Concentration-dependent vasoconstrictive effect of hyperoxia on hypercarbia-dilated retinal arterioles

Retinal vessel analysis to assess microvascular function in the healthy eye: A systematic review on the response to acute physiological and pathological stressors

The retina allows noninvasive in vivo assessment of the microcirculation. Autoregulation of the retinal microvasculature meets the changing requirements of local metabolic demand and maintains adequate blood flow. Analysis of the retinal vascular reactivity contributes to the understanding of regulatory physiology and its relationship to the systemic microcirculation. We conducted a literature review on the effect of different acute stimuli onto the retinal vasculature was conducted in accordance with the PRISMA guidelines. A literature search between 1-1-2005 and 17-10-2022 was performed in Medline, Embase, Web of Science and the Cochrane Library. We report the retinal vascular behavior of healthy individuals in response to both physiological and pathological stressors in 106 included articles. We provide ables of methodological characteristics for each stressor. Hypoxia, hypercapnia, high altitude, flicker light stimulation, rise of core temperature, blood pressure lowering, and the condition immediately after endurance exercise associate with larger retinal vessels. Hyperoxia, hypocapnia, blood pressure rise (Bayliss effect), and the condition during isometric exercise associate with smaller retinal vessels. The retinal vasculature is highly reactive to physiological and pathological stressors. This autoregulatory capacity is hypothesized to be a source of biomarkers for vascular health. Dynamic and static retinal vessel analysis are noninvasive methods to assess this (micro)vascular function. Exploring its diagnostic potential and application into clinical practice requires the development of standardized assessment methods, for which some recommendations are made.

The value of a shorter-delay arterial spin labeling protocol for detecting cerebrovascular impairment

Background

The aim of this study was to determine the relationship between blood oxygen level dependent (BOLD) cerebrovascular reactivity (CVR) and cerebral blood flow (CBF) obtained from arterial spin labeling (ASL) using different post labeling delays (PLD).

Methods

Forty-two patients with steno-occlusive diseases and impaired CVR were divided into two groups, one scanned with a 1.5-second (1.5-s) and the other with a 2.5-second (2.5-s) PLD ASL protocol. For all patients, a region of interest (ROI) was drawn around the CVR impairment. This affected ROI was then left-right flipped across the brain midline to obtain the control ROI. For both groups, the difference in grey matter CVR between affected and control ROI was first tested to confirm significance. The average grey matter CBF of affected and control ROIs were then compared. The same analysis method was used to compare affected and control hemispheres.

Results

In both groups of 1.5-s and 2.5-s PLD, CVR values in the affected ROI (-0.049±0.055 and -0.042±0.074%/mmHg, respectively) were significantly lower compared to that in the control ROI (0.152±0.054 and 0.152±0.053%/mmHg, respectively, P<0.0001). In the group with the 1.5-s PLD, CBF in the affected ROI (37.62±11.37 mL/100 g/min) was significantly lower compared to CBF in the control ROI (44.13±11.58 mL/100 g/min, P<0.05). However, in the group with the 2.5-s PLD, no significant differences could be seen between CBF in the affected ROI (40.50±14.82 mL/100 g/min) and CBF in the control ROI (39.68±12.49 mL/100 g/min, P=0.73). In the hemisphere-based analysis, CBF was significantly lower in the affected side than in the control side for the group with the 1.5-s PLD (P<0.05) when CVR was impaired (P<0.0001), but not for the group with the 2.5-s PLD (P=0.49).

Conclusions

In conclusion, our study reveals and highlights the value of a shorter-PLD ASL protocol, which is able to reflect CVR impairment. At the same time, we offer a better understanding of the relationship between BOLD CVR and CBF obtained from ASL.

Retinal oximetry and systemic arterial oxygen levels

PURPOSE

Continuous peripheral pulse oximetry for monitoring adequacy of oxygenation is probably the most important technological advance for patients' monitoring and safety in the last decades. Pulse oximetry has the disadvantage of measuring the peripheral circulation, and the only mean to measure oxygen content of the central circulation is by invasive technology. Determination of blood oxyhaemoglobin saturation in the retinal vessels of the eye can be achieved noninvasively through spectrophotometric retinal oximetry which provides access to the central nervous system circulation. The aim of the thesis was to determine whether retinal oximetry technique can be applied for estimation of the central nervous system circulation which until now has only been possible invasively. This was achieved by measuring oxyhaemoglobin saturation in three adult subject study groups: in people with central retinal vein occlusion (CRVO) to observe local tissue hypoxia, in patients with severe chronic obstructive pulmonary disease (COPD) on long-term oxygen therapy to observe systemic hypoxaemia and in healthy subjects during hyperoxic breathing to observe systemic hyperoxemia. In addition, the fourth study that is mentioned was performed to test whether retinal oximetry is feasible for neonates.

