses the importance of physiological noise correction and provides a summary of evidence demonstrating that functional connectivity does have a neuronal underpinning and cannot purely be the result of physiological noise.

Magnetic resonance imaging of resting OEF and CMRO₂ using a generalized calibration model for hypercapnia and hyperoxia

We present a method allowing determination of resting cerebral oxygen metabolism (CMRO₂) from MRI and end-tidal O₂ measurements acquired during a pair of respiratory manipulations producing different combinations of hypercapnia and hyperoxia. The approach is based on a recently introduced generalization of calibrated MRI signal models that is valid for arbitrary combinations of blood flow and oxygenation change. Application of this model to MRI and respiratory data during a predominantly hyperoxic gas manipulation yields a specific functional relationship between the resting BOLD signal M and the resting oxygen extraction fraction OEF₀. Repeating the procedure using a second, primarily hypercapnic, manipulation provides a different functional form of M vs. OEF₀. These two equations can be readily solved for the two unknowns M and OEF₀. The procedure also yields the resting arterial O₂ content, which when multiplied by resting cerebral blood flow provides the total oxygen delivery in absolute physical units. The resultant map of oxygen delivery can be multiplied by the map of OEF₀ to obtain a map of the resting cerebral metabolic rate of oxygen consumption (CMRO₂) in absolute physical units. Application of this procedure in a group of seven human subjects provided average values of 0.35 ± 0.04 and 6.0 ± 0.7% for OEF₀ and M, respectively in gray-matter (M valid for 30 ms echo-time at 3T). Multiplying OEF₀ estimates by the individual values of resting gray-matter CBF (mean 52 ± 5 ml/100 g/min) and the measured arterial O₂ content gave a group average resting CMRO₂ value of 145 ± 30 μmol/100 g/min. The method also allowed the generation of maps depicting resting OEF, BOLD signal, and CMRO₂.

Quantitative measurement of cerebral physiology using respiratory-calibrated MRI

Functional magnetic resonance imaging typically measures signal increases arising from changes in the transverse relaxation rate over small regions of the brain and associates these with local changes in cerebral blood flow, blood volume and oxygen metabolism. Recent developments in pulse sequences and image analysis methods have improved the specificity of the measurements by focussing on changes in blood flow or changes in blood volume alone. However, FMRI is still unable to match the physiological information obtainable from positron emission tomography (PET), which is capable of quantitative measurements of blood flow and volume, and can indirectly measure resting metabolism. The disadvantages of PET are its cost, its availability, its poor spatial resolution and its use of ionising radiation. The MRI techniques introduced here address some of these limitations and provide physiological data comparable with PET measurements. We present an 18-minute MRI protocol that produces multi-slice whole-brain coverage and yields quantitative images of resting cerebral blood flow, cerebral blood volume, oxygen extraction fraction, CMRO(2), arterial arrival time and cerebrovascular reactivity of the human brain in the absence of any specific functional task. The technique uses a combined hyperoxia and hypercapnia paradigm with a modified arterial spin labelling sequence.

Monitoring of breathing phases using a bioacoustic method in healthy awake subjects

OBJECTIVE

To test the ability of a microphone recording system, located distal to the respiratory outflow tract, to track the timing of the inspiratory and expiratory phases of breathing in awake healthy subjects.

METHODS

Fifteen subjects participated. Breath sounds were recorded using a microphone embedded in a face frame in a fixed location in relation to the nostrils and mouth, while simultaneously recording respiratory movements by respiratory inductance plethysmography (RIP). Subjects were studied while supine and were instructed to breathe normally for 2 min: through their noses only (nasal breathing), during the first min, and through their mouths only (oral breathing) during the second min. Five subjects (test group) were chosen randomly to extract features from their acoustic data. Ten breaths (5 nasal and 5 oral breaths) from each subject were studied. Inspiratory and expiratory segments of breath sounds were determined and extracted from the acoustic data by comparing it to the RIP trace. Subsequently, the frequency spectrum of each phase was then determined. Spectral variables derived from the 5 test subjects were applied prospectively to detect breathing phases in the remaining 10 subjects (validation group).

RESULTS

Test group data showed that the mean of all inspiratory spectra peaked between 30 and 270 Hz, flattened between 300 and 1,100 Hz, and peaked again with a center frequency of 1,400 Hz. The expiratory spectra peaked between 30 and 180 Hz and its power dropped off exponentially after that. Accordingly, the bands ratio (BR) of frequency magnitudes between 500 and 2500 Hz to frequency magnitudes between 0 and 500 Hz was chosen as a feature to distinguish between breathing phases. BR for the mean inspiratory spectrum was 2.27 and for the mean expiratory spectrum was 0.15. The route of breathing did not affect the BR ratio within the same phase. When this BR was applied to 436 breathing phases in the validation group, 424 (97%) were correctly identified (Kappa = 0.96, P < 0.001) indicating strong agreement between the acoustic method and the RIP.

CONCLUSION

Frequency spectra of breathing sounds recorded from a face-frame, reliably identified the inspiratory and expiratory phases of breathing. This technique may have various applications for respiratory monitoring and analysis.

