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Cerebrovascular reactivity (CVR), defined as the ability of the cerebral vasculature to dilate or constrict in response to a vasoactive stimulus, is an important indicator of the brain’s vascular health.1 Impaired CVR is present in numerous disease states, including cerebrovascular disease,2–8 stroke,9–12 cardiovascular disease,13,14 cardiac arrest,15 traumatic brain injury,16–19 diabetes,20 and sleep apnea.21 Moreover, several studies have suggested that impaired CVR may serve as a prognostic biomarker of functional outcome.22–25 Despite mounting evidence for the relevance of CVR as a potential disease biomarker, mechanisms of cerebrovascular dysregulation are poorly understood, and effective treatment strategies for impaired CVR are lacking. Preclinical murine models provide an excellent platform for interrogating mechanisms underlying vascular dysregulation and determining novel therapeutics that restore impaired CVR. However, quantification of CVR in mice is challenging. Common techniques to assess CVR include intravenous injection of the potent vasodilator acetazolamide (ACZ) and induction of hypercapnia via inhalation of a high-concentration (typically 5% to 6%) carbon dioxide gas mixture. For both of these techniques, cerebral blood flow is monitored before and after intervention, and CVR is defined as the relative percent change in blood flow from preintervention levels normalized to either the dose of ACZ or to the change in arterial partial pressure of carbon dioxide. Although these assessments are routinely used in humans, delivery of the vasoactive stimulus and reliable monitoring of the stimulus response is more challenging in mice. In the case of ACZ, although intravenous tail vein injection is possible, longitudinal assessment of CVR with this approach is challenging because repeated tail vein injections without the use of a catheter cause inconsistent drug delivery and increase risk of infection and vein collapse.26 Alternatively, hypercapnia requires anesthesia for gas delivery in mice, which can induce significant confounding effects on the cerebrovasculature that lead to errors in the estimation of CVR.27–32 Moreover, monitoring the amount of carbon dioxide that has reached the blood stream to accurately estimate CVR during hypercapnia requires either invasive insertion of an arterial catheter to sample blood or intubation to measure end tidal carbon dioxide concentrations. Finally, both interventions to assess CVR (ACZ/hypercapnia) require quantification of cerebral blood flow, which is non-trivial in mice due to their small size. Although a handful of techniques exist to measure CBF in mice, e.g., autoradiography, perfusion magnetic resonance imaging, laser Doppler flowmetry, laser speckle contrast imaging, and microultrasound, these techniques typically involve invasive surgery, anesthesia, and/or manual restraint.33 In sum, challenges in both stimulus delivery and monitoring response to the stimulus make CVR measurements in murine models technically challenging, time consuming, and preclude longitudinal assessment. Herein, we present a novel means of assessing CVR in mice that overcomes several of these technical limitations and enables longitudinal monitoring of CVR in awake, free behaving animals. In this approach, mice are given ACZ intraperitoneally, and the cerebral blood flow response is continuously monitored with diffuse correlation spectroscopy (DCS) using a minimally invasive sensor that is secured to the intact skull. Given the relative ease of both intraperitoneal injections and assessment of blood flow with DCS, multiple longitudinal measurements of CVR can easily be acquired. In this work, we characterize average CVR in healthy mice, and we quantify the spatial and temporal heterogeneity of the CVR response.

Background: Several case series and a recent meta-analysis indicate that intra-arterial thrombolysis (IAT) is effective for the treatment of acute central retinal artery occlusion (CRAO). Methods: A total of 37 patients with acute monocular blindness because of unilateral thromboembolic CRAO were treated with IAT using urokinase within six hours of the onset of symptoms. Visual outcome was compared with a control group of 19 patients, also seen within six hours, who did not undergo thrombolytic treatment. In both groups some patients were treated by paracentesis and/or acetazolamide. Predictors of visual outcome were evaluated. Results: Visual improvement was more likely with IAT (p = 0.01) as were the chances to regain visual acuity of >0.6 significantly better (p = 0.04): 8/37 patients (22%) regained visual acuity of >0.6 in the IAT group and none (0/19) in the control group. Younger patients were more likely to regain some vision with (p = 0.012) or without IAT (p = 0.026). Three patients had minor treatment related cerebral ischaemic events, two had transient ischaemic attacks and one a minor stroke. There were no haemorrhagic complications. Conclusions: This series of patients with CRAO demonstrated that IAT enhanced the chances of visual improvement compared with conventional treatment only. Furthermore, younger patients have a better chance to achieve some visual recovery.

It has been proposed that cerebral blood flow (CBF) response to acetazolamide may be reduced according to the degree of autoregulatory vasodilation in regions with normal oxygen extraction fraction (OEF), whereas the CBF response may be absent in regions with increased OEF where vasodilation may be maximal in response to reduced perfusion pressure. The objective of this study was to test this hypothesis.Positron emission tomography (PET) was used to study 30 symptomatic patients with carotid artery steno-occlusive lesions. CBF at baseline and 10 minutes after an intravenous injection of 1 g acetazolamide was measured. The correlation between the change in CBF during acetazolamide administration and the baseline value of OEF in the affected hemisphere was examined.The baseline OEF value was inversely and non-linearly correlated with the percentage change in CBF during acetazolamide administration (R(2) = 0.25, p = 0.02). There was an upward trend of OEF with diminishing acetazolamide response below a critical level around zero response. Acetazolamide response less than 6.65% over baseline (sensitivity 100%, specificity 89%, positive predictive value 50%, negative predictive value 100%) was established as most helpful in predicting abnormally high OEF.The inverse, non-linear relationship between OEF and CBF response to acetazolamide suggests that these two measurements may not identify haemodynamic impairment in the same patients.

SUMMARY A theoretical and experimental analysis of facilitated transport of CO2 across membranes containing NaHCO3 and the enzyme carbonic anhydrase (carbonate hydro-lyase EC 4.2.1.1) is presented. The necessary diffusion reaction equations are derived and the system constraints defined. For the CO2-HCO3- system, mathematical simplifications based on the magnitude of various reaction and concentration terms are made to make the equations tractable to solution. The resultant equations are solved by a number of analytical and numerical techniques, each having a limited, though useful, range of validity. The experimental arrangement consists of a liquid membrane (created by soaking a porous filter paper in the test solution), a diffusion chamber, and gas metering and analysis equipment. Conditions were selected to cover the range from diffusionto reaction-dominated behavior. The flux of CO2 across a membrane containing ! M NaHCO3 was measured at various partial pressures of CO2 (2-28 in Hg) and with membrane thicknesses of 0.02, 0.06 and 0.10 cm. The extent of facilitation (defined as the ratio of reactionrelated flux to the expected Fick's Law flux in the absence of reaction) ranged from near zero to nearly 5 in these experiments. The agreement between model calculations and experimental observation was found to be excellent over the entire range of neardiffusion to near-equilibrium behavior. In the presence of enzyme carbonic anhydrase (0.10 mg/ml, activity approx. 80%) and 1 M NaHCO3, the CO2 flux across a 0.02 cm membrane was 3-10-fold higher than the corresponding flux in the absence of enzyme. From experiments at various enzyme concentrations and membrane thicknesses, it appeared that the apparent CO2 reaction rate was directly proportional to the enzyme concentration. The model calculations for the enzyme-catalyzed reactions agreed with the experimental observations to within ± 10 %.