Andrzej F. Frydrychowski

Theoretical foundations for noninvasive measurement of variations in the width of the subarachnoid space.

Numerical modeling was used for the theoretical analysis of the propagation of optical radiation in the tissues of the human head, generated by a single source placed on the surface of the scalp. Of special interest and importance is the propagation of radiation within the layer of cerebrospinal fluid contained in the subarachnoid space (SAS), which is the only low absorption/high transmittance medium whose width can vary rapidly. Qualitative and quantitative assessment of changes in propagation of radiation within the SAS could become a source of information on changes in the geometry of this anatomical compartment playing a crucial role in cranio-spinal physiology and pathology. Essential for the idea of the possible noninvasive assessment of changes in width of the SAS by an optical method is the dependence of intensity of radiation reaching a photodetector located at a certain distance from the source on changes in the width of this fluid layer, which acts like a biological optical waveguide. Monte Carlo modeling and numerical analysis confirmed the feasibility of assessing changes in the width of the subarachnoid space optically. Presented here are details of the Monte Carlo simulation of light propagation in the tissues of human head and the results of such simulation as a function of the width of the subarachnoid space, calculated for different distances between the source and detector and for a few selected values of bone thickness. Results of numerical modeling were then compared with those of experiments on a mechanical-optical model.

Influence of Acute Jugular Vein Compression on the Cerebral Blood Flow Velocity, Pial Artery Pulsation and Width of Subarachnoid Space in Humans

Purpose The aim of this study was to assess the effect of acute bilateral jugular vein compression on: (1) pial artery pulsation (cc-TQ); (2) cerebral blood flow velocity (CBFV); (3) peripheral blood pressure; and (4) possible relations between mentioned parameters. Methods Experiments were performed on a group of 32 healthy 19–30 years old male subjects. cc-TQ and the subarachnoid width (sas-TQ) were measured using near-infrared transillumination/backscattering sounding (NIR-T/BSS), CBFV in the left anterior cerebral artery using transcranial Doppler, blood pressure was measured using Finapres, while end-tidal CO2 was measured using medical gas analyser. Bilateral jugular vein compression was achieved with the use of a sphygmomanometer held on the neck of the participant and pumped at the pressure of 40 mmHg, and was performed in the bend-over (BOPT) and swayed to the back (initial) position. Results In the first group (n = 10) during BOPT, sas-TQ and pulse pressure (PP) decreased (−17.6% and −17.9%, respectively) and CBFV increased (+35.0%), while cc-TQ did not change (+1.91%). In the second group, in the initial position (n = 22) cc-TQ and CBFV increased (106.6% and 20.1%, respectively), while sas-TQ and PP decreases were not statistically significant (−15.5% and −9.0%, respectively). End-tidal CO2 remained stable during BOPT and venous compression in both groups. Significant interdependence between changes in cc-TQ and PP after bilateral jugular vein compression in the initial position was found (r = −0.74). Conclusions Acute bilateral jugular venous insufficiency leads to hyperkinetic cerebral circulation characterised by augmented pial artery pulsation and CBFV and direct transmission of PP into the brain microcirculation. The Windkessel effect with impaired jugular outflow and more likely increased intracranial pressure is described. This study clarifies the potential mechanism linking jugular outflow insufficiency with arterial small vessel cerebral disease.

New aspects in assessment of changes in width of subarachnoid space with near-infrared transillumination/ backscattering sounding, part 2: clinical verification in the patient

The study presents comparison of near-infrared light propagation and near-infrared backscattered radiation power, as simulated with numerical modeling and measured live in a patient in clinical conditions with the use of the near-infrared transillumination-backscattering sounding (NIR-TBSS) technique. A unique chance for such precise comparative analysis was available to us in a clinical case of a female patient with scalp removed from one half of the head due to injury. The analysis performed indicates that the difference between the intensity of the signals in numerical modeling and live measurements is less than 4 dB. Analysis of the theoretical model also provides hints on the positioning of the two detectors relative to the source of radiation. Correctness of these predicted values is confirmed in practical application, when changes of signals received by the detectors are recorded, along with changes of the width of the subarachnoid space. What is more, the power distribution of the spectrum of near-infrared backscattered radiation returning to the detectors is confirmed in the real recording in the patient. An abridged description of the new method of NIR-TBSS is presented.