Respiratory, cerebrovascular and cardiovascular responses to isocapnic hypoxia

We simultaneously measured respiratory, cerebrovascular and cardiovascular responses to 10-min of isoxic hypoxia at three constant CO(2) tensions in 15 subjects. We observed four response patterns, some novel, for ventilation, middle cerebral artery blood flow velocity, heart rate and mean arterial blood pressure. The occurrence of the response patterns was correlated between some measures. Isoxic hyperoxic and hypoxic ventilatory sensitivities to CO(2) derived from these responses were equivalent to those measured with modified (Duffin) rebreathing tests, but cerebrovascular sensitivities were not. We suggest the different ventilatory response patterns reflect the time course of carotid body afferent activity; in some individuals, carotid body function changes during hypoxia in more complex ways than previously thought. We concluded that isoxic hyperoxic and hypoxic ventilatory sensitivities to CO(2) can be measured using multiple hypoxic ventilatory response tests only if care is taken choosing the isocapnic CO(2) levels used, but a similar approach to measuring the cerebrovascular response to isocapnic hyperoxia and hypoxia is unfeasible.

Severely impaired cerebrovascular reserve in patients with cerebral proliferative angiopathy

OBJECT

Cerebral proliferative angiopathy (CPA) has been morphologically distinguished from classically appearing brain arteriovenous malformations (AVMs) by exhibition of functional brain parenchyma that is intermingled with abnormal vascular channels. The presence of oligemia in this intralesional brain tissue may suggest ischemia, which is not detected in classic brain AVMs. The authors hypothesized that patients with CPA would exhibit a greater impairment of cerebrovascular reserve in neuronal tissue surrounding the true nidus compared with those with brain AVMs.

METHODS

Four patients with CPA, 10 patients with brain AVMs and seizures, and 12 young healthy individuals were studied. The 4 patients with CPA underwent blood oxygen level-dependent MR imaging examinations while applying normoxic step changes in end-tidal CO(2) to obtain quantitative cerebrovascular reactivity measurements.

RESULTS

Patients with a CPA lesion exhibited severely impaired perilesional cerebrovascular reserve in comparison with patients with brain AVMs and seizures (0.10 ± 0.03 vs 0.16 ± 0.03, respectively; p < 0.05), and young healthy individuals (0.10 ± 0.03 vs 0.21 ± 0.06, respectively; p < 0.01).

CONCLUSIONS

This study demonstrated severely impaired cerebrovascular reserve in the perilesional brain tissue surrounding the abnormal vessels of patients with CPA. This finding may provide an additional means to distinguish CPA from classic brain AVMs.

Supplementary oxygen for nonhypoxemic patients: O2 much of a good thing?

Supplementary oxygen is routinely administered to patients, even those with adequate oxygen saturations, in the belief that it increases oxygen delivery. But oxygen delivery depends not just on arterial oxygen content but also on perfusion. It is not widely recognized that hyperoxia causes vasoconstriction, either directly or through hyperoxia-induced hypocapnia. If perfusion decreases more than arterial oxygen content increases during hyperoxia, then regional oxygen delivery decreases. This mechanism, and not (just) that attributed to reactive oxygen species, is likely to contribute to the worse outcomes in patients given high-concentration oxygen in the treatment of myocardial infarction, in postcardiac arrest, in stroke, in neonatal resuscitation and in the critically ill. The mechanism may also contribute to the increased risk of mortality in acute exacerbations of chronic obstructive pulmonary disease, in which worsening respiratory failure plays a predominant role. To avoid these effects, hyperoxia and hypocapnia should be avoided, with oxygen administered only to patients with evidence of hypoxemia and at a dose that relieves hypoxemia without causing hyperoxia.

End-inspiratory rebreathing reduces the end-tidal to arterial PCO2 gradient in mechanically ventilated pigs

PURPOSE

Noninvasive monitoring of the arterial partial pressures of CO(2) (PaCO(2)) of critically ill patients by measuring their end-tidal partial pressures of CO(2) (PETCO(2)) would be of great clinical value. However, the gradient between PETCO(2) and PaCO(2) (PET-aCO(2)) in such patients typically varies over a wide range. A reduction of the PET-aCO(2) gradient can be achieved in spontaneously breathing healthy humans using an end-inspiratory rebreathing technique. We investigated whether this method would be effective in reducing the PET-aCO(2) gradient in a ventilated animal model.

METHODS

Six anesthetized pigs were ventilated mechanically. End-tidal gases were systematically adjusted over a wide range of PETCO(2) (30-55 mmHg) and PETO(2) (35-500 mmHg) while employing the end-inspiratory rebreathing technique and measuring the PET-aCO(2) gradient. Duplicate arterial blood samples were taken for blood gas analysis at each set of gas tensions.

RESULTS

PETCO(2) and PaCO(2) remained equal within the error of measurement at all gas tension combinations. The mean ± SD PET-aCO(2) gradient (0.13 ± 0.12 mmHg, 95% CI -0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO(2) measurements at all PETCO(2) and PETO(2) combinations (0.19 ± 0.06, 95% CI -0.32, -0.06).