METHODS

Retinal oximetry in central retinal vein occlusion: Sixteen subjects with central retinal vein occlusion participated in the study. The oxyhaemoglobin saturation of the central retinal vein occlusion affected eye was compared with the fellow unaffected eye. Retinal oximetry in healthy people under hyperoxia: Thirty healthy subjects participated in the study, and the oxyhaemoglobin saturation of retinal arterioles and venules was compared between normoxic and hyperoxic breathing. Retinal oximetry in severe chronic obstructive pulmonary disease: Eleven patients with severe chronic obstructive pulmonary disease participated in the study. Retinal oximetry measurements were made with and without their daily supplemental oxygen therapy. Retinal arteriolar oxyhaemoglobin saturation when inspiring ambient air was compared with blood samples from the radial artery and finger pulse oximetry and healthy controls. The healthy control group was assembled from our database for comparison of oxyhaemoglobin saturation of retinal arterioles and venules during the ambient air breathing. The retinal oximeter is based on a conventional fundus camera and a specialized software. A beam splitter coupled with two high-resolution digital cameras allows for simultaneous acquisition of retinal images at separative wavelengths for calculation of oxyhaemoglobin saturation. In addition, retinal images of 28 full-term healthy neonates were obtained with scanning laser ophthalmoscope combined with modified Oxymap analysis software for calculation of the optical density ratio and vessel diameter RESULTS: Retinal oximetry in central retinal vein occlusion: Mean retinal venous oxyhaemoglobin saturation was 31 ± 12% in CRVO eyes and 52 ± 11% in unaffected fellow eyes (mean ± SD, n = 14, p < 0.0001). The arteriovenous oxygen difference (AV-difference) was 63 ± 11% in CRVO eyes and 43 ± 7% in fellow eyes (p < 0.0001). The variability of retinal venous oxyhaemoglobin saturation was considerable within and between eyes affected by CRVO. There was no difference in oxyhaemoglobin saturation of retinal arterioles between the CRVO eyes and the unaffected eyes (p = 0.49). Retinal oximetry in healthy people under hyperoxia: During hyperoxic breathing, the oxyhaemoglobin saturation in retinal arterioles increased to 94.5 ± 3.8% as compared with 92.0 ± 3.7% at baseline (n = 30, p < 0.0001). In venules, the mean oxyhaemoglobin saturation increased to 76.2 ± 8.0% from 51.3 ± 5.6% (p < 0.0001) at baseline. The AV-difference was markedly lower during hyperoxic breathing as compared with the normoxic breathing (18.3 ± 9.0% versus 40.7 ± 5.7%, p < 0.0001). Retinal oximetry in severe chronic obstructive pulmonary disease: During ambient air breathing, chronic obstructive pulmonary disease subjects had significantly lower oxyhaemoglobin saturation than healthy controls in both retinal arterioles (87.2 ± 4.9% versus 93.4 ± 4.3%, p = 0.02, n = 11) and venules (45.0 ± 10.3% versus 55.2 ± 5.5%, p = 0.01) but the AV-difference was not markedly different (p = 0.17). Administration of their prescribed oxygen therapy significantly increased the oxyhaemoglobin saturation in retinal arterioles (87.2 ± 4.9% to 89.5 ± 6.0%, p = 0.02) but not in venules (45.0 ± 10.3% to 46.7 ± 12.8%, p = 0.3). Retinal oximetry values were slightly lower than finger pulse oximetry (mean percentage points difference = -3.1 ± 5.5) and radial artery blood values (-5.0 ± 5.4). Retinal oximetry study in neonates: The modified version of the retinal oximetry instrument estimated the optical density ratio in retinal arterioles to be 0.256 ± 0.041 that was significantly different from the 0.421 ± 0.089 in venules (n = 28, p < 0.001, paired t-test). The vascular diameter of retinal arterioles was markedly narrower than of venules (14.1 ± 2.7 and 19.7 ± 3.7 pixels, p < 0.001).

CONCLUSION

The results of this thesis indicate that spectrophotometric retinal oximetry is sensitive to both local and systemic changes in oxyhaemoglobin saturation. Retinal oxyhaemoglobin saturation values are slightly lower than radial artery blood sample and finger pulse oximetry values. The discrepancies between the different modalities are expected to derive from countercurrent exchange between central retinal artery and vein within the optic nerve but calibration issues cannot be excluded as contributing to this difference. Despite these differences, the findings indicate the potential of retinal oximetry for noninvasive real-time measurements of oxyhaemoglobin saturation in central nervous system vessels. Following calibration upgrade and technological improvement, verification retinal oximetry may potentially be applied to critically ill and anaesthesia care patients. The study on combined scanning laser ophthalmoscope and retinal oximetry supports the feasibility of the technique for oximetry analysis in newly born babies.