New aspects in assessment of changes in width of subarachnoid space with near-infrared transillumination/backscattering sounding, part 1: Monte Carlo numerical modeling

A modified Monte Carlo method was used for numerical modeling of the propagation of near-infrared radiation (NIR) within the anatomical layers of the human head. The distribution of NIR transmission between particular anatomical layers in the measurement region (frontal tubers) of the head was obtained. The study demonstrates the effect of the cardiac pump function-dependent changes in the width of the subarachnoid space (SAS) on the intensity of the backscattered radiation. It was proved that the influence of this factor increases with increasing distance between the observation point and the location of the NIR source placed on the surface of the head. Moreover, with sufficiently small NIR detector-source distance, the contribution of the optic radiation propagated within the SAS to the total signal received is negligibly low, which gives a basis for estimation of the modulatory influence of blood circulation within the superficial skin layer on the total intensity of the backscattered radiation. The dimensions of anatomical layers used in the study are real values measured in a female patient, in whom--due to unique circumstances--it was possible to make measurements followed by recordings in clinical conditions, a situation essential for verification of the results of numerical modeling.

Cerebral blood flow, sympathetic nerve activity and stroke risk in obstructive sleep apnoea. Is there a direct link?

Abstract Obstructive sleep apnoea (OSA) is significantly associated with the risk of stroke, and this association is independent of other risk factors, including hypertension, atrial fibrillation and diabetes mellitus. Therefore, additional pathogenic mechanisms may exist, which contribute to the increased risk of stroke. OSA is characterized by prolonged sympathetic overactivity; however the role of the sympathetic nervous system in regulating cerebral circulation remains a matter of controversy. Converging data indicate that brain perfusion is significantly distorted in OSA, with reported decreases in cerebral blood flow as well as intermittent surges in blood pressure and cerebral blood flow velocity. Based on recent research, there is accumulating evidence that sympathetic nerve activity is an important element in brain protection against excessive increases in perfusion pressure during blood pressure surges and flow during rapid eye movement sleep. The aim of this article was to review: (i) the current physiological knowledge related to the role of the sympathetic system in the regulation of cerebral blood flow, (ii) how the influence of the sympathetic system on cerebral vessels is affected by apnoea (increased PaCO2) and (iii) the potential significance of the pathological sympathetic system/PaCO2 interplay in OSA. Sympathetic system seems to be at least partially involved in pathogenesis of distorted haemodynamics and stroke in OSA patients. However, there are still several open questions that need to be addressed before the effective therapeutic strategies can be implemented.

Effects of the Valsalva maneuver on pial artery pulsation and subarachnoid width in healthy adults

AIM The objective was to characterize the effects of Valsalva maneuver (VM) on the amplitude of cerebrovascular pulsation (CVP), and to explore the direct interactions between the cerebral vasculature and the cerebrospinal fluid compartment in VMIII. METHODS Twenty-nine healthy volunteers between the ages of 25 and 40 (29.3 ± SE 4.0) were studied. Changes in the amplitude of CVP (cc-TQ) and width of subarachnoid space (SAS; sas-TQ) were recorded with NIR-T/BSS sensor. Changes in arterial blood pressure (ABP) and heart rate were measured using Finapres. Cerebral blood flow velocity (CBFV) in the left middle cerebral artery was recorded with transcranial doppler. RESULTS sas-TQ remained unchanged, while cc-TQ increased in VMI (+40% vs. baseline). In VMIIa, sas-TQ increase (+3.4% vs. baseline) and deep decrease in cc-TQ (-81% vs. baseline) were observed. sas-TQ decrease started in VMIIb (-2.7% vs. baseline), with simultaneous slight increase in cc-TQ (-58% vs. baseline). In VMIII deep sas-TQ decrease (-6.2% vs. baseline) was associated with huge increase in cc-TQ (+110% vs. baseline; r=-0.56, p<0.01). During VMIV sas-TQ increased (-4.8% vs. baseline) while cc-TQ decreased (+38% vs. baseline). The drop of cc-TQ in VMIIa was significantly greater than corresponding changes in CBFV and ABP. Increase in cc-TQ in VMIII preceded CBFV and ABP changes in VMIV. CONCLUSION The VM evokes significant changes in the amplitude of CVP. Changes in small vessel pulsation precede changes in CBFV. There are direct interactions between cc-TQ and sas-TQ in VMIII. NIR-T/BSS allows for continuous, non-invasive monitoring of the amplitude of CVP and width of the SAS.

