Cerebrovascular physiology – article alert #54

Alzheimer’s disease and the brain

260- Impaired Cerebral Autoregulation and Vasomotor Reactivity in Sporadic Alzheimer’s in Disease – Den Abeelen et al.

Hypercapnia and the brain

261- Regional cerebral blood flow and arterial blood volume and their reactivity to hypercapnia in hypertensive and normotensive rats – Kim et al.

Vasopressors and the brain

262- Under general anesthesia arginine vasopressin prevents hypotension but impairs cerebral oxygenation during arthroscopic shoulder surgery in the bear chair position – Cho et al.

Syncope and the brain

263- Hyperventilation, Cerebral Perfusion and Syncope – Immink et al.

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Cerebrovascular physiology – article alert #53

Altitude and the brain

249- AltitudeOmics: Effect of ascent and acclimatization to 5260 m on regional cerebral oxygen delivery – Subudhi et al.

Cardiopulmonary resuscitation and the brain

250- A Feasibility Study of Cerebral Oximetry During In-Hospital Mechanical and Manual Cardiopulmonary Resuscitation – Parnia et al.

Brain blood flow in the neonate

251- Cerebral blood flow in the neonate – Vutskits

252- Effect of oxygen inhalation on cerebral blood flow velocity in premature neonates – Basu et al.

Brain function and cognitive impairments

253- Changes in Brain Function Occur Years before the Onset of Cognitive Impairment – Beason-Held et al.

Brain blood flow and cardiovascular risks

254- Use of total cerebral blood flow as an imaging biomarker of known cardiovascular risks – Jennings et al.

One-lung ventilation and the brain

255- One-lung ventilation does not result in cerebral desaturation during application of lung protective strategy if normocapnia is maintained – Végh et al.

Brain function in preeclamptic women

256- Cerebro vascular reactivity (CVR) of middle cerebral artery in response to CO2 5% inhalation in preeclamptic women – Sariri et al.

Critical closing pressure and the brain

257- Cessation of Diastolic Cerebral Blood Flow Velocity: The Role of Critical Closing Pressure – Varsos et al.

Brain autoregulation

258- Variability of the autoregulation index decreases after removing the effect of the very low frequency band – Elting et al.

259- Comparison of Non-invasive and Invasive Arterial Blood Pressure Measurement for Assessment of Dynamic Cerebral Autoregulation – Petersen et al.

Is it fine to assume that cerebrovascular reactivity to CO2 is similar between anterior and posterior cerebral circulations?

When we want to examine cerebrovascular function in humans (cerebral blood flow, cerebral autoregulation, cerebrovascular reactivity to carbon dioxide (CO2), the middle cerebral artery is usually insonated. We assume that autoregulation and reactivity are similar between anterior and posterior regions. But are we right ?

Skow et al. compared cerebrovascular reactivity to CO2 from both the anterior (middle cerebral artery) and posterior (basilar artery and posterior cerebral artery) circulations using the hyperoxic rebreathing method in healthy volunteers. The authors hypothesized that reactivity to CO2 would be comparable between basilar artery and posterior cerebral artery. However, the middle cerebral artery and posterior cerebral artery would have different reactivity to CO2.

Thus, they compared reactivity to CO2 in basilar artery and posterior artery in a first protocol (n=13) and compared reactivity in the middle cerebral artery and posterior cerebral artery in a second protocol (n=17).

The main findings of this study are that both absolute and relative cerebrovascular reactivity to CO2 between basilar artery and posterior cerebral artery were not different. Also, absolute, but not relative, cerebrovascular reactivity to CO2 of the middle cerebral artery was higher compared to the posterior cerebral artery.

In light of these results, there exist differences in cerebrovascular reactivity to CO2 between anterior and posterior circulations. In addition, the authors suggest that the posterior cerebral artery represents an adequate surrogate index of basilar artery reactivity in studies where the latter is difficult to insonate. This is interesting because the basilar artery measurements are usually obtained by a hand held probe while the posterior cerebral artery measurements are obtained by a probe installed on a headpiece. This is an important methodological consideration for transcranial Doppler, since it will allow for stable and continuous measurements.

Particularly appreciated in this manuscript are the methodological and analysis considerations. Usually, these sections are not necessarily explicit. However, in this manuscript, the authors provide an in-depth discussion of important issues related to the study and its calculations. They have to be congratulated for the inclusion of such a detailed section.