Cerebrovascular reactivity and white matter integrity

OBJECTIVE

To compare the diffusion and perfusion MRI metrics of normal-appearing white matter (NAWM) with and without impaired cerebrovascular reactivity (CVR).

METHODS

Seventy-five participants with moderate to severe leukoaraiosis underwent blood oxygen level-dependent CVR mapping using a 3T MRI system with precise carbon dioxide stimulus manipulation. Several MRI metrics were statistically compared between areas of NAWM with positive and negative CVR using one-way analysis of variance with Bonferroni correction for multiple comparisons.

RESULTS

Areas of NAWM with negative CVR showed a significant reduction in fractional anisotropy by a mean (SD) of 3.7% (2.4), cerebral blood flow by 22.1% (8.2), regional cerebral blood volume by 22.2% (7.0), and a significant increase in mean diffusivity by 3.9% (3.1) and time to maximum by 10.9% (13.2) (p < 0.01), compared to areas with positive CVR.

CONCLUSIONS

Impaired CVR is associated with subtle changes in the tissue integrity of NAWM, as evaluated using several quantitative diffusion and perfusion MRI metrics. These findings suggest that impaired CVR may contribute to the progression of white matter disease.

Preliminary investigation of multispectral retinal tissue oximetry mapping using a hyperspectral retinal camera

Oximetry measurement of principal retinal vessels represents a first step towards understanding retinal metabolism, but the technique could be significantly enhanced by spectral imaging of the fundus outside of main vessels. In this study, a recently developed Hyperspectral Retinal Camera was used to measure relative oximetric (SatO2) and total hemoglobin (HbT) maps of the retina, outside of large vessels, in healthy volunteers at baseline (N = 7) and during systemic hypoxia (N = 11), as well as in patients with glaucoma (N = 2). Images of the retina, on a field of view of ∼30°, were acquired between 500 and 600 nm with 2 and 5 nm steps, in under 3 s. The reflectance spectrum from each pixel was fitted to a model having oxy- and deoxyhemoglobin as the main absorbers and scattering modeled by a power law, yielding estimates of relative SatO2 and HbT over the fundus. Average optic nerve head (ONH) saturation over 8 eyes was 68 ± 5%. During systemic hypoxia, mean ONH saturation decreased by 12.5% on average. Upon further development and validation, the relative SatO2 and HbT maps of microvasculature obtained with this imaging system could ultimately contribute to the diagnostic and management of diseases affecting the ONH and retina.

Assessing the effect of unilateral cerebral revascularisation on the vascular reactivity of the non-intervened hemisphere: a retrospective observational study

OBJECTIVES

Unilateral haemodynamically significant large-vessel intracranial stenosis may be associated with reduced blood-oxygen-level-dependent (BOLD) cerebrovascular reactivity (CVR), an indicator of autoregulatory reserve. Reduced CVR has been associated with ipsilateral cortical thinning and loss in cognitive function. These effects have been shown to be reversible following revascularisation. Our aim was to study the effects of unilateral revascularisation on CVR in the non-intervened hemisphere in bilateral steno-occlusive or Moyamoya disease.

STUDY DESIGN

A retrospective observational study.

SETTING

A routine follow-up assessment of CVR after a revascularisation procedure at a research teaching hospital in Toronto (Journal wants us to generalise).

PARTICIPANTS

Thirteen patients with bilateral Moyamoya disease (age range 18 to 52 years; 3 males), seven patients with steno-occlusive disease (age range 18 to 78 years; six males) and 27 approximately age-matched normal control subjects (age range 19-71 years; 16 males) with no history or findings suggestive of any neurological or systemic disease.

INTERVENTION

Participants underwent BOLD CVR MRI using computerised prospective targeting of CO2, before and after unilateral revascularisation (extracranial-intracranial bypass, carotid endarterectomy or encephaloduroarteriosynangiosis). Pre-revascularisation and post-revascularisation CVR was assessed in each major arterial vascular territory of both hemispheres.

RESULTS

As expected, surgical revascularisation improved grey matter CVR in the middle cerebral artery (MCA) territory of the intervened hemisphere (0.010±0.023 to 0.143±0.010%BOLD/mm Hg, p<0.01). There was also a significant post-revascularisation improvement in grey matter CVR in the MCA territory of the non-intervened hemisphere (0.101±0.025 to 0.165±0.015%BOLD/mm Hg, p<0.01).