CONCLUSIONS

The end-inspiratory rebreathing technique is capable of reducing the PET-aCO(2) gradient sufficiently to make the noninvasive measurement of PETCO(2) a useful clinical surrogate for PaCO(2) over a wide range of PETCO(2) and PETO(2) combinations in mechanically ventilated pigs. Further studies in the presence of severe ventilation-perfusion (V/Q) mismatching will be required to identify the limitations of the method.

Noninvasive MRI measures of microstructural and cerebrovascular changes during normal swine brain development

The swine brain is emerging as a potentially valuable translational animal model of neurodevelopment and offers the ability to assess the impact of experimentally induced neurological disorders. The goal for this study was to characterize swine brain development using noninvasive MRI measures of microstructural and cerebrovascular changes. Thirteen pigs at various postnatal ages (2.3-43.5 kg) were imaged on a 1.5-Tesla MRI system. Microstructural changes were assessed using diffusion tensor imaging measures of mean diffusivity and fractional anisotropy. Cerebrovascular changes were assessed using arterial spin labeling measures of baseline cerebral blood flow (CBF) and the cerebrovascular reactivity (CVR) of the blood-oxygen level dependent (BOLD) MRI signal to CO2. We found a positive logarithmic relationship for regional tissue volumes and fractional anisotropy with body weight, which is similar to the pattern reported in the developing human brain. Unlike in the maturing human brain, no consistent changes in mean diffusivity or baseline CBF with development were observed. Changes in BOLD CVR exhibited a positive logarithmic relationship with body weight, which may impact the interpretation of functional MRI results at different stages of development. This animal model can be validated by applying the same noninvasive measures in humans.

Measurement of cerebrovascular reactivity in pediatric patients with cerebral vasculopathy using blood oxygen level-dependent MRI

BACKGROUND AND PURPOSE

Cerebrovascular reactivity (CVR) is an indicator of cerebral hemodynamics. In adults with cerebrovascular disease, impaired CVR has been shown to be associated with an increased risk of stroke. In children, however, CVR studies are not common. This may be due to the difficulties and risks associated with current CVR study methodologies. We have previously described the application of precise control of end-tidal carbon dioxide partial pressure for CVR studies in adults. Our aim is to report initial observations of CVR studies that were performed as part of a larger observational study regarding investigations in pediatric patients with cerebral vascular disease.

METHODS

Thirteen patients between the ages of 10 and 16 years (10 with a diagnosis of Moyamoya vasculopathy and 3 with confirmed, or suspected, intracranial vascular stenosis) underwent angiography, MRI, and functional blood oxygen level-dependent MRI mapping of CVR to hypercapnia. The results of the CVR study were then related to both the structural imaging and clinical status.

RESULTS

Sixteen blood oxygen level-dependent MRI CVR studies were performed successfully in 13 consecutive patients. Twelve of the 13 patients with angiographic abnormalities also had CVR deficits in the corresponding downstream vascular territories. CVR deficits were also seen in 8 of 9 symptomatic patients and 2 of the asymptomatic patients. Notably, in patients with abnormalities on angiography, the reductions in CVR extended beyond the ischemic lesions identified with MR structural imaging into normal-appearing brain parenchyma.

CONCLUSIONS

This is the first case series reporting blood oxygen level-dependent MRI CVR in children with cerebrovascular disease. CVR studies performed so far provide information regarding hemodynamic compromise, which complements traditional clinical assessment and structural imaging.

Surgical revascularization reverses cerebral cortical thinning in patients with severe cerebrovascular steno-occlusive disease

BACKGROUND AND PURPOSE

Chronic deficiencies in regional blood flow lead to cerebral cortical thinning without evidence of gross tissue loss at the same time as potentially negatively impacting on neurological and cognitive performance. This is most pronounced in patients with severe occlusive cerebrovascular disease in whom affected brain areas exhibit "steal physiology," a paradoxical reduction of cerebral blood flow in response to a global vasodilatory stimulus intended to increase blood flow. We tested whether surgical brain revascularization that eliminates steal physiology can reverse cortical thinning.

METHODS

We identified 29 patients from our database who had undergone brain revascularization with pre- and postoperative studies of cerebrovascular reactivity using blood oxygen(ation) level-dependent MRI and whose preoperative study exhibited steal physiology without MRI-evident structural abnormalities. Cortical thickness in regions corresponding to steal physiology, and where applicable corresponding areas in the normal hemisphere, were measured using Freesurfer software.

RESULTS

At an average of 11 months after surgery, cortical thickness increased in every successfully revascularized hemisphere (n=30). Mean cortical thickness in the revascularized regions increased by 5.1% (from 2.40 ± 0.03 to 2.53 ± 0.03; P<0.0001).

CONCLUSIONS

Successful regional revascularization and reversal of steal physiology is followed by restoration of cortical thickness.

