Correction for blood pressure improves correlation between cerebrovascular reactivity assessed by breath holding and 6% CO(2) breathing

Estimation of cerebrovascular reactivity amplitude and lag using breath-holding fMRI and the global BOLD signal: Application in diabetes and hypertension

In this work, we demonstrate a data-driven approach for estimating cerebrovascular reactivity (CVR) amplitude and lag from breathhold (BH) fMRI data alone. Our approach employs a frequency-domain approach that is independent of external recordings. CVR amplitude is estimated from the BOLD frequency spectrum and CVR lag is estimated from the Fourier phase using the global-mean BOLD signal as reference. Unlike referencing to external recordings, these lags are specific to the brain. We demonstrated our method in detecting regional CVR amplitude and lag differences across healthy (CTL), hypertensive (HT) and hypertension-plus-type-2-diabetes (HT + DM) groups of similar ages and sex ratios, with a total N of 49. We found CVR amplitude to be significantly higher in CTL compared to HT + DM, with minimal difference between CTL and HT. Also, voxelwise CVR lag estimated in the Fourier domain is a more sensitive marker of vascular dysfunction than CVR amplitude. CVR lag in HT is significantly shorter than in CTL, with minimal difference between CTL and HT + DM. Our results support the importance of joint CVR amplitude and lag assessments in clinical applications.

Cerebral Hemodynamics and Intracranial Compliance Impairment in Critically Ill COVID-19 Patients: A Pilot Study

Introduction: One of the possible mechanisms by which the new coronavirus (SARS-Cov2) could induce brain damage is the impairment of cerebrovascular hemodynamics (CVH) and intracranial compliance (ICC) due to the elevation of intracranial pressure (ICP). The main objective of this study was to assess the presence of CVH and ICC alterations in patients with COVID-19 and evaluate their association with short-term clinical outcomes. Methods: Fifty consecutive critically ill COVID-19 patients were studied with transcranial Doppler (TCD) and non-invasive monitoring of ICC. Subjects were included upon ICU admission; CVH was evaluated using mean flow velocities in the middle cerebral arteries (mCBFV), pulsatility index (PI), and estimated cerebral perfusion pressure (eCPP), while ICC was assessed by using the P2/P1 ratio of the non-invasive ICP curve. A CVH/ICC score was computed using all these variables. The primary composite outcome was unsuccessful in weaning from respiratory support or death on day 7 (defined as UO). Results: At the first assessment (n = 50), only the P2/P1 ratio (median 1.20 [IQRs 1.00-1.28] vs. 1.00 [0.88-1.16]; p = 0.03) and eICP (14 [11-25] vs. 11 [7-15] mmHg; p = 0.01) were significantly higher among patients with an unfavorable outcome (UO) than others. Patients with UO had a significantly higher CVH/ICC score (9 [8-12] vs. 6 [5-7]; p < 0.001) than those with a favorable outcome; the area under the receiver operating curve (AUROC) for CVH/ICC score to predict UO was 0.86 (95% CIs 0.75-0.97); a score > 8.5 had 63 (46-77)% sensitivity and 87 (62-97)% specificity to predict UO. For those patients undergoing a second assessment (n = 29), after a median of 11 (5-31) days, all measured variables were similar between the two time-points. No differences in the measured variables between ICU non-survivors (n = 30) and survivors were observed. Conclusions: ICC impairment and CVH disturbances are often present in COVID-19 severe illness and could accurately predict an early poor outcome.

[Carboxytherapy - an innovative trend in resort medicine]

Carboxytherapy (the treatment based on carbon dioxide injections) is a multipurpose and widely used medical technology. The use of CO₂ injections (intracutaneous, subcutaneous, and pneumopuncture) have substantially supplemented and increased the practical relevance of carboxytherapy as a method for the treatment of many diseases. Thanks to it physiological properties, CO₂ has antihypoxic, antioxidant, vasodilatory, anti-inflammatory, analgesic, and spasmolytic activities; moreover, it improves blood viscosity, stimulates neoangiogenesis, and regenerative processes. Carbon dioxide is a sort of biochemical 'peacemaker' in tissue oxygenation: when blood cells are exposed to high CO₂ concentrations (Bohr effect), the rate of gas exchange (CO₂ and O₂) increases. The human organism interprets carboxytherapy (local hypercapnia) as oxygen deficiency and responses to it by boosting not only the blood flow, but also the vascular endothelial growth factor which stimulates neoangiogenesis and in the long run improves blood supply and tissue trophism. The multiple mechanisms of action, polymodal efficacy, a tool kit with a wide range of detectors and various modes of treatment make carboxytherapy a popular medical technology all over the world, namely in cosmetology, dermatology, aesthetic medicine, angiology, orthopaedics, cardiology, neurology, pulmonology, gynaecology, urology, proctology, plastic and general surgery, and other areas. Carboxytherapy provides a perfect example of the off-label usage in medicine that made it one of the most extensively applied medical technology for the treatment of various diseases despite the lack of the preclinical data and scarce relevant information available in textbooks, reference books and booklets.

Effects of short-term continuous positive airway pressure withdrawal on cerebral vascular reactivity measured by blood oxygen level-dependent magnetic resonance imaging in obstructive sleep apnoea: a randomised controlled trial

Impaired cerebral vascular reactivity (CVR) increases long-term stroke risk. Obstructive sleep apnoea (OSA) is associated with peripheral vascular dysfunction and vascular events. The aim of this trial was to evaluate the effect of continuous positive airway pressure (CPAP) withdrawal on CVR.

