The effect of hypertension on cerebrovascular carbon dioxide reactivity in atrial fibrillation patients

Atrial fibrillation (AF) and hypertension (HTN) are both associated with impaired cerebrovascular carbon dioxide reactivity (CVRCO2), an indicator of cerebral vasodilatory reserve. We hypothesised that CVRCO2 would be lower in patients with both AF and HTN (AF + HTN) compared to normotensive AF patients, due to an additive effect of AF and HTN on CVRCO2. Forty AF (68 ± 9 years) and fifty-seven AF + HTN (68 ± 8 years) patients underwent transcranial Doppler ultrasound measurement of middle cerebral artery blood velocity (MCA Vm) during stepped increases and decreases in end-tidal carbon dioxide (PETCO2). A cerebrovascular conductance index (CVCi) was calculated as the ratio of MCA Vm and mean arterial pressure (MAP). CVRCO2 was determined from the linear slope for MCA Vm and MCA CVCi vs PETCO2. Baseline MAP was higher in AF + HTN than AF (107 ± 9 vs. 98 ± 9 mmHg, respectively; p < 0.001), while MCA Vm was not different (AF + HTN:49.6 [44.1-69.0]; AF:51.7 [45.2-63.3] cm.s-1; p = 0.075), and CVCi was lower in AF + HTN (0.46 [0.42-0.57] vs. 0.54 [0.44-0.63] cm.s-1.mmHg-1; p < 0.001). MCA Vm CVRCO2 was not different (AF + HTN: 1.70 [1.47-2.19]; AF 1.74 [1.54-2.52] cm/s/mmHg-2; p = 0.221), while CVCi CVRCO2 was 13% lower in AF + HTN (0.013 ± 0.004 vs 0.015 ± 0.005 cm.s-1.mmHg-1; p = 0.047). Our results demonstrate blunted cerebral vasodilatory reserve (determined as MCA CVCi CVRCO2) in AF + HTN compared to AF alone. This may implicate HTN as a driver of further cerebrovascular dysfunction in AF that may be important for the development of AF-related cerebrovascular events and downstream cognitive decline. We demonstrated reduced cerebrovascular CO2 responsiveness in atrial fibrillation with hypertension (AF+HTN) vs. atrial fibrillation (AF). Furthermore, AF per se (as opposed to normal sinus rhythm) predicts reduced cerebrovascular CO2 responsiveness. Our findings suggest additional cerebrovascular dysfunction in AF+HTN vs. AF.

Comprehensive investigations of cerebral hemodynamic responses in CSVD patients with mental disorders: a pilot study

Although a portion of patients with cerebral small vessel disease (CSVD) present mental disorders, there is currently a lack of appropriate technologies to evaluate brain functions that are relevant to neurovascular coupling. Furthermore, there are no established objective criteria for diagnosing and distinguishing CSVD-induced mental disorders and psychiatric diseases. In this study, we report the first comprehensive investigation of the cerebral hemodynamics of CSVD patients who also presented with mental disorders. Two CSVD patients with similar magnetic resonance imaging (MRI) outcomes but with non-identical mental symptoms participated in this study. The patients were instructed to perform the verbal fluency task (VFT), high-level cognition task (HCT), as well as voluntary breath holding (VBH). A functional near-infrared spectroscopy (fNIRS) was used to measure the cerebral oxygenation responses. Additionally, a diffuse correlation spectroscopy (DCS) was used to measure the cerebral blood flow (CBF) responses. Both technologies were also applied to a healthy subject for comparison. The fNIRS results showed that both CSVD patients presented abnormal cerebral oxygenation responses during the VFT, HCT, and VBH tasks. Moreover, the patient with cognition impairment showed fluctuations in CBF during these tasks. In contrast, the patient without cognition impairment mostly presented typical CBF responses during the tasks, which was consistent with the healthy subject. The cognitive impairment in CSVD patients may be due to the decoupling of the neurons from the cerebrovascular, subsequently affecting the autoregulation capacity. The results of the fNIRS and DCS combined provide a comprehensive evaluation of the neurovascular coupling and, hence, offer great potential in diagnosing cerebrovascular or psychiatric diseases.