Quantitative measurement of cerebrovascular reactivity by blood oxygen level-dependent MR imaging in patients with intracranial stenosis: preoperative cerebrovascular reactivity predicts the effect of extracranial-intracranial bypass surgery

BACKGROUND AND PURPOSE

CVR is a measure of cerebral hemodynamic impairment. A recently validated technique quantifies CVR by using a precise CO(2) vasodilatory stimulus and BOLD MR imaging. Our aim was to determine whether preoperative CO(2) BOLD CVR predicts the hemodynamic effect of ECIC bypass surgery in patients with intracranial steno-occlusive disease.

MATERIALS AND METHODS

Twenty-five patients undergoing ECIC bypass surgery for treatment of intracranial stenosis or occlusion were recruited. CVR was measured preoperatively and postoperatively and expressed as %ΔBOLD MR signal intensity per mm Hg ΔPetCO(2). Using normative data from healthy subjects, we stratified patients on the basis of preoperative CVR into 3 groups: normal CVR, reduced CVR, and negative (paradoxical) CVR. Wilcoxon 2-sample tests (2-sided, α = 0.05) were used to determine whether the 3 groups differed with respect to change in CVR following bypass surgery.

RESULTS

The group with normal preoperative CVR demonstrated no significant change in CVR following bypass surgery (mean, 0.22% ± 0.05% to 0.22% ± 0.01%; P = .881). The group with reduced preoperative CVR demonstrated a significant improvement following bypass surgery (mean, 0.08% ± 0.05% to 0.21 ± 0.08%; P < .001), and the group with paradoxical preoperative CVR demonstrated the greatest improvement (mean change, -0.04% ± 0.03% to 0.27% ± 0.03%; P = .028).

CONCLUSIONS

Preoperative measurement of CVR by using CO(2) BOLD MR imaging predicts the hemodynamic effect of ECIC bypass in patients with intracranial steno-occlusive disease. The technique is potentially useful for selecting patients for surgical revascularization.

Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function

There is considerable utility in the use of transcranial Doppler ultrasound (TCD) to assess cerebrovascular function. The brain is unique in its high energy and oxygen demand but limited capacity for energy storage that necessitates an effective means of regional blood delivery. The relative low cost, ease-of-use, non-invasiveness, and excellent temporal resolution of TCD make it an ideal tool for the examination of cerebrovascular function in both research and clinical settings. TCD is an efficient tool to access blood velocities within the cerebral vessels, cerebral autoregulation, cerebrovascular reactivity to CO(2), and neurovascular coupling, in both physiological states and in pathological conditions such as stroke and head trauma. In this review, we provide: (1) an overview of TCD methodology with respect to other techniques; (2) a methodological synopsis of the cerebrovascular exam using TCD; (3) an overview of the physiological mechanisms involved in regulation of the cerebral blood flow; (4) the utility of TCD for assessment of cerebrovascular pathology; and (5) recommendations for the assessment of four critical and complimentary aspects of cerebrovascular function: intra-cranial blood flow velocity, cerebral autoregulation, cerebral reactivity, and neurovascular coupling. The integration of these regulatory mechanisms from an integrated systems perspective is discussed, and future research directions are explored.

An improved method for acquiring cerebrovascular reactivity maps

This study aims to improve the method used to produce cerebrovascular reactivity (CVR) maps by MRI. Previous methods have used a standard boxcar presentation of carbon dioxide (CO(2)). Here this is replaced with a sinusoidally modulated CO(2) stimulus. This allowed the use of Fourier analysis techniques to measure both the amplitude and phase delay of the BOLD CVR response, and hence characterize the arrival sequence of blood to different regions of the brain. This characterization revealed statistically significant relative delays between regions of the brain (ANOVA < 0.0001). In addition, post hoc comparison showed that the frontal (P < 0.001) and parietal (P = 0.004) lobes reacted earlier than the occipital lobe.

Exaggerated increase in cardiac output during exercise in patients with peripheral high-flow arteriovenous malformations

PURPOSE

To test the hypothesis that cardiac output (Q) in patients with arteriovenous malformations (AVMs) is normal at rest and increases disproportionately during exercise.

MATERIALS AND METHODS

Q was measured in eight patients with large peripheral AVMs and in nine healthy subjects using a noninvasive carbon dioxide (CO₂)-based differential Fick method. Subjects were tested while seated at rest and during mild exercise (repeated leg straightening while sitting). Oxygen consumption (VO₂) was monitored as an index of the degree of exercise.

RESULTS

Average resting Q was similar between AVM patients and healthy subjects (7.40 L/min ± 3.29 vs 6.13 L/min ± 0.94, respectively, P = .29). During exercise, AVM patients showed a smaller increment in VO₂ (0.50 L/min ± 0.11 vs 0.78 L/min ± 0.26, P = .012) but with more apparent effort and shortness of breath compared with healthy subjects. The change in Q per unit change in VO₂ (ΔQ/ΔVO₂) was greater in AVM patients than in healthy subjects (16.00 L/min ± 6.50 vs 9.79 L/min ± 5.33, P < .045).

CONCLUSIONS

Exercise intolerance in AVM patients may be due to an imbalance in ΔQ/ΔVO₂ resulting from increased shunting through the AVM. Exercise provocation may increase the sensitivity of Q in the clinical evaluation of AVM patients.