Reference

Skow RJ, Mackay CM, Tymko MM, Willie CK, Smith KJ, Ainslie PN, Day TA. Differential cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations. Respir Physiol Neurobiol 189(1):76-86, 2013

Cerebrovascular physiology – article alert #52

High positive end-expiratory pressure and the brain

246- Effects of high positive end-expiratory pressure on haemodynamics and cerebral oxygenation during pneumoperitoneum in the Trendelenburg position – Jo et al.

Central artery stiffness and the brain

247- Central artery stiffness, neuropsychological function, and cerebral perfusion in sedentary and endurance-trained middle-aged adults – Tarumi et al.

248- Aortic stiffening, cerebral resistance vessel function and structure, and cerebral perfusion – Izzard (editorial comment accompanying paper from Tarumi et al.

Why cerebral oxygenation doesn’t necessarily decrease during anesthesia-induced hypotension?

Regular readers know my interest for the influence of vasopressors, to correct mean arterial pressure during surgeries (whether it is during anesthesia or cardiopulmonary bypass), on cerebral oxygenation.

In some cases, vasopressors may not be necessary because evidence shows that cerebral oxygenation doesn’t always decrease during anesthesia-induced hypotension. We still don’t know the reason though.

In a recent study, Meng et al. tested the hypothesis that the reduction in brain perfusion, associated with hypotension during anesthesia, would be counterbalanced by a decrease in brain metabolism. As a consequence, frontal lobe oxygenation (or cerebral tissue oxygen saturation in this paper) measured by near-infrared spectroscopy would not be lowered.

To test this hypothesis, the authors recruited patients (data for 30 patients are presented) scheduled for elective non-neurological surgery under general anesthesia with tracheal intubation.

Anesthesia was induced with propofol and fentanyl. Cerebral tissue oxygen saturation, mean arterial pressure, heart rate, cardiac output and bispectral index were monitored before induction of anesthesia and after tracheal intubation.

Mean arterial pressure and cardiac output decreased from pre-anesthesia induction to tracheal intubation. Frontal lobe oxygenation after tracheal intubation remained similar to baseline (pre-anesthesia induction) in these patients.

Cerebral metabolism was not measured in this study. Based on previous findings showing that cerebral metabolism is decreased following induction of anesthesia, the authors inferred that brain oxygen demand and supply remained matched after induction of anesthesia with propofol and fentanyl, leading to an absence of significant change in cerebral tissue oxygen saturation in their patients.

Reference

Meng L, Gelb AW, McDonagh DL. Changes in cerebral tissue oxygen saturation during anaessthetic-induced hypotension: an interpretation based on neurovascular coupling and cerebral autoregulation. Anaesthesia 2013; 68(7):736-41.

Cerebrovascular physiology – article alert #51

Brain hemodynamics

234- Time-Varying Modeling of Cerebral Hemodynamics – Marmarelis et al.

Cardiopulmonary bypass and the brain

235- Normothermic cardiopulmonary bypass increases cerebral tissue oxygenation during combined valve surgery: a single-centre, randomized trial – Lenkin et al.

Brain perfusion pressure

236- Brief report: a comparison of clinical and research practices in measuring cerebral perfusion pressure: a literature review and practitioner survey – Kosty et al.

Cerebrovascular reactivity

237- Cerebrovascular reactivity in the brain white matter: magnitude, temporal characteristics, and age effects – Thomas et al.

Brain autoregulation

238- Reproducibility and variability of dynamic cerebral autoregulation during passive cyclic leg raising – Elting et al.

239- Dynamic cerebral autoregulation after bed rest: effects of volume loading and exercise countermeasures – Jeong et al.

Aging and the brain

240- Total cerebral blood flow and mortality in old age: A 12-year follow-up study – Sabayan et al.

Altitude and the brain

241- Cerebral pressure-flow relationship in lowlanders and natives at high altitude – Smirl et al.

Cardiac arrest and theb

242- Effects of Viscosity on Cerebral Blood Flow After Cardiac Arrest – Bisschops et al.

Sepsis and the brain

243- Lipopolysaccharide infusion enhances dynamic cerebral autoregulation without affecting cerebral oxygen vasoreactivity in healthy volunteers – Berg et al.

244- Sepsis Is Associated With Altered Cerebral Microcirculation and Tissue Hypoxia in Experimental Peritonitis – Taccone et al.

One-lung ventilation and the brain

245- Cerebral oxygen desaturation during one-lung ventilation: correlation with hemodynamic variables – Brinkman et al.