On the use and misuse of cerebral hemodynamics terminology using Transcranial Doppler ultrasound: a call for standardization

Cerebral hemodynamics (e.g., cerebral blood flow) can be measured and quantified using many different methods, with Transcranial Doppler ultrasound (TCD) being one of the most commonly utilized approaches. In human physiology, the terminology used to describe metrics of cerebral hemodynamics are inconsistent, and in some instances technically inaccurate; this is especially true when evaluating, reporting, and interpreting measures from TCD. Therefore, this perspectives article presents recommended terminology when reporting cerebral hemodynamic data. We discuss the current use and misuse of the terminology in the context of using TCD to measure and quantify cerebral hemodynamics and present our rationale and consensus on the terminology that we recommend moving forward. For example, one recommendation is to discontinue use of the term "cerebral blood flow velocity" in favor of "cerebral blood velocity" with precise indication of the vessel of interest. We also recommend clarity when differentiating between discrete cerebrovascular regulatory mechanisms, namely cerebral autoregulation, neurovascular coupling, and cerebrovascular reactivity. This will be a useful guide for investigators in the field of cerebral hemodynamics research.

Functional optical coherence tomography at altitude: retinal microvascular perfusion and retinal thickness at 3,800 meters

Cerebral hypoxia is a serious consequence of several cardiorespiratory illnesses. Measuring the retinal microvasculature at high altitude provides a surrogate for cerebral microvasculature, offering potential insight into cerebral hypoxia in critical illness. Additionally, while sex-specific differences in cardiovascular diseases are strongly supported, few have focused on differences in ocular blood flow. We evaluated the retinal microvasculature in males (n=11) and females (n=7) using functional optical coherence tomography at baseline (1,130m) (Day 0), following rapid ascent (Day 2) and prolonged exposure (Day 9) to high altitude (3,800m). Retinal vascular perfusion density (rVPD; an index of total blood supply), retinal thickness (RT; reflecting vascular and neural tissue volume) and arterial blood were acquired. As a group, rVPD increased on Day 2 vs. Day 0 (p<0.001) and was inversely related to PaO₂ (R²=0.45; p=0.006). By Day 9, rVPD recovered to baseline, but was significantly lower in males vs. females (p=0.007). RT was not different on Day 2 vs. Day 0 (p>0.99) but was reduced by Day 9 relative to Day 0 and Day 2 (p<0.001). RT changes relative to Day 0 were inversely related to changes in PaO₂ on Day 2 (R²=0.6; p=0.001) and Day 9 (R²=0.4; p=0.02). RT did not differ between sexes. These data suggest differential time course and regulation of the retina during rapid ascent and prolonged exposure to high altitude and are the first to demonstrate sex-specific differences in rVPD at high altitude. The ability to assess intact microvasculature contiguous with the brain has widespread research and clinical applications.

Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO₂ and O₂) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO₂ and O₂ and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (P_ET)CO₂ and P_ETO₂. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as P_ETCO₂/P_ETO₂ during RB. For VA, where P_ETCO₂ and P_ETO₂ could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.

Regional differences in cerebrovascular reactivity in response to acute isocapnic hypoxia in healthy humans: Methodological considerations

The cerebrovasculature responds to blood gas challenges. Regional differences (anterior vs. posterior) in cerebrovascular responses to increases in CO₂ have been extensively studied. However, regional cerebrovascular reactivity (CVR) responses to low O₂ (hypoxia) are equivocal, likely due to differences in analysis. We assessed the effects of acute isocapnic hypoxia on regional CVR comparing absolute and relative (%-change) responses in the middle cerebral artery (MCA) and posterior cerebral artery (PCA). We instrumented 14 healthy participants with a transcranial Doppler ultrasound (cerebral blood velocity), finometer (beat-by-beat blood pressure), dual gas analyzer (end-tidal CO₂ and O₂), and utilized a dynamic end-tidal forcing system to elicit a single 5-min bout of isocapnic hypoxia (∼45 Torr P_ETO₂, ∼80 % SpO₂). During exposure to acute hypoxia, absolute responses were larger in the anterior compared to posterior cerebral circulation (P < 0.001), but were not different when comparing relative responses (P = 0.45). Consistent reporting of CVR to hypoxia will aid understanding normative responses, particularly in assessing populations with impaired cerebrovascular function.

Cardiorespiratory plasticity in humans following two patterns of acute intermittent hypoxia

NEW FINDINGS

What is the central question of this study? Do cardiorespiratory experience-dependent effects (EDEs) differ between two different stimulus durations of acute isocapnic intermittent hypoxia (IHx; 5-min vs. 90-s cycles between hypoxia and normoxia)? What is the main finding and its importance? There was long-term facilitation in ventilation and blood pressure in both IHx protocols, but there was no evidence of progressive augmentation or post-hypoxia frequency decline. Not all EDEs described in animal models translate to acute isocapnic IHx responses in humans, and cardiorespiratory responses to 5-min versus 90-s on/off IHx protocols are largely similar.