Cerebral blood flow and oxygenation at maximal exercise: the effect of clamping carbon dioxide

During exercise, as end-tidal carbon dioxide (P(ET)(CO₂)) drops after the respiratory compensation point (RCP), so does cerebral blood flow velocity (CBFv) and cerebral oxygenation. This low-flow, low-oxygenation state may limit work capacity. We hypothesized that by preventing the fall in P(ET)(CO₂) at peak work capacity (W(max)) with a newly designed high-flow, low-resistance rebreathing circuit, we would improve CBFv, cerebral oxygenation, and W(max). Ten cyclists performed two incremental exercise tests, one as control and one with P(ET)(CO₂) constant (clamped) after the RCP. We analyzed , middle cerebral artery CBFv, cerebral oxygenation, and cardiopulmonary measures. At W(max), when we clamped P(ET)(CO₂) (39.7 ± 5.2 mmHg vs. 29.6 ± 4.7 mmHg, P < 0.001), CBFv increased (92.6 ± 15.9 cm/s vs. 73.6 ± 12.5 cm/s, P < 0.001). However, cerebral oxygenation was unchanged (ΔTSI -21.3 ± 13.1% vs. -24.3 ± 8.1%, P = 0.33), and W(max) decreased (380.9 ± 20.4W vs. 405.7 ± 26.8 W, P < 0.001). At W(max), clamping P(ET)(CO₂) increases CBFv, but this does not appear to improve W(max).

Improved fMRI calibration: precisely controlled hyperoxic versus hypercapnic stimuli

The calibration of functional magnetic resonance imaging (fMRI) for the estimation of neuronal activation-induced changes in cerebral metabolic rate of oxygen (CMRO(2)) has been achieved through hypercapnic-induced iso-metabolic increases in cerebral blood flow (CBF). Hypercapnia (HC) has been traditionally implemented through alterations in the fixed inspired fractional concentrations of carbon dioxide (F(I)CO(2)) without otherwise controlling end-tidal partial pressures of carbon dioxide (P(ET)CO(2)) or oxygen (P(ET)O(2)). There are several shortcomings to the use of this manual HC method that may be improved by using precise targeting of P(ET)CO(2) while maintaining iso-oxia. Similarly, precise control of blood gases can be used to induce isocapnic hyperoxia (HO) to reduce venous deoxyhaemoglobin (dHb) and thus increase BOLD signals, without appreciably altering CMRO(2) or CBF. The aim of our study was to use precise end-tidal targeting to compare the calibration of BOLD signals under an isocapnic hyperoxic protocol (HOP) (rises in P(ET)O(2) to 140, 240 and 340 mm Hg from baseline) to that of an iso-oxic hypercapnic protocol (HCP) (rises in P(ET)CO(2) of 3, 5, 7 and 9 mm Hg from baseline). Nine healthy volunteers were imaged at 3T while monitoring end-tidal gas concentrations and simultaneously measuring BOLD and CBF signals, via arterial spin labeling (ASL), during graded HCP and HOP, alternating with normocapnic states in a blocked experimental design. The variability of the calibration constant obtained under HOP (M(HOP)) was 0.3-0.5 that of the HCP one (M(HCP)). In addition, M-variances with precise gas targeting (M(HCP) and M(HOP)) were less than those reported in studies using traditional F(I)CO(2) and F(I)O(2) methods (M(HC) and M(HO), respectively). We conclude that precise controlled gas delivery markedly improves BOLD-calibration for fMRI studies of oxygen metabolism with both the HCP and the more precise HOP-alternative.

Impaired cerebrovascular reactivity with steal phenomenon is associated with increased diffusion in white matter of patients with Moyamoya disease

BACKGROUND AND PURPOSE

Reduced cerebrovascular reactivity (CVR) with steal phenomenon is an independent predictor for stroke and may indicate tissue exposed to episodic low-grade ischemia. The apparent diffusion coefficient (ADC) calculated using diffusion-weighted MRI is effective in characterizing focal brain ischemia and subtle structural changes in normal-appearing white matter (WM). We hypothesized that regions of steal phenomenon are associated with increased ADC in normal-appearing WM of patients with Moyamoya disease.

METHODS

Twenty-two patients with unilateral CVR impairment secondary to Moyamoya disease and 12 healthy control subjects underwent diffusion-weighted MRI and functional MRI mapping of the cerebrovascular response to hypercapnia. Parametric maps of ADC and CVR were calculated, coregistered, and segmented using automated image processing methods. ADC of normal-appearing WM was compared between hemispheres, and between WM with negative CVR (ie, steal phenomenon) and WM with positive CVR.

RESULTS

In patients, ADC of normal-appearing WM was elevated in the hemisphere ipsilateral to the CVR impairment compared with the contralateral hemisphere (P<0.005) and in WM with negative CVR compared with WM with positive CVR (P<0.001). WM in regions of steal phenomenon within the affected hemisphere had higher ADC than homologous contralateral WM (P<0.005). In control subjects, negative CVR in WM was not associated with elevated ADC.