Subarachnoid Space: New Tricks by an Old Dog

Purpose The purpose of the study was to: (1) evaluate the subarachnoid space (SAS) width and pial artery pulsation in both hemispheres, and (2) directly compare magnetic resonance imaging (MRI) to near-infrared transillumination/backscattering sounding (NIR-T/BSS) measurements of SAS width changes in healthy volunteers. Methods The study was performed on three separate groups of volunteers, consisting in total of 62 subjects (33 women and 29 men) aged from 16 to 39 years. SAS width was assessed by MRI and NIR-T/BSS, and pial artery pulsation by NIR-T/BSS. Results In NIR-T/BSS, the right frontal SAS was 9.1% wider than the left (p<0.01). The SAS was wider in men (p<0.01), while the pial artery pulsation was higher in women (p<0.01). Correlation and regression analysis of SAS width changes between the back- and abdominal-lying positions measured with MRI and NIRT-B/SS demonstrated high interdependence between both methods (r = 0.81, p<0.001). Conclusions NIR-T/BSS and MRI were comparable and gave equivalent modalities for the SAS width change measurements. The SAS width and pial artery pulsation results obtained with NIR-T/BSS are consistent with the MRI data in the literature related to sexual dimorphism and morphological asymmetries between the hemispheres. NIR-T/BSS is a potentially cheap and easy-to-use method for early screening in patients with brain tumours, increased intracranial pressures and other abnormalities. Further studies in patients with intracranial pathologies are warranted.

Flow-induced changes in pial artery compliance registered with a non-invasive method in rabbits☆

AIM This study was performed: 1) to assess the relationship between blood flow velocity in the internal carotid artery (CBF(ICA)) and pial artery pulsation (cc-TQ) and 2) to evaluate flow-induced changes in pial artery compliance. METHODS Experiments were performed on 10 crossbred male rabbits. Heart rate (HR), blood pressure (BP), left ventricle ejection fraction (LVEF), CBF(ICA), the systolic-diastolic blood volume fraction in the brain circulation (CBF(SDF)) and cc-TQ were recorded after glucagon and acetazolamide administration. cc-TQ was measured with near-infrared transillumination back scattering sounding (NIR-T/BSS), LVEF and CBF(SDF) with gated scintigraphy and BP and CBF(ICA) with electromagnetic pressure and flow transducers, respectively. Doses of drugs were chosen to exert a haemodynamic effect but not change BP or intracranial pressure. RESULTS Acetazolamide and glucagon evoked significant increases in cc-TQ, CBF(SDF), CBF(ICA), LVEF and HR. The following interdependencies were found: 1) changes after acetazolamide administration compared to baseline: CBF(SDF) vs. LVEF (r=0.73, p<0.05), cc-TQ vs. CBF(SDF) (r=-0.67, p<0.05), cc-TQ vs. LVEF (r=-0.76, p<0.05), 2) changes after glucagon administration compared to baseline: CBF(ICA) vs. BP (r=0.73, p<0.05), CBF(SDF) vs. LVEF (r=0.87, p<0.05), cc-TQ vs. HR (r=0.85, p<0.05), cc-TQ vs. CBF(ICA) (r=-0.74, p<0.05). CONCLUSION In the absence of systemic BP changes, pial arteries are significantly affected by changes in CBF(ICA). Pial arteries counteract changes in CBF(ICA) and CBF(SDF). The ability of pial arteries to stabilise CBF(ICA) is impaired after acetazolamide administration. Changes in cardiac output directly affect the brains microcirculation. NIR-T/BSS recordings allow for non-invasive assessment of changes in pial artery compliance.