ABSTRACT

Peripheral respiratory chemoreceptors monitor breath-by-breath changes in arterial CO₂ and O₂ , and mediate ventilatory changes to maintain homeostasis. Intermittent hypoxia (IHx) elicits hypoxic ventilatory responses, with well-described experience-dependent effects (EDEs), derived mostly from animal work involving intermittent 5-min cycles of hypoxia and normoxia. These EDEs include post-hypoxia frequency decline (PHxFD), progressive augmentation (PA) and long-term facilitation (LTF). Comparisons of these EDEs between animal models and humans using similar IHx protocols are lacking. In addition, it is unknown whether shorter bouts of hypoxia, which may be more relevant to clinical conditions, elicit EDEs of similar magnitudes in humans. Respiratory (frequency, tidal volume and minute ventilation ( V ̇ I ) and cardiovascular (heart rate and mean arterial pressure (MAP)) variables were measured during and following two patterns of acute isocapnic IHx in 14 healthy human participants (four female): (1) 5 × 5 min and (2) 5 × 90 s on/off hypoxia. Participants' end-tidal P O 2 was clamped at 45 Torr during hypoxia and 100 Torr during normoxia. We found that (1) PHxFD and PA were not present in either IHx pattern (P > 0.14), (2) LTF was present in V ̇ I following both 5-min (P < 0.001) and 90-s isocapnic IHx trials (P < 0.001), and (3) LTF was present in MAP following 5-min isocapnic IHx (P < 0.001), and trended towards significance following 90-s IHx (P = 0.058). We demonstrate that acute isocapnic IHx alone may not elicit all of the EDEs that have been described in animal models. Additionally, ventilatory LTF occurred regardless of the length of hypoxia-normoxia cycles.

Bilateral regional extracranial blood flow regulation to hypoxia and unilateral duplex ultrasound measurement error

NEW FINDINGS

What is the central question of this study? Is blood flow regulation to hypoxia different between the internal carotid arteries (ICAs) and vertebral arteries (VAs), and what is the measurement error in unilateral extracranial artery assessments compared to bilateral? What is the main finding and its importance? ICA and VA blood flow regulation to hypoxia is comparable when factoring for vessel type and vessel side. Compared to bilateral assessment, vessels assessed unilaterally had individual measurement errors of up to 37%. Assessing the vessel with the larger resting blood flow, not the left or right vessel, reduces unilateral measurement error.

ABSTRACT

Whether blood flow regulation to hypoxia is similar between left and right internal carotid arteries (ICAs) and vertebral arteries (VAs) is unclear. Extracranial blood flow is regularly calculated by doubling a unilateral assessment; however, lateral artery differences may lead to measurement error. This study aimed to determine extracranial blood flow regulation to hypoxia when factoring for vessel type (ICAs or VAs) and vessel side (left or right) effects, and to investigate unilateral assessment measurement error compared to bilateral assessment. In a repeated-measures crossover design, extracranial arteries of 44 participants were assessed bilaterally by duplex ultrasound during 90 min of normoxic and poikilocapnic hypoxic (12.0% fraction of inspired oxygen) conditions. Linear mixed model analyses revealed no Condition × Vessel Type × Vessel Side interaction for blood flow, vessel diameter and flow velocity (all P > 0.05) indicating left and right ICA and VA blood flow regulation to hypoxia was similar. Bilateral hypoxic reactivity was comparable (ICAs, 1.4 (1.0) vs. VAs, 1.7 (1.1) Δ%·Δ S p O 2 ^-1 ; P = 0.12). Compared to bilateral assessment, unilateral mean measurement error of the relative blood flow response to hypoxia was up to 5%, but individual errors reached 37% and were greatest in ICAs and VAs with the smaller resting blood flow due to a ratio-scaling problem. In conclusion, left and right ICA and VA regulation to hypoxia is comparable when factoring for vessel type and vessel side. Assessing the ICA and VA vessels with the larger resting blood flow, not the left or right vessel, reduces unilateral measurement error.