CONCLUSIONS

Regions of steal phenomenon are spatially correlated with elevated ADC in normal-appearing WM of patients with Moyamoya disease. This structural abnormality may reflect low-grade ischemic injury after exhaustion of the cerebrovascular reserve capacity.

Global cerebral oxidative metabolism during hypercapnia and hypocapnia in humans: implications for BOLD fMRI

The effect of carbon dioxide (CO(2)) on cerebral metabolism is of tremendous interest to functional imaging. In particular, mild-to-moderate hypercapnia is routinely used in calibrated blood oxygen-level dependent (BOLD)-functional magnetic resonance imaging (fMRI)-based quantification of cerebral oxidative metabolism changes (DeltaCMRO(2)), and relies on the assumption of a stable CMRO(2) during CO(2) challenges. However, this assumption has been challenged by certain animal studies, necessitating its verification in humans and under conditions customary to fMRI. We report, for the first time, on global DeltaCMRO(2) measurements made noninvasively in humans during graded hypercapnia and hypocapnia. We used computerized end-tidal CO(2) modulation to minimize undesired concurrent changes in oxygen pressure, and our findings suggest that no significant change in global CMRO(2) is expected at the levels of end-tidal CO(2) changes customary to calibrated BOLD.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Quantification of cerebrovascular reactivity by blood oxygen level-dependent MR imaging and correlation with conventional angiography in patients with Moyamoya disease

BACKGROUND AND PURPOSE

BOLD MR imaging combined with a technique for precision control of end-tidal pCO(2) was used to produce quantitative maps of CVR in patients with Moyamoya disease. The technique was validated against measures of disease severity by using conventional angiography; it then was used to study the relationship between CVR, vascular steal, and disease severity.

MATERIALS AND METHODS

A retrospective analysis comparing conventional angiography with BOLD MR imaging was performed on 11 patients with Moyamoya disease. Iso-oxic cycling of end-tidal pCO(2) between 2 target values was performed during BOLD MR imaging. CVR was calculated as the BOLD signal difference per Delta pCO(2). CVR was correlated with the presence of Moyamoya or pial collaterals and the degree of Moyamoya disease as graded by using a modified Suzuki score.

RESULTS

A good correlation between mean CVR and Suzuki score was found for the MCA and ACA territories (Pearson correlation coefficient, -0.7560 and -0.6140, respectively; P < .0001). A similar correlation was found between mean CVR and the presence of pial and Moyamoya collateral vessels for combined MCA and ACA territories (Pearson correlation coefficient, -0.7466; P < .0001). On a voxel-for-voxel basis, there was a greater extent of steal within vascular territories with increasing disease severity (higher modified Suzuki score). Mean CVR was found to scale nonlinearly with the extent of vascular steal.

CONCLUSIONS

Quantitative measures of CVR show direct correlation with impaired vascular supply as measured by the modified Suzuki score and enable direct investigation of the physiology of autoregulatory reserve, including steal phenomenon, within a given vascular territory.

The change in cerebrovascular reactivity between 3 T and 7 T measured using graded hypercapnia

Mapping cerebrovascular reactivity (CVR) to hypercapnia is important both clinically and for improved understanding of the haemodynamic properties of the BOLD effect. In this work, BOLD/R2 CVR was investigated by using a device which provided small, repeatable and stable steps in PETCO2, whilst maintaining a steady PETO2 level. Significant CVR was observed in both grey and white matter at both 3 and 7 T, whilst an approximately linear relationship found between R2 CVR and field strength has implications for BOLD models and calibration. Grey matter R2 CVR was 0.066+/-0.004 s(-1) mm Hg(-1) at 3 T and 0.141+/-0.008 s(-1) mm Hg(-1) at 7 T. White matter R2 CVR was 0.021+/-0.003 s(-1) mm Hg(-1) at 3 T and 0.040+/-0.007 s(-1) mm Hg(-1) at 7 T.

A simple and portable breathing circuit designed for ventilatory muscle endurance training (VMET)

BACKGROUND

Ventilatory muscle endurance training (VMET) involves increasing minute ventilation (V (E)) against a low flow resistance at rest to simulate the hyperpnea of exercise. Ideally, VMET must maintain normocapnia over a wide range of V (E). This can be achieved by providing a constant fresh gas flow to a sequential rebreathing circuit. The challenge to make VMET suitable for home use is to provide a source of constant fresh gas flow to the circuit without resorting to compressed gas.

METHODS

Our VMET circuit was based on a commercial sequential gas delivery breathing circuit (Pulmanex Hi-Ox, Viasys Healthcare, Yorba Linda, CA USA). Airflow was provided either by a small battery-driven aquarium air pump or by the entrainment of air down a pressure gradient created by the recoil of a hanging bellows that was charged during each inhalation. In each case, fresh gas flow was adjusted to be just less than resting V (E). Eight subjects then breathed from the circuit for three 10min periods consisting of relaxed breathing, breathing at 20 and then at 40L/min. We monitored V (E), end-tidal PCO2 (PetCO2) and hemoglobin O2 saturation (SpO2).