Effect of Maximal Apnoea Easy-Going and Struggle Phases on Subarachnoid Width and Pial Artery Pulsation in Elite Breath-Hold Divers

Purpose The aim of the study was to assess changes in subarachnoid space width (sas-TQ), the marker of intracranial pressure (ICP), pial artery pulsation (cc-TQ) and cardiac contribution to blood pressure (BP), cerebral blood flow velocity (CBFV) and cc-TQ oscillations throughout the maximal breath hold in elite apnoea divers. Non-invasive assessment of sas-TQ and cc-TQ became possible due to recently developed method based on infrared radiation, called near-infrared transillumination/backscattering sounding (NIR-T/BSS). Methods The experimental group consisted of seven breath-hold divers (six men). During testing, each participant performed a single maximal end-inspiratory breath hold. Apnoea consisted of the easy-going and struggle phases (characterised by involuntary breathing movements (IBMs)). Heart rate (HR) was determined using a standard ECG. BP was assessed using the photoplethysmography method. SaO2 was monitored continuously with pulse oximetry. A pneumatic chest belt was used to register thoracic and abdominal movements. Cerebral blood flow velocity (CBFV) was estimated by a 2-MHz transcranial Doppler ultrasonic probe. sas-TQ and cc-TQ were measured using NIR-T/BSS. Wavelet transform analysis was performed to assess cardiac contribution to BP, CBFV and cc-TQ oscillations. Results Mean BP and CBFV increased compared to baseline at the end of the easy phase and were further augmented by IBMs. cc-TQ increased compared to baseline at the end of the easy phase and remained stable during the IBMs. HR did not change significantly throughout the apnoea, although a trend toward a decrease during the easy phase and recovery during the IBMs was visible. Amplitudes of BP, CBFV and cc-TQ were augmented. sas-TQ and SaO2 decreased at the easy phase of apnoea and further decreased during the IBMs. Conclusions Apnoea increases intracranial pressure and pial artery pulsation. Pial artery pulsation seems to be stabilised by the IBMs. Cardiac contribution to BP, CBFV and cc-TQ oscillations does not change throughout the apnoea.

Effects of acute hypercapnia on the amplitude of cerebrovascular pulsation in humans registered with a non-invasive method.

AIM The aim of the study was to assess non-invasively the effects of acute hypercapnia on the amplitude of cerebrovascular pulsation (CVP) in humans. METHODS Experiments were carried out in four male volunteers aged 25, 26, 31 and 49. Changes in blood flow through the pial arteries were induced using two interventions: (A) breathing a gas mixture containing 5% CO(2) for 2 minutes and (B) intravenous administration of 1g acetazolamide. The amplitude of CVP and width of subarachnoid space (SAS) were measured non-invasively using near-infrared transillumination/backscattering sounding (NIR-T/BSS), while cerebral blood flow was assessed by single photon emission computed tomography (SPECT) and mean blood flow velocity in the left anterior cerebral artery by transcranial Doppler. RESULTS Inhalation of a gas mixture containing 5% CO(2) evoked an increase in the amplitude of CVP (202.5% ± SE 10.1), normalized number of counts (22.6% ± SE 3.5%) and mean blood flow velocity in the left cerebral anterior artery (37.6%± SE 11.7%), while resistive index decreased (-8.7% ± SE 2.3%) and the width of SAS decreased (-8.0 ± SE 0.45). Acetazolamide also produced an increase in the amplitude of CVP (23.7% ± SE 5.4%), normalized number of counts (7.9% ± SE 1.1%), and mean blood flow velocity in the left cerebral anterior artery (62.8% ± SE 13.7%), while resistive index decreased (-7.9% ± SE 1.7%), and the width of SAS decreased (-13.4% ± SE 3.4%). CONCLUSION Acute hypercapnia causes an increase in the amplitude of CVP pulsation in humans. NIR-T/BSS allows for non-invasive bedside monitoring of the amplitude of CVP. NIR-T/BSS is consistent with transcranial Doppler and SPECT.