Dynamic cerebral autoregulation and cerebrovascular carbon dioxide reactivity in middle and posterior cerebral arteries in young endurance-trained women

The integrated responses regulating cerebral blood flow are understudied in women, particularly in relation to potential regional differences. In this study, we compared dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity to carbon dioxide (CVRco₂) in the middle (MCA) and posterior cerebral arteries (PCA) in 11 young endurance-trained women (age, 25 ± 4 yr; maximal oxygen uptake, 48.1 ± 4.1 mL·kg^-1·min^-1). dCA was characterized using a multimodal approach including a sit-to-stand and a transfer function analysis (TFA) of forced blood pressure oscillations (repeated squat-stands executed at 0.05 Hz and 0.10 Hz). The hyperoxic rebreathing test was utilized to characterize CVRco₂. Upon standing, the percent reduction in blood velocity per percent reduction in mean arterial pressure during initial orthostatic stress (0-15 s after sit-to-stand), the onset of the regulatory response, and the rate of regulation did not differ between MCA and PCA (all P > 0.05). There was an ANOVA effect of anatomical location for TFA gain (P < 0.001) and a frequency effect for TFA phase (P < 0.001). However, normalized gain was not different between arteries (P = 0.18). Absolute CVRco₂ was not different between MCA and PCA (1.55 ± 0.81 vs. 1.30 ± 0.49 cm·s^-1/Torr, P = 0.26). Relative CVRco₂ was 39% lower in the MCA (2.16 ± 1.02 vs. 3.00 ± 1.09%/Torr, P < 0.01). These findings indicate that the cerebral pressure-flow relationship appears to be similar between the MCA and the PCA in young endurance-trained women. The absence of regional differences in absolute CVRco₂ could be women specific, although a direct comparison with a group of men will be necessary to address that issue.NEW & NOTEWORTHY Herein, we describe responses from two major mechanisms regulating cerebral blood flow with a special attention on regional differences in young endurance-trained women. The novel findings are that dynamic cerebral autoregulation and absolute cerebrovascular reactivity to carbon dioxide appear similar between the middle and posterior cerebral arteries of these young women.

Impact of mandibular advancement device therapy on cerebrovascular reactivity in patients with carotid atherosclerosis combined with OSAHS

PURPOSE

Obstructive sleep apnea-hypopnea syndrome (OSAHS) may affect cerebrovascular reactivity (CVR), representing cerebrovascular endothelial function, through complex cerebral functional changes. This study aimed to evaluate the change of CVR after 1-month and 6-month mandibular advancement device (MAD) treatment of patients with carotid atherosclerosis (CAS) combined with OSAHS.

METHODS

Patients with carotid atherosclerosis combined with OSAHS who voluntarily accepted Silensor-IL MAD therapy were prospectively enrolled. All patients underwent polysomnographic (PSG) examinations and CVR evaluation by breath-holding test using transcranial Doppler ultrasound at baseline (T0), 1 month (T1), and 6 months (T2) of MAD treatment.

RESULTS

Of 46 patients (mean age 54.4 ± 12.4 years, mean body mass index [BMI] 27.5 ± 4.5 kg/m²), 41 patients (responsive group) responded to the 1-month and 6-month treatment of MAD, an effective treatment rate of 89%. The remaining 5 patients (non-responsive group) were younger (47.4 ± 13.5 years) and had a higher BMI (35.8 ± 1.8 kg/m²). The responsive group had an improvement of apnea-hypopnea index (AHI) (events/h) from 33.0 ± 25.0 (T0) to 12.4 ± 10.4 (T1) and 8.7 ± 8.8 (T2), P < 0.001; minimum arterial oxygen saturation (minSpO₂) (%) increased from 79.8 ± 9.1 (T0) to 81.8 ± 9.4 (T1) and 85.2 ± 5.4 (T2), P < 0.01; longest apnea (LA) (s) decreased from 46.5 ± 23.1 (T0) to 33.3 ± 22.7 (T1) and 29.4 ± 18.5 (T2), P < 0.001; T90 (%) decreased from 10.3 ± 14.9 (T0) to 6.1 ± 11.8 (T1) and 3.3 ± 7.5 (T2), P < 0.05. Sleep architecture of these patients also improved significantly. The responsive group had a significant increase in left, right, and mean breath-holding index (BHI): left BHI(/s) from 0.52 ± 0.42 (T0) to 0.94 ± 0.56 (T1) and 1.04 ± 0.64 (T2), P < 0.01; right BHI(/s) from 0.60 ± 0.38 (T0) to 1.01 ± 0.58 (T1) and 1.11 ± 0.60 (T2), P < 0.01; mean BHI(/s) from 0.56 ± 0.38 (T0) to 0.97 ± 0.55 (T1) and 1.07 ± 0.59 (T2), P < 0.01), suggesting improved CVR.

CONCLUSION

Effective MAD therapy is beneficial for restoring cerebrovascular endothelial function in patients with CAS and OSAHS in a short period (1 month and 6 months).