RESULTS

During hyperpnea at 20 and 40L/min, PetCO2 did not differ significantly from resting levels with either method of supplying fresh gas. SpO2 remained greater than 96% during all tests.

CONCLUSION

Isocapnic VMET can be reliably accomplished with a simple self-regulating, sequential rebreathing circuit without the use of compressed gas.

The effects of reduced end-tidal carbon dioxide tension on cerebral blood flow during heat stress

Passive heat stress reduces arterial carbon dioxide partial pressure (P(aCO2)) as reflected by 3 to 5 Torr reductions in end-tidal carbon dioxide tension (P(ETCO2)). Heat stress also reduces cerebrovascular conductance (CBVC) by up to 30%. While is a strong regulator of CBVC, it is unlikely that the relatively small change in during heating is solely responsible for the reductions in CBVC. This study tested the hypothesis that P(aCO2), referenced by P(ETCO2), is not the sole mechanism for reductions in CBVC during heat stress. Mean arterial blood pressure (MAP), P(ETCO2), middle cerebral artery blood velocity (MCA V(mean)), and calculated CBVC (MCA V(mean)/MAP) were assessed in seven healthy individuals, during three separate conditions performed sequentially: (1) normothemia, (2) control passive heat stress and (3) passive heat stress with P(ETCO2) clamped at the normothermic level (using a computer-controlled sequential gas delivery breathing circuit). MAP was similar in the three thermal conditions (P = 0.55). Control heat stress increased internal temperature approximately 1.3 degrees C, which resulted in decreases in P(ETCO2), MCA V(mean) and calculated CBVC (P < 0.001 for all variables). During heat stress + clamp conditions internal temperature remained similar to that during the control heat stress condition (P = 0.31). Heat stress + clamp successfully restored to the normothermic level (P = 0.99) and increased MCA V(mean) (P = 0.002) and CBVC (P = 0.008) relative to control heat stress. Despite restoration of P(ETCO2), MCA V(mean) (P = 0.005) and CBVC (P = 0.03) remained reduced relative to normothermia. These results indicate that heat stress-induced reductions in , as referenced by P(ETCO2), contribute to the decrease in MCA V(mean) and CBVC; however, other factors (e.g. perhaps elevated sympathetic nerve activity) are also involved in mediating this response.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Retinal arteriolar and capillary vascular reactivity in response to isoxic hypercapnia

The aim of the study was to compare the magnitude of vascular reactivity of the retinal arterioles in terms of percentage change to that of the retinal capillaries using a novel, standardized methodology to provoke isoxic hypercapnia. Ten healthy subjects (mean age 25 years, range 21-31) were recruited. Subjects attended a single visit comprising two study sessions separated by 30 min. Subjects were fitted with a sequential re-breathing circuit connected to a computer-controlled gas blender. Each session consisted of breathing at rest for 10 min (baseline), increase of P(ET)CO(2) (maximum partial pressure of CO(2) during expiration) by 15% above baseline whilst maintaining isoxia for 20 min, and returning to baseline conditions for 10 min. Retinal hemodynamic measurements were performed using the Canon Laser Blood Flowmeter and the Heidelberg Retina Flowmeter in random order across sessions. Retinal arteriolar diameter, blood velocity and flow increased by 3.3%, 16.9% and 24.9% (p<0.001), respectively, during isoxic hypercapnia. There was also an increase of capillary blood flow of 34.8%, 21.6%, 24.9% (p< or =0.006) at the optic nerve head neuroretinal rim, nasal macula and fovea, respectively. The coefficient of repeatability (COR) was 5% of the average P(ET)CO(2) both at baseline and during isoxic hypercapnia and was 10% and 7% of the average P(ET)O(2) (minimum partial pressure of oxygen at end exhalation), respectively. The overall magnitude of retinal capillary vascular reactivity was equivalent to the arteriolar vascular reactivity with respect to percentage change of flow. The magnitude of isoxic hypercapnia was repeatable.

Non-invasive prospective targeting of arterial P(CO2) in subjects at rest

Accurate measurements of arterial P(CO(2)) (P(a,CO(2))) currently require blood sampling because the end-tidal P(CO(2)) (P(ET,CO(2))) of the expired gas often does not accurately reflect the mean alveolar P(CO(2)) and P(a,CO(2)). Differences between P(ET,CO(2)) and P(a,CO(2)) result from regional inhomogeneities in perfusion and gas exchange. We hypothesized that breathing via a sequential gas delivery circuit would reduce these inhomogeneities sufficiently to allow accurate prediction of P(a,CO(2)) from P(ET,CO(2)). We tested this hypothesis in five healthy middle-aged men by comparing their P(ET,CO(2)) values with P(a,CO(2)) values at various combinations of P(ET,CO(2)) (between 35 and 50 mmHg), P(O(2)) (between 70 and 300 mmHg), and breathing frequencies (f; between 6 and 24 breaths min(-1)). Once each individual was in a steady state, P(a,CO(2)) was collected in duplicate by consecutive blood samples to assess its repeatability. The difference between P(ET,CO(2)) and average P(a,CO(2)) was 0.5 +/- 1.7 mmHg (P = 0.53; 95% CI -2.8, 3.8 mmHg) whereas the mean difference between the two measurements of P(a,CO(2)) was -0.1 +/- 1.6 mmHg (95% CI -3.7, 2.6 mmHg). Repeated measures ANOVAs revealed no significant differences between P(ET,CO(2)) and P(a,CO(2)) over the ranges of P(O(2)), f and target P(ET,CO(2)). We conclude that when breathing via a sequential gas delivery circuit, P(ET,CO(2)) provides as accurate a measurement of P(a,CO(2)) as the actual analysis of arterial blood.