CONCLUSIONS

Not only does CVR improve in the hemisphere ipsilateral to a flow restoration procedure, but it also improves in the non-intervened hemisphere. This highlights the potential of CVR mapping for staging and evaluating surgical interventions.

Measurement of retinal blood flow in the rat by combining Doppler Fourier-domain optical coherence tomography with fundus imaging

A wide variety of ocular diseases are associated with abnormalities in ocular circulation. As such, there is considerable interest in techniques for quantifying retinal blood flow, among which Doppler optical coherence tomography (OCT) may be the most promising. We present an approach to measure retinal blood flow in the rat using a new optical system that combines the measurement of blood flow velocities via Doppler Fourier-domain optical coherence tomography and the measurement of vessel diameters using a fundus camera-based technique. Relying on fundus images for extraction of retinal vessel diameters instead of OCT images improves the reliability of the technique. The system was operated with an 841-nm superluminescent diode and a charge-coupled device camera that could be operated at a line rate of 20 kHz. We show that the system is capable of quantifying the response of 100% oxygen breathing on the retinal blood flow. In six rats, we observed a decrease in retinal vessel diameters of 13.2% and a decrease in retinal blood velocity of 42.6%, leading to a decrease in retinal blood flow of 56.7%. Furthermore, in four rats, the response of retinal blood flow during stimulation with diffuse flicker light was assessed. Retinal vessel diameter and blood velocity increased by 3.4% and 28.1%, respectively, leading to a relative increase in blood flow of 36.2%. The presented technique shows much promise to quantify early changes in retinal blood flow during provocation with various stimuli in rodent models of ocular diseases in rats.

Retinal blood flow in response to an intravitreal injection of ranibizumab for neovascular age-related macular degeneration

PURPOSE

To assess the hemodynamic response of retinal arterioles and venules following a single intravitreal injection of ranibizumab in neovascular age-related macular degeneration (NV-AMD) patients and to assess the influence of the number of prior injections on this response.

METHODS

Fifteen NV-AMD patients were prospectively recruited and grouped according to the dosage of ranibizumab previously received. Group 1 NV-AMD patients (n = 7) had previously received 1.50 mg or less, and group 2 patients (n = 8) had received more than 1.50 mg in the study eye. A group of 12 non-NV AMD patients were also recruited for control comparison. Vessel diameter, centreline blood velocity and blood flow were assessed with the Canon Laser Blood Flowmeter immediately prior to an injection and at a mean follow-up of 37.7 and 36.7 days for group 1 and group 2 patients, respectively.

RESULTS

The NV-AMD patients as a whole and the group 1 cohort had a significantly greater arteriolar diameter at baseline than the non-NV AMD patients. There was a significant reduction in arteriolar diameter, velocity and blood flow in group 1 but not in group 2 NV-AMD patients at follow-up. There was only an insignificant decrease in measured parameters of the retinal venules. At follow-up, there was no difference in the diameter, velocity or flow between AMD patients.

CONCLUSION

Intravitreal ranibizumab treatment for NV-AMD induces a reduction in arteriolar diameter, velocity, and blood flow in patients who have received <1.50 mg of ranibizumab.

In vivo assessment of peripheral vascular function by tcpO₂ and skin blood flow modelling

There are multiple techniques and methods to assess peripheral vascular function in vivo but not without limitations. More discriminative, sensitive and also practical evaluation strategies are needed to fully characterize the peripheral vascular function. In the present work, a new quantitative descriptor, the 'elimination half-life time' was developed from flow-related variables as a non-invasive microcirculatory rate parameter to describe vascular dynamics. Fifty-four healthy volunteers and six type 2 diabetic patients, both genders, were submitted to a dynamical procedure consisting in the inhalation of a 100% saturated atmosphere of oxygen for 10 min. The tcpO(2) and microcirculatory blood flow [Laser Doppler Flowmetry (LDF)] were measured in a randomly selected leg with a Periflux 5000 system before, during and after the procedure. A monocompartmental model was adjusted to tcpO(2) and LDF data. The tcpO(2) constant elimination rate, expressed as the Oxygen elimination half-life, was used as an indicator of the vulnerability of peripheral tissue and compared in healthy versus non-healthy individuals. Under normal conditions, the saturated ventilation increases the tissue's O(2) availability, as an expression of the natural capacity to adjust the tissue hemodynamics to new metabolical/perfusion conditions. Diabetic patients are expected to suffer vascular impairment and ischemia. Under O(2) overloading conditions, those hypoxic territories tend to uptake all the delivered oxygen, expressed as a significant increase in the O(2) elimination half-life. This approach allows to propose 'elimination half-life time' as the first quantitative descriptive parameter combining miogenic, hemodynamic and metabolic aspects of the microcirculatory physiology and to help to identify the individual's vascular vulnerability.