TRIAL REGISTRATION

Clinical trial registration number: NCT03665818. September 11, 2018.

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

The reliability of a breath-hold protocol to determine cerebrovascular reactivity in adolescents

PURPOSE

Cerebrovascular reactivity (CVR) is impaired in adolescents with cardiovascular disease risk factors. A breath-hold test is a noninvasive method of assessing CVR, yet there are no reliability data of this outcome in youth. This study aimed to assess the reliability of a breath-hold protocol to measure CVR in adolescents.

METHODS

Twenty-one 13 to 15 year old adolescents visited the laboratory on two separate occasions, to assess the within-test, within-day and between-day reliability of a breath-hold protocol, consisting of three breath-hold attempts. CVR was defined as the relative increase from baseline in middle cerebral artery mean blood velocity following a maximal breath-hold of up to 30 seconds, quantified via transcranial Doppler ultrasonography.

RESULTS

Mean breath-hold duration and CVR were never significantly correlated (r < .31, P > .08). The within-test coefficient of variation for CVR was 15.2%, with no significant differences across breath-holds (P = .88), so the three breath-hold attempts were averaged for subsequent analyses. The within- and between-day coefficients of variation for CVR were 10.8% and 15.3%, respectively.

CONCLUSIONS

CVR assessed via a three breath-hold protocol can be reliably measured in adolescents, yielding similar within- and between-day reliability. Analyses revealed that breath-hold length and CVR were unrelated, indicating the commonly reported normalization of CVR to breath-hold duration (breath-hold index) may be unnecessary in youth.

Cerebrovascular reactivity assessment with O2-CO2 exchange ratio under brief breath hold challenge

BACKGROUND

Hypercapnia during breath holding is believed to be the dominant driver behind the modulation of cerebral blood flow (CBF). However, increasing evidence show that mild hypoxia and mild hypercapnia in breath hold (BH) could work synergistically to enhance CBF response. We hypothesized that breath-by-breath O2-CO2 exchange ratio (bER), defined as the ratio of the change in partial pressure of oxygen (ΔPO2) to that of carbon dioxide (ΔPCO2) between end inspiration and end expiration, would be able to better correlate with the global and regional cerebral hemodynamic responses (CHR) to BH challenge. We aimed to investigate whether bER is a more useful index than end-tidal PCO2 to characterize cerebrovascular reactivity (CVR) under BH challenge.

METHODS

We used transcranial Doppler ultrasound (TCD) to evaluate CHR under BH challenge by measuring cerebral blood flow velocity (CBFv) in the middle cerebral arteries. Regional changes in CHR to BH and exogenous CO2 challenges were mapped with blood oxygenation level dependent (BOLD) signal changes using functional magnetic resonance imaging (fMRI). We correlated respiratory gas exchange (RGE) metrics (bER, ΔPO2, ΔPCO2, end-tidal PCO2 and PO2, and time of breaths) with CHR (CBFv and BOLD) to BH challenge. Temporal features and frequency characteristics of RGE metrics and their coherence with CHR were examined.

RESULTS

CHR to brief BH epochs and free breathing were coupled with both ΔPO2 and ΔPCO2. We found that bER was superior to either ΔPO2 or ΔPCO2 alone in coupling with the changes of CBFv and BOLD signals under breath hold challenge. The regional CVR results derived by regressing BOLD signal changes on bER under BH challenge resembled those derived by regressing BOLD signal changes on end-tidal PCO2 under exogenous CO2 challenge.

CONCLUSION

Our findings provide a novel insight on the potential of using bER to better quantify CVR changes under BH challenge.

Confounding of Cerebral Blood Flow Velocity by Blood Pressure During Breath Holding or Hyperventilation in Transient Ischemic Attack or Stroke