Selective reduction of blood flow to white matter during hypercapnia corresponds with leukoaraiosis

BACKGROUND AND PURPOSE

Age-related white matter disease (leukoaraiosis) clusters in bands in the centrum semiovale, about the occipital and frontal horns of the lateral ventricles, in the corpus callosum, and internal capsule. Cerebrovascular anatomy suggests that some of these locations represent border zones between arterial supply territories. We hypothesized that there are zones of reduced cerebrovascular reserve (susceptible to selective reductions in blood flow, ie, steal phenomenon) in the white matter of young, healthy subjects, the physiological correlate of these anatomically defined border zones. Furthermore, we hypothesized that these zones spatially correspond with the regions where the elderly develop leukoaraiosis.

METHODS

Twenty-eight healthy volunteers underwent functional MR mapping of the cerebrovascular response to hypercapnia. We studied 18 subjects by blood oxygen level-dependent MRI and 10 subjects by arterial spin labeling MRI. We controlled both end-tidal pCO(2) and pO(2). All functional data was registered in Montreal Neurological Institute space and generated composite blood oxygen level-dependent MR and arterial spin labeling MR maps of cerebrovascular reserve. We compared these maps with frequency maps of leukoaraiosis published previously.

RESULTS

Composite maps demonstrated significant (90% CI excluding the value zero) steal phenomenon in the white matter. This steal was induced by relatively small changes in end-tidal pCO(2). It occurred precisely in those locations where elderly patients develop leukoaraiosis.

CONCLUSIONS

This steal phenomenon likely represents the physiological correlate of the previously anatomically defined internal border zones. Spatial concordance with white matter changes in the elderly raises the possibility that this steal phenomenon may have a pathogenetic role.

Mapping cerebrovascular reactivity using blood oxygen level-dependent MRI in Patients with arterial steno-occlusive disease: comparison with arterial spin labeling MRI

BACKGROUND AND PURPOSE

Blood oxygen level-dependent MRI (BOLD MRI) of hypercapnia-induced changes in cerebral blood flow is an emerging technique for mapping cerebrovascular reactivity (CVR). BOLD MRI signal reflects cerebral blood flow, but also depends on cerebral blood volume, cerebral metabolic rate, arterial oxygenation, and hematocrit. The purpose of this study was to determine whether, in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia is directly related to changes in cerebral blood flow.

METHODS

Thirty-eight patients with stenoocclusive disease underwent mapping of CVR by both BOLD MRI and arterial spin labeling MRI. The latter technique was used as a reference standard for measurement of cerebral blood flow changes.

RESULTS

Hemispheric CVR measured by BOLD MRI was significantly correlated with that measured by arterial spin labeling MRI for both gray matter (R=0.83, P<0.0001) and white matter (R=0.80, P<0.0001). Diagnostic accuracy (area under receiver operating characteristic curve) for BOLD MRI discrimination between normal and abnormal hemispheric CVR was 0.90 (95% CI=0.81 to 0.98; P<0.001) for gray matter and 0.82 (95% CI=0.70 to 0.94; P<0.001) for white matter. Regions of paradoxical CVR on BOLD MRI had a moderate predictive value (14 of 19 hemispheres) for spatially corresponding paradoxical CVR on arterial spin labeling MRI. Complete absence of paradoxical CVR on BOLD MRI had a high predictive value (31 of 31 hemispheres) for corresponding nonparadoxical CVR on arterial spin labeling MRI.

CONCLUSIONS

Arterial spin labeling MRI confirms that, even in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia predominantly reflects changes in cerebral blood flow.

Measuring the ventilatory response to hypoxia

After defining the current approach to measuring the hypoxic ventilatory response this paper explains why this method is not appropriate for comparisons between individuals or conditions, and does not adequately measure the parameters of the peripheral chemoreflex. A measurement regime is therefore proposed that incorporates three procedures. The first procedure measures the peripheral chemoreflex responsiveness to both hypoxia and CO(2) in terms of hypoxia's effects on the sensitivity and ventilatory recruitment threshold of the peripheral chemoreflex response to CO(2). The second and third procedures employ current methods for measuring the isocapnic and poikilocapnic ventilatory responses to hypoxia, respectively, over a period of 20 min. The isocapnic measure is used to determine the time course characteristics of hypoxic ventilatory decline and the poikilocapnic measure shows the ventilatory response to a hypoxic environment. A measurement regime incorporating these three procedures will permit a detailed assessment of the peripheral chemoreflex response to hypoxia that allows comparisons to be made between individuals and different physiological and environmental conditions.