41 OSA patients (88% male, mean age 57±10 years) were randomised to either subtherapeutic or continuation of therapeutic CPAP. At baseline and after 2 weeks, patients underwent a sleep study and magnetic resonance imaging (MRI). CVR was estimated by quantifying the blood oxygen level-dependent (BOLD) MRI response to breathing stimuli.

OSA did recur in the subtherapeutic CPAP group (mean treatment effect apnoea–hypopnoea index +38.0 events·h−1, 95% CI 24.2–52.0; p<0.001) but remained controlled in the therapeutic group. Although there was a significant increase in blood pressure upon CPAP withdrawal (mean treatment effect +9.37 mmHg, 95% CI 1.36–17.39; p=0.023), there was no significant effect of CPAP withdrawal on CVR assessedviaBOLD MRI under either hyperoxic or hypercapnic conditions.

Short-term CPAP withdrawal did not result in statistically significant changes in CVR as assessed by functional MRI, despite the recurrence of OSA. We thus conclude that, unlike peripheral endothelial function, CVR is not affected by short-term CPAP withdrawal.

The Breath-Hold Acceleration Index: A New Method to Evaluate Cerebrovascular Reactivity using Transcranial Doppler

BACKGROUND AND PURPOSE

Cerebrovascular reactivity (CR) is an ideal biomarker to detect cerebrovascular damage. CR can be quantified by measuring changes in cerebral blood flow velocity (CBFV) resulting from a CO₂  vasodilatory stimulus, often using the breath-holding index (BHI). In this method, transcranial Doppler (TCD) ultrasound is used to measure CBFV changes in the middle cerebral artery (MCA) during a breath-hold maneuver. Despite its convenience, BHI has high variability. Changing body position may contribute to potential variability. It is important to determine if CR differs with body position. The aims of this study were, first, to propose an alternative, more robust index to evaluate CR using a breath-hold maneuver; second, investigate the effect of body position on CR measured with conventional (BHI) and a new proposed index.

METHODS

Ten healthy young volunteers held their breath for 30 seconds on a tilt table. CR was calculated at five different angles using two indices: the conventional BHI, and the breath-hold acceleration index (BHAI), a new index obtained by linear regression of the most linear portion of the mean velocity change during the breath-hold maneuver. The regression represents acceleration (change in blood flow velocity per unit of time) sampled at each cardiac cycle.

RESULTS

The mean coefficient of variation was 43.7% lower in BHAI in comparison with BHI. Neither index was statistically significant between body positions (P > .05).

CONCLUSIONS

BHAI has less variability in comparison with the conventional standard BHI. Additionally, neither index showed statistical significance in CR based on change in body position.

Respiratory challenge MRI: Practical aspects

Respiratory challenge MRI is the modification of arterial oxygen (PaO₂) and/or carbon dioxide (PaCO₂) concentration to induce a change in cerebral function or metabolism which is then measured by MRI. Alterations in arterial gas concentrations can lead to profound changes in cerebral haemodynamics which can be studied using a variety of MRI sequences. Whilst such experiments may provide a wealth of information, conducting them can be complex and challenging. In this paper we review the rationale for respiratory challenge MRI including the effects of oxygen and carbon dioxide on the cerebral circulation. We also discuss the planning, equipment, monitoring and techniques that have been used to undertake these experiments. We finally propose some recommendations in this evolving area for conducting these experiments to enhance data quality and comparison between techniques.

•

Oxygen and carbon dioxide affect cerebral blood flow and metabolism.

•

This can be imaged with various MRI sequences.

•

The practicalities of these techniques are reviewed.

•

Examples of how this has been used to understand disease mechanisms.

Correlations between endothelial function in the systemic and cerebral circulation and insulin resistance in type 2 diabetes mellitus

Insulin resistance is associated with endothelial dysfunction in type 2 diabetes mellitus, which can lead to impaired vascular reactivities of both systemic and cerebral circulations. Appropriate 'correction' of vascular reactivity results for non-endothelium-dependent systemic effects avoids misinterpretation of endothelial function. Therefore, we 'corrected' vascular reactivity results and explored the potential correlations between systemic vascular reactivity, cerebrovascular reactivity and insulin resistance. In 34 patients, 'systemic vascular reactivity' was assessed by quantifying reactive hyperaemia. Cerebrovascular reactivity was assessed by quantifying changes in cerebral blood flow velocity during hypercapnia. To minimize the influence of non-endothelium-dependent systemic effects on vascular reactivity results, 'corrected systemic vascular reactivity' was calculated by normalizing systemic vascular reactivity using the measurements from the contralateral side; and cerebrovascular reactivity results were corrected by calculating percentage and absolute changes in cerebrovascular conductance index ('percent cerebrovascular conductance index' and 'delta cerebrovascular conductance index', respectively). Insulin resistance was estimated by homeostatic model assessment. Correlation between conventional cerebrovascular reactivity and systemic vascular reactivity was not significant. But correlations between 'corrected systemic vascular reactivity' and 'percent cerebrovascular conductance index' (r = 0.51; p = 0.002) and 'corrected systemic vascular reactivity' and 'delta cerebrovascular conductance index' (r = 0.50; p = 0.003) were significant. Among all vascular reactivity parameters, only 'delta cerebrovascular conductance index' was significantly correlated with homeostatic model assessment of insulin resistance (r = -0.38; p = 0.029). In conclusion, endothelial function in the systemic and cerebral circulations is moderately correlated, provided that vascular reactivity estimates are corrected for non-endothelium-dependent influences.