Background and Purpose- Breath holding (BH) and hyperventilation are used to assess abnormal cerebrovascular reactivity, often in relation to severity of small vessel disease and risk of stroke with carotid stenosis, but responses may be confounded by blood pressure (BP) changes. We compared effects of BP and end-tidal carbon dioxide (etCO₂) on middle cerebral artery mean flow velocity (MFV) in consecutive transient ischemic attack and minor stroke patients. Methods- In the population-based, prospective OXVASC (Oxford Vascular Study) phenotyped cohort, change in MFV on transcranial Doppler ultrasound (ΔMFV, DWL-DopplerBox), beat-to-beat BP (Finometer), and etCO₂ was measured during 30 seconds of BH or hyperventilation. Two blinded reviewers independently assessed recording quality. Dependence of ΔMFV on ΔBP and ΔetCO₂ was determined by general linear models, stratified by quartiles. Results- Four hundred eighty-eight of 602 (81%) patients with adequate bone windows had high-quality recordings, more often in younger participants (64.6 versus 68.7 years; P<0.01), whereas 426 had hyperventilation tests (70.7%). During BH, ΔMFV was correlated with a rise in mean blood pressure (MBP; r²=0.15, P<0.001) but not ΔCO₂ (r²=0.002, P=0.32), except in patients with ΔMBP <10% (r²=0.13, P<0.001). In contrast during hyperventilation, the fall in MFV was similarly correlated with reduction in CO₂ and reduction in MBP (ΔCO₂: r²=0.13, P<0.001; ΔMBP: r²=0.12, P<0.001), with a slightly greater effect of ΔCO₂ when ΔMBP was <10% (r²=0.15). Stratifying by quartile, MFV increased linearly during BH across quartiles of ΔMBP, with no increase with ΔetCO₂. In contrast, during hyperventilation, MFV decreased linearly with ΔetCO₂, independent of ΔMBP. Conclusions- In older patients with recent transient ischemic attack or minor stroke, cerebral blood flow responses to BH were confounded by BP changes but reflected etCO₂ change during hyperventilation. Correct interpretation of cerebrovascular reactivity responses to etCO₂, including in small vessel disease and carotid stenosis, requires concurrent BP measurement.

Impaired cerebrovascular reactivity in chronic obstructive pulmonary disease

Impaired cerebrovascular reactivity (CVR) is associated with stroke. Cerebrovascular diseases are common comorbidity in chronic obstructive pulmonary disease (COPD) patients. The aim of our study was to quantify CVR in the anterior and posterior cerebral circulation during voluntary breath-holding in COPD patients according to airflow limitation severity. In this cross-sectional study, we compared 90 COPD patients without previous cerebrovascular disease and 30 age- and sex-matched healthy volunteers (mean age 67 ± 7.9, 87 males). Using transcranial Doppler ultrasound and breath-holding index (BHI), we analysed baseline mean flow velocities (MFV) and CVR of middle cerebral artery (MCA) and basilar artery (BA). Our results demonstrated that COPD patients had lower baseline MFV of both MCA and BA than controls. COPD patients had significantly lower BHI_mMCA and BHI_mBA than controls (0.8 and 0.7 versus 1.24 and 1.07, respectively; p < 0.001). With the severity of airflow obstruction, there were significant declines of BHI_mMCA and BHI_mBA in mild (0.94 and 0.83), moderate (0.8 and 0.7) and severe to very severe COPD (0.7 and 0.6), respectively (p < 0.001). For all participants, we found a significant and positive correlation between forced expiratory volume in one second (FEV₁) and BHI_mMCA (Rho = 0.761, p < 0.001) and between FEV₁ and BHI_mBA (Rho = 0.409, p < 0.001). COPD patients have impaired CVR in anterior and posterior cerebral circulation. Impairment of CVR increase with the airflow limitation severity. CVR is an appropriate marker to identify vulnerable COPD subjects at high risk to develop cerebrovascular disease. Prospective studies are needed for further evaluation.

What Is the Point of the Peak? Assessing Steady-State Respiratory Chemoreflex Drive in High Altitude Field Studies

Measurements of central and peripheral respiratory chemoreflexes are important in the context of high altitude as indices of ventilatory acclimatization. However, respiratory chemoreflex tests have many caveats in the field, including considerations of safety, portability and consistency. This overview will (a) outline commonly utilized tests of the hypoxic ventilatory response (HVR) in humans, (b) outline the caveats associated with a variety of peak response HVR tests in the laboratory and in high altitude fieldwork contexts, and (c) advance a novel index of steady-state chemoreflex drive (SS-CD) that addresses the many limitations of other chemoreflex tests. The SS-CD takes into account the contribution of central and peripheral respiratory chemoreceptors, and eliminates the need for complex equipment and transient respiratory gas perturbation tests. To quantify the SS-CD, steady-state measurements of the pressure of end-tidal (P_ET)CO₂ (Torr) and peripheral oxygen saturation (SpO₂; %) are used to quantify a stimulus index (SI; P_ETCO₂/SpO₂). The SS-CD is then calculated by indexing resting ventilation (L/min) against the SI. SS-CD data are subsequently reported from 13 participants during incremental ascent to high altitude (5160 m) in the Nepal Himalaya. The mean SS-CD magnitude increased approximately 96% over 10 days of incremental exposure to hypobaric hypoxia, suggesting that the SS-CD tracks ventilatory acclimatization. This novel SS-CD may have future utility in fieldwork studies assessing ventilatory acclimatization during incremental or prolonged stays at altitude, and may replace the use of complex and potentially confounded transient peak response tests of the HVR in humans.

Reactivity of larger intracranial arteries using 7 T MRI in young adults

The larger intracranial conduit vessels contribute to the total cerebral vascular resistance, and understanding their vasoreactivity to physiological stimuli is required when attempting to understand regional brain perfusion. Reactivity of the larger cerebral conduit arteries remains understudied due to a need for improved imaging methods to simultaneously assess these vessels in a single stimulus. We characterized reactivity of basal intracranial conduit arteries (basilar, right and left posterior, middle and anterior cerebral arteries) and the right and left internal carotid arteries, to manipulations in end-tidal CO₂ (PetCO₂). Cross-sectional area changes (%CSA) were evaluated from high-resolution (0.5 mm isotropic) images collected at 7 T using a T1-weighted 3D SPACE pulse sequence, providing high contrast between vessel lumen and surrounding tissue. Cerebrovascular reactivity was calculated as %CSA/ΔPetCO₂ in eight healthy individuals (18-23 years) during normocapnia (41 ± 4 mmHg), hypercapnia (48 ± 4 mmHg; breathing 5% CO₂, balance oxygen), and hypocapnia (31 ± 8 mmHg; via hyperventilation). Reactivity to hypercapnia ranged from 0.8%/mmHg in the right internal carotid artery to 2.7%/mmHg in the left anterior cerebral artery. During hypocapnia, vasoconstriction ranged from 0.9%/mmHg in the basilar artery to 2.6%/mmHg in the right posterior cerebral artery. Heterogeneous cerebrovascular reactivity to hypercapnia and hypocapnia was characterized across basal intracranial conduit and internal carotid arteries.

Mechanisms of sympathetic regulation during Apnea

Volitional Apnea produces a robust peak sympathetic response through several interacting mechanisms. However, the specific contribution of each mechanism has not been elucidated. Muscle sympathetic activity was collected in participants (n = 10; 24 ± 3 years) that performed four maximal volitional apneas aimed at isolating lung-stretch (mechanical) and chemoreflex drive: (Ainslie and Duffin ) end-expiratory breath-hold, (Ainslie et al. ) end-inspiratory breath-hold, (Alpher et al. ) prehyperventilation breath-hold, and (Andersson and Schagatay ) prehyperoxia breath-hold. A final repeated rebreathe breath-hold protocol was performed to measure the peak sympathetic response during successive breath-holds at increasing chemoreflex stress. Finally, the influence of dynamic ventilation was assessed through asphyxic rebreathe. Muscle sympathetic activity was calculated as the change in burst frequency (burst/min), burst incidence (burst/100 heart-beats), and amplitude (au) between baseline and prevolitional breakpoint. Rebreathe was analyzed at similar chemoreflex stress as inspiratory breath-hold. All maneuvers increased muscle sympathetic activity compared to baseline (P < 0.01). However, prehyperoxia exhibited a smaller increase (+22.18 ± 9.13 burst/min; +25.52 ± 11.7 burst/100 heart-beats) compared to inspiratory, expiratory, and prehyperventilation breath-holds. At similar chemoreflex strain, rebreathe sympathetic activity was blunted compared to inspiratory breath-hold (P < 0.01). Finally, muscle sympathetic activity was not different between the repeated rebreathe trials, despite elevated chemoreflex stress and lower breath-hold duration with each subsequent breath-hold. We have demonstrated an obligatory role of the peripheral, but not central, chemoreflex (prehyperventilation vs. prehyperoxia) in producing peak sympathetic responses. At similar chemoreflex stresses the act of dynamic ventilation, but not static lung stretch per se, blunts muscle sympathetic activity. Finally, similar peak sympathetic responses during successive repeated breath-holds suggest a sympathetic ceiling may exist.

Cerebrovascular Reactivity during Prolonged Breath-Hold in Experienced Freedivers

BACKGROUND AND PURPOSE

Experienced freedivers can endure prolonged breath-holds despite severe hypoxemia and are therefore ideal subjects to study apnea-induced cerebrovascular reactivity. This multiparametric study investigated CBF, the spatial coefficient of variation as a correlate of arterial transit time and brain metabolism, dynamics during prolonged apnea.

MATERIALS AND METHODS

Fifteen male freedivers (age range, 20-64 years; cumulative previous prolonged breath-holds >2 minutes and 30 seconds: 4-79,200) underwent repetitive 3T pseudocontinuous arterial spin-labeling and ³¹P-/¹H-MR spectroscopy before, during, and after a 5-minute breath-hold (split into early and late phases) and gave temporally matching venous blood gas samples. Correlation of temporal and regional cerebrovascular reactivity to blood gases and cumulative previous breath-holds of >2 minutes and 30 seconds in a lifetime was assessed.

RESULTS

The spatial coefficient of variation of CBF (by arterial spin-labeling) decreased during the early breath-hold phase (-30.0%, P = .002), whereas CBF remained almost stable during this phase and increased in the late phase (+51.8%, P = .001). CBF differed between the anterior and the posterior circulation during all phases (eg, during late breath-hold: MCA, 57.3 ± 14.2 versus posterior cerebral artery, 42.7 ± 10.8 mL/100 g/min; P = .001). There was an association between breath-hold experience and lower CBF (1000 previous breath-holds reduced WM CBF by 0.6 mL/100 g/min; 95% CI, 0.15-1.1 mL/100 g/min; P = .01). While breath-hold caused peripheral lactate rise (+18.5%) and hypoxemia (oxygen saturation, -24.0%), cerebral lactate and adenosine diphosphate remained within physiologic ranges despite early signs of oxidative stress [-6.4% phosphocreatine / (adenosine triphosphate + adenosine diphosphate); P = .02].

CONCLUSIONS

This study revealed that the cerebral energy metabolism of trained freedivers withstands severe hypoxic hypercarbia in prolonged breath-hold due to a complex cerebrovascular hemodynamic response.

Investigation of ischemic and demyelinating lesions by cerebral vasoreactivity based on transcranial Doppler sonography: a comparative study

PURPOSE

Variations of cerebral blood flow in response to hypoxia and hyperoxia in different disease conditions can provide new insights into disease etiopathogenesis. This study aimed to determine the characteristics of cerebral vasoreactivity for ischemia and demyelination.

MATERIALS AND METHODS

This case-control study included: 28 patients with lacunar infarctions verified by history, physical examination, and MRI; 28 age- and sex-matched healthy controls; 28 patients with relapsing-remitting multiple sclerosis (MS), based on McDonald criteria; and 28 age- and sex-matched healthy controls for the MS group. Transcranial Doppler sonography was undertaken in all subjects to calculate the mean flow velocity (MFV) of the right middle cerebral artery (MCA) and, after a breath-holding (BH) maneuver, the breath-holding index (BHI) was determined.

RESULTS

There was no significant difference of BHI and changes of MFV of the MCA in MS patients compared to controls (1.02 ± 0.4 vs 1.02 ± 0.3, p = 0.993; and 16.8 ± 8.1 vs 11.3 ± 10.8, p = 0.057). BHI in patients with lacunar infarctions was significantly lower (0.8 ± 0.4 vs 1.2 ± 0.3, p < 0.001) compared to controls. The BHI (p = 0.040) and variations of MFV of MCA (p = 0.007) in MS patients were significantly higher than in patients with lacunar infarctions. The vasoreactivity of demyelinating lesions was higher than that of ischemic ones.

CONCLUSION

Therefore, cerebral vasoreactivity determined by transcranial Doppler could be utilized for differentiating demyelinating from ischemic lesions.

Assessing chemoreflexes and oxygenation in the context of acute hypoxia: Implications for field studies

Carotid chemoreceptors detect changes in PO₂ and elicit a peripheral respiratory chemoreflex (PCR). The PCR can be tested through a transient hypoxic ventilatory response test (TT-HVR), which may not be safe nor feasible at altitude. We characterized a transient hyperoxic ventilatory withdrawal test in the setting of steady-state normobaric hypoxia (13.5-14% F_IO₂) and compared it to a TT-HVR and a steady-state poikilocapnic hypoxia test, within-individuals. No PCR test magnitude was correlated with any other test, nor was any test magnitude correlated with oxygenation while in steady-state hypoxia. Due to the heterogeneity between the different PCR test procedures and magnitudes, and the confounding effects of alterations in CO₂ acting on both central and peripheral chemoreceptors, we developed a novel method to assess prevailing steady-state chemoreflex drive in the context of hypoxia. Quantifying peak hypoxic/hyperoxic responses at low altitude may have minimal utility in predicting oxygenation during ascent to altitude, and here we advance a novel index of chemoreflex drive.